WO2003083419A1

WO2003083419A1 - Method and spectrometer for measuring a raman spectrum

Info

Publication number: WO2003083419A1
Application number: PCT/FI2003/000227
Authority: WO
Inventors: Pentti NIEMELÄ; Janne Suhonen
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2002-03-28
Filing date: 2003-03-25
Publication date: 2003-10-09
Also published as: FI115072B; FI20020612A; AU2003216947A1; FI20020612A0

Abstract

The invention relates to a method and a spectrometer for implementing the method of measuring a Raman spectrum, the method comprising generating at least two monochromatic optical excitation beams (330, 332) having known, different frequencies, generating a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and measuring a Raman spectrum induced by the excitation beams (330, 332) from the radiation (340, 342) emitted from each radiation target (382, 384) by using a silicon-based radiation detector. The method enables the measurement of a broad-band Raman spectrum at a good effective signal-to-noise ratio and enables the use of different measuring head constructions.

Description

METHOD AND SPECTROMETER FOR MEASURING A RAMAN SPECTRUM

FIELD

[0001] The invention relates to a method and a spectrometer suitable for wideband optical spectroscopy, particularly for Raman spectroscopy.

BACKGROUND

[0002] When monochromatic excitation radiation is directed to a Raman active material at measuring wavelength, polychromatic radiation, called Raman scattered radiation, is emitted from the material. The intensity of Raman scattered radiation is typically several orders of magnitude lower as compared with the intensity of the excitation radiation. Raman scattered radiation is generated by energy interchange between radiation quanta and the molecules in the material. The frequency of Raman quanta, which obtain energy from the molecules, is lower than the excitation frequency. Instead, the frequency of Raman quanta, which deliver energy to the molecules, is higher than the excitation frequency. Measurements of the Raman phenomenon yield information about the molecular structure of the material measured, for example.

[0003] In Raman spectroscopy, an excited energy state of the object measured is excited at a strong electromagnetic excitation frequency VE, which excited state is discharged emitting a Raman scattering frequency VR such that VR « VE-V_C, wherein v_c is a characteristic frequency of the object measured. The characteristic frequency is also generally called a Raman shift. In Raman spectroscopy, the aim is to define the Raman scattering frequencies v_R, from which the characteristic frequencies vc can be calculated provided the excitation frequency VE is known.

[0004] Usually characteristic frequencies vc are within the infrared range of the electromagnetic spectrum, the measurements carried out within which being technically demanding mainly because of the thermal noise of detectors and the low energy of the radiation quanta detected. In Raman spec- troscopy, the information contained in the characteristic frequency is transferred to the Raman scattering frequency v_R, which, by a suitable choice of the excitation frequency v_E, can be placed within the range of an electromagnetic spectrum that is easier to measure by spectroscopy, for instance within an optical spectral range. Raman spectroscopy is utilized for instance in quantitative analyses of the constituents in chemical samples, each constituent having a characteristic Raman spectrum.

[0005] Low resolution Raman spectrometers are preferably implemented by means of laser radiation sources, a grating spectrograph and matrix detectors, in which implementation the electromagnetic radiation measured is divided into wavelengths spatially to different parts of the detector.

[0006] In known CCD Raman spectrometers, an optical beam, containing the excitation frequency VE, is directed to the object measured. The object to be measured may be a solid-state material, gas, or a mixture thereof. In prior art solutions; the bandwidth of a Raman spectrum is determined mainly on the basis on the bandwidth of the characteristic spectrum of the object measured. When the characteristic spectrum of the object is determined by means of a CCD Raman scattering spectrum, the factor limiting the band often turns out to be the finite spectral response of the CCD detector, which weakens significantly outside the visible electromagnetic spectral range. This means that such measuring objects are difficult to detect, the Raman scattering frequencies associated with whose characteristic frequencies at a given excitation frequency are in the fringe areas or outside of the operating range of the CCD detector.

[0007] In known CCD Raman spectrometer implementations, the determination of a wide characteristic spectral band is subject to a sufficiently high excitation frequency VE, which causes multiple-component sample fluorescence spectrum in the spectral range to be measured, the spectrum generating a strong background component at high frequencies of the Raman spectrum thus impairing the effective signal-to-noise ratio of the Raman spectrum. [0008] Furthermore, prior art solutions require that the number of pixels in the CCD detector in the dimension measured correspond to the spatial bandwidth of the spectrograph and the desired resolution. In this case, the measurement of a wideband spectrum is subject to either a significantly large and expensive CCD detector or special optical and mechanical solutions. These include for instance adjustable gratings or the division of a grating into portions having different grating constants, whereby the different spectral ranges are imaged to the same detector. Some embodiments combine gratings and prisms, the different spectral ranges being combined by software. A common feature in said solutions for saving the number of pixels in a CCD de- tector is the use of mobile parts or optical components, which reduce the reliability of the spectrometer and increase its price. BRIEF DESCRIPTION

[0009] The object of the invention is to provide a measuring method for alleviating the above problems associated with the measurement of a broadband characteristic spectrum. This is achieved by a method of measuring a Raman spectrum, in which method monochromatic optical excitation radiation having a known frequency is used to measure an object to be measured, and the Raman spectrum characteristic of the object to be measured is measured by means of radiation emitted from the object to be measured. The method comprises generating at least two monochromatic optical excitation beams having known, different frequencies; generating a radiation target for each excitation beam in the object to be measured; and measuring a Raman spectrum induced by the excitation beams from the radiation emitted from each radiation target by using a silicon-based radiation detector.

[0010] The invention also relates to a spectrometer for measuring a Raman spectrum to implement the method, the spectrometer comprising an excitation line for directing monochromatic optical excitation radiation having a known frequency to an object to be measured, and a spectrum analyzer comprising a radiation detector, the spectrum analyzer measuring a Raman spectrum received with the radiation detector from the object to be measured and characteristic of the object to be measured by using radiation emitted from the object. The spectrometer is characterized in that the excitation line is configured to generate at least two monochromatic optical excitation beams having known, different frequencies, and that the excitation line is configured to generate a radiation target for each excitation beam in the object to be measured, and that the spectrum analyzer is configured to measure the Raman spectrum induced by the excitation beams from the radiation emitted from each radiation target by using a silicon-based radiation detector.

[0011] The preferred embodiments of the invention are disclosed in the dependent claims. [0012] The invention is based on the Raman spectrometer utilizing two or more excitation frequencies selected such that the desired Raman scattering frequencies VR corresponding to the characteristic frequency vc are within the effective operating range of the detector employed.

[0013] The solution of the invention provides a plurality of advan- tages. The use of two or more excitation frequencies and selecting each excitation frequency suitably enables optical transfer of the information included in the characteristic frequencies of the sample be measured to the desired spectral band of electromagnetic radiation. The method enables the implementation of the optical arrangement for measuring the broadband Raman spectrum with one apparatus optimized for measuring the desired spectral band. In this way, the solution simplifies the structure of the spectrometer and lowers the manufacturing costs of the spectrometer.

LIST OF THE FIGURES

[0014] In the following, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings, in which Figure 1 shows the mechanism of an emission shift by means of an energy level diagram,

Figure 2A shows the mechanism of a Raman spectrum, Figure 2B shows noise sources in Raman spectrum measurement, Figure 2C shows the mechanism of a Raman spectrum, Figure 3 is a diagram of a preferred embodiment of the solution presented,

Figure 4 shows an example of measuring the spectrum of optical radiation,

Figure 5A shows an example of implementing an optical measuring head,

Figure 5B shows a second example of implementing an optical measuring head,

Figure 5C shows a third example of implementing an optical measuring head, and Figure 5D shows a fourth example of implementing an optical measuring head.

