Orquid "Phoenix"

Orquid "Phoenix"

by ceinmart

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From what I understand most digital cameras have a sensor where each pixel-sensor has three sub-sensors, each one with an R,G and B filter. RGB is obviously the more fundamental colour model since it directly corresponds with the receptors in the human eye.

However, RGB filters necessarily cut out two thirds of white light to get their component. Surely cameras would benefit from shorter exposure times if the filters where instead CYM where each element cuts out only one third of the light? The camera's processor can still save the image in whatever format the consumer wants since a CYM datapoint can be converted easily to an RGB one.

I know this is sometimes done in astrophotography where three seperate B&W photos are taken with CYM filters.

Am I just wrong and this is, in fact, what's already done - or is there a good reason for an RGB sensor?

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In long exposure astrophotography, one of the few areas where the total amount of light captured is a major concern, a common approach is to use a pure monochrome sensor and combine (relatively) short exposures periods taken with red, green, and blue filters to color the image, with a much longer interval where no filter is used to maximize the detail in the final result. –  Dan Neely Oct 16 '13 at 13:37
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2 Answers

up vote 9 down vote accepted

First, a little background to clear up a slight misunderstanding on your part.

The vast majority of color digital cameras have a Bayer filter that masks each pixel for only a single color: Red, Green, or Blue. The RAW data does not include any color information, but only a luminance value for each pixel. Just as with the human eye, almost all Bayer filters include twice as many Green pixels as they do Red or Blue pixels. In other words every other pixel is masked with Green and the remaining half are split between Red and Blue. So a 20MP sensor would have roughly 10M Green, 5M Red, and 5M Blue pixels. When the luminance values from each pixel are interpreted by the camera's processing unit the difference between adjacent pixels masked with different colors are used to interpolate a Red, Green, and Blue value for each pixel. Each color is additionally weighted to roughly the sensitivity of the human eye, so the Red pixels carry a little more weight than the Blue ones do. The process of converting monochrome luminance values from each pixel into an interpolated RGB value for each pixel is known as demosaicing. Since most camera manufacturers use proprietary algorithms to do this, using third party RAW convertors such as Adobe Camera RAW or DxO Optics will yield slightly different results than using the manufacturer's own RAW convertor. There are some sensor types, such as the Foveon, that do have three color sensitive layers stacked on top of each other. But the manufacturers claim such a sensor with three 15MP layers stacked on each other is a 45MP sensor. In reality such an arrangement yields the same amount of detail as an approximately 30MP conventional Bayer masked sensor. The problem with Foveon type sensors, at least thus far, has been poorer noise performance in low light environments.

So why don't most digital cameras use CYM filters instead of RGB filters? The primary reason is color accuracy. It is much more difficult to interpolate color values accurately using values from adjacent pixels when using a CYM mask than when using an RGB mask. So you give up a little light sensitivity to gain color accuracy. After all, most commercial photography at the highest levels is either done with controlled lighting (such as a portrait studio where it is easy enough to add light) or from a tripod (which allows longer exposure times to collect more light). And the demands of professional photographers are what drives the technology that then finds its way down to the consumer grade products.

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Thanks for the Bayer filter information. As someone who lives in a gloomy, low-light country I'd always optimise for sensitivity, but I understand the argument for colour accuracy. Great answer! –  Robin Oct 17 '13 at 13:06
    
Does your gloomy, low light country not have electricity and light bulbs? :-) –  Michael Clark Oct 17 '13 at 23:42
    
We are the most CCTV covered country in the world, so I guess some cameras work. As for light bulbs... I prefer to harness the glorious power of the sun. –  Robin Oct 18 '13 at 9:00
    
Even the glorious power of the sun needs a reflector sometimes to cast light on the shadow side of a subject. And the sun is 'asleep' half the time over the course of a year, no matter where on this planet you are. –  Michael Clark Oct 23 '13 at 7:47
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Cyan magenta yellow sensors have been made, along with red green cyan and a few other variations.

The main problem is that even with RGB sensors there is significant overlap between the spectral response of each of the dyes, i.e. the "green" pixels are sensitive to red and blue light to a certainer extent. This means the results require complex calculations to obtain accurate colours, the relative responses of adjacent red and blue pixels are used to judge how much of the green response was really the result of red and blue light.

With CMY the problem is much worse. You're essentially trading light efficiency for colour accuracy. This may be fine for astronomical photography where you don't always have crisp colour boundaries, hence you can reduce colour noise by blurring, but it's not good for landscape or fashion photography.


Amongst RGB chips, the exact choice of filters varies by manufacturer. Canon for instance uses weak dyes with broad response in order to chase low light performance, but the specific dyes used are also tuned toward discerning colours under fluorescent lighting, for the benefit of the army of sports and news photographers who use Canon cameras.

Sony on the other hand with the A900 tried to break into the professional fashion market by providing very high colour accuracy. Colour filter arrays used in medium format digital backs are tuned to provide pleasing (though not necessarily accurate) skintones.

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