The eye "sees" the colors in quite a weird way finally! To sum up, the light-sensitive cells covering the retina only see in blue, red and green. And yet, we see thousands or millions of colors. Everything seems to happen in the brain. And by the way, how many colors are we really able to see?
Key points if you are a beginner ...
Here are the key points to remember on eye and colors. The rest of this page will be dedicated to those who want to go into more details.
How does the human eye sees colors ? The human eye sees colors through the light-sensitive cells at the back of the retina. it is lined with two types of photosensitive cells:
cells sensitive to colors : cones,
cells sensitive to light quantity (they see in B&W) : rods.
The cells sensitive to colors are in fact particularly sensitive to the color red, green and blue. So all colors are "constructed" by our brain and are not seen directly.
First of all, let's talk about colors, but how many color is an eye of a "standard" human being able to see ? A "standard" eye can distinguish about 200 nuances per color. The human eye sees therefore approximately: 200 x 200 x 200 = 8.000.000 colors ! (Note that we are far from the billions of colors, that some adds on monitors or televisions are talking about).
The eye is an extremely complex organ, fruit of a very long evolution. This is what enables us to see the world in colors and see millions of them. But how does it work? What is a color and how many different colors can the eye see? Why and when do we talk about gamma? How to represent it digitally? These are questions that I will try to answer now.
The eye and the vision of light
Before talking about how the eye sees colors, let's have a look at what light is.
Properties of the light
Color is a property of the light that is an electromagnetic wave. Like all waves, it can have different frequencies that give it its "color", when speaking of colors visible by humans. When the wavelengths are short - around 380 nm - we perceive them as blue-violet and when they are longer - around 700 nm - we perceive them as red. The whole set of visible waves is called the visible light spectrum. Beyond the visible light spectrum for humans, on the sides of short wavelengths, are ultraviolet, X-rays and gamma rays and on the side of long wavelengths, infrared and radio waves. Traditionally you can obtain a spectrum of the sunlight, if it passes through a prism or through water droplets. This creates a rainbow in certain weather conditions where one perceives all colors below.
Light spectrum visible for humans, from ultraviolet to infrared.
An image is formed by the cornea (transparent) and projected at the back of the eye, on the retina. This one is literally covered with nerve cells - the photoreceptors - sensitive to the quantity of light - the brightness - thanks to the rods, but also to its quality - the colors - thanks to the cones. Cones (therefore sensitive to the colors) are not all sensitive to light color in the same way. Some are especially sensitive to red, others to green and finally the last kind to blue.
However, the eye sees all the colors of the visible spectrum - above - from red to violet in a continuous way, so it is through the mixture of these three colors that the brain is able to "produce", to build all other colors. That is why the RGB color model was invented: to match the functioning of human eye. That is why monitors or colorimeters only have pixels in Red-Green-Blue or RGB and only see in RGB! They reproduce the human eye's way of functioning with three types of pixels. The screen just needs to turn on, more or less, each pixel of the display to reproduce ALL the colors! If you get really close to a computer monitor, you will see the RGB pixels separatly but they are made small enough so that, seen by a human being at a normal distance, they are seen as amalgamated, into a single pixel with multiple colors.
The eye is not sensitive to all colors in the same way, just like it is more sensitive to small differences in brightness in low lights than in high lights, by the way. Scientists noticed that the eye has a more extended sensitivity towards greens than towards the other two primary colors (R and B).That's why digital cameras sensors have two green pixels for a red and a blue pixel when you look at it with a magnifyer.
It's the same with screens or digital sensors. When you look at them with a magnifyer or a microscope, you can see that they have two green pixels for a red pixel and a blue pixel.
Millions of colors with three or four!
A monitor or printer does not manage colors in the same way : the screen, like the eye, works with so-called additive colors - Red, Green, Blue or RGB to obtain white by addition as you can see in the center of this color animation below - while a printer works in substractive mode - Cyan, Magenta, Yellow, Black or CMYK to obtain black by subtraction (below).
Concretely, with a screen that is thus working in additive mode, if one does not send any RGB signal to the pixels (0, 0, 0), the screen remains black. It is black by default. For white (255, 255, 255) each Red, Green and Blue pixel must be lit, as shown in the figure on the right. Other colors are obtained by changing the value of each primary color. To display yellow (255, 255, 0) for instance, the blue pixels must be turned off (the screen is yellow at the intersection between green and red as blue pixels are off). Only three colors require only one pixel. The three primary colors and their variations in brightness (from 0 to 255). All others, that is to say, the millions of others, will be a mixture of at least two primary color pixels, more or less bright.
With a printer, you start with a sheet of paper already white by default because it reflects all wavelengths of visible light equally. So here, it is the opposite, if you want black you will have to spray inks of complementary colors on this sheet, that is to say, Cyan, Magenta and Yellow which have the characteristic of absorbing each time a portion of visible light. In practice, you will also have to add Black Ink, as 100% of the other three colors do not create a deep black, due to impurities in the inks but a very dark brown. You can clearly see in the picture on the left that the center is brown and not black. Those inks projected in the same area will have the property of absorbing all light radiations. This area will thus now appear black. If you want the paper to look blue, then magenta and cyan must be projected over the same area. All light rays except blue will then be absorbed. Only blue wavelengths of light will be reflected there.
By convention, it was decided by the International Commission on Illumination - CIE - that the primary color red is a light with a 700 nm wavelength, green 546 nm and 436 nm for blue. Whatever the wavelength of their RGB pixels, screen manufacturers still try to reproduce colors as best as they can...
And finally, it is important to know how a color is "digitally" described. Typically, the colors are characterized by three or four digits, depending on whether you are working in RGB or CMYK, still with the aim to reproduce the functioning of the human eye. Therefore a color will be recorded as - 158, 128, 84 - in RGB and - 80%, 64%, 21%, 12% - in CMYK. But why these figures and to what color perceived by an eye does it correspond to?
To be remembered!
Light is an electromagnetic wave, which visible wavelengths for humans range from about 380 to 700 nm (from violet to red).
The eye "sees" thanks to sensitive cells, some to colors and others to brightness only. The color-sensitive cells are called cones. They're especially sensitive towards red (ground), green (chlorophyll leaves) and blue (sky). There is not a cone for each color.
Cones are sensitive to about 200 shades for each color, from the darkest to the lightest : it thus represents 200x200x200 = 8 millions different colors for a "standard" eye.
By recomposing or mixing these three so-called primary colors, the brain "sees" eight million colors.
Most people "only" see one to two million colors! And that is already a lot.
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