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How is the brightness of a light measured?

How is the brightness of a light measured?

Lumens, Lux, Watts? Here is our simple guide to some of the basics in understanding how brightness and light are measured in modern electric lighting.


Many people still think of the brightness of a light in terms of watts, and would have a good idea of the difference in brightness of 40, 60, or 100 watt lightbulb. This worked better in the past as most lights were incandescent and the variation in efficiency between different manufacturers was small. Nowadays with more energy efficient lights, watts are not such a useful way to consider brightness.  A watt is actually a unit of power, or the rate of energy used or generated over time. Energy companies use watts to track the amount of electricity we use in Kilowatts per hour (kWh), which is 1,000 watts of power over one hour of use. So if we have a 100w lightbulb switched on for 1 hour, this would be 0.1 Kilowatt hours or 1 watt hour (Wh).   


In basic terms, Lumens (lm) are a measure of how much visible light is emitted by a light source, this is described technically as its luminous flux. The total power of the visible light is represented by the total number of lumens given off. In short, when looking for the power of the lamp within a lighting fixture, look for the total number of Lumens.


Lux (lx) is used to measure the amount of visible light (luminous flux) spread over a surface area, this is known technically as illuminance. Illuminance is a fundamental consideration for lighting designers and architects when specifying luminaires for an installation. Another way of expressing this unit is as lumens per square metre, as 1 lux unit equals 1 lumen per square metre, or 1 lx = 1 lm/m2. In buildings, lux can be measured with a device called lux meter which will give an accurate reading of the illuminance. 


In addition to light itself, there are other factors to consider when measuring the amount of light emitted. Most lamps are encased inside a light fitting consisting of elements such as diffusers and reflectors. These elements, along with things including the shape and number of lamps, will have an effect on the brightness and distribution of light from the luminaire. Luminaire efficacy factors these elements in to give a more accurate measurement of the lumens given off by the entire luminaire, in a fully assembled state.


This ratio is basically the number of lumens a light gives off for each watt of power it consumes, referred to technically as luminous efficacy. As an example, a light emitting 1,000 lumens that consumes 100 watts of power, would offer 10 lm/W. Most lighting manufacturers will provide a lm/W figure on their luminaires and lamps as a guide to their overall efficiency. 


The candela (cd) is the standard unit of luminous intensity or brightness of a light in a specific direction. The name candela is derived from the fact that a common candle produces approximately 1 candela of brightness in a given direction. The candela is mostly used when dealing with focused light, such as a torches or spotlights, and as such often isn’t stated on luminaires. 



how the brightness of a light is measured

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