Color measurement

The color impression an observer gets from a sample depends on three interrelated factors. 

Different light sources (e.g. daylight, incandescent lamp) have different spectral power distributions ( Color and intensity) and thus generate a different illumination of the sample. 

The composition of the sample is crucial for determining the fractions of absorption, transmission, and reflection of the incident light from the light source. Accordingly, the spectral composition of the reflected and/or transmitted light that ultimately reaches the eye of the observer is changed by the sample. 

Due to the difference in the sensitivity of the three light-sensitive receptors in the retina, even normal-sighted individual observers have slightly varying color perception. In order to determine an objective measure of the spectral change due to the sample (i.e. the "color effect" of the sample), the influence of both illumination and individual observer needs to be determined.

For this purpose, standard illuminants and two defined "standard observers" have been defined on an international level. Accordingly, all calculated color values should be reference these factors based on their standardized derivation. For this reason, objective color numbers always need to be accompanied by specification of the standard illuminant and standard observer for which the color values are to be calculated. This is usually done by means of a corresponding setting in the software of the device.

The eye possesses three types of color receptors (cones) on the retina. These differ in their spectral sensitivities. Whereas One type of cone is particularly sensitive to red-orange (X), the second and third types are more sensitive for green (Y) and blue (Z), respectively. 

This was used as the basis for definition of a standardized "normal human eye" with defined spectral sensitivities (standard spectral value functions) for three receptors for a defined observation angle. This definition was established in 1931 for the CIE 2° standard observer and in 1964 for the now most commonly used 10° standard observer. This generally accepted definition of color stimuli provides the foundation for most other color coordinate systems. By means of appropriate transformations, the tristimulus values can be converted into other derived color coordinates (e.g. CIE Lab, Hunter Lab, CIE LCh), which define color and color differences that can be understood by humans better than with XYZ coordinates.

To allow the nature of human perception to be integrated into a measuring result in a controlled manner, it was necessary to define a standard for human vision. This standardized vision was then defined in the so-called CIE standard observers.
Firstly, the standard observer is based on the premise that the observer perceives colors without any interference. Moreover, CIE took into account that humans perceive colors most exactly in the eye if the colors impinge in the region of the fovea (fovea centralis).

Since this region, for a standard observation distance from a color sample, deviates from the optical axis of the eye by approx. 2°, the angle under which the standard observer sees was defined to be exactly these 2°. Accordingly, the so-called CIE 1931 2° standard observer was defined as early as in 1931.

However, the eye sees objects that are more distant under a different viewing angle. Therefore, another standard viewing angle was defined in 1964, the CIE 1964 10° standard observer, in order to consider these circumstances as well.

Since the color of objects looks different for different light sources, the type of light source always needs to be defined. For unambiguous definition of the illumination conditions during the determination of colors, the spectral composition of the light source must be known.

Standard illuminants D65 and D75 are used very commonly. They correspond to the spectral composition of average daylight (color temperature 6,500 K) or of a sunny day with blue sky (7,500 K) and take into consideration the corresponding UV fraction of this light.

The measuring geometry plays a crucial role in the measurement and assessment of color samples. Due to structural and surface features of the samples, the incident radiation reflected or transmitted in various directions shows spectral differences. The angle of incidence and the aperture angle of the illuminating beam of rays as well as the direction and aperture angle of the radiation detected by the detector have an impact on the colorimetric result.

CIE recommends four measuring geometries for colorimetry, including the 0°/45° and 45°/0° geometries.

CIE recommends four measuring geometries for colorimetry, including the 0°/d and d/0° geometries.

In this context, d stands for observation of the light that is diffusely scattered by the sample by means of the wall of an integrating sphere or diffuse illumination of the sample by means of an integrating sphere

According to CIE definition, the axis of the beam of rays must be tilted by no more than 10° with respect to the surface normal upon perpendicular illumination or observation of the sample.