The colour rendering index (CRI) is a measure that has been widely adopted and used by the lighting industry to characterise the quality of a light source.
CRI is a specification to assist designers in making comparison between different lamp sources and is a relative comparison between a lamp source and a reference source.
It is related to the idea that if a red object is illuminated with red light it appears red, but if it is illuminated with blue light it appears grey or black – the colour perceived will change with the colour of the source used.
The CRI is used to measure these subtle shifts in colour of a source. In order to quantify this, the CRI uses 14 (or more) standard coloured tiles and measures the shift in colour when the illumination changes from a reference to the test source.
The reference source used is the sun (or a black body), and by definition it has a CRI of 100. Everything else is measured from that point downwards. Sunlight is composed of a continuous spectrum of colours from the darkest reds through to the deepest violets with particular weighting at each wavelength, known as its spectral power distribution (SPD).
If white light generated from an LED does not contain the same wavelength components then its SPD is different and it will have poor colour rendering properties. Variation of CRI with different types of white LED Deficiencies in the use of CRI for LEDs.
The CRI was originally developed nearly 40 years ago, and even after a few revisions it still uses outdated formulae and methods, and it is known to have deficiencies:
• The problem is prominent with narrowband sources, and the use of CRI for RGB white LEDs or multichip white LEDs can mislead design directions.
• Studies carried out with white LED light sources have shown that CRI can have no correlation to people’s colour preferences. CRI is just a figure of merit to show how close a source matches a standard.
• LED spectral power distribution is sensitive to temperature effects. Effect of LED source characteristics on the CRI.
Currently, white LED light can be generatedusing one of two basic methods:
• Phosphor conversion: where a phosphor is excited with light from a blue or near-UVlight LED.
• LED colour mixing: where light from three or more coloured LEDs is mixedtogether.
Surely, Each method has its merits and disadvantages.