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
