WO2011038826A1

WO2011038826A1 - Flash lamp or gas discharge lamp with integrated reflector

Info

Publication number: WO2011038826A1
Application number: PCT/EP2010/005445
Authority: WO
Inventors: Jeremy Woffendin; Juergen Weber; Sven Linow
Original assignee: Heraeus Noblelight Gmbh
Priority date: 2009-10-01
Filing date: 2010-09-04
Publication date: 2011-04-07
Also published as: GB2474032A; GB2474032B; GB0917205D0

Abstract

The invention relates to a device for the radiation of a substrate (37), comprising - a gas discharge lamp with a lamp bulb (31) arranged in a cooling medium (34), - a reflector (32) arranged on the outside of the lamp bulb, wherein the reflector arranged on the outside of the lamp bulb is in direct contact with the cooling medium, as well as an optical system comprising a device.

Description

Patent Application

Flash Lamp or Gas Discharge Lamp with Integrated Reflector

This invention relates to a device for the irradiation of a substrate by means of a gas discharge lamp, as well as an optical system and a reflector on a gas discharge lamp.

Gas discharge lamps have usually a noble gas filling and are light sources using a gas discharge for generating light and utilizing the spontaneous emission by atomic or molecular electronic transitions and the recombination radiation of a plasma generated by electric discharge.

There is a great variety of applications for such light sources. For example, continuous gas discharge lamps are typically used in the area of pumping laser crystals. Pulsed gas discharge lamps or flash lamps are also used for pumping pulsed lasers; however, they are also used in the areas of semiconductor production, the sterilization of water and foods. Pulsed emitters are used for illumination in photography, for drying and hardening of print colors, in solar simulation and the testing of solar cells, as well as in cosmetics for skin rejuvenation and the removal of tattoos and hairs. Potential applications are so manifold that the lamps must meet the most diverse requirements.

In most applications, the light of the lamp must be guided onto the substrate by an optical system. A high efficiency of this optical system and of all its components is important. Furthermore, high demands are made on the homogeneity of the radiation with reference to the angular distribution of the radiation and the power distribution of the radiation on the substrate.

Powerful discharge lamps or those in which much energy is converted into radiation in a small volume must be directly cooled with a cooling medium to dissipate the heat released by the lamp. Accordingly, a so-called fluid duct is arranged around the lamp which becomes a part of the optical system. Deionized water is mostly used as the cooling fluid; however, other fluids or media which ensure adequate cooling are conceivable as well.

Tests with reflectors arranged extremely closely around the bulb of the lamp which direct the major part of the radiation back into the light generating discharge show that, , due to the high self-absorption in the plasma of highly charged lamps, only a slight increase in directional radiated power is to be expected [I.S. Marshak: Pulsed Light Sources. Consultants Bureau, New York: 1984]. This is explained by extra heating of the plasma due to self-absorption and connected effects.

Accordingly, state of the art is that the light emitted by the lamp - by means reflectors, that are generously spaced apart from the lamp bulb and which frequently form at the same time the cooling duct - is guided onto the substrate, onto a laser rod, or into a fiber optic conductor. Frequently, involutes are used or, respectively, M or W shaped reflectors which minimize self- absorption as e.g. shown in Fig. 3b of US 6,867,547.

US 2008/0223441 A1 discloses, for example, a device for radiating a solar cell which images the light of an argon flash lamp on the solar cell. The entire optical system is composed of 26 individual, separately adjustable mirrors, which are arranged around the centrally arranged cooled flash lamp.

US 6,849,831 presents a device for "flash assisted RTP" method.

US 7,109,443 describes the problem of achieving homogeneous irradiation or heating of a substrate and thus minimizing the thermal stress of the heated substrate. An external complex reflector is proposed which is subdivided into concentric zones. Such lamps and reflector arrangements have been known for some time already; e.g. from US 4,789,771.

The requirements on the homogeneity of the radiation are lower for the application of discharge lamps in the area of cosmetic irradiation; however, the requirements are all the more higher regarding the minimization of the size of the radiation unit. Since the radiation head including the radiation unit which consist of the lamp mount, cooling etc, and the optical system is to be easily moved by hand, This radiation unit must accordingly be light-weight and have small dimensions to insert it into the radiation head. US 5,620,478 explains, for example, the principle of a simple radiation unit with a low-power flash lamp which is operated at a low frequency and therefore also does not require any cooling. The power and frequency thus realized is not economical for most applications, however.