DESCRIPTION OF THE EMBODIMENTS

[0015] The method presented is particularly suitable for measuring Raman scattering, but can be applied to any spectroscopic method wherein the band of the spectrum to be measured depends on the frequency of the radiation directed to the object to be measured.

[0016] Figure 1 shows the physical mechanism of an emission spectrum, such as a Raman spectrum, which at its simplest can be explained by means of an energy level system 100. The vertical axis 102 of the energy level system 100 shows the unit of energy, for example wavenumber. The wavenumber will be defined later. The energy level system 100 describes the property of the object measured of interacting with an external stimulus, such as electromagnetic radiation, directed thereto, its details depending on the physical properties and environment of the object measured. The object measured comprises one or more energy level systems of the kind shown in Figure 1, which may differ from each other.

[0017] Each energy level system 100 is composed of three or more energy level groups 120, 130 and 140, each comprising one or more energy sublevels 122, 132 and 142. The energy sublevels of energy level group 130 are virtual levels, typically having a very short lifetime and not necessarily being identifiable. However, as for the solution presented, identification is of no significance. In Figure 1 , for the sake of clarity, the order of magnitude of the distances between the energy sublevels in each energy level group 120, 130 and 140 is set smaller than the order of magnitude of the distances between the energy level groups, but this does not usually have to be the case.

[0018] Quantities definable in the energy level system 100 include at least three spectral transitions 126, 136 and 146, each of which occurs between an initial level and final level associated with each transition. In the solution presented, transition 146 is a computational auxiliary variable, which is not directly detected in the measurements, but which can be defined by means of transitions 126 and 136. Corresponding spectral transitions occur simultaneously or almost simultaneously in other energy level systems of the object and the detectable transition spectra characteristic of the object are generated as a combination of the spectral transitions. The initial level of a transition is an en- ergy sublevel of an energy level group 120 to 140, on which the object to be measured is before the spectral transition. The final level of a transition is similarly the energy sublevel of an energy level group 120 to 140, on which the object to be measured is after the spectral transition.

[0019] In a preferred embodiment of the solution presented, optic radiation is directed to the object to be measured, the radiation being called excitation radiation and the vibration frequencies of its electromagnetic field including two or more frequencies VH, called excitation frequencies. The band of the optical radiation is determined as the wavelength range 1 mm to 40 nm of the electromagnetic spectrum, corresponding to a wavelength band (in units of cm^"1) 10 cm^"1 to 250000 cm^"1 and which unambiguously correspond to optical frequencies. A wavenumber unit is defined as the reciprocal 1/λ of wave- length λ when wavelength λ is expressed in centimetres. In this context, the wavenumber unit is used as the unit of frequency and energy. Frequency in a hertz unit (Hz = 1/s) is obtained from the wavenumber unit by multiplying the wavenumber unit by coefficient 29979.2458 Hz/cm^"1, when a vacuum consti- tutes the optical medium.

[0020] As the energy VH of each excitation frequency approaches the difference between one or more energy sublevels 122 of two energy level groups 120 and one or more energy sublevels 132 of energy level group 130, the probability of a spectral transition 126 between energy sublevels 122 and 132 increases strongly. The object now receives an amount of energy corresponding to the excitation frequency VH, and settles at energy sublevel 132 of energy level group 130. Usually the object to be measured then tends to be transferred to energy sublevel 142 of energy level group 140. This may also be preceded by a transition inside energy level group 140 from one energy sub- level to another, which is not detected in the measurements. The frequency VR of the emitted radiation, in this context called Raman frequency and constituting the quantity to be measured, corresponds to the difference 136 between energy sublevels 132 and 142. At the same time, fluorescence radiation having an almost continuous spectral distribution is emitted from the object, but its mechanism is not shown in Figure 1. Figure 2B shows the characteristics of fluorescence radiation in detail.

[0021] Energy level group 140 and the energy sublevel structure of the energy level group are typically caused by energy forms, which are caused by the elementary structure, such as for instance the molecular structure, of the object, the energies associated with which are known and which characterize the object to be measured or parts thereof in a manner allowing the object measured or parts thereof to be identified. Characteristic frequencies vc can be determined as the difference between the employed excitation frequency VH and the detected Raman frequency VR. [0022] Figures 2A to 2C show the generation of a Raman spectrum according to a preferred embodiment of the solution presented, when optical radiation including excitation frequencies is directed to the object measured, the object being Raman active. In Figures 2A to 2C, horizontal axis 202 shows the frequencies of transitions resembling the spectral transitions 126, 136 and 146 shown in diagram 1 on a wavenumber scale. Vertical axis 200 shows the intensity of each transition allowing each transition to be graphically identified between pictures. The intensities are not real intensities. The origins and scaling of axes 200 and 202 are selected arbitrarily.

[0023] Figure 2A shows at least two characteristic spectral lines 204, 206, an excitation spectral line 212 and at least two Raman spectral lines 208, 210 caused by excitation frequency 212. In this context, a quantity detected in the measurement is a spectral line, from which the frequency can be unambiguously determined in a manner known per se. This means that the spectral lines can be understood as frequencies and vice versa. The characteristic spectral lines 204, 206 are hypothetical lines that can also be understood as two or more spectral bands that have different frequencies and are possibly partially overlapping, and each of which includes at least one characteristic frequency. Indicator 214 shows at least one Raman frequency 208 generated from characteristic frequency 204. Indicator 216 indicates at least one Raman spectral line 210 corresponding to characteristic frequency 206. Indicators 214, 216 also show how characteristic spectral frequencies 204, 206 are obtained from the detected Raman frequencies 208, 210. Indicators 218, 220 show at least two Raman spectral lines 208, 210 induced by excitation frequency 212. By suitable selection of excitation frequency 212, the preferred embodiment of the solution presented enables the shift of spectral group 222 of lines on fre- quency scale 202.

