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.