Damn Ken. That hurt my head. I think I need a beer then give it another
go.
Charlie
On Fri, Nov 20, 2009 at 2:08 PM, Ken Norton <ken@xxxxxxxxxxx> wrote:
> >
> > > They can’t be “identical” and go on to have “slightly more red
> > sensitivity.”
> > Thanks for that. I hate to always be the one.
>
>
> Simple explanation, actually. But you know that "simple" ain't suitable,
> therefore here is a typical Schnozz answer.
>
> 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. (Leica M8).
>
> 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.
>
> 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.
>
> AG
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