WO 02/069825 A2 discloses some embodiments of a radiation unit each of which comprises water-cooled flash lamps. The optical system for guiding the radiation from the lamp to the skin consists of numerous components and is accordingly rather complex, large and heavy.

DE 10 2004 051 846 A1 discloses a reflector which has - especially at ambient temperature - excellent reflectivity ranging from the VUV to the mid IR. The inciding light is scattered at the interphase surfaces between quartz glass and the nano-scaled inclusions of air or other gases present at high density in the quartz glass. The density and the size of the inclusions is crucial for the bulk reflectivity of the material, as well as the difference of the refractive index between solid body and pores. Usually the pore material is air which has a refractive index of 1.0 whereas the refractive index of quartz glass is 1.45. Even the use of other gases or glasses does not result in any substantial change of the difference of the refractive index between pore material and glass.

The reflection of the bulk material disclosed in the DE 10 2004 051 846 A1is caused by numerous reflections on nano scaled air/glass surfaces. According to the Fresnel formulas for calculating the reflection on such surfaces [Bergmann, Schafer: Lehrbuch der Experimentalphysik Band 2, Optik. Walter de Gruyter, Berlin: 1993, 4% reflectance is typically obtained for air / quartz glass and 0.4% reflectance for water / quartz glass. This results from the refractive index of water which, at 1.32, is close to that of quartz glass.

This reduction of the underlying reflection by one order of magnitude should dramatically limit the effectivity of such a porous opaque reflector in water. It was expected that the required layer thicknesses for an industrially utilizable reflectivity of more than 50% would be ar least 5 mm. Such a thick layer can no longer be used on a gas discharge lamp since it wouldbasicallyserve as a thermal insulator and could result in a premature total loss of the lamp. In addition, the lamp body would become highly unsymmetrical, bulky and heavy.

It would be possibile to provide the opaque reflector with a glass coating on the outer layer bordering to the cooling medium. This encloses the gas in the pores and prevents the pores from filling with water. This measure does not seem sensible since an additional work step is neces- sary, which presents extreme requirements on the quality of workmanship since the inner glass tube may not deform so as not to jeopardize the burst strength of the tube, and because the additionally required layer results in a significant increase in the thickness of the reflective layer and thus a deterioration of thermal conductivity. Thus the lamp is loosing thermal resistivity in operation, and the manufacturing costs increase significant.

It has also been shown that metallic layers on gas discharge lamps are disadvantageous since they are interact with the electric field of the discharge and influence the geometry of the discharge. Metallic layers cannot be applied before the manufacture of the lamp since they would make the heat processes of the the manufacturing process impossible. Subsequently burnt-in metallic layers are difficult to realize since the sealing must be protected against oxidation. Likewise, it is almost impossible to apply coatings, such as a CVD coatings, on round forms with a uniform thickness of the coating.

Accordingly, it is the object of the invention to provide an optical system which can be manufactured simply and cost-effectively and has a compact design. Furthermore, it is the object of the invention to provide a reflector which has an efficient effect on the radiation of the lamp and can especially be used in coolants, as well as simply mounted on a gas discharge lamp.

The problem is surprisingly solved already with the features of the independent claims.

Preferred embodiments are to be taken from the relevant sub-claims.

The device according to the invention for the irradiation of a substrate comprises a gas discharge lamp with a lamp bulb arranged in a medium, as well as a reflector arranged on the outside of the lamp bulb. The reflector arranged on the outside of the lamp bulb is directly in contact with the medium. The medium is preferably a fluid, particularly preferably a cooling water which thus washes directly around the reflector. The cooling water is provided in a jacket tube and thus forms a part of an optical system. Such a device has the advantage that highly directional radiation is rendered possible and high reflection values of the system are measurable.

It has been shown that it is advantageous if the device is additionally provided with a filter which is arranged in the beam path. In a preferred embodiment, the device provides a reflector which consists of an opaque material scattering in volume. The scattering material has a refractive index which differs from that of air and from water, as well as a plurality of pores whose diameter is comparable in diameter or larger than the wavelength of the light emitted from the lamp and transmitted from the radiator tube. This effects that high reflection values are possible, and thus makes the directional emission of the lamp by means of a simple, small and light-weight optical system possible and enables the optimum irradiation of a substrate.

