Composing white light from colored components in an optimum way has been a key problem of the lighting industry since the introduction of fluorescence lamps in the 1930s. Presently, the ability of white light to properly render the colors of illuminated objects is optimized by maximizing the general color rendering index, Ra, a figure of merit introduced by the International Commission of Illumination (Commission Internationale d'Éclairage, CIE) in 1974 and updated in 1995 (CIE Publication No. 13.3, 1995). A trichromatic system with a maximized Ra composed of red (610 nm), green (540 nm) and blue (450 nm) components (W. A. Thornton, U.S. Pat. No. 4,176,294, 1979) is widely accepted in lighting technology as a white light standard.
The development of efficient LEDs radiating in the short-wavelength range of the visible spectrum has resulted in the emergence of solid-state lighting. Since LEDs employ injection electroluminescence and potentially offer radiant efficiency that exceeds the physical limits of other sources of light, solid-state lighting is a tremendous lighting technology with the promise of the highest electric power conservation and vast environmental benefits.
Composite white light from LEDs can be obtained by means of partial or complete conversion of short-wavelength radiation in phosphors, using a set of primary LED chips with narrow-band emission spectrums or a complementary use of both phosphor-conversion and colored LEDs. The multichip approach based on colored LEDs offers an unsurpassed versatility in color control, since the peak wavelengths of the LEDs can be tailored by varying the chemical contents and thickness of the active layers.
Using a large number of colored LEDs with different wavelengths allows for tailoring continuous illumination spectra similar to those of blackbody radiators or sunlight, which are widely accepted as the ultimate-quality sources of white light. This requires the determination of the LED wavelengths providing the best possible quality of light for a given number of colored LEDs comprising a white light source, the number of colors that can be rendered by a white light source composed of a particular number of colored LEDs, and the minimal number of LEDs required for attaining the ultimate quality of white light. However, the existing approach of designing composite white light sources relies on the CIE 1995 procedure (CIE Publication No. 13.3, 1995), which employs the general color rendering index Ra based on eight test color samples and an additional six special color rendering indexes. This number of colors (eight to fourteen) is much smaller than that resolved by human vision.