Color Quality Metrics Research Articles

Colour-rendition properties of solid-state lamps

The applicability of colour-quality metrics to solid-state light sources is validated and the results of the assessment of colour-rendition characteristics of various lamps are presented. The standard colour-rendering index metric or a refined colour-quality scale metric fails to distinguish between two principle colour-rendition properties of illumination: the ability to render object colours with high fidelity and the ability to increase chromatic contrast, especially when the spectra of light sources contain a few narrow-band electroluminescence components. Supplementing these metrics by the known figures of merit that measure the gamut area of a small number of test colour samples does not completely resolve this issue. In contrast, the statistical approach, which is based on sorting a very large number of test colour samples in respect of just-perceivable colour distortions of several kinds, offers a comprehensive assessment of colour-rendition properties of solid-state light sources. In particular, two statistical indices, colour-fidelity index (CFI) and colour-saturation index (CSI), which are the relative numbers of object colours rendered with high fidelity and increased saturation, respectively, are sufficient to reveal and assess three distinct types of solid-state light sources. These are (i) high-fidelity lamps, which cover the entire spectrum with the spectral components present in the wavelength ranges of both 530–610 nm and beyond 610 nm (e.g. trichromatic warm white phosphor-converted (pc) light-emitting diodes (LEDs), red–amber–green–blue LED clusters, complementary clusters of white and coloured LEDs); (ii) colour-saturating lamps, which lack power in the 530–610 nm wavelength range (e.g. red–green–blue or red–cyan–blue LED clusters) and (iii) colour-dulling lamps, which lack power for wavelengths longer than 610 nm (dichromatic daylight pc LEDs and amber–green–blue LED clusters). Owing to a single statistical format, CSI and CFI can be used for design and optimization of multiwavelength LED clusters providing ‘smart’ illumination with a trade-off between different colour-rendition characteristics.

New Statistical Figures of Merit for Color Quality of Solid-State Lamps

The existing color quality metric, which relies on color rendering indices, is in conflict with subjective rating of lamps based on light-emitting diodes (LEDs). To resolve the conflict, we introduce an approach, which relies on the statistical analysis of color shifts for a large number of color test samples. The analysis has shown that solid-state lamps have at least two distinct color rendering properties, namely, the color fidelity ability and color saturation ability. Using this approach, we performed optimization of trichromatic sources of light in terms of color fidelity ability or color saturation ability, or a trade-off between these two characteristics. In particular, the highest color fidelity index can be attained in phosphor-conversion LEDs with multiphospor blends or tetrachromatic or pentachromatic clusters of colored LEDs, whereas the highest color saturation index can be realized by using common trichromatic (red-green-blue) clusters.