It has been shown that such a reflector layer can be simply applied and is very cost-effective in manufacturing.

The invention furthermore comprises an optical system with a device for irradiating a substrate which has a gas discharge lamp with a lamp bulb arranged in a medium, wherein a reflector is arranged on the outside of the lamp bulb, and the reflector is in direct contact with a cooling medium. The optical system here comprises of a block of glass which picks up the part of the radiation emitted from the surface area of the lamp, which is not enclosed by the reflector, transmits the light further by means of internal total reflection and thereafter allows it to emerge onto a substrate. A reflector according to the invention here renders it possible that an extra layer - for example glass - limiting the cooling medium will become redundant since the reflector is designed such that it can be in direct contact with the cooling medium without losing its properties as a reflector.

The invention furthermore provides a reflector for use in a device or a system, as stated above, wherein the reflector consists of an opaque scattering material. It is here advantageous if the bulk material of the reflector (without pores) has a refractive index n > 1.4 or 1.1 < n < 1.2.

Prior to the manufacture of the lamp from the lamp bulb, the reflector is apllied onto the surface of the lamp bulb. This is particularly simple in manufacture and thus cost-effective. Such a reflector can also be applied on round forms without adversely affecting the surface. As opposed to metallic layers on gas discharge lamps, the reflector according to the invention does not produce any interactions with the electric field of the lamp discharge and thus does not negatively influence the discharge or its geometry.

The reflector according to the invention which is of an opaque material and applied on a gas discharge lamp will effect highly directional emitted radiation. The reflector according to the invention is particularly well suited for the application in small devices and models and can be specifically adjusted to the specific requirements, depending on the device and the field of application. Conversely, the reflector according to the invention enables the miniaturization of devices or models in which such lamps are used, e.g. by increasing the utilizable emission and thus a reduction of the required power.

Illustrations:

Figure 1 shows the reflectivity of a plain water/glass surface as a function of the refractive index of the glass.

Figure 2 shows a cross-section through a discharge lamp covered with a reflector, with the reflector covering an angle a of the surface.

Figure 3 shows a module for irradiation with a lamp according to the invention and an optical system according to the invention.

The reflectivity as function of the refractive index of the glass as presented in Figure 1 for a quartz glass / water interface was determined with the Fresnel formulas for the calculation of the reflectivity at such interfaces [Bergmann, Schafer: Lehrbuch der Experimentalphysik Band 2, Optik. Walter de Gruyter, Berlin: 1993].

Embodiment 1

A quartz tube (31) with the dimensions of 80 mm length, 6 mm diameter, and 0.5 mm wall thickness is provided with an opaque porous, nano-structured layer (32) of quartz glass. This is sprayed on as a slurry, with the areas not to be coated being covered by means of masks. The layer is subsequently sintered in a furnace so that the layer is mechanically stable.

The reflector layer (32) covers— the lamp tube (31 ) in the area between the two electrodes over an angle a = 180°.

From this tube (31), a gas-filled discharge lamp for pulsed operation with an arc length of 50 mm is subsequently manufactured by means of the standard methods. Special attention is here given to the reflector (32) during the processing steps using flames, such as sealing the electrodes or cleaning the glass tube. The temperature at the reflector (32) may no longer exceed the sintering temperature. This lamp is operated in a standard cooling duct for flash lamps without internal reflector on the inside wall of the duct. As compared to the non-coated lamp, an increase in radiant power by 50% can be directly observed with a thickness of 0.8 mm of the reflective layer. A layer thickness of 0.5 mm of the reflector results in an increase in radiant power of 35%. On the reflector side of the lamp, a reduction of radiant power to 25% is observed as compared to the non- coated lamp.

Embodiment 2

A irradiation module which can be hand-held for the irradiation particularly of skin areas or of a substrate (37). A flash lamp, as described in embodiment 1 , is inserted into a cooling duct of aluminum with a small gap width. This cooling duct or cooling housing (33) is also provided with openings through which the ends of the lamp are guided out of the duct and with inlet and outlet ducts for the cooling water. The lamp is sealed against the housing at the openings so that no cooling water (34) can escape here. Furthermore, an optical conductor (35) made of glass, preferably quartz glass, is inserted into the cooling housing (33). Said conductor trapps the largest possible part of radiation emitted by the lamp and only allows it to emerge on the opposite face again by means of total internal reflection on the sides. An anti glare filter is additionally installed between lamp and optical conductor (35), or better still, integrated into the conductor. The optic conductor is sealed into the cooling housing (36).

