Last edited 22 May 2020

Colour Rendering Index CRI

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Comparing the Colour Rendering Index (CRI) of incandescent and LED lamps.

Contents

[edit] Introduction

Colour can only be perceived due to the presence of light. Most light sources produce different wavelength combinations.

Visual light comprises a wide range of colours. This can be seen by using a prism to split it into its constituent parts. This range of colours is called a ‘spectrum’ and the colours, each with a different wavelength, are always in the same order, from red at one end through orange, yellow, green, blue, indigo to violet at the other end. Red light has a longer wavelength and blue shorter.

In reality, there are many, many different colours in the spectral range because it is seamless, as one colour gradually merges into the next. Each wavelength stimulates the cone cells (colour receptors) in the human eye in a different way to produce the sensation of colour. So, when we say that ‘the banana is yellow’, it would be more accurate to say a sensation of yellow is generated by an area of the retina (at the back of the eye) that corresponds to where light rays from the banana are being received.

[edit] Colour appearance

Colour appearance refers to the visual sensation correlated with the ‘warmth’ or ‘coolness’ of the light emitted by a lamp. There are a wide variety of different types of lamps available. These include filament lamps, discharge lamps (including fluorescent, xenon arc lamps and high intensity discharge or HID lamps), light-emitting diodes (LEDs) and so on. For more information, see Types of lamp.

Each type of lamp has an impact on colour appearance. The metric used to characterise the colour appearance of the light emitted by a light lamp is the correlated colour temperature (CCT), expressed in Kelvin (K). Warm white light is produced by lamps with a colour temperature below 3,000 K (reddish hues), whereas 4,000 K and above (bluish) is cool to cold.

In some instances, the colour of light can trigger certain behaviours. For instance, in outdoor applications, artificial light with bluish tones may have a disruptive impact on wildlife and plants that depend on warmer tones to maintain their circadian rhythm.

[edit] Colour rendering

BRE's 'The essential guide to retail lighting' notes, '...colour appearance is quite separate from the colour rendering of the lamp, which is the ability of a light source to show surface colours as they should be, usually in comparison with a tungsten or daylight source. Measured on the colour rendering index (CRI) scale. A value of 0 means it is impossible to discern colours at all, and a score of 100 means no colour distortion.’

CRI measures the output of lamps (artificial light) compared to natural light (daylight). It also calculates and analyses any colour that is reflected from an object that is illuminated by an artificial source.

[edit] CRI rating scale

90 - 100 Excellent
85 - 65 Good (daylight)
65 Good (cool white light)
55 Poor (warm white light)
50 - 0 Poor

A lamp’s spectrum determines its CRI. A low CRI can make it difficult to distinguish colour, so lamps that fall in this range are not ideal for installations where this characteristic is a priority. An incandescent lamp has a continuous spectrum, which means it should have a high CRI and will deliver a clear visual experience with little distortion.

The preferred CRI rating will relate to the mood or experience that is required in the area being lit. If the accuracy of colour is vital, a higher CRI rating should be selected. For most indoor lighting applications a CRI value of at least 80 is recommended, according to BRE.

LED lamps typically fail to register a CRI higher than 90. However, they have emerged as a popular replacement, especially in Europe, for incandescent and halogen lamps (which are disappearing from the marketplace).

CRI is a general term used to label the colour rendering characteristics of many commercial lighting products. However, the correct international term is the CIE Ra value, which is the international standard colour rendering index as determined by the International Commission on Illumination (or Commission internationale de l'éclairage, CIE).

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