Correlated Colour Temperature (CCT) and Colour Rendering Index (CRI)

There are two parameters which define the colour of LED light. – colour temperature and colour rendering.

Colour temperature is the perceived colour of the light source.  It is compared with a glowing black body source.  When heated to around 3000K it will glow a warm yellow (think about a 40 watt filament lamp).  When heated to 4000K you get whiter (neutral white) and when heated to 5-7000K you get a bluey white (daylight or cool white).

Warm white is most useful in domestic environments and tends to provide a “homely” feel.  Neutral white is more suited to the office environment as it is slightly brighter than warm white and easier to see by.  Cool white provides the most light for a given amount of power but may appear harsher so but is ideal where good visibility is required such as a gym or car park.

Colour rendering is the ability of the light source to allow colours to be seen accurately and is not directly related to colour temperature.

White light is made from a mixture of colours.  A white LED has a blue source with a yellow phosphor overlay which emits white light when illuminated by the blue source.  Needless to say, the colour rendering may not be great.  Reds will appear purple and oranges rather muddy.  

This is because there is no red within the light source to reflect from the red surface.

In simple terms, the solution is the addition of red phosphors to the yellow overlay.  This provides the red to reflect from the red surface and give a good colour rendering.  It would be desirable if this could be added to all LED coatings but, as always, the issue is cost.  Red phosphors are significantly more expensive than yellow so the cost of the LED goes up.

Does it matter?  In many cases no, but it may have safety implications.

For example, old low pressure sodium (LPS) street lights have a colour rendering index of 20 – pretty poor.  They may be bright but a red car will appear black and may simply be invisible at night.

A moderate to good colour rendering of 70-80 is almost certainly adequate.  The fact that a royal blue shirt may appear simply as dark blue probably isn’t important.  It’s also reasonable for security cameras as it will provide good contrast.

A high colour rendering of 90 or more is probably only required if comparing absolute colours, perhaps in paint or fabric manufacture or in retail.  With a really high colour rendering oranges will look really orange and may sell better than your competitors and you can match a shirt and tie exactly so they will look the same in daylight. (although it is still wise to ask the lady in your life).

So, it’s a choice between budget and application.  You could pay 15-20% more for a high colour rendering so it’s worth asking the question…is it necessary?

At Earlsmann we use CREE LEDs for most of our products.  These are available from 2700K to 6500K and with a CRI up to 95 so we can accommodate most requirements