Another color characteristic of the light source is its Spectral Power Distribution (SPD). SPD of light source displays knowledge about the color of the light and the illuminated object (Fotios, 2001). It shows the radiant power emitted by the source at each wavelength or band of wavelengths over the visible region (380 to 760 nm). Correlated Color Temperature (CCT) is the spectral power distribution which distills to a simple number. According to SPD graph, the sources that emit more strongly at the middle or longer (red) wavelengths appear as warm white and often aesthetically preferred.
However, the light sources with shorter wavelengths which appear cool white give an impression of harsher and colder light for aesthetic purposes (Luginbuhl et al., 2010). The light sources can be divided into four main types according to their spectral intensities. These are monochromatic radiation, near monochromatic radiation, continuous spectra, and band spectra. Lasers are the common example of monochromatic radiation light source. Light emitting diodes (LED) and filtered light sources are the examples of near monochromatic radiation light sources. Incandescent bulbs and daylight are continuous spectra light sources which cover a relatively large range of wavelengths in their SPD.
Further, in a band spectrum, there are dispersion radiations. Gas and electric charge light sources are the examples of band spectrum (Gilchrist & Nobbs, 2017). SPD of daylight varies particularly in the ultraviolet spectral region, as a function of season, time of day, and geographic location. Considering this issue, International Commission on Illumination (CIE) has defined simulators of the standard daylight illuminants which their SPD are defined for color measurement purposes. These simulators provide electric light sources such as filtered tungsten-halogen or filtered fluorescent lamps.
The color temperatures of CIE simulators vary from 4000k to 25000k. The most common illuminants are D65 which the label D represents daylight and the number of the color temperature in hundreds (65=6500 K). The SPD of simulators is continuous band spectrum but not exactly the same as daylight SPD.
D65 simulator with the filtered fluorescent lamp is now in use for many color measurement purposes (Broadbent, 2017; Gilchrist & Nobbs, 2017; CIE, 2004) (see figure 2).Elvidge, Keith, Tuttle, & Baugh, (2010)