Optics: measuring and testing – By dispersed light spectroscopy – For spectrographic investigation
Patent
1997-01-21
1999-01-12
Evans, F. L.
Optics: measuring and testing
By dispersed light spectroscopy
For spectrographic investigation
356328, G01J 318
Patent
active
058597026
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention comprises an apparatus for carrying out spectral analysis of optical radiation emitted from a light source, more particularly an apparatus comprising a spectral detector with an entrance aperture for the radiation from a light source, a first means for optical imaging, a diffraction grating for wavelength dispersion of the radiation, an order sorting means for separation of the spectral orders from the diffraction grating, and for dispersion of the radiation in a direction orthogonal to the direction of dispersion of the diffraction grating, a second means for optical imaging, and detecting means for recording the spectrum of the light source, the spectrum being divided into spectral orders by the order sorting means.
DISCUSSION OF THE BACKGROUND
At present new high-sensitive image sensors having wide spectral recording capabilities and many picture elements ("pixels") are available on the market for the field of spectral analysis. One category of these image sensors is the CCD-sensor (Charge Coupled Device sensor). Modem electronic techniques offer both rapid data conversion (A/D-conversion) and rapid processing of large amounts of information that image sensors generate. In order to take advantage of these techniques for optical spectral analysis, it is required that the optical arrangements preceding the sensor can be optimzed in such a way that the techniques are fully utilized in the spectral analysis. This is not the case with the techniques available today.
The use of diffraction gratings for wavelength dispersion, i.e. separation of optical radiation into wavelength components, is a well known technique. Diffraction gratings are very effective optical elements for carrying out wavelength dispersion. However, diffraction gratings have the disadvantage that spectra from several spectral orders result in ambiguities in the analysis of a spectrum.
Modern Optical sensors are sensitive to optical radiation simultaneously in the wavelength range from vacuum ultra violet ("VUV"), at wavelengths below 200 nm, to the near infrared ("NIR"), at wavelengths above 700 nm. In order to use modern optical sensors, in a single instrument, for simultaneous recording of the whole spectral range, the problem of ambiguities from other spectral orders has to be solved. One solution to the problem is to place several optical filters suitably in the vicinity of the focal plane of the optical instrument. Through this arrangement one can in principle extend the recording capability in a single spectral order. However, diffraction gratings manufactured either holographically or through ruling show a variation of the efficiency by which the optical radiation is dispersed. This diffraction efficiency, i.e. the ratio between the intensity of the dispersed light and the intensity of the incident light at a certain wavelength, has a maximum value at a wavelength given generally in the first spectral order.
The wavelength, called "the blaze wavelength", is a quantity characteristic for the diffraction grating, and depends on the distance between the grating grooves, the shape of the grating grooves, and the polarization of the optical radiation. In selecting a diffraction grating the blaze wavelength is chosen within the wavelength range of the instrument. When the wavelength to be recorded diverges considerably from the blaze wavelength, the diffraction efficiency of the grating is substantially reduced.
The method discussed above to filter disturbing wavelengths from other spectral orders to decrease the degree of ambiguity in a single spectral order results in a considerable reduction in the efficiency of the spectral recording at wavelengths that diverge from the blaze wavelength. This reduction in efficiency makes it difficult to cover a large wavelength range in one single spectral order of a diffraction grating.
In order to avoid the reduction in efficiency, G. R. Harrison in J. Opt. Soc. Am. No. 39 (1949) page 522, proposed using diffraction gratings with coarse rulings, or g
REFERENCES:
patent: 3922089 (1975-11-01), Danielsson et al.
patent: 4684253 (1987-08-01), Lindblom et al.
patent: 5139335 (1992-08-01), Lundeen et al.
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