How RGB light works

I’m getting some questions about ‘digital light’. How does it actually work, and how on earth can an RGB light source be suitable for photo and video? The answer might surprise you. Lemme explain.


The sun has been around for a long time, it shone down upon us as we staggered out of the primordial ooze. Because our eyes evolved under the sun, we expect to see stuff in sunlight. Colors look ‘right’ when seen under sunlight.
Intuitively you would think that any light for photography must be as similar to sunlight as possible. Sunlight is after all the gold standard. Well, that certainly sounds reasonable, but it is not the whole story. Not even close.

From Wikipedia

Normalized responsivity spectra of human cone cells. From Wikipedia

How do we actually see colors?

Humans eyes contains 3 types of color sensitive cells called cones. One type is sensitive to red, one is sensitive to green and one is sensitive to blue. Physically, we can sense three colors and three colors only. So how are we able to see the other colors, for example purple? Or yellow? The eye has no sensors for purple or yellow or any of the millions of colors we can distinguish.

The explanation is this: Light are waves. Actually they have a wave-particle duality, but let’s not even go there. Some lightwaves, the blue ones, have a short wavelength, some have long wavelengths, those are the red ones. Different wavelengths gives different colors, you can see them all in a rainbow. Yellow light has a wavelength half way between red and green. The sensitivity of the cones in our eyes overlap a bit, so yellow light will stimulate both the red and the green cones in our eye. The brain knows that an equal amount of red and green means yellow. So we experience a mix of red and green as the color yellow.

Let’s do an experiment. The eye see yellow because both the red and green cones are stimulated. So what happens if you shine, not a yellow light, but a mix of red and green light into the eye. Well, the eye again senses an equal amount of red and green, and again sees yellow! Interestingly, the eye can not see a difference between actual yellow light and a mix of equal amounts of red and green light. That requires of course that the lights mix. That they overlap or are so small and close together that the eye can’t resolve the individual lights or pixels, like on your computer monitor.

Proof is right in front of you

Still sceptical? Think about it. Each pixel on the computer screen you are watching right now is made up of three colors only, red, green and blue. So there is no way for your computer screen to send out yellow light. But you are still able to see these yellow dots right here: • • • , right? If you look at the screen with a magnifying glass, you will see that the red and green in each pixel is lit.

This is called additive color mixing. Colors of light add together in the eye. Mix green and red, you see a third color, yellow. Mix red and blue and you see purple. White is what you see when you mix equal amounts of all three. Additive color mixing is not a physical property of light, it is how the eye interprets light.

Additive color mixing, From Wikipedia

But if there is a yellow object on a table, say a pencil, and you shine an RGB light on it, there is no yellow wavelengths in the light. The pencil can’t reflect yellow light. Won’t the pencil look black? No, and that is because we don’t see an object as yellow because it reflects yellow light. We see it as yellow because it absorbs blue. Your head spinning yet? Take an aspirin and watch this.

The ability to produce any color from mixing three primary colors are at the core of digital video and photography. Your eyes senses red green and blue (RGB), your camera sensor is a big array of RGB sensors, your computer screen is made up of RGB pixels. Here we are making digital lights with RGB emitters. Makes sense.

Neat system, yes?


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