[0024] Figure 2B shows curves including a response curve 224 of a radiation indicator of the type described in the solution presented, and at least two fluorescence radiation spectral distributions 226, 228, which are obtained by directing at least two excitation frequencies 212 of Figures 2A, having dif- ferent frequencies but similar intensities, to the object to be measured. Radiation indicator response curve 224 describes the ability of the radiation detector to generate a signal, from which the intensity of the radiation directed to the detector can be determined. Response curve 224 can also be understood as an optical response curve of the spectrometer according to the solution pre- sented, the curve describing the ability of the entire measuring device to transform an optical signal into a measurable form. Consequently, curve 224 can be interpreted as spectral information generated by the measuring device, when radiation having an almost even spectrum distribution is directed to the measuring device. [0025] Of the at least two fluorescence curves 226, 228, curve 228 corresponds to excitation frequency 212 of Figure 2A, for example. The excita- tion generating fluorescence curve 226 is higher than the excitation frequency 212. The fluorescence curves describe the frequency distribution of the fluorescence radiation emitted from the object, of which at least part can be located in the frequency band of the Raman spectrum in accordance with Fig- ures 2A and 2B. Fluorescence spectrum 226 generates a background component in the detectable Raman spectrum, under which the Raman spectrum may remain partly or entirely. The effects of the fluorescence spectrum are difficult to eliminate, and thus the fluorescence spectrum can be understood as one factor affecting the signal-to-noise ratio of the spectrum measurement. [0026] Figures 2A and 2B show that the excitation frequency 212 according to Figure 2A enables the determination of Raman spectral line 210, since Raman spectral line 210 is located in a favourable part of detector response curve 224, and thus fluorescence curve 228 corresponding to excitation frequency 212 does not cover Raman spectral line 210. However, at exci- tation frequency 212, the at least one Raman spectral line 208 corresponding to characteristic frequency 204 remains in an unfavourable range of the radiation detector response curve 224, and is not detected with sufficient intensity. If excitation frequency 212 is raised sufficiently, group 222 of lines can be shifted to a higher frequency. In this case, Raman spectral line 208 shifts to a favour- able part of the radiation detector response curve 224, but at the same time, Raman spectral line 210 is covered under fluorescence curve 226 corresponding to a higher excitation frequency, and remains undetected. In other words, raising the excitation frequency does not increase the spectral band to be measured. [0027] Figure 2C shows a preferred embodiment of the solution presented, wherein two or more excitation frequencies 212, 234 are used instead of one excitation frequency, whereby fluorescence curve 226 corresponds to excitation frequency 234. In this embodiment, excitation frequency 234 generates a Raman spectral line 230 corresponding to characteristic fre- quency 204 in accordance with indicators 236 and 232 in a favourable operating range of radiation detector response curve 224. In this case, Raman spectral lines 230, 210 corresponding to each characteristic frequency 204, 206 can be detected with the same detector in accordance with response curve 224, and characteristic frequencies 204, 206 can be determined when Raman spec- tral lines 210, 230 and excitation frequencies 212, 234 corresponding thereto are known in accordance with indicators 216, 232, 220 and 236. The detect- able effective fluorescence curve is composed of part 238 corresponding to excitation frequency 212 and part 240 corresponding to excitation frequency 234. The fluorescence curves 238, 240 in Figure 2C also take into account radiation detector response curve 224. The above example shows that by se- lecting suitable excitation frequencies, the effective signal-to-noise ratio of the frequency band measured can be affected. In this case, the effective signal-to- noise ratio can be determined as the ratio of effective signal to effective noise, wherein the effective signal is determined for instance by the strengths of the Raman spectral lines located on each frequency band. Effective noise, in turn, is composed of noise generated in the spectrometer, such as detector noise, and noise generated by the fluorescence of the sample. In an embodiment, the excitation frequencies are selected in a manner optimizing the effective signal- to-noise ratio of the measurement, whereby the selection of the excitation frequencies allows at least one Raman spectral line to be placed at a point in the detectable spectral band allowing the effective signal-to-noise to be maximized. The explanation for this is that increasing the excitation frequency decreases the noise caused by the spectrometer, but, on the other hand, increases the noise caused by the fluorescence, whereby an excitation frequency exists that allows the effective signal-to-noise ratio to be maximized. [0028] In an embodiment of the solution presented, the radiation generated by a radiation source is used to form at least two monochromatic excitation frequencies 212, 234 having known, different frequencies, which enable the detection of the Raman spectral lines corresponding to the different frequencies of the object measured. Each excitation frequency 212, 234 is then directed to the object to be measured separated either in time or in place, in a manner allowing the spectra generated at different excitation frequencies to be detected separately from each other to eliminate fluorescence effects. The Raman spectral lines 210, 230 corresponding to each excitation frequency 212, 234 are then determined from the radiation emitted from the object, the excitation frequencies 212, 234 and Raman spectral lines enabling the determination of the characteristic frequencies characteristic of the object to be measured. The characteristic frequencies can be used to determine for instance the chemical composition of the object to measured.

[0029] Let us study an embodiment of the solution presented, wherein the object to be measured contains paper coating pasta. The Raman active main components of paper coating pasta include carbonate, kaolin and SB latex. The main components of the characteristic spectrum of carbonate are 278 cm^"1, 712 cm^"1 and 1084 cm^"1. The main components of kaolin are 430 cm^"1, 474 cm^"1, 3620 cm^"1 and 3700 cm^"1. The main components of SB latex are 1002 cm^"1, 2940 cm^"1 and 3060 cm^"1. The characteristic spectral band to be measured based on the above spectral lines is composed of a lower part 200 cm^"1 to 1100 cm^"1 and an upper part 3000 cm^"1 to 4000 cm^"1. The desired measuring range is between 10000 cm^"1 to 20000 cm^"1, which corresponds to a wavelength range between 1000 nm and 500 nm. In an embodiment of the solution presented, the characteristic spectrum of the object is determined us- ing two excitation frequencies. In this case, the lower part of the characteristic spectrum is determined at excitation frequency 12048 cm^"1 (830 nm), whereby the Raman spectral band becomes 10784 cm^"1 to 11648 cm^"1. This enables the detection of carbonate lines 712 cm^"1 and 1084 cm^"1, and SB latex line 1002 cm^"1, whereas weak kaolin lines 430 cm^"1 and 474 cm^"1 remain entirely covered by the kaolin fluorescence spectrum. The upper part of the characteristic spectrum, in turn, is determined by using excitation frequency 14286 cm^"1 (700 nm), the detectable Raman spectrum being measured in the band 10286 cm^"1 to 11286 cm^"1. In this case, kaolin lines 3620 cm^"1 and 3700 cm^"1 and SB latex lines 2940 cm^"1 and 3060 cm^"1 are located in the detectable spectral range. Thus, at least one measurement is performed on all components of the paper coating paste. In another embodiment of the solution presented, the excitation frequencies are 12739 cm^"1 (785 nm) and 14706 cm^"1 (680 nm). In this case, the detectable Raman spectral bands are 11639 cm^"1 to 12539 cm^"1 , corresponding to excitation frequency 12739 cm^"1 and the lower part of the char- acteristic spectrum, 200 cm^"1 to 1100 cm^"1. Raman spectral band 10706 cm^"1 to 11706 cm^"1 corresponds to excitation frequency 14706 cm^"1 and the upper part of the characteristic spectrum. In the solution presented, the composition of the paper coating paste can be determined by measuring one or more spectral lines of the main components of said paper coating paste. [0030] The above mechanism of an emission spectrum was illustrated only to specify and clarify the terminology required for illustrating the solution presented. It is therefore apparent that the solution presented is not limited to the details described therein.

[0031] Figure 3 shows a diagram of a preferred embodiment of the solution presented for measuring a Raman spectrum. The measuring arrangement comprises a spectrometer 300, an object 380 to be measured, and a system 364 for processing, managing and storage of the measurement results.

[0032] The spectrometer 300 comprises an excitation line 326 for generating optical radiation 320, 322 and directing it to an object 380, and a spectrum analyzer 368 for analyzing optical radiation 340, 342 emitted from the object 380. The excitation line 326 comprises a radiation source 310 and transmitter optics 324. The spectrum analyzer 368 comprises reception optics 350 for applying the optical radiation 340, 342 emitted from the object 380 measured to a spectrograph 356, wherein the optical radiation emitted from the object 380 is divided into different frequencies. In addition, the spectrum analyzer 368 comprises a detector 362 for detecting optical radiation.

