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Lux, Lumens and Watts,What is the difference

Brillianz Electronic Systems has Brillianz Electronic Systems has a passion to help anyone understand
further lighting terminology. With this in mind please find the attached PDF file containing Brillianz Electronic Systems view of LED Lighting ' A UK Perspective'
Light is such an obvious – “taken for granted” – experience that we naturally think Lighting is straightforward as well …. Not necessarily so.

In this short article we examine the terminology of lighting and try to unravel the important light properties of LEDs – the solid state and Green friendly answer to the humble light bulb.

The Lux (symbol: lx) is the SI derived unit of illuminance or illumination. It is equal to one lumen per square metre.
· sunlight on an average day ranges from 32 000 to 100 000 lux
· TV studios are lit at about 1 000 lux [i.e. 1000 lumens per square metre]
· a bright office has about 500 lux of illumination
· At sunset and sunrise, ambient outdoor light is also about 400 lux (if the sky is clear).
· moonlight represents about 1 lux
· starlight measures a mere 0.00005 lux

Lumens measure "luminous flux". This is a measure of the total number of packets (or quanta) of light
produced by a light source (e.g. a globe or fluorescent tube). This is the “quantity” of light emitted by the light source.

The purpose of lux is intended to tell you how many lumens you need given the area you are trying to illuminate. Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 1200 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area with the same number of lux requires a larger number of lumens.
The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux (light) is spread. 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 a dimmer illuminance of only 100 lux.

Now to the issue of watts vs lumens produced. This is a difficult one because the wattage of a light source refers to the power consumed to drive the source while lumens refer to the brightness of that source as the human eye perceives it. The wattage of course would be the sum of the heat generated as well the energy of the light emitted.

So, what of LEDs?

When evaluating LEDs we are often asked for the wattage of LED lamps when what is really meant is: “How bright are they?” One can only compare the brightness of the various light sources on the market by knowing the number of lumens they produce. We have all developed expectations about the how bright a 20-,40-, 60- or 100-watt bulb should be and we tend to apply that frame of reference to LED lamps.

To clarify the relationship of watts to light output and LED bulbs, we talk about the number of ‘lumens per watt’ (lm/w) that the device produces – this is called the efficacy. Incandescent bulbs are very inefficient and only generate around 12 lm/w, whereas halogen sources have an efficacy of 20 lm/w. Fluorescent tubes generate around 60 lm/w, whilst metal Halide lamps have quite high efficacies of up to 80 lm/w. LEDs now reach over 100 lm/w but in lighting applications 90 lm/w is probably more typical. It is always better to talk about lumens (and lux required) than watts when it comes to lighting decisions.

Just to make things a bit more complex, lumens relate to the eye’s sensitivity and depend on the wavelength (colour) of the light. The true point to measure lumens is at 555nm – yellow green – which is where the eye is most sensitive. White light is a mix of many colours and to get a proper measure of ‘brightness’ is a complex process involving integration of lumens over each colour with an ‘eye sensitivity’ weighting factor ….

Another point to bear in mind is that unlike incandescent light bulbs that illuminate in a 360-degree spherical pattern regardless of the shape of the bulb, LEDs depend on the shape and composition of their lenses and package structure to direct the light where it is needed. When we illuminate an area with a ‘spherical bulb’ source, even with a reflector, only a small proportion (perhaps 50% at best) of the lumens is delivered as lux to the surface we are lighting – the rest is lost. With LEDs, properly applied, over 80% of the lumens can be directed to provide lux at the surface.

That is why LEDs can seem such bright pinpoints when you look at them. So - LEDs are the brightest per watt of all the common sources of light today – much more efficient than halogen and brighter than fluorescent or metal Halide. When properly, applied they can deliver more light per watt than any of these onto merchandise or a display floor. Low wattage – but very high brightness LEDs have now taken a leading position in commercial and retail lighting. As this trend moves into the household market over the next few years more than a few will be wondering what watts they need for their lumens ….

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