For a white light source to be suitable for illumination applications, it must have a high colour rendering index. The CRI (colour rendering index) is a measure of how well a white light source renders a set of standardised colour tiles as compared with a reference source. The reference source is a Planckian black body source, which by definition has a CRI of 100. The reference source used has a direct impact on the CRI measurement, and should ideally have the same chromaticity co-ordinates and luminous flux as the white light source under test. Standard white LEDs containing a blue chip with yellow phosphor have limited CRI due to the relatively low power content at red wavelengths. CRI values can be improved at the cost of efficiency for white LEDs comprising of a blue/UV chip with a blend of several phosphors. The human element in CRI calculation makes it a subjective metric, and work is underway to develop a more quantitative measure of light quality.