What are Lux, Lumens and Watts?

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Lux, Lumens & Watts - Our Guide

This guide helps you to understand what the expressions "lux" and "lumens" mean and how they relate to the power consumption & light output of commercial & industrial light sources.

First we’ll begin with the definitions of lux and lumens – what do they actually mean?

Definition of Lux

Lux is a standardised unit of measurement of the light intensity (which can also be called “illuminance” or “illumination”) - as an example for reference purposes - 1 lux is equal to the illumination of a surface one metre away from a single candle. Some further examples of average illuminance (as measured in lux) are shown below:

  • Outdoor average sunlight ranges from 32 000 to 100 000 lux
  • Warehouse aisles are lit to approx 100-200 lux
  • A bright office requires about 400 lux of illumination
  • At sunset and sunrise (with a clear sky), ambient outdoor light is also about 400 lux
  • Building corridors can be lit adequately at around 100 lux
  • Moonlight represents about 1 lux

Definition of Lumen

The lumen is a standardised unit of measurement of the total "amount" of light (packets or quanta if you want to get technical!) that is produced by a light source - such as a lamp or tube.

This total amount of light may also be referred to by lighting engineers as “luminous flux”. Some examples of total light output (as measured in lumens) from common commercial & industrial light sources are given below, (NOTE: these are sample figures for example purposes only – the actual output can vary quite a bit):

  • A 400W Metal Halide lamp for high bay warehouse lighting: 38000 Lumens output
  • A 100W Incandescent bulb – for general task lighting applications: 1700 Lumens output
  • A 32W T5 or T8 Fluorescent tube – for office ceiling lighting 1600 Lumens output
  • A 150W High pressure sodium bulb – for street/area lighting 12000 Lumens output

The relationship between lumens & lux

One (1) lux is defined as being equivalent to one lumen spread over an area of one square metre, or to put it another way – a measurement of lux (light intensity) tells you how many lumens (total light output) you need given the area you are trying to illuminate.

So 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces an illuminance of only 100 lux.

Therefore, lighting a larger area to the same necessary level of lux requires a larger number of lumens – this is usually achieved by increasing the number of light fixtures (and hence the power consumed).

The relationship of lumens & Watts

The power required to operate a light fitting is measured in Watts (Joules of energy per second) - includes:

  • The energy of the visible light emitted from the bulb or tube
  • The heat generated
  • Other parasitic power consumption which depend on the type of lighting device (such as control gear/ballast)

Therefore - the rated wattage of a light source refers to the entire power consumed - lumens only refers to the light output of that source.

A light engineering term exists for the measurement of the rate at which a lamp is able to convert electrical power (watts) to light (lumens) – this is referred to as “luminous efficacy” – and is expressed in lumens per watt (LPW) - or sometimes lumens per circuit Watt.

Some examples of luminous efficacy in common business & industry light sources are given below (NOTE: again for example purposes only):

  • A 400W Metal Halide lamp - high bay lighting in warehouses: 95 lumens/Watt
  • A 100W Incandescent bulb – general task lighting applications: 17 lumens/Watt
  • A 32W T5 or T8 Fluorescent tube – general office ceiling lighting: - 50 lumens/Watt
  • A 150W high pressure sodium bulb – street lighting: 80 lumens/Watt

The "actual" output of light fittings

Although this article has covered the basics of lux, lumens & watts it wouldn’t be quite right to leave it there – it is wrong to assume that 100% of the light source output is available for the lifetime duration of the light fitting.

The Light Output Ratio of a light fitting

The actual useful light provided by a light fitting will depend on the Light Output Ratio – this term is defined as the ratio of the total amount of light output of a light fitting (containing a lamp) to that of just the bare lamp. When the lamp is placed in a fitting (or luminaire) losses of light will occur – this is because with most artificial light sources on the market light is radiated in all directions. Therefore, a great deal of the light is lost because it goes upwards or sideways rather than down, where it is needed.

Lumen Depreciation of the light source

Lumen depreciation is a process of the gradual decline in light output that is observed from most light sources over time due to gradual filament or electrode deterioration and blackening of the lamp.

Both Light Output Ratio and Lumen Depreciation must be taken into account when calculating the required number of light fittings to maintain a set level of light intensity for a commercial or industrial setting.

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