Standard illuminator and standard light source

Standard illuminator：
Within the wavelength range that affects the color of objects, there is a specific relative spectral power (energy) distribution, which is defined as “illuminant”. It refers to a specific “spectral power distribution”, which may or may not be provided by a specific artificial light source. “Light source” refers to the physical radiator that can emit light, such as lamp, sun, etc. Standard illuminants refer to illuminants recommended by CIE for colorimetry, including standard illuminants A, B, C and D.

Standard illuminant A: represents the relative spectral power distribution of the complete radiator when the absolute temperature of the 1990 international practical temperature scale is 2856K. Its chromaticity coordinate points fall on the black body locus of u ‘- v’ chromaticity graph.

Blackbody trajectory and A, C, D illuminants

“Complete radiator” is also called “blackbody” or “Planck radiator”. The temperature in bold is expressed in “K” (Kelvin), which is equal to the temperature in Celsius plus 273

Standard illuminant B: represents direct sunlight with relevant color temperature of about 4874K. Its light color is equivalent to the sunlight at noon, and its chromaticity point is close to the black body track.

Standard illuminant C: represents the average daytime light with the relevant color temperature of 6774K. Its light color is similar to the white light of thin cloud sky, and its chromaticity point is located below the black body track, close to the black body track.

Standard illuminant D65: represents the time phase daylight with the relevant color temperature of 6774K, and its chromaticity point is in the green direction of the black body track (as shown in Figure 1 above)

Other standard illuminant D represents other time phase daylight other than standard illuminant D65. For example, D50, D55 and D75, their relevant color temperatures are 5000K, 5500K and 7500K respectively.

Relative spectral power distribution of standard illuminant

Figure 2 shows the relative spectral power distribution curve of standard illuminants A, B, C, D65.

The practice shows that the standard illuminants B and C cannot correctly represent the daylight of the corresponding phase. Due to historical reasons, except that the Lovibond colorimeter still uses the B light source, the B illuminant has been eliminated, and the standard illuminant D represents daylight. The standard illuminant C is still widely used in measurement. In fact, in the field of chromaticity, A and D65 are standard illuminants used.

Standard light source：
Light source refers to the observable object that generates radiation power, such as the sun, lamp, etc.

The standard light source uses artificial light source to realize the relative spectral power distribution of CIE standard illuminant to meet the application needs of colorimetry. The recommended standard light sources are A and C.

Standard light source A: It is realized by the gas filled spiral tungsten filament lamp with the relevant color temperature of 2856K. If more accurate relative spectral power distribution of ultraviolet radiation of standard illuminant A is required, it is recommended to use fused quartz glass shell or lamp with quartz window.

Tungsten filament lamp is a widely used artificial light source, which is convenient to use and widely used by many color measuring instruments. However, it is easy to be damaged. At present, some color measuring instrument enterprises with strong R&D teams have used LED lamps, which use CLEDs (full band balanced LED light sources).

Standard light source C: It is realized by combining standard light source A with a specific set of Davis Gibson liquid filters. The filter consists of two solutions C1 and C2, which are respectively installed in a liquid tank made of 1cm thick colorless optical glass.

In practice, the liquid filter needs very careful configuration, and it is inconvenient to use. As a result, approximate glass filters are often used instead. Of course, all the spectral transmission characteristics obtained cannot be as accurate as expected.

Because the power of standard light sources A and C in the ultraviolet region is very small, different from the natural sunlight in which people usually observe colors, the influence is not obvious in the observation of non fluorescent colors, but it cannot meet the needs in the observation of fluorescent colors. In view of the increasing use of fluorescent dyes, it is necessary to have a standard illuminant containing ultraviolet radiation power to approximately represent sunlight. The D65 illuminant represents the average sunlight within the visible spectrum, with short wave distribution up to 300 nm. In the region of about 380-300nm, illuminant D65 has a much higher power distribution than illuminant C. The main purpose of D65 illuminant is to calculate tristimulus value and other chromaticity data of object color by weighting. It can be widely used in textile, printing and dyeing, clothing, leather, footwear, plastics, electrical appliances, spraying, electroplating, coating, ink, pigment, chemical industry, printing, packaging, furniture, building materials, photography and other pigment management fields.

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