Colour Temperature and Colour Rendering Index

Colour temperature and Colour Rendering Index are the standards for measuring light. Because these terms tend to cause a fair amount of confusion among everyday consumers when it comes to choosing the best light source for their needs, we will explain the terms and relationship so it can be easily understood and used appropriately.

Colour temperatures (Kelvin) 

Colour Temperature is a term used to describe the colour of a given light and represented in Kelvin degrees (k). The Colour temperature of a light source is derived from heating a “blackbody radiator” (think of this as a piece of black metal) until it glows/radiates light of a similar appearance to that of the light source. It is based on the colour given off by a glowing hot piece of platinum. For instance, heat the metal to “red hot” and its temperature describes the red colour. As more heat is applied, it glows “yellow hot”, and then “white hot”, and ultimately “blue hot”. The measurement of colour temperature is in Kelvin degrees.

Colour temp and CRIThe colours radiating from the blackbody are correlated to colours we are familiar with in our daily lives. Therefore colour temperature is a measurement that is used to define the colour of a light source. This is also used to indicate the ‘whiteness’ or ‘warmness’ of a light source. The colour emitted from a tungsten lamp in your living room is identical to the yellow-white glow when the blackbody radiator temperature is approximately 3200 degrees Kelvin. When the temperature rises to 5500 degrees, the quality of white light is identical to the colour of the sun at midday. The bluish quality of twilight just before dark is similar to the colour of the blackbody at about 12,000 degrees Kelvin.

Colour temperatures (2,000–3,000 K) are called warm colours (yellowish white through red).

Colour temperatures over 5,000K are called cool colours (bluish white)

Colour rendering index (CRI)

Colour rendering index (CRI) is a quantitative measure of the ability of a light source to reproduce the colours of various objects faithfully in comparison with a compared to the same colours light of the same Colour Temperature. As a rule, artificial light should enable the human eye to perceive colours correctly, as it would in natural daylight. Obviously, this depends to some extent on the location and purpose for which light is required.

CRIA light source with a Ra value of 100 displays all colours exactly as they appear under a natural light source. The lower the Ra value, the worse the colours are rendered. Below are Spectral Emission Distribution charts. These charts visually show you the colours you will see with sunlight; you get a full array of the colour spectrum

Relationship of Colour temperature and Colour Rendering Index

light comparison

CRI in conjunction with Colour Temperature is the best way to draw comparisons between different light sources. A common misconception is that colour temperature and colour rendering (CRI) both describes the same properties of the lamp. This is not true. Colour temperature describes the colour appearance of the light source and the light emitted from it. Colour rendering describes how well the light renders colours in various objects by a reference source of the same Colour Temperature. An ideal light source for colour rendering will have both a colour temperature similar to day (light above 5000K, it is an agreed upon form of daylight) and a high CRI value.

KONICA MINOLTA DIGITAL CAMERAWritten By Rachel McCann

Project Manager Photovoltaic Lighting Group

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Categories: Lighting Technology

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