Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-12-30
2003-07-15
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S226000, C250S214100
Reexamination Certificate
active
06593558
ABSTRACT:
BACKGROUND
Full color film must sense in three color dimensions. These three dimensions are sensed by three monochrome, or black and white emulsions, each acting as a sensor for a different spectral sensitivity, or color. In digital imaging terminology, each of these emulsions produces a separate color channel, or component of the full color image. Historically, there have been several topological arrangements of these sensors. The first color film, Duffycolor, used colored rice grains to create a red, green, and blue matrix over black and white film like a modern CCD matrix. Polaroid made an instant transparency film that lay red, green, and blue stripes over a black and white film, similar to the shadowmask in a color CRT. The original Technicolor process exposed three spatially separate color images on black and white film, created a separate dye transfer matrix from each, and dyed a single layer of receiving film with three dyes from the three matrices to make the theatrical print. It wasn't until the advent of Eastman's multilayer Kodachrome that a color film suitable for the mass market became practical.
Today virtually all color film includes multiple layers stacked on top of each other Light impinging on the film passes through all the layers. The layers have different spectral sensitivities, so depending on the color of the light, it will expose a specific layer. In most films, each layer also is given at manufacture a unique color coupler, or piece of a dye molecule, that will react with byproducts of development to form a full dye appropriate to the color sensitivity of that layer. After development, the silver image is bleached away, leaving a color image composed of dyes in layers. If you abrade a color film, black areas will first turn blue as the yellow layer is removed, then cyan, as the magenta layer is removed, and finally white as all layers are removed.
Kodachrome has multiple layers, but uses a unique process limited to reversal transparency film that does not require couplers to be stored in the undeveloped film. After a first development to expend to silver the exposed silver halide, the unexposed halide is flash exposed and processed in a developer containing its own color coupler. In Kodachrome the flash exposure is done one color at a time to flash one color layer at a time, followed after each flash by a developer with a coupler specific to the color sensitivity of that layer. Kodachrome development is very difficult, and only a few labs in the world process Kodachrome. However, by eliminating color couplers from the film during exposure, light scattering in the emulsion is reduced, giving Kodachrome an extra measure of clarity. Negative film gives a much wider latitude than reversal film, but in the prior art the Kodachrome process was limited to reversal film, and there was no way to obtain the latitude advantage of a negative film and at the same time the sharpness advantage of a film without couplers.
Prior art color film was limited to operate in the RGB color space because each layer had to map to a specific dye that would develop an image, or color channel, that could be viewed or printed directly without color space conversion. Thus, a red sensitive layer was needed to generate cyan dye to modulate the amount of red light passing through the developed film, a green sensitive layer generated magenta dye, and a blue sensitive layer generated yellow dye. This traditional requirement of direct viewability of the chemically developed image placed a restriction on prior art color film to sense light in the RGB color space. Furthermore for pure colors to be recorded and viewed without color space conversion, the layers had to sense relatively pure red, green, and blue, without cross, contamination of colors. For example, if the magenta forming layer sensed blue light in addition to green, and the red sensitive layer sensed blue in addition to red, then a blue flower would expose not only the blue layer, but also the green and red, forming a shade of gray when developed conventionally and viewed directly as a color image.
Further, the depth ordering of sensitive layers of conventional color film was limited by the universal sensitivity of silver halide to blue light. Silver halide always is blue sensitive. In addition to this blue sensitivity, dyes may be added to trap the photons of other colors and couple them into the halide crystals. Thus a green sensitive layer is actually sensitive to both blue and green, and a red sensitive layer is sensitive to both blue and red. Green only and red only sensitive layers, which are needed for direct control of the magenta and cyan dyes, can only be realized by filtering out blue light with a yellow filter. In color film this is accomplished by adding a yellow filtering layer. This yellow layer must of course, be placed above the red and green sensitive layers in order to filter blue light from those layers, and must be under the blue sensitive layer so as not to occlude blue light from that blue sensing layer. Thus in the prior art, the blue sensitive layer had to be on top, over the yellow filter and therefore over the red and green sensitive layers.
Undeveloped silver halide scatters light with its milky consistency. When held up to the light, undeveloped film acts as a diffuser and attenuator. This can be observed while loading film into a camera. Each sensitive layer in color film degrades the image for lower layers, both by diffusing and thus blurring the light, and also by using, reflecting, and absorbing some of the light, thus dimming the light to lower layers, and requiring those layers to be more sensitive, and hence grainier. Only the top layer receives the full unattenuated, unblurred light.
Because the human eye senses detail almost totally in the luminance, ideally the full luminance should be sensed in that top layer. Unfortunately only one layer can be on top. The next choice would be to make that one layer the green sensor because green is responsible for over half the luminance. But as we have just shown, the blue layer must be on top, followed by a yellow filtering layer prior to the green. Blue is responsible for only about 10% of luminance, therefore the mandatory requirement that blue be on top means that almost all the luminance is sensed at lower layers where the image is dimmed and blurred. Most of the advancement in film technologies has been in color film, and yet today a fine-art black and white print has a clarity and vivaciousness that is not matched by prior art color film.
There have been historic and niche attempts to place green or red on top. Most interesting is color print paper which places blue at the bottom and red on top. The immediate question is how the red sensing layer is shielded from blue. Actually it isn't shielded, and in fact the red sensing layer is nearly as sensitive to blue light as to red. Several conditions unique to printing paper make this practical. First, because the high contrast paper views a low contrast negative, the density range of exposure needed to go from white to black is only about 10:1, as opposed to camera film that must respond over a 1000:1 range, and a separation of 100:1 between red and blue is therefore adequate for print paper. Printing paper sees light that typically emanates from an orangish incandescent light, filtered by an orangish filter pack that removes typically 80% of the blue from the lamp, and is then focused through a negative that has the base orange cast of the coupler mask. The deep orange of the resulting light takes the place of the yellow filter level in camera film for the low contrast image of negative printing, and the blue sensitive layer is made about 100 times more sensitive than the red layer to compensate for the orange light, which is possible because of the relatively low base sensitivity of printing paper compared to camera film.
The approach used in printing paper to put the red on top would not work with camera film because, first, the film must respond over a range of 1000:1, not 10:1,
Allen Stephone B.
Applied Science Fiction Inc.
Dinsmore & Shohl LLP
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