Glossary of Terms

The rapid growth and adoption of low energy LED lighting and the bold claims made by many manufacturers means that comparative performance measures backed by supporting evidence have become essential.

This requires some degree of understanding of the technology to enable the consumer to make the most informed decision.

LED Evolution reports its performance data in full, usually backed by independent testing or approvals, and most of the information required is available on this web-site. Definitions of some of the key performance measurement criteria and units are provided below by way of assistance. Below are many generally used terms within the industry and specific to describing LED type lighting.

CRI (or Colour Rendering Index)

Colour Rendering Index (CRI) measures the ability of a light source to accurately render all frequencies of its colour spectrum when compared to a perfect reference light of a similar type. Generally speaking, the higher the CRI the more rich and colourful the surrounding environment that is being illuminated due to the more truthful representation of colouring.

Colour Temperature

Colour Temperature is measured in °K (degrees Kelvin) and describes the colour characteristics of light from warm (yellow) to cool (blue) light and is illustrated below:

Kelvin Temperature Chart


A lower colour temperature (e.g. 3,000 °K) will produce a warmer more yellow light and a higher temperature (e.g. 5,000 °K) will produce a cooler more blue light. Colour temperature of around 4,250 °K is often referred to as Neutral white light.


A Driver is the electronics used to power illumination sources. Also referred to as a Ballast or Power Supply.

Efficacy (or Luminous Efficacy)

The light output of a light source divided by the total electrical power input to that source, expressed in lumens per watt (lm/W).

Kilowatt-hour (or kWh)

A Kilowatt-hour is a unit of energy equivalent to 1 Kilowatt of energy consumed for 1 Hour (usually abbreviated as kWh). The kWh is usually used by electrical power distribution companies for the purposes of billing.


A lumen (lm) is a unit of measure of the total amount of visible light emitted by a source.

Lumens per Circuit Watt

Lumens per Circuit Watt is a measure of the efficiency of the light source. It represents the amount of light produced per Circuit Watt of energy consumed. It considers various conversion efficiencies from delivered mains power to actual amount of light made by a light fixture and Ac to Dc conversion as well as thermal and optical losses and is particularly relevant to building standards such as BSI part L.

Luminous Flux

Luminous Flux (or Luminous Power) is the measure of the perceived power of light and is quoted in Lumen (lm)


Lux (lx) is the measure of Luminous Flux per unit area. One Lux is equivalent to one Lumen per square metre (i.e. 1 lx = 1 lm/m²).

Photometric Performance Data

This term usually refers to the data charts that detail the angular spread and distance of the light
output from a particular source. These are generally reported in a number of different formats (see below):

Polar Luminous Intensity Graph:

Polar Luminous Intensity Graph
The diagram illustrates the distribution of luminous intensity, in candelas, for the transverse (solid line) and axial (dashed line) planes of the luminaire. The curve shown provides a visual guide to the type of distribution expected from the luminaire e.g. wide, narrow, direct, indirect etc. in addition to intensity.


Cartesian Luminous Intensity Graph:

Cartesian Luminous Intensity Graph
The diagram indicates the distribution of luminous intensity, in candelas of the luminaire. The curve shown provides a visual guide to the type of distribution expected from the luminaire e.g. narrow or wide beam etc, in addition to intensity. This diagram is useful when light intensity changes rapidly within a small angular area.


Illuminance Cone Diagram:

Illuminance Cone Diagram
Usually used for spotlights or lamps with reflectors, the diagram indicates the maximum illuminance at different distances away from the lamp.



A Watt is a measure of energy consumption. It represents power which is the rate at which energy is generated or consumed.





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