LED Guide


LED is now firmly established as a light source for many different applications and represents over 50% of sales of all luminaires in the UK market. The improvement in efficiencies and costs have resulted in significant benefits to end-users and installers as they look to take advantage of longer life and advantages in lighting maintenance.

Light output

Choices for which model are no longer made on power options or physical size (e.g. 4ft or 5ft) but on lumen output. Some manufacturers will quote LED module output, but every part of a light fixture will affect the final light output and so luminaire lumens is the key figure to identify.


LEDs are often installed to save energy and so accurate saving projections can only be realised with accurate power ratings for the complete luminaire in its installed environment. Many suppliers are still quoting LED module power ratings only, when these can rise by up to 20% when driver and thermal losses are taken into account.

Colour Rendering

The ability of any light source to show colours to their true effect is the basis of the Ra, colour rendering index. Halogen lamps previously had an index of 100 which means that they are showing colours as accurately as natural daylight. Most LED sources are now providing an Ra of 80 which is equivalent to fluorescent Tri-phosphor tubes.


Standards for quoting expected life has only starting to be fully understood. In many cases claims of 30,000hrs or 50,000hrs are not supported by any testing. However, the first step is to understand what those values represent.
In most cases quoted life hours represent the point at which the output of the LED’s reaches 70% of it’s initial output. Any projections of life for luminaires must be supported by appropriate test data and as most products are not tested for the lifetimes quoted.

– 30,000 hours = over 3 years – then the international standards TM21-11 and LM-80-08 are used to help provide long-term projections TM21-11 is not a test itself but is a mathematical calculation used to project LM-80 test data to periods up to 6 times the test period. So, quotations for 50,000 hours of life must be supported by LM-80 tests for at least 8500 hours.

Colour Temperature

LED lights are available in many different colour appearances – this is known as the colour temperature and is measured by the Correlated Colour Temperature or CCT. There are different terms and values for warm white, neutral white etc. so it is vital that the specific CCT value is used so that consistency across a site can be achieved when designing with a range of different products.


The efficiency of a LED fixture is now dependant on 2 main factors: how much light is provided and how much energy is being used. Once the measures used to identify these 2 factors are agreed (see opposite) then a simple division of each provides an efficacy rating.

  • Luminaire lumens = 3000
  • Luminaire total power = 30
  • Efficacy = 100 lumens per watt (lm/W)