Optics: measuring and testing – By dispersed light spectroscopy – With monochromator structure
Patent
1984-01-03
1987-08-04
Evans, F. L.
Optics: measuring and testing
By dispersed light spectroscopy
With monochromator structure
356334, G01J 314, G01J 318
Patent
active
046842535
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an apparatus for carrying out spectral analysis comprising means for leading the radiation into the apparatus, means for recording the produced spectrum and diffraction grating means for dispersing the incident radiation.
It is previously known to use in spectral analysis so-called diffraction gratings as wavelength dispersing optical means for dividing optical radiation into wavelength components. These gratings therefore are commonly included in spectral apparatuses which, in addition to the grating, usually have an entrance slit through which the optical radiation to be analyzed is admitted to the apparatus, whereafter the diffraction grating together with other optical means produces an image of the entrance slit on a focal plane so that optical radiation of different wavelengths is reproduced in different locations on said plane. The recording of the spectrum, i.e. the measurement of the intensity of the different wavelength components is usually carried out photoelectrically. Spectral instruments provided with photoelectric recording are usually called spectrometers. In certain types of spectrometers a slit, a so-called secondary slit, is used also in the focal plane behind which a photoelectric detector is positioned. The recording of the spectrum then takes place so that by rotating, for example, the grating the different wavelength components are made to pass the slit sequentially and are recorded by the detector. A spectrometer of this kind is called monochormator. Spectrometers provided with several secondary slits and detectors are analogically called polychromators.
An important property in a spectral apparatus is its spectral resolution which is a measure of how close to each other two wavelength components can be located so that they can barely be distinguished in the apparatus. Another important characteristic is the so-called dispersion which can be described in two ways, viz. either as angular or as linear. The wavelength dispersing properties of a diffraction grating are determined by the so-called grating equation reading as follows ##EQU1## Therein the quantity .alpha. denotes the so-called angle of incidence, i.e. the angle which the optical radiation incident on the grating forms with the normal to the grating surface, hereinafter called grating normal. The quantity .beta. analogically denotes the angle which the outgoing optical radiation forms with the grating normal after diffraction in the grating surface. The quantity .lambda. denotes the wavelength of the optical radiation, the quantity d the distance between the grooves in the grating surface, and m the so-called spectral order. This is an integer implying that the same wavelength can be dispersed through diffraction in different well-defined diffraction angles corresponding to different values of the integer m. In the same spectral order different wavelength components give rise to diffraction at different diffraction angles .beta.. In the equation (1) n indicates the refractive index of the gas which surrounds the grating surface. This has approximately the value 1 when the grating surface is surrounded by air at normal air pressure.
By differentiating the equation (1) the expression (n=1) is obtained ##EQU2## The quantity d.beta./d.lambda. indicates the change in the diffraction angle per change in wavelength caused by the grating. This quantity is called the angular dispersion.
A measure of what a grating can resolve is obtained in the following manner. With a perfect optical image of a very narrow entrance slit in the focal plane, the width of the image will be mainly determined by the diffraction in the often rectangular aperture that the grating or the focusing optical means defines. Using a diffraction grating, the width of this aperture is given by the expression W cos .beta., wherein the quantity W denotes the width of the grating. It is known that a criterion of which angle change d.beta. thereby barely can be distinguished is d.beta.=.lambda./W cos .beta.. The smallest wavelength di
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Murty, Appl. Optics, V. 11, N. 10, 10/72, p. 2286.
Engman Sonja A.
Lindblom Karl P. C.
Evans F. L.
Scanoptics Oy
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