Contents > Colour basics 2 / 10
 

Human eye
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The human eye and colour perception
Published on April 15, 2015 / Updated on October 22, 2018

 

The eye "sees" - we are talking about perception - colours in a very strange way after all! The photosensitive cells that line the retina see, in short, only in blue, red and green ! And yet we see thousands or millions of colours. So everything seems to be happening in the brain. But in fact, how many colours do we perceive ?


The eye is an extremely complex organ, the result of a very long evolution. It allows us to see the world in colour and to perceive thousands of colours. But how does it do it ? What is colour and how much can the eye perceive from different colours ? Why and when are we talking about gamma ? How to represent it digitally ? These are all questions I will try to answer now.


 

The visible light

Before we understand how the eye sees colours, let's take a look at light.

Light properties

Light is an electromagnetic wave and "its color" depends on its wavelength. Like all waves, it has an infinity of different wavelengths that give it its "color" when we talk about colors visible to man. 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. All visible waves are called the visible light spectrum. Beyond the spectrum visible to man are ultraviolet, X-rays and other gamma rays on the sides of the even shorter waves and infrared and then radio waves on the side of the longer waves. Traditionally, a spectrum of sunlight is obtained by passing it through a prism or through water droplets. This creates a rainbow in certain weather conditions where all the colors below are perceived.

   
Spectrum of light
  Spectrum of visible lisible to the human eye
 

 

The human eye and light

The back of the human eye - the retina - is lined with light-sensitive cells but with unique properties. Here they are...

Spectral sensitivity of the three main categories of human cones

An image is formed by the cornea (which is transparent except when you get older) and projected onto the retina at the back of the eye. It is literally lined with nerve cells - the photoreceptors - sensitive to light in quantity - clarity - thanks to the rods, but also in quality - colors - thanks to the cones. Not all cones (the only ones capable of perceiving colours) are sensitive to the colour of light in the same way and have a striking singularity. Indeed, some cones are mainly sensitive in red, others in green and finally the last ones in blue.

Key point ! Therefore, there is not one type of cone per color at the back of our eye. The colours are well "reconstituted" and "perceived" in the brain thanks to the "mixture" of the perception of light, mainly blue, green and red (yellow if we look closely at the diagram below). Let us note in passing that the "blue" cones are really centered on the blue, the "green" cones are really centered on the green but the so-called "red" cones are more centered on the yellow. Finally, note that the "green" and "red" cones perceive many common colors.

   
  Spectral sensitivity of human cones and batonets
  Spectrum of colour absorption of a human eye.
 

 

The pixels of a screenHowever, the eye can see all the colors of the visible spectrum, from purple to red continuously, so it is by mixing these three "colors" that the brain is able to "make", to reconstitute, and therefore to perceive all the "other" colors through its brain. That's why we invented an RGB color model: to get closer to the way the human eye works. Thus, the screens or sensors of the APNs have Red-Green-Blue or RGB pixels and only see in RGB! They approach the functioning of the human eye through three "types" of pixels. It is enough for the screen to turn on more or less each pixel of the screen to reproduce ALL the visible colors! If you get too close to a computer screen (picture above right), you see the RGB pixels separated but they are made small enough that, seen by a human being at a normal distance, they are seen as amalgamated into a single pixel with multiple colors.

   
 
 

Funny remark !


Bayer grid of cameraThe eye is not sensitive to all colours in the same way, nor is it more sensitive to small differences in brightness in low light than in high light. Scientists have noticed that the eye has a more extensive sensitivity in green than in the other two primary colours (R and B). This is simply why the APN sensors have two green pixels for a red pixel and a blue one when viewed closely with a very magnifying glass.
The same applies to displays or digital sensors. When you look at them with a strong magnifying glass or a microscope you can see that they have two green pixels for a red pixel and a blue pixel.

 
     
   
 

Additive or subtractive synthesis ?

Before trying to measure the number of colors that a human being is able to see, let's try to understand how it perceives colors since it does not have a cone that is sensitive to each color. To do this, let's analyze how a screen and a printer work.
A monitor or printer does not work with colors in the same way : the screen works with so-called additive colors - Red, Green, Blue - or RGB to give white by addition as shown in the center of this color animation below - while a printer works in subtractive mode - Cyan, Magenta, Yellow, Black or CMYK to give black by subtraction - (below). Well, it turns out that the human eye works a little like a screen (it's an image of course!), i.e. in an additive mode.

