Optics: measuring and testing – By dispersed light spectroscopy – With monochromator structure
Patent
1982-01-26
1986-05-27
Evans, F. L.
Optics: measuring and testing
By dispersed light spectroscopy
With monochromator structure
356334, 35016217, 35016223, G01J 318
Patent
active
045912700
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a dispersive optical device for use such as a spectroscope, polarizer, monochromator or the like, of for use as a basic component for a monochromator, spectograph, spectophotometer, polarizer or the like.
BACKGROUND
Conventional dispersive optical devices, such as those utilizable as spectroscopes or monochromators, have relatively large physical dimensions, are fragile, and contain slits and focussing elements which result in high production costs.
OBJECT
One object of the invention is to propose a dispersive device which can have small physical dimensions, is robust, can be produced at a relatively low cost, and has relatively high light strength and high resolution. Another object of the invention is to propose a dispersive device which furthermore allows through-sighting with transmission of an image which is in scale. Further objects and advantages of the invention will be apparent from the following.
SUMMARY OF THE INVENTION
The inventive dispersive optical device includes a dispersive optical member comprising a first and a second grating applied to planar parallel substrate surfaces, preferably reflection gratings which have the same grating frequency and have parallel grating scores or rulings, the second grating being adapted for being struck by a portion of the light defracted by the first grating, so that light which is defracted by the second grating is parallel with light incident on the first grating.
Since a greater defraction angle gives a greater angular dispersion, the angle between a normal to a grating and a line between the gratings should attain at least 60.degree. and preferably at least 70.degree.. The upper limit of the angle is 90.degree., for natural reasons, but attains in practice to about 88.degree.. The angle is selected to advantage within the interval 80.degree.-85.degree..
By selecting, for example, a relatively large distance between the gratings and a relatively narrow width for them, the angular variation will be small for rays between the gratings. This signifies that only a correspondingly small spectrum portion of parallel light incident on the first grating will strike the second grating. Since the gratings are mutually the same, light defracted by the second grating will be parallel to the light incident on the first grating. By measuring the angle of the member (i.e. the grating) to incident light, an indication can be obtained of the wavelength of the light observed at the member output. In a corresponding mode, when the member is utilized as a monochromator, for example, it can be arranged for light of a predetermined wavelength range to leave the output of the member by setting a given angle between incident light and the member. When the member is used as a monochromator, a focussing lens can be arranged behind the member output with a slit after the lens, suitably at its focal point, so that aligning errors are eliminated, as well as extraneous light wavelengths, only the whole of the pertinent pencil of rays being looked after.
It should further be clear that the dispersive member can be provided with wavelength scale which is readable at the output from the second grating, so that the wavelength of the light observed at the output of the second grating can be read off by eye directly.
In one embodiment of the invention, the member is built up on a transparent substrate with planar parallel surfaces, e.g. a plate of so-called float glass, a grating being arranged on each chief surface of the substrate at opposed ends thereof. The gratings are preferably protected by some kind of covering on their surfaces facing away from the substrate. It is further suitable for the whole member to be enclosed by a light-absorbing coating, e.g. a black paint, the refractive index of which is close to that of the substrate, except at the surfaces of the substrate used as input, output and total reflection surfaces.
According to another embodiment of the invention, the gratings can be arranged opposite, and adjacent each oth
REFERENCES:
patent: 3698795 (1972-10-01), Flint
Evans F. L.
Institutet for Optisk Forskning
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