Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2002-01-02
2003-07-08
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S334000
Reexamination Certificate
active
06590649
ABSTRACT:
The present invention relates to a spectrophotometer, comprising a housing in which a measuring system is arranged, which housing has a measuring opening, via which light is passed to the measuring system, and whereby the measuring system comprises a grating monochromator, an autocollimator cooperating therewith, and detection means for the light originating from the grating monochromator. More in particular, the invention relates to such a spectrophotometer, which also comprises an illuminator in a 45°/0° configuration with a light source formed by a lamp and an illuminator optic, whereby after reflection light emitted by the lamp is passed via a measuring opening into a housing.
A spectrophotometer as defined in the opening paragraph is known from the international patent application WO82/0163. For such a measuring system a spectrophotometer ISO standards have been fixed; in the most recent ISO standards for color measurement an optical band width of 10 nm is recommended and a maximum value of 20 nm is prescribed. Moreover, the measuring system must be sensitive to collimated light to comply with these standards, whereby the rays entering the housing are allowed to deviate from the optical axis by up to 5°. The sensitivity to light from other directions must be minimal.
As indicated, light refraction takes place by means of an optical reflection grating. The detection often takes place by means of an array of photosensitive cells, in particular an integrated circuit with an array of photodiodes. In addition, other methods of light refraction and detection are used in practice. Known is, or instance, light refraction by means of a prism or a number of constant color filters or a linearly variable color filter. The detection may also occur with one single measuring cell, whereby for measuring different wavelengths the optical grating or the color filter device is rotated, for instance with a stepping motor with which the whole measuring range can be scanned within a few seconds.
Measuring techniques by means of grating monochromators are extensively described in E. G. Loewen, E. Popov;
Diffraction gratings and applications
(Marcel Dekker, Inc., New York, 1997), in particular in Chapter 12 thereof. Such techniques can be divided into two main categories: constructions with plane reflection gratings and constructions with concave reflection gratings. Nearly all the constructions with plane reflection gratings make use of one or more concave mirrors for collimating the light. A known exception is the so-called Littrow arrangement, which, however, is designated in the above literature as out-of-date (page 444). The concave reflection gratings are relatively expensive, but offer various advantages, in particular because of their applicability in the UV range and because of their simple construction. As a special advantage it holds that the functions of dispersion and autocollimation are combined therein; other optical components are not required therefor, which also prevents lining-up and stability problems.
For plane reflection gratings the Monk-Gillison arrangement is designated as the simplest and cheapest system (see the above literature, paragraph 12.5). In this system only two components are used for dispersion and collimation, namely a concave mirror and a reflection grating.
The object of the invention is to provide the spectrophotometer with a measuring system which means a further simplification with respect to the Monk-Gillison arrangement, which is additionally inexpensive and also complies with the above ISO standards.
According to the invention the spectrophotometer as defined in the preamble is characterized in that the grating monochromator and the autocollimator form a grating lens which, as a single, physical structure, has on one side convex collimator lens structure and on the other side an externally mirrored grating structure, the plane of the grating structure being inclined at a relative small angle to the optical axis of the measuring system and/or to the optical axis of the collimator lens. The edges of the grating lens are blackened to extinguish undesired reflections.
From U.S. Pat. No. 4,838,645 a reflecting diffraction grating is known in which a grating monochromator and an autocollimator which form one whole optical system, but not a single physical structure.
In the simplest form the grating lens is made of an optically bright plastic, preferably acrylate glass (PMMA). By using inexpensive manufacturing methods such as injection molding or pressing, the cost price of such a lens can be low. It is also possible to co-form fastening edges which facilitate the mounting, Also, the mirroring of the grating lens with aluminum evaporated under vacuum can be realized in an inexpensive manner in a mass production.
The grating structure can be made in a mold for the lens in the form of a so-called ruled grating. The original master grating can be notched with a diamond chisel in an optically pure plane substrate by means of a so-called ruling engine. Ruled gratings have a higher efficiency than holographically made gratings. By making the proper selection of the blaze angle, the efficiency can be optimized for the desired measuring range, in this case the visible light spectrum. This is an advantage over concave gratings, with which it is difficult to obtain a comparable efficiency.
In a preferred embodiment the measuring system comprises a Littrow arrangement whereby light enters the housing in a first direction (Y-direction) and falls therein on the grating lens via an entrance slit and a reflecting element in a direction substantially perpendicular thereto, the negative Z-direction, while reflected light from the grating lens substantially falls on the detection means in the positive Z-direction. The measuring system thereby depicts the slit on the detection means.
To keep the rays falling on and reflected by the grating lens separated, the angle therebetween in the YZ-plane is in the order of 15 to 20°. It will be bright that this angle value is only motivated by practical considerations. The optical axis of the lens curvature in the X-direction, perpendicular to the YZ-plane, falls into the YZ-plane, namely in the Z-direction. The plane of the grating is inclined at an angle in the order of 6° to the XY-plane.
The detection means are formed by an array of photosensitive cells, the dimensions of which are in the order of 0.2×0.2 mm or less, while, furthermore, a cylindrical lens is present to converge the light from the grating lens on the photocells. The entrance slit extending in the Z-direction often has a size of about 2 mm, while the width thereof extending in the X-direction is about 0.2 mm, which corresponds with a band width of about 10 nm, so that, when the dimensions of the photocells is of the same order as the size of the projected entrance slit, an array length of 6 to 9 mm is necessary to enable depicting of the visible spectrum. Arrays of narrow long photocells in the size of the above slit are certainly manufactured, but they are considerably more expensive than the more conventional arrays, the cells of which have a smaller and substantially square cross-section, often in the order of 0.1×0.1 mm. For reasons of cost price, it is favorable to use these arrays, but the light-sensitivity of such small photocells is often lower. To compensate this drawback, the array in the device according to the invention is provided with a cylindrical lens which converges the about 2 mm high picture on the small photocells, which causes the local light intensity to be increased proportionally. Because of the small dimensions of the array the focal distance of the grating lens may also be small and thus the whole optical system. The whole housing of the measuring system can therefore be kept within the dimensions of 3×3×5 cm.
Scattered light and radiation of higher orders must be reduced or stopped in the conventional manner by providing stop filters in the optical path. The position of such stop filters, which have to act on a pa
Evans F. L.
Spectrostar B.V.
Swanson & Bratschun L.L.C.
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