Colour Rendering Index (CRI) Explained
The Colour Rendering Index (CRI) is the ability of a light source to replicate colours. It is measured against light generated by a reference light source of the same colour temperature. It is measured as a number between 0 and 100.
The colour rendering index is a measure of a light source’s ability to render the colours of an object ‘realistically’ or ‘naturally.’ This is done by comparing the lamp with a reference source – which is usually either incandescent light or daylight. The Ra value averages the first eight sample ratios and that provides the single value approximation of the colour quality of the light source. So, the worst types of light source for colour rendering, such as the low-pressure sodium lamp, colours rendered result in a poor illumination quality (low CRI value).
Gaps in a light source’s spectra lead to poor colour rendering: For example, shining ‘warm’ low-pressure sodium light on a green apple will make the apple look grey. This is due to the fact there is a shortage of colour in the light source that can faithfully reflect the apple’s green part of its spectrum.
Examples of Colour Temperature and CRI
“The Colour Rendering Index (CRI) (sometimes called Colour Rendition Index), is measures a light source’s ability to reproduce colours of various objects being lit by the source. A CRI scoring of 100 is best, and a CRI of zero being the worst. For a source like a low-pressure sodium vapour lamp, which is ‘black- and-white’, the CRI is almost 0, whereas an incandescent light bulb, which emits almost ‘black body’ radiation, is nearly one hundred. The CRI is measured by comparing the colour rendering of the test source to that of a “perfect” source which is generally a black body radiator, except for sources with colour temperatures above 5000K, in which case a simulated daylight (e.g. D65) is used. For example, a standard “cool white” fluorescent lamp will have a CRI near 63. Newer “triphosphor” fluorescent lamps often claim a CRI of 80 to 90.
CRI is a quantitatively measurable index, not a subjective one. A reference source, such as black body radiation, is defined as having a CRI of 100 (this is why incandescent lamps have that rating, as they are, in effect, almost blackbody radiators), and the test source with the same colour temperature is compared against this. Both sources are used to illuminate eight standard samples. The perceived colours under the reference and test illumination (measured in the CIE 1931 colour space) are compared using a standard formula, and averaged over the number of samples taken (usually eight) to get the final CRI. Because eight samples are usually used, manufacturers use the prefix “octo-” on their high-CRI lamps.”
Definition of LED Colour Temperature:
LED colour temperature, measured in degrees Kelvin, determines the colour of an LED light bulb. It is the amount of red, yellow, blue and pure white light given off by the lamp. This effect is often expressed as to how ‘warm’ or ‘cool’ the white light being produced is.
This concept can be simplified if you imagine the light given off from the sun at different times of day. At midday the light is at its brightest and whitest. At sunrise and sunset the light takes a yellower or more reddish tint. Thus, lower temperature LEDs have more yellow and higher colour temperatures are purer white.
LED Colour Temperature
The following boundaries should heed a better understanding of the various colour temperature definitions:
• Warm White: typically from 2600 Kelvin to 3500 Kelvin
• Natural White: typically from 4000 Kelvin to 4500 Kelvin
• Daylight White: typically from 5000 Kelvin to 5500 Kelvin
• Commercial or Cool White: typically above 6000 Kelvin
Colour Temperature & Kelvin Explained
Colour temperature is an aesthetic choice and you may have different preferences to match different applications. The following is a very general guide:
• Warmer Whites are preferable in living and dining areas as well as reception areas to create a more relaxed environment.
• Natural Whites are preferable for kitchens and bathrooms where tasks are performed.
• Daylight Whites are favoured by retailers and offices, though natural whites could be utilised here also.
• Cool Whites are found mainly in industrial and hospital areas.
Older eyes often react better in cooler colour temperatures. Women often prefer warmer colours than men. Task lighting is better if cooler. Cooler whites can raise attention/awareness. Warmer whites can soften environments and make for a more relaxed space. Warmer whites a more likely to mask the true colour of objects, adding a add yellow tint.