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Lux, Lumens and Watts – Get The Best Luminaire

There is a lot of confusion regarding Lux, Lumens and Watts which is worth clarifying so that the best luminaire for a given use can be correctly specified.

Asking the right questions
I am often asked if we can supply a luminaire in Watts.  Do you have a 50 Watt flood light?  Do you have a 100 Watt low bay light?  The answer is “Yes” but the question could be put better.

I need 150 lux in my aisle, do you have a light that can do that?  Or – I need 700 lux at working height in my workshop, can I do that?

Lux (lx)
The wattage of the luminaire, or the lumen output, aren’t important to answer the lux level required but are used to indicate how efficient the luminaire is.  Usually the minimum recommended lux level needed (illuminance at the required surface) is specified in health and safety guidelines, by CIBSE or a regulatory body.
For example, the HSE suggests a minimum average illuminance of 50 lux in loading bays. This is to provide adequate light to safely see other users and provide a comfortable light level to do the required task without causing eye strain.

The luminaire light output required to achieve this will depend on several factors – lux level required, mounting height, area to be illuminated and spacing of the luminaires.  Once this information has been established it is possible to work out the light output required from the luminaire to achieve the desired lux level.

Lumens (lm) is the light output from a given luminaire and
Watts is the power required to achieve it.

Once the lumen output has been established is the time to start considering the power (wattage) required and selecting the most suitable luminaire.

Efficacy (lm/W)
and Watts determine how efficient a luminaire is and is quoted in lumens/Watt.
A good luminaire nowadays will be at least 80lm/W.  The more efficient it is the fewer will be required and the less power used.  Make sure the efficiency quoted refers to the complete luminaire not just the LED.  An inefficient power supply can end u being inefficient.

ExampleBrighton High Bay Light For the best luminaire
We recently supplied some Brighton High Bay lights to illuminate a production area to 700lx.  It was determined that we could achieve this using our 120 watt luminaire with a system efficacy of 90 lm/W.  We required 42 fittings.  If the efficacy of the luminaire had only been 60lm/W we would have needed to use either 42 x 180 watt luminaires or increased the number of fittings to 60.  Either the cost of the fitting increases significantly or the installation costs would increase.  Another important thing to note is that 50% more power would be required.

If you would like to discuss further please contact us at Earlsmann.

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