Know your Lux from your Lumens

Decoding the terminology used to measure light levels, or illumination, can be a daunting and confusing task for those unsure of the vocabulary used. It’s important to stress straight away that we are not talking about quantities of radiant flux here, such as watts; nor are we mentioning radiant energy, measured in joules. These units of power are used to determine radiometry levels, referring to the measurement of light levels at all wavelengths, which aren’t always relevant to humanly perceptible light levels.
Both lux and lumens are involved with photometrical measurements – i.e. light levels in terms of their perceived brightness to the human eye – and are far more relevant to everyday use.
So what exactly are they?
Well, as simply put as possible, lux is the “throw” of light; the light intensity, determining how much light can reach a certain point, which will obviously decrease as distance increases. For example, a light source 1 metre away will have a far higher lux reading than that of one 100 metres away.
Lumens, or “luminous flux”, describe the overall level of light emitted from a light source, regardless of how far from the source a reading is being taken. A useful reference point would be to consider lumens as similar to a car’s brake horsepower, in that the lumen level doesn’t necessarily correspond to the perceived light level at a particular distance, just as an engine’s brake horsepower doesn’t directly relate to the maximum speed a vehicle is capable of.
The relationship between lux and lumens can be expressed as a simple formula: 1 lx = 1 lm/m2 (1 lux (lx) is equal to 1 lumen (lm) per square metre. So, for example, a luminous flux of 500 lumens measured at one square metre illuminates that entire square metre with a luminance of 500 lux. If the distance were to increase, however, the lux reading would decrease incrementally, while the lumens level would remain constant: the same light source covering 100 square metres would create a lux level of 5 lx.
Lumens are generated by energy measured in watts: greater wattage = greater lumens. The energy saving benefits seen in advanced LED lights are due to their ability to use fewer watts to generate the same lumen level of a standard incandescent bulb. On a day-to-day basis, lumens are a useful way to compare the efficiency of all LEDs, incandescent and fluorescent bulbs. But remember: the lumen level does not determine the perceived brightness of a lamp – the lux level is far more informative in this case.  
London Electric Design is a forward thinking LED installations company in London.
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