[0033] A data processing device 364 converts the quantity expressing the amount of light generated by the detector 362 into a numerical form, generates spectrum data and stores the spectrum data. In a preferred em- bodiment of the solution presented, the data processing device 364 is a computer whereto are coupled the required converters for converting an analog signal into digital form, the required computer programs and the required user interfaces.

[0034] In an embodiment, the radiation source 310 is configured to be able to produce monochromatic optical radiation 320, 322 having at least two different frequencies, each having at least the desired intensity. The radiation source 310 comprises at least one optical power source 312, 314.

[0035] In principle, monochromatic radiation includes only one frequency and thus only one wavelength. However, at present, no optical radia- tion source produces such radiation, but, for instance, the monochromatic radiation of the best laser is a narrow spectral band instead of one wavelength. The excitation radiation bandwidth can be determined for instance as the full width at half maximum (FWHM) of the spectral line corresponding to the excitation frequency. In this context, monochromatic excitation radiation means such a radiation spectral band (= frequency band), which is of the same magnitude or narrower than the resolution of the spectrometer used in the measurement.

[0036] The at least two optical radiations 320, 322 generated by the radiation source 310 are directed to the transmitter optics 324, which gener- ates excitation beams 330, 332 from the optical radiations 320, 322. In this context, the concept beam is a form of radiation or part thereof, with which controllability is associated. Because of optical filtering, a beam may also deviate from the radiation associated therewith. Thus, the excitation beams 330, 332 are said to be generated by the radiation source 310.

[0037] In a preferred embodiment, the frequencies of the excitation beams 320, 322 generated by the optical power sources 312, 314 are selected in a manner allowing the information content, of the Raman spectra induced by the excitation beams 330, 332 to cover the desired characteristic spectral range of the object 380. The frequencies of the excitation beams 330, 332 correspond to the excitation frequencies 212, 234 shown in Figure 2C, for exam- pie.

[0038] In another embodiment, the frequencies of the excitation beams 320, 322 generated by the radiation source 310 are of the desired magnitude for instance when the Raman spectrum corresponding to each excitation beam 330, 332 is located in the desired optical frequency band. The de- sired frequency band may be determined for instance by the desired operating range of the spectrometer 300, which again can be determined for instance by the response curve 224 of the radiation detector 362 employed.

[0039] In an embodiment, the frequencies of the excitation beams 320, 322 generated by the radiation source 310 are of the desired magnitude when the amount of fluorescence radiation emitted from the object 380 at the desired frequency band to be measured is controlled. In this case, for instance the fluorescence spectrum corresponding to each excitation beam 330, 332 is at minimum in the desired spectral part.

[0040] In a preferred embodiment of the solution presented, the fre- quencies of the excitation beams 330, 332 are selected such that the frequency difference of the excitation beams 320, 322 is at least in the order of magnitude of the resolution of the spectrometer 300. The frequency difference of the excitation beams 330, 332 can be also determined from the fact that the different excitation beams 330, 332 are capable of producing a Raman spec- trum having a different spectral information content, each spectrum being located in the desired frequency band. The spectral information content includes the information obtained from the characteristic spectrum of the object at each particular excitation frequency.

[0041] The intensity of the radiation 320, 322 generated by the ra- diation source 310 is of the desired magnitude for instance when the Raman scattering achieved by each excitation radiation is of the desired strength. The intensity of the Raman scattering is affected not only by the intensity of the excitation radiation, but also the Raman cross section of the object 380, which typically depends on the Raman cross section and content of the Raman- active constituents of the object. The strength of Raman radiation must also fulfil the requirements set by the spectrometer 300, which are determined for instance by the optical response of the detector 362, the optical capacity of the spectrograph 356, the desired resolution of the measurement, and the targeted measuring time.

[0042] In an embodiment of the solution presented, the radiation source 310 comprises at least two monochromatic optical power sources 312, 314, each of which produces at least one excitation beam 330, 332.

[0043] In another preferred embodiment of the solution presented, the radiation source 310 comprises at least one optical power source 312, at least two optical excitation beams 330, 332 being generated by adjusting the frequency of said source.

[0044] In still another preferred embodiment of the solution presented, the radiation source 310 comprises at least one optical power source 312, which is capable of generating radiation having at least two different frequencies, from which the excitation beams 330, 332 are selected. [0045] In a preferred embodiment of the solution presented, the optical power sources 312, 314 are laser radiation sources. In this embodiment, the power sources 312, 314 are for instance diode lasers, whose bandwidth is typically 0,1 cm^"1 to 1 cm^"1 and whose radiation power varies between 100 mW and 300 mW. The laser radiation sources may also be gas lasers, dye lasers and chemical lasers. In a preferred embodiment, the optical power source is a laser arrangement, wherein at least one laser optically pumps another laser, wherein the occurring induced emission constitutes the radiation employed. The adjustability of the frequencies can be implemented for instance by means of diode lasers or lasers utilizing non-linear optics. [0046] The at least two beams 320, 322 produced by the radiation source 310 at different frequencies are transmitted to transmitter optics 324. In an embodiment, the transfer of at least two beams 320, 322 from the radiation source 310 to the transmitter optics 324 occurs along optical fibres, but beam transfer may also take place in a free space, such as air, or in a vacuum. [0047] From the at least two radiations 320, 322 having different frequencies and directed thereto, the transmitter optics 324 generates at least two beams 330, 332 that have different frequencies and can differ from the radiations 320, 322 for instance as a result of optical filtering, polarization or direction. In a preferred embodiment of the solution presented, the transmitter optics 324 comprises optical filters for optical filtering of the optical radiations 320, 322. The transmitter optics 324 may also comprise optics that are configured to focus optical beams and generate the beams 330, 332 from the radiations 320, 322. In a preferred embodiment of the solution presented, the transmitter optics comprises reflective optics for generating the beams 330, 332 from the radiations 320, 322. [0048] In an embodiment of the solution presented, the beams 330,

332 can be led out of the transmitter optics 324 along optical fibres to the object 380, but the medium may also be air or a vacuum.

[0049] The object 380 to be measured comprises at least two radiation targets 382, 384, which at least two beams 330, 332 generate in the object 380. The radiation targets 382, 384 per se are part of the object 380, but their places and dimensions are determined from the beams 330, 332. Consequently, one speaks of the radiation targets 382, 384 of the object 380. At least two radiation targets 382, 384 of the object 380 have spectral characteristics of the kind shown in Figure 1. The beams 330, 334 induce a Raman phenome- non in the object 380. The spectral characteristics of the kind shown in Figure 1 of at least two radiation targets 382, 384 may be different. The shape and size of at least two radiation targets 382, 384 are determined for instance by the diameters and shapes of the beams 330, 332 directed thereto. In the solution presented, the diameter of the radiation target is between 0.1 mm and 1 mm, but the solution presented is not limited thereto.

[0050] In a preferred embodiment of the solution presented, the transmitter optics have at least two operating modes, which determine the mutual modulation of the beams 330, 332 at different frequencies, directed to the object 380. [0051] In a space division mode, the transmitter optics are configured to generate at least two space-divided excitation beams 330, 332 at different frequencies, which are directed simultaneously to the radiation targets 382, 384. In this case, the radiation targets 382, 384 are substantially separate from each other. [0052] In a time division mode, the beams 330, 332 at different frequencies are directed to the radiation targets 382, 384, respectively, at differ- ent times. In this case, the radiation targets 382, 384 may overlap partly or entirely.

