What is meant by the Colour Rendering Index (CRI)?

Put simply, the Colour Rendering Index (CRI) measures the ability of a light source to accurately reproduce the colours of the object it illuminates.

This is a seemingly simple definition, but there is a lot going on, so we'll help break it down into three parts

Part 1: Colour Rendering Index (CRI) is a score with a maximum of 100

What does it mean to measure the ability of something? Like test scores, CRI is measured on a scale where a higher number represents higher ability, with 100 being the highest.

CRI is a convenient metric because it is represented as a single, quantified number.

CRI values that are 90 and above are considered excellent, while scores below 80 are generally considered poor. (More on this below).

Part 2: Colour Rendering Index (CRI) is used to measure artificial, white light sources

Light sources can be grouped into either artificial or natural light sources.

In most situations, we are concerned about the colour quality of artificial forms of lighting, such as LED and fluorescent lamps.

This is compared to a daylight or sunlight - a natural light source.

Part 3: Colour Rendering Index (CRI) measures and compares the reflected colour of an object under artificial lighting

First, a quick refresher on how colour works.

Natural light such as sunlight is a combination of all the colours of the visible spectrum. The colour of sunlight itself is white, but the colour of an object under the sun is determined by the colours that it reflects.