after each integration cycle (45)(46) and that the envelope 5. The method according to claim 1, which comprises
(47) has been extracted regardless of the modulation phase. modulating the light source at a constant frequency f=2v/l.
Another more robust way of linking subsequent frames is where v is a constant speed of the translation stage and A. is
to simply determine the absolute difference of corresponding a mean wavelength of the light source,
pixels in subsequent frames: 5 6 ^ method according to claim 1. which comprises
... (2) modulating the light source discontinuously by switching
This is the determination of the contrast of corresponding the light source on and off at a constant modulation fre
pixels in subsequent frames without normalization. quency f=2v/l, where v is a constant speed of the translation
The present invention has the decisive advantage of stage and A. is a mean wavelength of the light source,
enabling measurements at a continuous (high) speed without 10 7. The method according to claim 1, which comprises
limitations in terms of range of measurement and with no integrating the incoming intensity signal section by section
need to fully sample the modulation in accordance to the wjm solid-state imaging arrays having a given time of
Nyquist theorem. The measuring speed determines the dis- integration between subsequent readouts,
tance along which the object is moved within one integration g ^ method ^c^g t0 claim 7, whereill me solid.
cylce, i.e. the faster the object is moved the longer is ttie is ^ ^ ^ CCD.devices.
distance and toe lower is the accuracy of determining the The method according to claim 1 which further com
maximum of the envelope. In other words, the accuracy of , . . % .. ,. . . .. , .. ,.
the sensor can be adjusted by selecting an appropriate Pnses changing a phase of tight modulation of the light
measuring speed. This trade-off between accuracy and mea- ate each integration time of the photodetector.
suring speed only depends on the integration time of the ,n 10 Tte method according to claim 1. which comprises,
photodetector, which suggests the use of a high speed 20 shifting a phase of a modulation of a light output of the light
camera, source by Jt/2 after each integration time of the photodetec
If there is a need for a very precise measurement, the tor and adding corresponding pixels of two subsequent
object has to be moved rather slow. This becomes a problem, frames read out from the photodetector.
if the object is not moved along many wavelengths or along 11. The method according to claim 1, which comprises
a multiple of A/2 during one integration cycle. For that case 25 alternately shifting a phase of a modulation of a light output
the light modulation which is asynchronous to the video of the light source by n/2 and by -7t/2 after each integration
cycle leads to flickering in subsequent frames. For slow time of the photodetectors.
measurements the amount of light emitted during each 12. The method according to claim 1, which further
integration time has to be kept constant. comprises subtracting an average value of a pixel of a
The highest accuracy is achieved at a very low speed, 30 respective photodetector from a present value of the pixel,
which means that during one integration time the object is squaring a difference determined in the subtracting step, and
moved along less than A.. Since the amount of light emitted addi me squared differences of corresponding pixels of
during one integration cycle has to be kept constant, the subsequent frames
modulation frequency has to be set to zero for that case. „ ^ accordi t0 claim j wnich
However if a constant speed of A/6 per video cycle is 35 rises stori in addMon to a maximum value for a
chosen, the same image processing can be used as suggested ^of a re ctive photodetector, a last and a next value
in equation 2, which leads to at least two evaluable intensity fof each ^ and dsten^Ising a of me maximum
values m three subsequent frames. yalue fcy ^,^0^ wherein ^ position of the maximum.
We claim: 14. The method according to claim 13. which further
1. A method of profiting a surface of an object, which w comprises determining a maximum of an absolute difference comprises the steps of: of subsequent values for each pixel.
a) placing an object in vicinity of an interferometer having 15 ^ method according to claim 1. which comprises interferometer arms and a translation stage, and trans- maintaining the tight signal emitted by the light source mitting a low coherent tight signal from a light source constant during one integration time of the photodetector. with a mean wavelength A. to the object; 45 16 ^ meth0d according to claim 1, which comprises
b) varying an optical path in one of the interferometer setting a modulation frequency to zero at a translation stage arms with the translation stage; speed of A/6 per video cycle.
c) receiving a resulting intensity-modulated interferom- 17. A method of profiling a surface of an object, which eter signal with a photodetector having low pass char- comprises the steps of:
acteristics and outputting photodetector output signals; 50 a) placing an object in vicinity of an interferometer having
d) evaluating the photodetector output signals with an interferometer arms and a translation stage, and transimage processing system; and mitting a non-modulated low coherent light signal with
e) extracting a depth of modulation of the interferometer a mean wavelength A, from a light source to the object; signal by intensity-modulating the low coherent light b) varyjng an optical path in one of the interferometer source. 55 arms wral the translation stage;
2. The method according to claim 1, which furttier com- c) receiving a reflected signal with a photodetector having pnses shifting the ttanslation stage by respective A/2 shifts ^ characteristics &od outputting photodetector and, during each A/2 shift of the translation stage, varying an outout sienals
intensity of the light source between a high intensity and a . .' . . ,
low intensity 60 <0 evaluating the photodetector output signals with an
3. The method according to claim 2, wherein the step of processing system; and
varying the intensity of the light source comprises varying e) extracting a depth of modulation of the interferometer
the intensity sinusoidally. signal by modulating one of a flux of light which hits
4. The method according to claim 1, which comprises the photodetector, and a sensitivity of the photodetecintegrating a signal received by the photodetector section by 65 tor.
section and thereby extracting an envelope of the interfer-
ometer signal. *****