[0053] In the space division mode, at least two targets 382, 384 are substantially separate from each other. In a preferred embodiment of the solu- tion presented, the distance 386 between the at least two radiation targets 382, 384 is determined from the detected fluorescence spectrum 226. In a preferred embodiment of the solution presented, the distance 386 between the at least two radiation targets 382, 384 is 1 cm.

[0054] The reception optics 350 receives at least two optical radia- tions 340, 342 emitted from at least two radiation targets 382, 384, the radiations corresponding to the Raman frequencies 210, 230 of Figure 2C, for example. The reception optics 350 may comprise focusing optics required for processing the optical radiations 340, 342, optical filters and mirrors that may be semi-permeable. The at least two beams 352, 354 generated by the recep- tion optics from the optical radiations 340, 342 are applied to the spectrograph 356 for instance along an optical fibre, but the optical medium may also be air or a vacuum.

[0055] In the space division mode, the reception optics 350 collects simultaneously at least two space-divided optical radiations 340, 342 from the radiation targets 382, 384. In a preferred embodiment of the solution presented, at least two radiations 340, 342 are filtered optically to eliminate the desired frequency components, such as a component of the size of the excitation frequency. The at least two thus generated space-divided beams 352, 354 are applied to the spectrograph 356. [0056] In a preferred embodiment of the time division mode, the reception optics 350 collect at least two time-divided optical radiations 340, 342 from the radiation targets 382, 384 at different times. In a preferred embodiment of the time division mode, the radiations 340, 342 propagate along the same path, whereby they are not space-divided. In a preferred embodiment of the solution presented, optical filtering is directed to at least two radiations 340, 342 to eliminate the desired frequency components. The thus generated at least two time-divided beams 352, 354 are applied to the spectrograph 356.

[0057] Figure 4 shows a spectrograph 356 and a detector 362 according to a preferred embodiment of the solution presented. At least one beam 352 generated by the reception optics 350 is applied to an inlet 410 in the spectrograph 356, from where the beam 352 is applied to a dispersion element 420. The dispersion element 420 divides the arrived radiation 352 into frequencies thus enabling the elimination of the spectral distribution 358 of the radiation 352 from the dispersion element 420 in a manner allowing the different frequencies of the spectral distribution to be spatially separated from each other with the detector 362. The dispersion element may be for instance a transmission grating, a reflection grating or a holographic transmission grating. Each spectral distribution 358 leaving the dispersion element 420 may include at least one Raman spectral line 230, but the spectral distribution 358 may also not include spectral lines. [0058] The detector 362 receives at least two spectral distributions

358, 360 onto a surface that is sensitive to optical radiation. The detector 362 measures the amount of light directed thereto and generates a magnitude descriptive of the amount of light. In a preferred embodiment of the solution presented, the detector 362 comprises at least one detector element row 450, which comprises at least one detector element 460. The detector element may be for instance a pixel of a matrix detector. Each detector element 460 is a silicon-based semiconductor. In a preferred embodiment of the solution presented, the detector element 460 of the detector 362 is a silicon-based multiple element detector, such as a CCD detector (Charge Coupled Device). In an- other embodiment, the detector elements 460 of the detector are silicon-based diodes. In some cases, the detector 362 may also be a CID detector (Charge Injection Device) pixel, whose structure resembles a CCD cell, but wherein each pixel can be read individually. In the detector 362, the magnitude descriptive of the amount of radiation is typically a change in the electric state of the detector element. Such an electric state is for instance the resistance of the element or voltage or current induced in the element.

[0059] In the space division mode of the solution presented, at least two space-divided beams 352, 354 are applied to the inlet 410 of the spectrograph 356 such that at least two spectral distributions 358, 360 leaving the dis- persion element hit at least two detector element rows 440, 450 of the detector 362. In this case, only one spectral distribution 358, 360 is directed to each detector element row 450. Each spectral distribution 358, 360 may be divided into more than one detector element row 450.

[0060] In the time division mode, at least two time-divided optical beams 352, 354 are applied to the inlet 410 of the spectrograph 356 such that at least two spectral distributions 358, 360 leaving the dispersion element 420 hit at least one detector element row 440, 450 of the detector 362 at different times. Each spectral distribution 358, 360 corresponds to the excitation frequency employed. In this case, at least one spectral distribution 358, 360 is directed to each detector element row 450. Each spectral distribution 358, 360 can be divided into more than one detector element row 450. In a preferred embodiment of the time division mode, the optical beams 352, 354 are applied to the inlet 410 of the spectrograph 356 such that all spectral distributions 358, 360 hit at least one detector element row 450 of the detector at different times. In this case, the same spectral distribution 358 is directed to every one or more detector element rows 450 at each particular moment.

[0061] In the space division mode, the spectral distribution 358, 360 directed to each detector 362 element row 440, 450 is registered at the same time in a memory means of the data processing device 364, which may be for instance a random access memory (RAM) or a hard disk. The spectral data of each at least one spectral distribution 358 are stored in the memory means in a manner allowing the spectral data of each spectral distribution 358, 360 to be read independently of the spectral data of the other one or more spectral distributions 358, 360. One excitation frequency corresponds to each spectral distribution 358, 360. [0062] In the time division mode, the spectral distribution 358, 360 directed to each detector 362 element row 450 is registered at different times in the memory means of the data processing device 364. The spectral data of each at least one spectral distribution 358, 360 are stored in the memory means in a manner allowing the spectral data of each spectral distribution 358, 360 to be read independently of the spectral data of the other one or more spectral distributions 358, 360.

[0063] In a preferred embodiment of the solution presented, the spectral data of each at least one spectral distribution 358, 360 stored in the memory means of the data processing device can be combined in the form of a characteristic spectrum. In this case, the excitation frequency corresponding to each spectral distribution must be known. In a preferred embodiment of the solution presented, each spectral distribution 358 can be identified to its excitation frequency based on the location of the memory space of the memory means of the data processing device where said spectral data are stored. In this case, each excitation frequency is identified to said location in the memory space. Herein, the characteristic spectra determined at different excitation fre- quencies are combined by using one or more characteristic spectral lines that can be determined at two or more excitation frequencies.

[0064] In an embodiment, the transmitter optics 324 and the reception optics 350 are configured in such a way that the transmitter optics 324 and the reception optics 350 together constitute a measuring head 370, which is arranged to generate a radiation target 382, 384 for each excitation beam 330, 332 in the object 380 to be measured and to transfer the optical radiation 340, 342 emitted from each radiation target 382, 384 to the measurement. In an embodiment, the measuring head 370 comprises at least one optical guide for transferring the excitation beams 320, 322 to the measuring head 370 and a second optical guide for transferring the optical radiation 340, 342 emitted from the object 380 to be measured to the measurement. The optical guides are for instance optical fibres, and the measurement is performed with the spectrograph 356. Preferred embodiments 500A, 500B, 500C and 500D of the meas- uring head 370 are shown in Figures 5A, 5B, 5C and 5D. Optical guides enable the use of a measuring head several meters away from the spectrograph 356 and the radiation source 310.

[0065] Let us next study embodiments 500A, 500B, 500C and 500D of the measuring head 370. The measuring head 500A in Figure 5A shows a general measuring head construction, on which the embodiments 500B, 500C and 500D shown in Figures 5B, 5C and 5D are largely based. The same reference numbers are used in Figures 500A, 500B, 500C and 500D when the object of the reference number remains substantially unchanged.

