Colour rendering

Colour rendering is also important. This is the ability of a light source to show surface colours as they should be, usually in comparison to a tungsten or daylight source. Lamps with poor colour rendering will distort some colours.

Colour rendering is usually assessed by the CIE colour rendering index (CRI or Ra); a number between 0 and 100, where lower values indicate poor colour rendering and higher ones good colour rendering. CRI only works for approximately white sources.
Guidance in the CIBSE Code recommends lamps with CRI between 90 and 100 for inspection and colour matching. Where accurate colour judgement is required, such as in offices and shops, lamps with CRI over 80 are recommended.
Researchers at NIST in the USA have suggested that the colour rendering index is less suitable for LED spectra, especially in the way saturated colours like red and yellow are dealt with. They have proposed an alternative colour quality scale (CQS).

Colour Temperature

One aspect of lamp colour is its colour appearance; whether the light from the lamps looks ‘warm’ or ‘cool’, measured by the correlated colour temperature (CCT). Lamps with a warm appearance having a CCT of 2700 – 3000K are generally considered appropriate in a domestic setting. Lamps of 4000K and above are considered ‘cool’ and are more appropriate for office and some retail applications.


IRC halogen lamps are sources where the central halogen burner has been coated with an infra-red coating (hence IRC). This reflects the infra-red energy back onto the tungsten burner, which means it heats up and becomes more efficient as much of the reflected heat energy is turned into visible light.