Couleurs RVB additivesIn concrete terms, with a display that works in additive mode, if no RGB signal is sent to the pixels (0, 0, 0, 0), the display remains black. It is black by default. To obtain white (255, 255, 255, 255) it is necessary to illuminate each Red, Green and Blue pixel, as shown in the figure opposite at most. The other colours are obtained by changing the value of each primary colour. To display yellow for example (255, 255, 255, 0), you must stop illuminating the blue pixels (the screen is yellow at the intersection between green and red because the blue pixels are off). Only three colors require only one pixel. The three primary colours and their variations in brightness (from 0 to 255). All the others, i.e. the millions of others, will be a mixture of at least two pixels of more or less luminous primary colors.

Couleurs CMJ soustractivesWith a printer, you start from an already white sheet of paper by default because it reflects all wavelengths of visible light in equal parts. So there, it is the opposite, if we want black we will have to project ink with complementary colours on this sheet, i.e. Cyan, Magenta and Yellow which have the particularity of absorbing each time a part of the visible light. In practice it will also be necessary to add Black ink because 100% of the other three colours do not produce a deep black because of the impurities contained in the inks but a very dark brown. It is very clear from the image opposite that the centre is brown and not black. These inks projected in the same place will have the property of absorbing all the luminous radiations. So this place will look black now. If you want the paper to look blue, you have to project magenta and cyan on it in the same place. All light radiation, except blue, will be absorbed. Only the blue wavelengths of the light will be reflected at this location.

By convention, it has been decided by the International Commission on Illumination - CIE - that the primary red colour is light with a wavelength of 700 nm, green 546 nm and blue 436 nm. Whatever the wavelength of their RGB pixels, screen manufacturers still try to reproduce colours as well as possible...

How many colors are perceived by the human eye ?

It remained to be seen how many colored shades the human eye could perceive in these famous three so-called primary colors. This work was carried out in the 1930s under the aegis of the Compagnie Internationale de l'éclairage (La CIE). She asked very many people to carefully compare shades of blue, green and red to count how many different colours, on average, they were able to distinguish. Two important things emerged from these test series.

  • Human beings with the finest visual acuity are able to distinguish up to 200 shades per color,... but
  • never in the three primary colors !
  • And the gamut of the "average" eye is very wide : the famous L*a*b* space.


So if a simple arithmetic calculation gives us 200 x 200 x 200 x 200 or 8 million different perceived colors, in reality we are very far from that because color specialists (researchers, color marketing professionals) tell us that 300,000 (three hundred thousand) would be much more realistic ! If a high-performance eye is indeed able to distinguish up to 200 shades of the same primary colour, the same eye never achieves this feat in the other two primary colours.

Conclusion on color perception : Certainly, the human eye is sometimes able to distinguish many shades for a given color, but overall, a significant difference between two shades of the same color is required so that, without ambiguity, this person can affirm that he or she has indeed perceived two different colors. This does not prevent him from perceiving a very important and wide gamut, the famous L*a*b* space. We are beginning to guess that several RGB combinations will code for the same color perceived by an individual...

It is now time to know how a colour is described "computerically". Typically, colours are described using three or four digits depending on whether you work in RGB or CMYK, always with the objective of reproducing the functioning of the human eye. Thus such color will be noted - 158, 128, 84 - in RGB and - 80%, 64%, 21%, 12% - in CMYK. But why are these numbers and to which colour does it correspond exactly ?

The next page is about how computers treat these colors when they are only able to work with 0s and 1s - Colors and computer science - 3 / 10  


 
 

To be remembered !


Light is an electromagnetic wave whose wavelengths visible to humans range from about 380 to 700 nm (from purple to red).

 The eye "sees" - we speak of perception - thanks to cells that are photosensitive to colour and others only to brightness. Colour-sensitive cells are called cones. They are most sensitive in red (earth), green (chlorophyll from trees) and blue (sky). There is not a cone for every color.

 The cones are sensitive for each colour to about 200 shades, from the darkest to the lightest : this represents 200 x 200 x 200 x 200 = 8 million mixtures of different colours for an "average" eye.

 By recomposing or mixing these three so-called primary colours, the brain "sees" up to eight million colours... but...

 Most people see "only" one to two million colours (!) because it is rarely as effective in the three primary colours.

 
     
 
 
Through these 10 pages we will learn all the vocabulary related to colour management: colour spaces, ICC profiles, gamuts, etc...
- Introduction to colour management
- Eye and colour perception - 2 / 10
  - The visible light
- The eye and human vision

- Colours and computer science
- Gamma
- Colour spaces
- ICC profiles
- Assign an ICC profile
- Convert an image
- Relative and Perceptual Conversion Modes
- What is calibration ?!

 

- 2018 monitors buying guide !
- My 21 full monitor review


 

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