The colour rendering index defines the ability of a light source to identify colours, and is measured on a scale of 1 - 100. On this scale a rendering of 1 is monochromatic light, and a rendering of 100 is natural sunlight, so you can think of the scale as a measure of the quality of light produced by the source.

For example, have you ever stood under an old street light at night and wondered why the colour of a car or your wheelie bin doesn’t seem the same as it does during the day? Although the lamp used in the street light gives off a lot of light, the CRI is very low so the appearance of any colour is transformed.

On commercially available light sources the CRI scale may also be referred to as the Ra or CIE Ra value. This is the name of the international standard for the CRI scale.   

The example below demonstrates the change in how colours are rendered at decreasing values on the Ra scale.

Workplace lighting

To ensure a safe environment at work the European Union has set a standard stating that a minimum Ra 80 is required in the workplace.

Colour temperature and CRI

The colour temperature represents the visual appearance of a light source and is measured in degrees Kelvin. Low colour temperature light sources have a warm appearance while high colour temperature sources have a crisp, almost blue appearance. 

Consider the different use of light between a cosy coffee shop that has warm, low colour temperature, lamps to a dental practice where bright white, high colour temperature, lamps are needed for clarity. 

CRI shows us how accurately a colour is represented and colour temperature signifies the colour of the light being emitted.

The two can often be confused as both affect how we perceive colour. It also means that if you want to fairly compare the CRI of two different lamps they should both have the same colour temperature.

The CRI of lamps

The colour rendering index is just one of many different attributes that make up a lamp's specification. To find out more about the different attributes and the information listed against each lamp we’ve create a guide to light bulb terminology explaining what each feature means.