I'm trying high speed dark room photography with Logitec c270 simple CMOS web camera. I flash red or blue light and capture frames. Then I separate it to channels in photoshop. There, I can see all the channels have some exposure, instead of just the flasher color's channel. E.g. when a red flasher is used, the picture is clear in red channel, but green channel also has a shaded picture. This phenomenon get worse when the flasher is brighter.

Can someone please explain how this happens, and how I can get rid of this?

  • Can you explain further what you are trying to accomplish with this and why it is a problem? – mattdm Sep 27 '17 at 20:40
  • @mattdm an ongoing experiment on PIV – TRiNE Sep 28 '17 at 1:20
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    Your requirements may not really be suited for this site, which focuses on photography for the sake of photography rather than using cameras for measurements. – mattdm Sep 28 '17 at 1:25
  • @mattdm I don't want to get requirement fulfilled. and it can't be done. I want to figure out this specific thing. when people don't know specific thing they tend to bypass it and cover the requirment in other ways. I don't wan't to figure out other possible ways. it is not about PIV. it is specific question to photography. – TRiNE Sep 28 '17 at 1:29

There are at least two reasons this happens:

  1. There are very few pure (single / narrow frequency band) light sources. That is to say, your red light is not strictly confined to the "red" end of the visible light spectrum. It emits light in the yellow/green region of the spectrum (and probably just a little in the blue, as well). Similarly, your blue light is not confined to the blue region of the visible light spectrum.

    Lab-grade lasers emit very narrow light spectra. They are probably the closest "single pure color" light source you can use. Most other light sources have much broader emission spectra.

  2. There is substantial overlap in the light frequency sensitivities of the color filtered photosites in your CMOS sensor. That is, the red-sensing sensor pixels (more appropriately, "reddish"-sensing) overlap quite a bit into the greenish spectrum. The blueish-sensing pixels also respond a little bit to greenish and reddish light, as well.

    There is nothing you can do about this. Tri-color imaging sensors are made to mimic the human eye's response to light. The "average" or "normal" human eye color receptors, while described as red, green, and blue, have a lot of overlap in light frequency. This is especially so in the red and green receptors, to the point that the most common form of color deficiency or colorblindness is red-green. This is because the frequency response of the red cones and green cones in red-green deficient or colorblind people overlaps so much, that they can't distinguish reddish hues from greenish hues.

If you want more color separation, you will have to get better / more pure light color sources. But even if you use a single-frequency light-blue light source and a single-frequency red light source, they will probably be picked up in the green sensors to some degree.

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    Interesting bit of trivia, there's a company that makes glasses for color-blind individuals that puts a notch in the overlap region between red and green. By making those receptors in the eye less overlapping, some people can start seeing in full color for the first time! – Mark Ransom Sep 27 '17 at 22:31
  • @MarkRansom talking about Enchroma? Yeah, they have a pretty interesting thing going on, but as you said, it's basically just a narrow-band filter to increase separation. I've looked at them. I'm one of the people who would probably benefit pretty well (mild deutan) (r-g deficient). I just haven't pulled the trigger yet. Inertia, I suppose. – scottbb Sep 27 '17 at 22:35
  • I couldn't remember the company name, but Enchroma sounds familiar. It's one of those things that sounds too good to be true, but the theory actually seems plausible. If it works for you I'm sure the experience would be truly mind-blowing. – Mark Ransom Sep 27 '17 at 22:39
  • @MarkRansom If the videos of people seeing color for the first time are to be believed, then it looks like a great product. I should just get them... – scottbb Sep 27 '17 at 22:40
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    @TRiNE that could certainly be a contributor, but that's a tertiary explanation. My #2 point above is probably the primary reason for the red LED to show up in the green and even blue color component values. See the 2nd graph in this answer to the Photo.SE question, Why are sensors less sensitive to blue light? – scottbb Sep 28 '17 at 1:34

Why do color channels from a digital camera respond slightly to lights of other colors?

Because the filters placed in front of the silicon on a Bayer masked filter all allow some of all the wavelengths of visible light to pass through. They just allow more of the light at wavelengths near the color of each filter to pass.

Back in the days of B&W film photography color filters were often used. At first glance that might sound a bit ridiculous. But what color filters allow one to do with B&W photography is separate the similar brightness of two objects of different colors. If one has a red flower and a green flower in a scene and both are the same brightness, then an unfiltered B&W photo would show both as the same shade of gray. If one places a red filter in front of the camera, more of the light from the red flower is allowed to pass through the filter than the light from the green flower. Now the red flower will be a brighter shade of gray than the green one will in our photo. But we can still see details in the green flower, they're just darker than they were before. Everything that isn't the same exact shade of red as the filter does not become pure black in our photo. The red filter has not eliminated all green light, it has just reduced green light compared to red light of the same intensity.

"Everything that isn't the same exact shade of red as the filter does not become pure black in our photo."

Yet that is what it seems many people expect of the Bayer masks on digital image sensors - they seem to think that only a range of green wavelengths make it through the green filter, and only a range of red wavelengths make it through the red filter, and only a range of blue wavelengths make it through the blue filter.

The color filter arrays on Bayer masked imaging sensors work exactly the same way as color filters do with B&W photography. Each sensel (a/k/a pixel well) in an imaging sensor records a single brightness value. All light that passed through the filter above it is recorded equally, regardless of the wavelength of that light¹. The color filters of a Bayer sensor are also working similarly to the way the cones in our retinas work. That's why a Bayer masked sensor and an RGB display device can recreate colors that look "correct" to our eyes. They're using the same three channels that our eye/brain system does to create the colors we perceive.

¹ Technically speaking, the response will vary by the silicon wafer's response to various wavelengths of light when there is no color mask in front of it. But the response to a 'blue' photon that passes through the bayer filter will have the exact same effect on a red, green, or blue filtered sensel. It's just that fewer 'blue' photons that strike the front of the Bayer mask will get through the red and green filters than get through the blue one.

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