A numerical analysis of the light paths of scattered light inside the cooling housing shows that light which does not enter from the lamp directly into the optical conductor (35) has only a minor probability to reach the optical conductor via further reflections in the interior of the cooling duct. This concerns especially portions which are emitted away from the position of the conductor (35).

In the simplest case, the reflector (32) is applied over an angle of a = 180° onto the lamp tube. However, by means of suitable optimization of the reflector (32) and the geometry of the inside of the cooling duct, a higher portion of the light can be guided into the optical conductor (35) in addition to the effects described so far. Angles of 135° < a < 180° are preferred and, at the same time, suitably formed segments on the interior surface of the cooling duct to reflect parts of the scattered radiation into the optical conductor. Embodiment 3

A module for the homogeneous irradiation of substrates, like solar cells. An elongated, argon- filled flash lamp is manufactured with a reflector: A quartz tube with the dimensions of 280 mm length, 6 mm diameter, and 0.5 mm wall thickness is provided with an opaque porous, nano- structured layer of quartz glass. This is sprayed on as a slurry, with the areas not to be coated being covered by means of masks. The layer is subsequently sintered in a furnace so that the layer is mechanically stable.

The reflector layer (32) coversover an angle a (23) the tube in the area between the two electrodes.

From this tube, a gas-filled discharge lamp for pulsed operation with an arc length of 250 mm is subsequently manufactured by means of the standard methods. Special attention is given to the reflector during the processing steps using flames, such as sealing the electrodes or cleaning the glass tube. The temperature at the reflector may no longer exceed the sintering temperature.

The flash lamp is located centrically in a quartz glass tube with deionized cooling water flowing through. The radiation of the flash lamp is reflected via a parabolic mirror onto the substrate to be irradiated. This achieves a parallel and nearly homogeneous illumination of the substrate surface. Parallelity or homogeneity of the irradiation might be optimized by changing the exact position of the lamp in regard to the reflector. The reflector on the lamp tube (32) itself is oriented in the direction of the substrate to suppress the inhomogeneous and non-parallel direct emission of the lamp onto the substrate and to reinforce the light irradiating the substrate via the reflector.

The circumferential angle of the reflector 32 is selected such that the parabolic reflector 32 is illuminated over its entire surface. In particular, angles (23) a < 180° have proven well to achieve a particularly high irradiation surface power with simultaneously high homogeneity of the radiant power on the substrate. Angle a, orientation of the opaque reflector located on the radiator, position of the lamp in regard to the external reflector, and dimensions of the external parabolic reflector are to be coordinated with each other.

The invention is not limited to the examples presented; they only serve to illustrate the inventive idea. List of Reference Symbols

Angle 23

Lamp tube 31

Reflector 32

Cooling housing 33

Cooling water 34

optical conductor 35

Seal of the fiber optic conductor 36

Substrate 37

Claims

Patent Claims

1. Device for the irradiation of a substrate, comprising

- a gas discharge lamp with a lamp bulb, arranged in a cooling medium,

- a reflector arranged on the outside of the lamp bulb,

characterized in that the reflector arranged on the outside of the lamp bulb is in direct contact with the cooling medium.

2. Device according to claim 1 , characterized in that the medium comprises a fluid, preferably water.

3. Device according to any one of the preceding claims, characterized in that the reflector features an opaque scattering material.

4. Device according to any one of the preceding claims, characterized in that a filter is arranged between the object to be irradiated and the gas discharge lamp.

5. Optical system comprising a device according to any one of the preceding claims.

6. Optical system according to claim 5, characterized in that the optical system comprises a block of glass.

7. Reflector for use in a device or system according to any one of the preceding claims, characterized in that the reflector features an opaque scattering material.

8. Reflector according to claim 7, characterized in that the non-porous or bulk reflector material has a refractive index of n > 1.4 or 1.1 < n < 1.2.

9. Reflector according to any one of the preceding claims, characterized in that the reflector features a porous material.

10. Reflector according to claim 9, characterized in that the porous material of the reflector has 5% to 60%, preferably 15% to 40% pores.

11. Reflector according to claim 10, characterized in that the porous material of the reflector has a composition which deviates in the concentrations of the individual oxides in each case by not more than 3% from the composition of the jacket bulb of the lamp.