What is Luminous Flux?

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Luminous flux is a measure of the power of visible light produced by a light source or light fitting. It is measured in lumens (lm).  

Note that luminous flux only relates to the perceived power of light. In other words, it is only concerned with the visible spectrum, so that while a light source might emit a small amount ultra violet (UV) or infrared (IR) radiation (neither of which are visible by the human eye) these components will not contribute to the luminous flux.

For the technically minded

In physics, the lumen is defined in relation to the amount of light produced by 1W of radiated power at the wavelength of green light. Accordingly, 1W of radiated power at 555nm (or 540 x 10¹² Hz) is 683 lumens.

Lumens are calibrated to the sensitivity of the human eye. That is to say, 1W of radiated power at 555nm will appear to be brighter than 1W at 455nm or 755nm. Therefore, the relationship between watts of radiated power and lumens is not fixed at 683, but it varies according to the wavelength so that a light source with a luminous flux of 1,000 lumens will appear, to the human eye, to be delivering twice the amount of light as a similar source with a luminous flux of 500 lumens.

For the less technically minded

In the lighting industry we often speak of the “lumen output” or the “lumen package” of a light source or a light fitting. This is synonymous with luminous flux and is also expressed in lumens.

Another way to consider the lumen output of a light source is to ask “How much light does this produce?”, in the same way that you might ask “How much heat does my central heating boiler produce?” The answer to the central heating boiler question is in kilowatts (kW) or British Thermal Units (BTU); the equivalent answer in relation to a light fitting is in lumens (lm).

Measuring luminous flux in an LM-80 test

Luminous flux is one of the measurements taken during an LM-80 test.

At the start of the LM-80 test the luminous flux is measured and this is deemed to be 100%, usually written as L100 and known as the initial output.

At regular intervals through the test the luminous flux will be measured again. Typically, the luminous flux increases for the first 1,000 – 2,000 hours and then starts to decline gradually. This gradual decline is known as lumen depreciation and is written as L90 or L80, meaning that after, say, 10,000 hours the luminous flux of the device under test (DUT) has fallen to 90% or 80% of its initial value. The L value is the % of light output remaining, not the % that has been lost.

Luminous flux and temperature

After the initial 1,000 – 2,000 hours of running the luminous flux of an LED gradually declines. The rate of decline is mostly influenced by temperature, with the fastest rate of decline happening at the highest temperatures. This is why LM-80 requires testing to be carried out at 3 different temperatures. This also why manufacturers go to considerable lengths in luminaire design to ensure that the LEDs they use are kept within their specified temperature range.

Projecting luminous flux into the future

An LM-80 report will usually show data collected over 6,000 to 10,000 hours. However, when many lighting applications require the life of an LED to be many times this it is necessary to project the data into the future to determine how it might perform after several years of use. There are many ways that data can be projected, some of which would be downright misleading. To avoid this and enable the data of different LEDs and manufacturers to be compared the Illuminating Engineering Society of North America (the IES) has written another standard, TM-21. This defines precisely what arithmetic formulae are to be used to project data gathered over, say 9,000 hours, out to 30,000 or more hours.

Manufacturers will often talk of L80 or L70 (for example) as if they were points in time. They are. These are the points in the life of an LED when the light output has fallen to 80% or 70% respectively of the initial light output.

At NVC Lighting when we state the rated life of an LED fitting we mean the running hours that are projected to have elapsed by the time the fitting reaches L70, unless stated otherwise.

In general, when a light fitting reaches the end of its rated life its luminous flux will continue to fall gradually. Unless there is a sudden failure of another component, such as the driver, the output of an LED fitting will continue to fall gradually, unlike the old fluorescent tubes, HID lamps and other light sources we used in the past.