[0066] Let us next study the use of the measuring head 500A in the space division mode, wherein at least two separate measuring heads are needed; one for each excitation beam 320, 322. In an embodiment, an excitation beam 320 is first applied to one measuring head 500A along an optical guide 502, then applied to a lens 504, which further directs the excitation beam 320 to the optics of the measuring head 500A. In an embodiment, the measur- ing head 500A comprises a filter 506, by means of which the frequencies that may be located in the range of the Raman spectrum to be measured are filtered off from the optical beams 320 generated by the radiation source 310, the frequencies including for instance different modes of laser radiation sources and possibly also components of Raman radiation generated in the optical guide 502. The filter 506 may be for instance a narrow band-pass filter. The same effect is achieved also with a narrow-band notch filter, such as a holographic notch filter, which is located on the surface of the mirror 508 or a beam splitter 510. The optical radiation is then reflected by means of the mirror 508 to the beam splitter 510, which directs the excitation beam 330 to a lens 512, which generates the radiation target 382 from the beam 330. [0067] The radiation emitted from the object 380 is gathered with the lens 512, and the collimated optical radiation propagates through the beam splitter 510 to a second lens 514, which generates the beam 352 to be applied to the spectrograph 356. In another embodiment, the beam splitter 510 is a semi-permeable mirror. Before propagating to the lens 514, the focused radia- tion is filtered with a filter 516, which eliminates Rayleigh scattering at the excitation radiation frequency leaving the frequencies of the Raman radiation 352. [0068] In a preferred embodiment, the filter 516 is a narrow-band notch filter, e.g. a holographic notch filter. The beam splitter 510 may also be a holographic notch filter, which then acts as a dichroic beam splitter transmitting the Raman spectrum and efficiently reflects the frequency of the excitation beam. Advantages of this solution include smaller optical losses and a more efficient attenuation of Raleigh scattering.

[0069] In the space division mode, the second excitation beam 322 is applied to the measuring head 500A according to Figure 5A, whose filters 506 and 516 are selected to correspond to the frequency of the excitation beam 322. The excitation beam 322 generates, in the sample 380, a radiation target 384, which is substantially separate from the radiation target 382 generated by the excitation beam 320, since the generated Raman spectra 352 and 354 are measured at the same time. [0070] In an embodiment of the time division mode, the measuring head 370 can be constructed with the measuring head 500B according to Figure 5B. The at least two monochromatic beams 320, 322 generated in the radiation source 310 at different frequencies are applied at different times to at least two inputs 610, 622 of the optical measuring head 500B, which are opti- cal fibres, for example. In this case, the radiation targets 382, 384 constituted by the at least two excitation beams 330, 332 overlap partly or entirely. In a preferred embodiment of the solution presented, the at least two monochromatic radiations 320, 322 generated in the radiation source 310 at different frequencies are alternately applied to at least two different inputs 610, 622 in the optical measuring head 500B. In this case, the time-modulation of the beams 320, 322 is performed at the front ends of the optical fibres 610, 622 acting as inputs, near the radiation source 310, such as a laser.

[0071] The propagation of the excitation beams 320, 322 in the measuring head 500B corresponds to the propagation of the excitation beam in the space-divided measuring head 500A. The excitation beams 320, 322 are coupled by means of special beam splitters 620, 630 to a common collection optics axis between lenses 612 and 626. The beams 340, 342 emitted from radiation targets 382, 384 propagate via the same optics to an optical fibre 552, wherein the beams propagate to the spectrograph 356. In a time-divided measuring head, the same components can be used as in the corresponding pair of space-divided measuring heads, except that in this case they are packed in the same casing.

[0072] The general measuring head construction 500A of Figure 5A can also be used in the time division mode of two or more lasers, when the notch filter 516 is replaced with a high-pass filter, which transmits Raman ra- diation emitted from the object 380, but eliminates excitation wavelengths. In addition, the band-pass filter 506 is replaced with an abrupt low pass filter, which only just lets through the highest excitation wavelength, but eliminates emissions arriving at the Raman spectrum area. In this case, all excitation beams 330, 332 propagate to the sample and the back-scattered (Rayleigh) beams are absorbed in the high-pass filter 516. In this connection, the high- pass or low-pass bands of the filters are determined in the bands in terms of wavelengths. Consequently, the transmission of for instance a high-pass filter is high at long wavelengths.

[0073] In a preferred embodiment, the high-pass filter 516 is an ab- sorbing filter based on a direct bandcap semiconductor material, whose use in filtering Rayleigh scattering in a Raman measuring head and abilities to simplify a time-divided measuring head construction are studied next. In the case of one excitation beam, measuring head constructions based on said semiconductor filter are previously described in Finnish patent application No. 20002250.

[0074] The operation of a direct bandcap semiconductor material is based on the absorption of the material, a thin disc made from it operating as an almost ideal high-pass filter: attenuation in the cut-off region is high and the change in the transmission at the cut-on wavelength abrupt. Such a material is cadmium telluride (CdTe), which is suitable for use with an 830-nm laser.

[0075] The measuring head 500C of Figure 5C represents a pre- ferred embodiment of a measuring head construction employing the time division mode of two or more lasers based on a semiconductor filter 516. Excitation beams are denoted by reference numbers 320, 322, and the beams emitted from the object 380 are denoted by reference numbers 340, 342. In the construction, the optical fibre 502 on the excitation side is substantially thinner than the fibre 552 on the collection side, wherefore the excitation beams 330, 332 between the lenses 512 and 514 are narrower, and a small surface mirror 518 can be used for directing them, which mirror blocks only a small part of the beams 340, 342 collected by the lens 512 from the object 380. This eliminates problems caused by beam splitters, which include the high losses of a semi- permeable mirror and the high price and poor availability of a dichroic beam splitter. A semiconductor filter 516, in turn, can also be placed in a focused beam, whereby a smaller disc is sufficient. Once the excitation beams 330, 332 propagate along the same path in the measuring head, they form overlap- ping radiation targets 382, 384 in the object 380, and there is no need for a separate tuning.

[0076] The measuring head 500D shown in Figure 5D represents a second preferred embodiment, allowing the structure of the measuring head 500C to be further simplified and its size reduced. The excitation radiation 320, 322 time-divided from the radiation source 310 arrives along the optical guide 502, such as a fibre, to an optical arrangement 520, which is substantially at the optical axis of the measuring head. The optical arrangement 520 will be described in detail later. The radiation 330, 332 collimated from the optical arrangement 520 propagates through a low-pass filter in the optical arrangement 520 to a lens 512, which constitutes overlapping radiation targets 382, 384 in the object 380. The radiation 340, 342 emitted from the object 380 is collected and directed with lens 512 towards lens 514, which focuses the radiation 340, 342 to an optical guide 552, which again applies the radiation for measurement in the spectrograph 356. [0077] The optical arrangement 520 comprises a collimating GRIN lens (GRaded INdex) having a graded refractivity coefficient, which collimates the radiation 320, 322 coming from the optical guide 502. In addition, the optical arrangement 520 comprises a low-pass filter in front of the GRIN lens, the filter corresponding to the filter 506 of the measuring head 500C. Furthermore, the optical arrangement 520 comprises a semiconductor filter disc of the size of the lenses 512, 514, the GRIN lens and the low-pass filter being placed in a hole in the middle of the disc. The diameter of the GRIN lens is small, whereby it only blocks a small part of the beams 340, 342 collected by the lens 512 from the object 380.

