Before explaining the color management on the numerical basis, it is necessary to understand what are the characteristics of the light, the various types and sources of light, vision of the colors, the basics of colorimetry etc.

What is light?

        From a historical point of view, in the beginning of the year 1800, the Theory of the Emission was proposed, particularly by Newton. The simplified explanation of this theory says that a luminous element emits immaterial particles which, in contact with the retina of the human eye, causes a visual feeling. Also the Thomas Young (English doctor and physicist 1773-1829) who first showed that light as having a vibratory origin, i.e. due to a propagation of waves. A few years later Augustin Fresnel (physicist French 1788-1827) highlights the theory of an undulatory light. In 1865, James-Clerk Maxwell (physicist English 1831-1879) shows that the light is an electromagnetic radiation. That is equivalent to a propagation of energy resulting from the periodic variations of an electric and magnetic field.

         Twenty two years later, Heinrich Hertz (German physicist 1857-1894) establishes the theoretical concepts of Maxwell by producing and detecting electromagnetic waves.

Only the presence of radio waves and the visible light are shown at that time. Waves of different frequencies and different wavelengths were highlighted.

Figure 3: Mixed spectrum resulting from a measurement of the fluorescent source
of a scanner.

Since those are propagated in the vacuum at the speed of the light (300 000 ms), and are due to the movement of particles (photons).

To characterize the type of light, each parts of wavelength can be represented by the diagram. This diagram is called spectrum. For example the sunlight has a balanced spectrum, all the wavelengths are presented in the same manner. In the light of a bulb, the red fields are more presented in the spectrum. Thats why a light is hotter. In the coloured light, it misses parts of spectrum. In the red light of tricolour fire, the part of purple via green until the yellow is missing. Our perception of the colors thus depends very precisely on the spectrum.

         The diagram of the electromagnetic spectrum is shown below. The proportion which holds the visible light on the spectrum is extremely small.

Various types and Sources of light

          As we saw previously, the electromagnetic waves can be defined by their frequency and their wavelength. In visible spectrum the values of wavelengths is expressed in nanometers (Nm), ranging between 380 Nm and 780 Nm. We will see later, which is the qualitative composition of this visible spectrum.

Sources with continuous spectrum

         A source with continuous spectrum has an emission of electromagnetic radiation which extends the frequency practically continuous on the whole of the spectrum (part visible).

Figure 4: Mixed spectrum resulting from a measurement of a fluorescent source
of scanner with CCC

The thermal sources, which using heat to excite the electrons and to provide energy, are the principle sources of this family.

Sources with discontinuous spectrum

          The sources with discontinuous spectrum have a spectral distribution which is determined by a discrete wavelength succession, called emission lines. The presence of holes on the spectrum indicates there is no emission of energy.

Sources with mixed spectrum

         The sources with mixed spectrum are the result of the superposition of a continuous spectrum with a discontinuous spectrum.

Sources with specific lines of spectrum

         The spectrum of lines emit only in rare wavelengths.For example, lasers.

Concept of black body and color temperature

         The color temperature of a source light is an important concept that is necessary to understand before explaining the perception of colors phenomenon. The lighted objects are strongly influenced by the nature of the light which surrounds them. The preceding classification of the sources of light is partly explained by the concept of color temperature.

          When a metal part is heated until a certain temperature, color evolves and changes as it accumulates energy. It is Kelvin which measures the white light by heating a metal object and by changing the red with blue.

           The black body is a hollow object having a black inside cavity, with an opening to measure emission of energy. This black body is thus the reference, because it absorbs all incidental radiations and re-emits energy in the same proportions for each wavelength. The whole of the other sources is thus compared with this reference. Max Planck, German physicist, highlighted this phenomenon by explaining why a perfect black body emits a spectrum of temperature of about 2856 Kelvin degree. It puts an equation which describes maximum theoretical energy being able to be emitted for any wavelength at any temperature. Its luminous spectrum depends only on the temperature. From these results obtained from a black body, each thermal source has its own color temperature. The sources having a color temperature whose value is lower than 5500K have a yellowish color, whereas those having a color temperature higher have a bluish color. Thus a halogenous lamp (figure 2) has a color temperature approximately equivalent to 3400K, whereas a television screen has a color temperature around 9000K. Thus, when we decide to measure one coloured sample, the nature of light source must clearly defined and be included in the final result.

Figure 5: A black body - Represented in a
two-dimensional space CIELUV.

With examples of color temperature of different source , we can say the following:

Candle (1400K) - Early morning Sun (2000K)
Incandescent lamp 60W (2800K)
Daylight (5500K) - covered Sky (6500-12000K)

The position of the black bodies

          We can represent the position of the black bodies in a CIE Yxy or CIELUV colorimetric space (we will see more in detail, characteristics of these spaces in the later chapter). These two spaces are the representations of the colors based on a standard observer. We can thus make correspond to the color value to a black body whose temperature is known. The spectral data calculated by Planck can be converted into co-ordinates 'u' 'v', and thus placed in colorimetric space CIELUV, as shown in the below diagram.

         The ISO temperatures lines makes it possible to connect a precise color (determined by the co-ordinates resulting from CIELUV space), and the black bodies curve.
         The preceding graph shows how to find the color temperature, by plotting a perpendicular straight line instead of using the black bodies and passing it to the u and v co-ordinatespoint .

Illuminating Principle

          This concept of illuminating intervenes on the branch of the color science called colorimetry. Thus, we can know the spectral distribution of radiation of a light source and use that in particular for measurements of colors. This characteristic is called "Illuminating".

Illuminating B, represents the sunlight at midday ( color temperature 4870K).
Illuminating C (2.), represents the average daylight without the zone of the ultraviolet rays (color temperature 6774K),
Illuminating D65 (1.), represents of average daylight (including the zone of the ultraviolet rays) with a color temperature of 6504K.
Illuminating F represents the fluorescent sources (examples F2 (4.), F7 (5.), F11 (6.), etc).

         After having highlighted the basics of the light and the elementary concepts of color temperature and illuminating, it is now necessary for us to approach the characteristics of the vision of the colors.