Color temperature, light source

Clearly, standardized light sources are desirable for color matching, particularly in view of the phenomenon of illuminant metamerism described below. Over the years CIE has defined several standard illuminants, some of which can be closely approximated by practical sources. In 1931 there was Source A, defined as a tungsten filament incandescent lamp at a color temperature of 2854 K. Sources B and C used filtering of A to simulate noon sunlight and north sky daylight, respectively. Subsequently a series of D illuminants was estabUshed to better represent natural daylight. Of these the most important is Illuminant E). ... 

Typical values of correlated color temperature (CCT) and color rendering index (CRl) for some common electric light sources. 

B. The comparison is made under an artifical light source of approximately 150 candela intensity and a color temperature of 7500 degrees Kelvin i 200 degrees. 

A color correction may also be achieved by using filters. Table 3.1 shows the type of filter used by professional photographers to achieve accurate color reproduction. The required filter depends on the type of illuminant and also on the type of film. The type of light source can be described using the temperature of a black-body radiator. A black-body radiator is a light source whose spectral power distribution depends only on its temperature (Jacobsen et al. 2000). The color temperature of a fight source is the temperature of a black-body radiator, which essentially has the same spectral distribution in the visible region. The concept of a black-body radiator is formally introduced in Section 3.5. 

For artificial light sources, two values are important the color temperature in degrees Kelvin and the relative spectral distribution of the light source S(k), where X shows that the radiant energy is dependent on wavelength. 

The CIE (Commission Internationale de l Eclairage) has selected a number of light sources from the wide range available with different spectral distribution and color temperature. The use of these standard illuminants is recommended and those used most commonly are listed here ... 

The actual color appearance of light that comes from a "black body" source is called its color temperature or chromaticity. Correlated color temperature (CCT) is the terminology used when referring to the color of an artificial source. Because the color temperature of an artificial source will most likely not fall on the normal black body curve, it is the accepted practice to refer to their temperature as the temperature of the closest CIE daylight (D) temperature value. Hence, a CCT of 6500 K means that its color temperature is closest to that of CIE D6500 daylight standard. 

Fluorescent lamps are rated in catalogs by two indices, their correlated color temperature (CCT) and color-rendering index (CRI). The CCT is the temperature of a black body whose chromaticity most nearly matches that of the subject light source. Because a fluorescent lamp only approximates a "black body," it is called correlated to distinguish it from actual. The CRI on the other hand is a subjective method developed by the CIE in which eight test colors are viewed under the test and a reference lamp(s) and its ability to reproduce the test colors numerically rated. Both numbers are not absolute but useful in selecting lamps for color rendering applications. 

The color of a light source is typacally characterized in terms of its color temperature. If the x,y coordinates of an illumination source do not exactly sit on the blackbody locus, the color of a light source is characterized in terms of its CCT. The CCT is the temperature of a blackbody radiator that has a colour that most closely matches the emission from a non-blackbody radiator. For high quality white light illumination the CCT should between 2500K and 6500 K. There is an accepted method (Wyszelki et al 1982) to determine lines of constant correlated color temperature in x, y space. CIE, CCT and CRI for common white light sources are given in Table 1 for comparison purpose (Misra et al 2006). 

It is well known that the spectral distribution and irradiance of the solar radiation at the Earth s surface depend on the location and is subjected to seasonal and diurnal variations. Therefore, a reference spectrum is needed as a basis for comparison with the spectral energy distribution of artificial light sources. Data from CIE No. 15 1971 (colorimetry official recommendations of the International Commission on Illumination) that recommend a standard illuminant D65 with a scheduled color temperature of approximately 6500 K have been used as a basis over the years. 

Figure 133. Stabilization of sulfisomidine tablet coloration (light source mercury lamp) by modifying film coating thickness and oxybenzone content. Samples were irradiated at a temperature below 27°C. Oxybenzone concentration , (0.5% A, 1% o, 2% A, 5% , 10%. (Reproduced from Ref. 557 with permission.)...

Color Temperature - A measure of the quality of a light source by expressing the color appearance correlated with a black body. 

Refractive index can be measured with a high precision to five decimal places using critical angle method with Bausch Lomb Precision Refractometer. This method can be used for the transparent and light colored liquids measured at temperatures from 20 to 30°C. Several light sources and filters are specified in order to provide monochromatic light. Also, calibration and calibration liquids are described to assure high precision of measnrement. 

The color temperature is used to characterize the color of a light source. Its unit is Kelvin, which is defined as the temperature a black body must have to emit light with the desired color. It is directly related to Planck s law of black body radiation. Sunlight, for example, has a color temperature of 5,500 K. Light with a lower color temperature appears more red, while light with a higher color temperature appears blue. 

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