It’s A Green World

That’s not an environmental statement. As far as our cameras are concerned, green is the “most important” color. I’ll explain why green is foundational to our photography.

Bayer filter

In my previous article I discussed the Bayer Filter and how it allows our digital cameras to reconstruct color. I made a cryptic comment that it was important that there were twice as many green cells as red and blue, but I did not explain. I’ll try to correct that. It is fascinating and highlights some of the brilliance of the Bayer filter design.

Bryce Bayer’s patent (U.S. Patent No. 3,971,065[6]) in 1976 called the green photosensors luminance-sensitive elements and the red and blue ones chrominance-sensitive elements. He used twice as many green elements as red or blue to mimic the physiology of the human eye. The luminance perception of the human retina uses M and L cone cells combined, during daylight vision, which are most sensitive to green light. ” This is quoted from Wikipedia. Let me try to unpack it a little.

Color description

There are several ways to describe color. Some, like the HSV or HSB or Lab models, separate the concepts of luminance and chrominance. Luminance is the tonal variation of a scene, the brightness range from black to white. Hue and saturation define the color value and purity.

It is all very complicated and, in reality, only interesting to color scientists. I strongly recommend you view this great video that explains how the CIE-1931 diagram was created and what it means. It answered a lot of my questions. As photographers and artists we have to be familiar with some of it. For instance, we have all seen a color wheel like this:

This is a simplified slice through the HSV space at a constant, maximum lightness. Such a model is useful to us because it shows all colors with their most saturated form at the outer edge and least saturated (white, colorless) in the center.

Our eyes

This is nice, but it is all possible colors, not what we really see. As the quote above about Bayer said, the eye is most sensitive to green. Green is right in the middle of the range of light we are sensitive to, the visible spectrum. Here is a plot of our sensitivity to visible color:

Subjective response of typical eye
From: https://lightcolourvision.org/wp-content/uploads/09550-0-A-BL-EN-Sensitivity-of-Human-Eye-to-Visible-Light-80.jpg

It is clear to see, just as Mr. Bayer said, we are most sensitive to green. This is why there are twice as many green cells in the Bayer filter as red and blue. The green is used to measure the luminance, the tone range of the image. This information is critical to deriving the image detail plus the color information through a complex set of transformations.

Why is is so important to get a good measure of luminance? Because of another interesting property of the eye. We are more sensitive to luminance than color. Luminance gives detail. Think of a black and white picture you like. That image is pure luminance information, no color information at all. Yet we see all the fantastic detail and subtle tones perfectly.

Color adds a lot of interest to some images, but we can recognize most subjects perfectly well without it. The opposite is not true in general. If you took all the luminance information out of one of your images it is basically unrecognizable.

Example

Here is a quick example of a typical outdoor scene here in the Colorado mountains. This is the original image:

If I convert it to Lab mode and take just the luminance channel (L) we get a black & white version containing all the detail and tone variation that makes it recognizable:

But now if I copy just the color information (the a and b channels) it is … surreal?:

Why green?

I hope I have demonstrated some of the reasoning behind the Bayer filter. It is a key to our ability to capture color information with our cameras.

The human eye really is most sensitive to green. Having half the color filters in the Bayer filter array as green allows maximum ability to construct the luminance data we are so sensitive to. The magic of the sophisticated built in data processing algorithms let the Raw file converters take all this information and derive the luninance and color information we rely on for our images.

Does this mean we should shoot more green subjects? No. I don’t. Many on my images have little discernible green in them. Take the image at the top of this article. I love the colors in this mountain stream. I don’t look at it and think “green”. The color range is very full, though.

As I write this it is the depth of winter here. Much of the shooting I do right now is very monochrome, almost black and white. The Bayer filter is not there to make our images more green. But if you look at your histogram or channels you may be surprised at how much green data is there. Think about it, a black and white image is 33% green.

Thank you Mr. Bayer and all the scientists and engineers who have done such a great job of perfecting our digital sensing over the decades. You are doing an excellent job!

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