Lumen (lm), lux (lx) and candela (cd) are units of measurement used to describe the output of bulbs and light fittings. With incandescent and halogen lighting it was always easy to equate wattage with light output, because the same wattage always produced the same amount of light.
Now that different technologies have entered into the equation, using wattage as an indicator of output has become increasingly obsolete. In retrofit CFLs or LEDs, and even energy-saving halogen lamps, an equivalent incandescent or halogen wattage is given by manufacturers to make life easier.
Like-for-like comparisons between modern light sources are complex, since their output per watt varies considerably from one product to the next. Therefore, more universal units of measurement are required – measurements that actually record light.
Lumen, lux, and candela are definable as follows:
Lumens measure the total amount of light energy from any source, regardless of direction. The lumen answers a simple question: how much light does this source produce?
When buying modern bulbs you’ll find the lumen measurement added to the packaging and stated prominently – usually at twice the size of the wattage.
Lumens are used to measure energy efficiency, using a lumens-per-watt formula. In the lighting industry this formula is called ‘luminous efficacy’. Efficacy refers to a ratio between two different units of measurement.
Sometimes lighting manufacturers give a ‘LOR’ measurement (Light Output Ratio), which accounts for the percentage of light in lumens wasted between the source and transmission through a light fitting.
A guide to the energy efficiency (or luminous efficacy) of various technologies is as follows:
- 5-10 lm/W – Incandescent
- 10-20 lm/W – Halogen
- 45-70 lm/W – CFL
- 50-100 lm/W – Linear fluorescent (T5)
- 80-150 lm/W – LED (subject to ongoing development)
- 80-120+ lm/W – HID
Lux is a unit of illuminance equivalent to 1 lumen per square metre. It is a measurement given in conjunction with distance, or a set of incremental distances, so that end-users can gauge whether a lamp or luminaire delivers an adequate intensity of light for specific applications, or to meet building regulations. For instance, corridors are often required to measure a minimum 100 lux at floor level (typical height around 3m), and 300-500 lux is recommended for reading.
The illustration below shows an example of decreasing light levels over distance: 0.75m = 3500lx, 1.5m = 800lx, 2.15m = 400lx, and 3m = 150lx.
Candela is a measurement of luminous intensity at source, and in a given direction. One candela is the approximate equivalent in output of a traditional candle. A candela measurement differs from lumens in that it measures directional intensity rather than the overall amount of light, so it takes no account of multidirectional output. For that reason candelas are used primarily to measure focused lamps, such as spotlights.
For more information, advice and ideas take a look at our Lighting Advice section.