Tristimulus Template-Type Colorimeter

1. Field of the Invention

The present invention relates to a tristimulus calorimeter which measures the tristimulus color content of light incident thereon. More particularly, the invention relates to a template-type colorimeter in which both diffraction means for diffracting incident light and templates for spatially filtering the diffracted light are mounted on a common light-transparent substrate.

2. Description of the Related Art

Tristimulus calorimeters are instruments whose outputs are directly proportional to the tristimulus values of one or another of the CIE standard colorimetric observers, such as the CIE 1931 XYZ tristimulus values or RGB tristimulus values. Such instruments are useful in measuring the color content of light and they are used in connection with color reproduction, color matching, color research, etc.

Wyszecki, et al., at pages 243-248 in "Color Science: Concepts and Methods", John Wiley & Sons (1982), describe two different types of tristimulus colorimeters: filter-type calorimeters and template-type calorimeters. In filter-type calorimeters, three tristimulus filters are provided to filter radiant light, the spectral transmittance of the filters matching the desired CIE color-matching function. Three filtered light components from respective ones of the filters are measured by photo cells to provide the desired CIE tristimulus values. Filter-type colorimeters are inexpensive but are usually less accurate because of the difficulty in obtaining filters having the proper spectral transmittance functions.

In template-type calorimeters, radiant light is diffracted by diffraction means such as a diffraction grating or a prism into its spectral components. Templates having spatial filters which match the desired CIE color-matching functions are arranged to block some of the spectral components and to transmit only selected spectral components in accordance with the CIE color-matching function. Detectors are provided behind each template to measure the transmitted spectral components, thereby to output the desired CIE tristimulus values. Template-type calorimeters are accurate but they are more expensive.

It is an object of the invention to address the foregoing situation and to provide a template-type colorimeter that is relatively inexpensive.

In one aspect, the invention is a template-type calorimeter comprising a light-transparent substrate having a diffraction grating on a surface thereof, the diffraction grating being operative to diffract light incident thereon from a spaced-apart aperture into its constituent spectral components. A template is provided on a surface of the substrate and arranged to receive the diffracted spectral components. The template has formed thereon at least three spatial filters, each for selectively transmitting the diffracted spectral components in accordance with respective ones of the desired color-matching functions such as the CIE x(.lambda.), y(.lambda.), z(.lambda.) or r(.lambda.), g(.lambda.), b(.lambda.) color-matching functions. Because both the diffraction grating and the spatial filter template are mounted on a common substrate, relative positioning between the two can be accomplished with great accuracy. The diffraction grating and the template may be mounted on opposite sides of the substrate, or they may be mounted in a laterally spaced-apart relationship on the same side of the substrate, in which case the opposite side of the substrate is coated with a reflective material. Detectors are arranged adjacent each spatial filter to detect the spectral components transmitted by the respective filters, and the entire assembly is mounted in a light-tight housing which houses the aperture and the assembly in the desired spaced-apart relationship.

This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.

FIG. 1 is a combined perspective view and functional view of a tristimulus colorimeter according to the invention.

FIG. 2 is a perspective view of a common substrate having a diffraction grating and a spatial filter template mounted thereon, FIG. 2A shows an enlarged inset from FIG. 2, and FIG. 3 is a cross-sectional view of the FIG. 2 arrangement.

FIG. 4 is a view showing the FIG. 3 spatial filter template.

FIG. 5 is a perspective view of an alternate embodiment of the invention in which a diffraction grating and a spatial filter template are mounted on a common substrate.

FIG. 6 is a graph for explaining the changing response of photodetectors as a function of different light wavelengths.

FIGS. 7 and 8 are views for alternative constructions of spatial filter templates.

FIG. 1 is a combined perspective and functional view of a tristimulus calorimeter according to the invention. As shown in FIG. 1, calorimeter 10 includes a light-tight housing 11 which supports an aperture 12 or other light restricting means in spaced-apart relationship from an assembly 14 in which both a diffraction grating and a spatial filter template are mounted on a common substrate. Preferably, three such spatial filters are provided, each transmitting spectral components corresponding to the desired CIE color-matching function such as the CIE 1931 x(.lambda.), y(.lambda.), and z(.lambda.) color-matching function. The outputs 15 of the spatial filters are provided to analog-to-digital converter 16 which digitizes those signals and provides them to compensator 17. Compensator 17 compensates for errors in outputs 15 such as non-linearities and photodetector errors. Compensator 17 need not be provided where the accuracy of outputs 15 is adequate, but preferably is provided in situations where higher accuracy is needed. The compensated outputs are provided to display 18 for display to a local operator, and may also be provided to digital computer interface 19 so that the measured tristimulus values may be provided to unshown computing equipment such as a local area network or personal computer.

In operation, colorimeter 10 is positioned so that radiant light is incident on aperture 12 which restricts the incident light and projects it onto assembly 14. For example, calorimeter 10 may be placed so that aperture 12 is adjacent a region on a color monitor so as to measure the tristimulus values of that region on the color monitor. Likewise, the colorimeter may be positioned so that it receives light reflected from a sample material so as to measure the color tristimulus values of the sample material.

FIG. 2 is a perspective view showing assembly 14 in more detail.

As shown in FIG. 2, assembly 14 includes a light-transparent substrate 20 such as hard glass, for example, Corning 7059, or plastic. Mounted on substrate 20 are both a diffraction grating 21 and a spatial filter template 22. In this embodiment, the diffraction grating 21 and spatial filter template 22 are mounted on opposite surfaces of the substrate. The diffraction grating 21 is effective to diffract light incident thereon from spaced-apart aperture 12 into its constituent spectral components, as shown schematically at 24 in which "VGR" refers to the violet, green and red regions of the spectrum, respectively. The thickness T of substrate 20 is selected so as to spread the spectrum over a suitably large region; with the above-referenced Corning 7059 hard glass, a thickness of T=2 mm is found to yield satisfactory results when the spacing "a" between lines of the diffraction grating is 10 micrometers.

