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Zone-10 Color Tester - Olympus E-1 MOST sensors use a three color sensor array consisting of Red, Green and Blue sensitive detectors. In MOST cases, the peak sensitivity is at the primary colors at or near 650nm (Red), 540nm (Green), and 450nm (Blue). As these are the primary colors, all other colors in the visible spectrum can be mixed from them. The IR cut filter kicks in just past 650nm which is a good thing because all detectors are equally sensitive to IR at 850nm. If you didn't cut off the IR wavelengths bad things happen...
In the case of the Kodak sensors found in the E-1, E-300, E-500 and E-400, all of the green detectors have a peak sensitivity at approximately 540nm and they are identical in how sensitive they are at that wavelength. What is different, however, is that half of the green detectors are slightly more sensitive to the longer wavelengths. In some documents, these particular detectors are referred to as "Orange" pixels, but that is a misnomer because the peak wavelength sensitivity is still pure green, but the anti-red filtering isn't quite as strong.
As a point of comparison, the typical human eye's cones are sensitive to three different wavelengths of light too, but definitely do not match the primary colors we are familiar with.The cones are sensitive to short (blue), middle (green) and long (red) wavelengths, but they peak at 425nm, 525nm and 570nm. The color "red" is not directly seen by the human eye, but is extrapolated based on the absence of green.
I know some of you must be thinking "what about film?" I'm glad you asked. Kodachrome's peak color sensitivities are 425nm, 550nm and 650nm with a very hard IR cut kicking in almost immediately with the sensitivity essentially nulled by 700nm. Fujicolor Reala has FOUR sensitivity layers with the standard color overlaps between the layers for the Red, Green and Blue layers, but the fourth layer is not sensitive past 575nm which is where the red sensitivity layer starts. This fourth layer is the Cyan layer.
What does all of this have to do with practical application? Well, I'm glad you asked that question too!!! How does a film, digital or the human eye see the color purple? And what exactly is the color purple? We all know that with paints, if you mix the primary colors of blue and red together you get purple, right? That is correct, but does that actually MAKE the color purple or is it fooling the eye into seeing purple? It doesn't actually form the color purple, but it blends to make the eye think it is the color purple. However, there are pigments which truly are purple with a reflectance in the 400nm area. If you filtered out red light, they would still reflect purple. An African Violet is a bit different. An African Violet has a peak reflectance of 450nm-475nm which is blue and another peak reflectance starting at 675nm and extending far into the IR band. An African Violet isn't actually purple, but is blue and red. Unfortunately for most sensors, the red portion is actually far into the IR cut wavelengths so most digital cameras only see blue. If the real color of a pigment or flower is purple with a peak reflectance around 400nm, not all is lost because the Red detectors in a digital camera have a second minor peak in the deep purple range (just beyond blue) so a true purple (instead of a mixture of blue and red) is actually visible to most cameras.
Which brings up a point. What about the Macbeth ColorChecker or an IT-8 target? Can you determine color response of a film or sensor with it? The answer is yes and no. It totally depends on the pigments or dyes used in the target itself. If they are printed with cyan, magenta and yellow dyes then the camera is ONLY seeing a mixture of cyan, magenta and yellow. That purple square in the target? It's NOT purple. It's actually magenta and cyan. You cannot create colors in printing or painting, just blend them. A blended paint or ink does not lose its native pigmentation--it just blends the color dots much smaller than a half-tone printed page. So, the reality is, we're judging how our sensors see blended colors, not true spectral response which would only be determined through refracted sunlight. (prism or DVD/CD). Fortunately, however, most colors in the natural world are blended colors anyway. To illustrate this, the two photographs in this article are taken with two different cameras, the Olympus E-1 and the Nikon D2X. The flowers appear purple in the E-1 shot, but blue in the D2X shot. To the human eye, the flowers were nearly identical to square K4 on the IT8 target.
The sources of the information above include: 1. Vision and Art, The Biology of Seeing, by Dr. Margaret Livingstone. 2. Kodak specification sheets for the KAF5101CE, KAF8300 sensors and Kodachrome. 3. Fujifilm Reala specification sheets. 4. Miscellaneous sources to support the above theories.
Zone-10 Color Tester, Nikon D2X
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