[0078] In the space division mode, the spectrometer comprises at least two measuring heads, one excitation beam 320 being directed to each of them. The structure of the measuring head is not decisive, so long as its filtering is selected to correspond to the frequency of the excitation beam. Herein, a space-divided measuring head is described by the general measuring head construction 500A. An optical beam 330 generated by each optical measuring head 500A is directed to different radiation targets 382, 384, substantially separated from one other physically, of each object to be measured. A beam of rays 354 generated by each measuring head 500A from its radiation target 382, 384 is led to the inlet of the spectrograph 356 in such a manner that each beam of rays 354 is directed to the dispersion element 420 of the spectrograph 356 such that the Raman spectrum 358, 360 measured from each radiation target 382, 384 is generated on a different row 440, 450 in the detector 362. The spectra 360, 358 measured from the different optical measuring heads 500A can be combined in the form of a characteristic spectrum by software in a data processing device 364. A space-divided measurement can be per- formed as a continuous measurement or for instance in periods of 10 seconds.

[0079] In the time division mode, the spectrometer comprises one measuring head, herein represented by the more general construction 500B and the constructions 500C and 500D based on an absorbent semiconductor filter. At least two excitation beams 320, 322 are directed to the measuring head separated from each other in time. The beams 352, 354, corresponding to each excitation beam 320, 322 and collected from the object, are directed to the inlet 410 of the spectrograph 356 in such a manner that the beams 352, 354 are directed to the spectrograph's dispersion element 420, which generates the spectral distribution 360. In the time division mode, the Raman spec- tra corresponding to the different excitation beams 330, 332 can be detected on the same at least one detector element row 440 in such a manner that the signals from the detector elements 360 are registered separated in time in an order corresponding to the sequence of the different excitation beams 320, 322. In an embodiment, a measuring sequence is performed at two different excitation frequencies, wherein the measurement takes place for instance during one second at each excitation frequency, the sequence including totally five measurements at each excitation frequency. In this case, totally five seconds are measured at each excitation frequency, the total measuring time being ten seconds. The five one-second Raman spectra measured at each excitation frequency are averaged and separately stored, and, after the measure- ment, the characteristic spectra determined at the different frequencies are combined by software in the data processing device 364.

[0080] Although the invention is described above with reference to the example according to the attached drawings, it is apparent that the invention is not limited thereto, but can be modified in a plurality of ways within the inventive idea disclosed in the appended claims.

Claims

1. A method of measuring a Raman spectrum, in which method monochromatic optical excitation radiation having a known frequency is used to measure an object (380) to be measured, and the Raman spectrum charac- teristic of the object (380) to be measured is measured by means of radiation emitted from the object (380) to be measured, characterized by generating at least two monochromatic optical excitation beams (330, 332) having known, different frequencies; generating a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured; measuring a Raman spectrum induced by the excitation beams (330, 332) from the radiation (340, 342) emitted from each radiation target (382, 384) by using a silicon-based radiation detector.

2. A method as claimed in claim ^characterized by gener- ating excitation beams (330, 332) having frequencies, the Raman spectra induced by which cover the desired characteristic spectral range of the object (380).

3. A method as claimed in claim 1, characterized by generating excitation beams (330, 332) having frequencies, the Raman spectrum induced by which is in the desired optical frequency band.

4. A method as claimed in claim 3, c h a r a c t e r i z e d by the desired frequency bands of the Raman spectra being within the desired operating range of a spectrometer (300).

5. A method as claimed in claim 1, characterized by using a measuring head (370, 500A, 500B, 500C, 500D) that generates a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and transfers the optical radiation (340, 342) emitted from each radiation target (382, 384) to a measurement, the measuring head (370, 500A, 500B, 500C, 500D) comprising: a first optical guide (502, 610, 622) for transferring the excitation beams (320, 322) to the measuring head (370, 500A, 500B, 500C, 500D); and a second optical guide (552) for transferring the optical radiation (340, 342) emitted from the object (380) to be measured to the measurement.

6. A method as claimed in claim 1, characterized by using a measuring head (370, 500A, 500B, 500C, 500D) that generates a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and transfers the optical radiation (340, 342) emitted from each radiation target (382, 384) to a measurement, the measuring head (370, 500A, 500B, 500C, 500D) comprising: an optical guide (552) for transferring optical radiation (340, 342) emitted from the object (380) to be measured to a measurement; and at least two lenses (512, 514, 612, 626) that focus the excitation beam (330, 332) to the object (380) to be measured, the lenses (512, 514, 612, 626) receiving and directing the optical radiation (340, 342) emitted from the object (380) to be measured to the optical guide (552).

7. A method as claimed in claim 1, characterized by using a measuring head (370, 500C, 500D) that generates a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and transfers the optical radiation (340, 342) emitted from each radiation target (382, 384) to a measurement, the measuring head (370, 500C, 500D) comprising a high-pass^' filter (516) of a direct bandgap absorbing semiconductor material for filtering off the frequencies of the excitation beams (330, 332) from the optical radiation (340, 342) emitted from the object (380) to be measured before the measurement.

8. A method as claimed in claim 1, characterized by using a measuring head (370, 500A) that generates a radiation target (382) for each excitation beam (330) in the object (380) to be measured, and transfers the optical radiation (340) emitted from each radiation target (382) to a measurement, the measuring head (370, 500A) comprising a holographic band-stop filter (516) for filtering off the frequencies of the excitation beams (330) from the optical radiation (340) emitted from the object (380) to be measured before the measurement.

9. A method as claimed in claim 7, characterized by using a measuring head (370, 500C, 500D) that generates a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and transfers the optical radiation (340, 342) emitted from each radiation target (382, 384) to a measurement, the measuring head (370, 500C, 500D) comprising a low pass filter (506) for generating excitation beams (330, 332) at the desired frequency.

10. A method as claimed in claim ^characterized by affecting the effective signal-to-noise ratio ofthe frequency band measured by selec- tion ofthe frequencies ofthe excitation beams (330, 332).

11. A method as claimed in claim ^ characterized by generating such excitation beams (330, 332) whose frequency difference is at least in the order of magnitude of the resolution of a spectrometer (300).

12. A method as claimed in claim ^ characterized by generating the excitation beams (330, 332) with at least two different optical power sources (312, 314), each of which generates at least one excitation beam (330, 332).

13. A method as claimed in claim 1, characterized by gen- erating at least two excitation beams (330, 332) with at least one optical power source (312, 314) by adjusting the frequency of at least one optical power source (312, 314).

14. A method as claimed in claim ^ characterized by generating the excitation beams (330, 332) with at least one optical power source (312, 314) by selecting the excitation beams (330, 332) from at least two radiations having different optical frequencies and being generated by at least one optical power source (312, 314).

15. A method as claimed in claim 1, characterized by directing at least two excitation beams (330, 332) to their radiation targets (382, 384) at different times.

16. A method as claimed in claim 15, characterized by at least two radiation targets (382, 384) being at least partly overlapping.

17. A method as claimed in claim 1, characterized by directing the excitation beams (330, 332) to the radiation targets (382, 384) at the same time, the radiation targets (382, 384) being physically separate.

18. A method as claimed in claim 1, characterized by measuring the Raman spectrum using a detector (362) including at least one detector element row (440, 450), each of which includes at least one detector element (460).

