Definition of the color

This chapter deals with basics of color, while bringing a certain number of answers to the questions related to the general study of the color, we will try to propose the anatomical and physiological characteristics of the human eye.

We can define the color as the interaction between several factors such as the source of light which lights the object, intrinsic characteristics of the object itself, and the observer. This definition utilizes the concept of triplet.

Isaak Newton highlighted the phenomenon of dispersion of the white light through a prism. The below diagram shows that, the colors forming an integral part of the visible spectrum are limited by values in wavelengths ranging between 380 to 780 Nm.

Concept of triplet

The observer

It is the brain which interprets what the eye perceives. This is why these two elements are indissociable. The eye does not have the same sensitivity to the luminosity on the whole wavelengths constituting the visible spectrum. Thus, the C.I.E. (Commission Internationale de l’Eclairage) developed the theory basics of colorimetry and the characteristics of the spectral curve.

Diagram 1: Prism of Newton - Dispersion of the light

standard observer, synthesized in the shape of a curve of sensitivity of the eye for an average person. We will see in this chapter, devoted to the basics of colorimetry and the characteristics of this spectral curve.

The object - the subject

This element is second constituent triplet. The physical and chemical characteristics of these components plays an important role in the perception of the colors. The reflexion and absorption occurs at the time of the interaction between the object and the source of light . The reflexion of the light can occur in various ways (diffuse, specular), which implies to have the same properties on the level of the measuring apparatus of the colors (what is often the case).

The source of light

It is the third component of the triplet. We have already explained the properties and the sources of light, from the historical as well as scientific point of view. It is the first chain link in the concept of color.

The eye: optical system

In spite of the complexity of the means implemented by the eye in the process of perception of the colors, we will try to understand it's guiding principle. The diagram below helps us to locate the majority of the parts of this body intervening in the vision of the colors. In the principle, the eye functions like a camera. The iris (associate with the pupil) plays the part of diaphragm, the transparent mediums constitute the objective and the retina represents the sensitive plate to the light. The cornea helps in focusing and the crystalline lens with the development. The light penetrating by the pupil and reaching the retina finally, cause an excitation of the retinal cells, which induces a nerve impulse that the optic nerve hastens to communicate to the visual part of the cerebral cortex. It is only at the time of this stage that the visual feeling appears. Here we do not enter the biochemical meanders, physics and physiological of each stage,

Diagram 2: Diagrammatic cut of the human eye

in the explanation of the electric phenomena which occurs at the time of creation from the nerve impulse, or in the roles which the optic nerve and the cerebral visual center play.
There are receivers located at the level of the retina, which makes it possible to distinguish two kinds of retinal sensitivities: night vision (or twilight) and diurnal vision. These types of receivers are sensitive cells called cones and sticks.

Roles and Operation of the cones and sticks

The cells rétinniennes the internal wall of the retina (area called the Fovea), makes it possible to distinguish a number of colors and different colors rather consequent. There are approximately 120 million cells photoréceptrices.

The sticks are receivers being used for the night vision (approximately 100 million) and have a sensitivity of about 510 Nm. Rhodopsine is a chemical substance present in these sensitive cells and the properties are directly related to those of coloured perception.

The cones (approximately 6 million) are used for the diurnal vision. These cones are subdivided into three families of cells sensitive to the red, green and blue, thus makes it possible to be able to distinguish around 20,000 different colours.
It should be noted that the sensitive cells miss area where the optic nerve begins. This area is called the blind point.

Defects of the vision of the colors

When a type of receiver misses the total composition of the cones, the vision of the colors is not correct any more. It is in the case of daltonism, a disease which comes from the abscence of red pigments constituting the photosensitive receivers. The number of colours perceived by the daltonism subject is only a few tens, whereas for a normal subject, this can reach over several thousands. The daltonism, whose scientific name is dyschromatopsie, has several alternatives which are the protanopes (insensitivity to red), deuteranopes (insensitivity to green) and tritanopes (insensitivity to blue). Generally, the anomalies of the vision of the colors are called chromatic anomalies.

Additive and subtractive synthesis of the colors

We can obtain all colors from three primary colors red, green and blue, by the established principle of the three-colour process. This concept of addition of colors between them bears the name of additive synthesis. As diagram 3 shows that, we can obtain all visible colors by the mixture of these primary colors. This principle of the additive synthesis is used in technologies of the monitor, the scanner and television.

Diagram 3: Additive synthesis of the colors (screen)



Subtractive synthesis of the colors (imrimante CMYK and RGB)

For reasons related to the process of impression, it may be that the colors are not reproduced exactly.

What is metamerism?

         After explaining the principle of the subtractive and additive syntheses of the colors, as well as the principal characteristics of a source of light, we can now approach the phenomenon of the metamerism. Let us take two objects which have the same spectral appearance under a source of light and with identical conditions. If we change source of light, their coloured appearances can differ. This phenomenon is called metamerism.

Coloured perception of an eye and an imagery system

Currently, the colors can be coded by digital imagery systems via the use of binary data (a number N of 0 and 1). However, there is a limit for this creation of the colors since each system of restitution treats the color in a different way. A system of restitution such as a screen does not reproduce the same colors as a silver system (photographic film color). This example can be applied to the whole links constituting the graphic chain. The ideal system of perception of the colors must approach more close to that of the average observer. In other words, it is enough that there is a correlation exists between the characteristics established by digital system or silver imagery with the sensitivities of the three types of cones present in the photosensitive cells of the human eye. They must also correspond in terms of intensity of light and more specifically in terms of variation of this intensity. The restitution of the colors must remain constant on the whole of the chain of reproduction. It is the problem on which we will lean in details in the following chapters. A system of imagery must, initially, separate the light received from the source (separation in three primary colors red, green and blue), before treating the signals electronically (phenomenon called treatment of the signal), in order to reconstitute the original color. We will not return in the principle of reproduction of the colors of a silver photographic film, however the principle characteristics of this process will be approached in a synthetic way with each time that is necessary.