The basic building blocks of color measurement
Visual color perception is influenced by our individual color preferences, which are dependent on personal factors (mood, age, gender etc.), environment (lighting, surrounding etc.) as well as our ability to communicate color and color differences. A color looks different in the department store (cool white fluorescent lighting) than at home (warm, incandescent lighting). In order to guarantee consistent color and appearance under all possible circumstances, it is essential to standardize light source, observer and understand the spectral remission data of the object. This information will be the basis for calculation of colorimetric data as it is used for color communication and color QC in production.
1. Standardized conditions for visual evaluation
Honey Yellow, Raspberry Red, Sapphire Blue or Moss Green are very appealing and descriptive color names. But are you sure that every person means the same color by it? Usually not. How do you clearly describe a color and guarantee the same color over time?
Our color perception is dependent on our individual “taste”, which is influenced by our mood, gender, age, but also the light source used, the viewing environment being light or dark, neutral or colorful as well as our deficiency to exactly remember and communicate one specific color.
For controlled visual and instrumental evaluation, the light source, the surrounding and the observer are to be defined. Colors may match under one light source (daylight), but not under another (fluorescent light). This effect is known as metamerism and is a crucial quality requirement for multicomponent products. Therefore, the match needs to be verified with the light source likely to be used.
2. Light source and illuminant
Color changes with the light source. Therefore, standard illuminants must be agreed upon and used. The prerequisite of a light source to be usable for color evaluation is to continuously emit energy throughout the visible spectrum (400 to 700 nm) (Fig. 1).
The CIE (Commission Internationale de l’Eclairage) standardized light sources by the amount of emitted energy at each wavelength (= relative spectral power distribution).
In practice, important illuminants are:
CIE Standard Illuminant D65 (Fig. 2)
D65 is intended to represent average daylight and has a correlated color temperature of approximately 6504 K. D65 corresponds roughly to the average midday light in Western Europe / Northern Europe, comprising both direct sunlight and the light diffused by a clear sky.
CIE Standard Illuminant C
An approximation of average daylight having a correlated color temperature of
approximately 6670 K; approximates daylight on a clear day at noon
CIE Standard Illuminant A (Fig. 2)
A tungsten filament lamp operated at a correlated color temperature of approximately 2856 K. Illuminant A has a peak in the red area of the spectrum. Accordingly, it is a more reddish light with a warm atmosphere
CIE Standard Illuminant F2 (Fig. 2)
Fluorescent lamp consisting of two semi-broadband emissions.
Formerly known as: CWF (Cool White Fluorescent)
CIE Standard Illuminant F11
Fluorescent lamp consisting of three narrowband emissions.
Formerly known as: TL84 (Phillips Ultralume 40).
Figure 1 White daylight dispersed into the spectral colors (rainbow)
Figure 2 CIE Standard Illuminants
3. Standard Observer
Reflected light from a colored object enters the human eye through the lens and strikes the retina. The retina is populated with three different types of light-sensitive receptors: one which reacts to red light, another to green light, and a third to blue light.
Together they stimulate the brain to produce the impression of color. To determine the sensitivity of the receptors, systematic visual tests were done by the CIE in 1931 and 1964. Based on the results (Fig. 3), the 2° and 10° observer were standardized, representing a small and large field of view, respectively.
The observer for instrumental color control was standardized with two different viewing fields (Fig. 4): 2° standard observer and 10° standard observer. The 2° standard observer should be used for viewing angles of 1° to 4°, the 10°standard observer should be used for viewing angles more than 4°. Today mainly the 10° observer functions are used as the eye integrates over a larger area.
Figure 3 Spectral response corresponding to the human eye
Figure 4 2° Standard Observer CIE 1931 10° Standard Observer CIE 1964
4. Surroundings for color measurement
ISO and ASTM standards define the surroundings as portion of the visual field immediately surrounding the specimens as well as the ambient visual field, when the observes glances away from the specimen, such as the interior surfaces of the light booth. It shall have the color with Munsell notation N5-N7 and a 60° gloss not greater than 15 GU. The observer for visual appraisal should have normal color vision and be trained in observing and classifying colors. Visual tests are recommended to check an observer’s color vision periodically as it can change over time (see Guide ASTM E1499).
5. Observer Spectral curves of objects
Light source and observer are defined by the CIE and their spectral functions are stored within color instruments. Optical properties of an object are the only variables that need to be measured. Modern color instruments measure the amount of light that is reflected by a colored sample. This is done at each wavelength and is called the spectral data. For example, a black object reflects no light across the complete spectrum (0% reflection), whereas an ideal white specimen reflects nearly all light (100% reflection). All other colors reflect light only in selected parts of the spectrum. Therefore, they have specific curve shapes or fingerprints, which are their spectral curves (Fig. 5).
In the following graphs, typical spectral curves for a red, blue and green sample are shown.
Figure 5 Spectral curves
6. Summary
Combining data of standardized illuminant, standardized observer and spectral reflection data are needed to calculate internationally standardized color systems, like the widely used CIELab system. Now, a color can be described with the terms of lightness, hue and saturation. Thus, color communication becomes objective i.e. independent of external circumstances and the personal judgement.
Standards and Literature
[1] DIN 5033-1 (1979-03-00) Colorimetry; basic concepts
[2] DIN 5033-2 (1992-05-00) Colorimetry; standard colorimetric systems
[3] ISO/CIE 10527; CIE 527 (1991-12-00) CIE standard colorimetric observers
[4] DIN 5033-7 (1983-07-00) Colorimetry; measuring conditions for object colours
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