19. A method as claimed in claim 1, characterized by measuring the Raman spectrum using a CCD detector or a CID detector.

20. A method as claimed in claim 1, characterized by the object (380) to be measured including paper coating paste.

21. A method as claimed in claim 1, characterized by the object (380) to be measured including carbonate, the characteristic spectral range comprising one or more of the following spectral lines: 278 cm^"1, 712 cm^"1 and 1084 cm^"1.

22. A method as claimed in claim ^ characterized by the object (380) to be measured including latex, the characteristic spectral range comprising one or more of the following spectral lines: 1002 cm^"1, 2940 cm^"1 and 3060 cm^"1.

23. A method as claimed in claim 1, characterized by the object (380) to be measured including kaolin, the characteristic spectral range comprising one or more of the following spectral lines: 430 cm^"1, 474 cm^"1, 3620 cm^"1 and 3700 cm^"1.

24. A method as claimed in claim 1, characterized by combining the characteristic spectra defined at different excitation frequencies together using one or more such characteristic spectral lines that are determin- able at two or more excitation frequencies.

25. A spectrometer for measuring a Raman spectrum, comprising: an excitation line (326) for directing monochromatic optical radiation having a known frequency to an object (380) to be measured; and a spectrum analyzer (368) comprising a radiation detector (362), the spectrum analyzer (368) measuring a Raman spectrum received with the radiation detector (362) from the object (380) to be measured and characteristic of the object (380) to be measured by using radiation emitted from the object (380), characterized in that: the excitation line (326) is configured to generate at least two monochromatic optical excitation beams (330, 332) having known, different frequencies; the excitation line (326) is configured to generate a radiation target

(382, 384) for each excitation beam (330, 332) in the object (380) to be measured; the spectrum analyzer (368) is configured to measure the Raman spectrum induced by the excitation beams (330, 332) from the radiation (340, 342) emitted from each radiation target (382, 384) by using a silicon-based radiation detector (362).

26. A spectrometer as claimed in claim 25, characterized in that the excitation line (326) is configured to generate the excitation beams (330, 332), the information content in the Raman spectra induced by which excitation beams (330, 332) covers the desired characteristic spectral range of the object (380).

27. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the frequencies of the excitation beams (330, 332) are such that the Raman spectrum induced by each excitation beam (330, 332) is in the desired optical frequency band.

28. A spectrometer as claimed in claim 27, c h a r a c t e r i z e d in that the desired frequency bands of the Raman spectra are in the desired operating range of a spectrometer (300).

29. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the spectrometer comprises a measuring head (370, 500A, 500B, 500C, 500D) arranged to generate a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and to transfer optical radiation (340, 342) emitted from each radiation target (382, 384) to a measurement, the measuring head (370, 500A, 500B, 500C, 500D) comprising: a first optical guide (502, 610, 622) for transferring the excitation beams (320, 322) to the measuring head (370, 500A, 500B, 500C, 500D); and a second optical guide (552) for transferring the optical radiation (340, 342) emitted from the object (380) to be measured to the measurement.

30. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the spectrometer comprises a measuring head (370, 500A, 500B, 500C, 500D) arranged to generate a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and to transfer optical radiation (340, 342) emitted from each radiation target (382, 384) to a measurement, the measuring head (370, 500A, 500B, 500C, 500D) comprising: an optical guide (552) for transferring the optical radiation (340, 342) emitted from the object (380) to be measured to the measurement; and at least two lenses (512, 514, 612, 626) arranged to focus each excitation beam (330, 332) to the object (380) to be measured, the lenses (512, 514, 612, 626) being arranged to receive and direct the optical radiation (340, 342) emitted from the object (380) to be measured to the optical guide (552).

31. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the spectrometer comprises a measuring head (370, 500C, 500D) arranged to generate a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and to transfer the optical radiation (340, 342) emitted from each radiation target (382, 384) to a measurement, the measuring head (370, 500C, 500D) comprising a high-pass filter (516) of a direct bandgap absorbing semiconductor material for filtering off the frequen- cies of the excitation beams (330, 332) from the optical radiation (340, 342) emitted from the object (380) to be measured before the measurement.

32. A spectrometer as claimed in claim 25, characterized in that the spectrometer comprises a measuring head (370, 500A) arranged to generate a radiation target (382) for each excitation beam (330) in the object (380) to be measured, and to transfer the optical radiation (340) emitted from each radiation target (382) to a measurement, the measuring head (370, 500A) comprising a holographic band-stop filter (516) for filtering off the frequencies of the excitation beams (330) from the optical radiation (340) emitted from the object (380) to be measured before measurement.

33. A spectrometer as claimed in claim 31, characterized in that the spectrometer comprises a measuring head (370, 500C, 500D) arranged to generate a radiation target (382, 384) for each excitation beam (330, 332) in the object (380) to be measured, and to transfer the optical radiation (340, 342) emitted from each radiation target (382, 384) to a measurement, the measuring head (370, 500C, 500D) comprising a low pass filter (506) for generating excitation beams (330, 332) at the desired frequency.

34. A spectrometer as claimed in claim 25, characterized in that the frequencies of the excitation beams (330, 332) are such that the effec- tive signal-to-noise ratio ofthe frequency band measured is optimized.

35. A spectrometer as claimed in claim 25, characterized in that the frequency difference between the excitation beams (330, 332) is at least in the order of magnitude ofthe resolution of a spectrometer (300).

36. A spectrometer as claimed in claim 25, characterized in that the excitation line (326) comprises at least two different optical power sources (312, 314), each of which is arranged to generate at least one excitation beam (330, 332).

37. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the excitation line (326) comprises at least one adjustable optical power source (312, 314) arranged to generate at least two excitation beams (330, 332).

38. A spectrometer as claimed in claim 25, characterized in that the excitation line (326) comprises at least one optical power source (312, 314) arranged to generate at least two excitation beams (330, 332) having dif- ferent optical frequencies.

39. A spectrometer as claimed in claim 25, characterized in that the excitation line (326) is arranged to direct at least two excitation beams (330, 332) to their radiation targets (382, 384) at different times.

40. A spectrometer as claimed in claim 39, c h a r a c t e r i z e d in that the excitation line (326) is arranged to generate at least two at least partly overlapping radiation targets (382, 384).

41. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the excitation line (326) is arranged to direct the excitation beams (330, 332) to the radiation targets (382, 384) at the same time, the radiation targets (382, 384) being physically separate.

42. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the spectrum analyzer (368) comprises a detector (362) including at least one detector element row (440, 450), each detector element row (440, 450) including at least one detector element (460).

43. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the detector (362) is a CCD detector or a CID detector.

44. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the spectrometer is arranged to measure paper-coating paste.

45. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the spectrometer is arranged to measure carbonate, the characteristic spectral range comprising one or more of the following spectral lines: 278 cm^"1, 712 cm^"1 and 1084 cm^"1.

46. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the spectrometer is arranged to measure latex, the characteristic spectral range comprising one or more of the following spectral lines: 1002 cm^"1, 2940 cm^"1 and 3060 cm^"1.

47. A spectrometer as claimed in claim 25, c h a r a c t e r i z e d in that the spectrometer is arranged to measure kaolin, the characteristic spectral range comprising one or more of the following spectral lines: 430 cm^"1, 474 cm^"1, 3620 cm^"1 and 3700 cm^"1.