Both diffraction grating 21 and spatial filter template 22 may be formed by deposition, by etching or by photolithographic techniques directly on substrate 20, or they may be formed independently and laminated directly to substrate 20. In particular, if photolithographic techniques are used, then an emulsion such as the one used in Kodak.RTM. Technical Pan Film 6415 developed in Kodak Technidol LC developer will provide a resolution of 3.12 micrometers, which is suitable for formation of both the diffraction grating and the spatial filter template.

Although diffraction grating 21 is preferred for use as diffraction means in the FIG. 2 assembly, other diffraction means such as a prism may also be used so long as the diffraction means diffracts light incident thereon from spaced-apart aperture 12 into its constituent spectral components.

FIG. 3 is a cross-sectional view of the FIG. 2 assembly taken along line 3--3. As shown in FIG. 3, spatial filter template 22 has three filter areas 25, 26 and 27, spaced longitudinally along the template so that each receives the full spectrum 24. Adjacent and behind each spatial filter are respective photosensitive detectors 39, 40 and 41. As shown in FIG. 3, the photosensitive detectors are wide-area detectors so as to detect light from a wide area on the template. Of course, should narrow-area detectors be used instead, then collecting means or other suitable means for collecting and integrating light and channeling light to the detectors may be interposed between the template and the detector.

FIG. 4 is a cutaway view showing representative spatial filters used to provide CIE x(.lambda.), y(.lambda.), z(.lambda.) color-matching functions. As described above in connection with FIG. 3, spatial filter template 22 includes three areas 25, 26 and 27, each exposed to the full spectrum 24 and each for providing a different one of the tristimulus color-matching functions. The template includes an opaque area 30 which blocks light, and three transparent areas 31, 32 and 33, each of which selectively transmits the diffracted spectral components 24 in accordance with respective ones of the color-matching functions.

In FIG. 4, filters for CIE x(.lambda.), y(.lambda.), z(.lambda.) color-matching functions are shown; other filters such as RGB filters may also be provided. In addition, the filters can also be in the shape of the spectral sensitivity functions for cones in the retina of an eye. In this latter case, it is preferable also to provide a fourth template relating to the spectral sensitivity function for the rods in the retina. With this arrangement, display 18 displays cone and rod responses instead of tristimulus values.

In operation, light incident on diffraction grating 21 from spaced-apart aperture 12 is diffracted into its constituent spectral components 24. The spectral components are transmitted through substrate 20 where they are filtered by spatial filter template 22. Each spatial filter 31, 32 and 33 on the template transmits predetermined spectral components in accordance with respective ones of the color-matching functions. The transmitted spectral components are detected by respective ones of detectors 39, 40 and 41 and the detected outputs are provided via outputs 15 to analog-to-digital converter 16, as described above.

FIG. 5 is a perspective view of an alternative embodiment of assembly 14. As shown in FIG. 5, assembly 114 includes diffraction grating 121 or other diffraction means mounted on the same substrate 120 as spatial filter template 122. Both diffraction grating 121 and spatial filter template 122 are mounted on the same side of the substrate in a laterally spaced-apart relationship. On the opposite side of the substrate, reflector 123 reflects diffracted spectral components 124 from grating 121 to the spatial filter template 122. The FIG. 5 embodiment is advantageous since it allows the spectral components to traverse the substrate 120 twice, thereby effectively increasing the spectral width of spectral components 124.

The spatial filters used in the practice of the invention may correspond exactly to the desired tristimulus color-matching functions, but they may also be any linear combination thereof. In particular, certain situations require tristimulus difference values, and the spatial filter templates may be modified so as to provide the desired difference values rather than the precise tristimulus values.

Preferably, the spatial filters also compensate for the detection sensitivity characteristics of the photodetectors 39, 40 and 41. More particularly, as shown in FIG. 6, typical photodetector response is not constant over all wavelengths of visible light, but rather varies in accordance with the wavelength of light incident thereon. As shown in FIG. 6, relative response for shorter wavelength light (450 nanometers, corresponding to the violet region of the spectrum) is lower than the relative response for longer wavelengths of light (750 nanometers, corresponding to the red end of the spectrum). In accordance with the relative spectral response of the photodetectors, the spatial filter characteristics may be altered. Thus, if photodetectors having the characteristics shown in FIG. 6 are used, then the spatial filters should be modified so as to increase the amount of light transmitted from the violet end of the spectrum with respect to the red end of the spectrum.

FIGS. 7 and 8 show alternative arrangements for spatial filters. As shown in these figures, it is not strictly necessary to provide spatial filters which "look" exactly like the desired spectral response. Rather, as shown in FIG. 7, which is a spatial filter suitable for the x(.lambda.) color-matching function, mirror images 33a and 33b of the desired spectral transmittance are provided on opposite sides of opaque area 30. Likewise, in FIG. 8, rather than using fully opaque and fully transmissive areas, partially transmissive areas are used. Thus, regions 33c are dense regions allowing relatively little light to be transmitted therethrough, while regions 33d and 33e are less dense regions allowing respectively greater amounts of light to be transmitted therethrough. The embodiments of FIGS. 7 and 8 are advantageous relative to the templates in the FIGS. 4 and 5 embodiments in that they transmit selected spectral components with greater diffusion and with greater smoothness, thereby allowing detection with greater ease and possibly without the need for collecting means.