On luminaires, lamps and lighting systems, you will sometimes find lux values or watt specifications. But often only one value is known and you would like to convert lux to watts or watts to lux. But is this physically possible at all? Basically yes, but some points have to be considered. These important points and a helpful online calculator can be found in the article.
Lux to Watts Conversion
Current luminaires and lighting systems have a large number of technical data that need to be taken into account when planning or retrofitting your lighting installation. But sometimes not all of the data is known, so you have to calculate the missing parameters yourself. Converting lumens to watts and lux to lumens are the most common calculations.
On the other hand, if you want to calculate the wattage from the lux value, it gets a little bit tricky. This is because there is no direct connection between these two values. Lux is the unit for the illuminance, which hits an illuminated surface. Watt is the unit for electrical power.
Lux is a received quantity
1 lux (lx) corresponds to a luminous flux of 1 lumen per square meter. A specification in lux is therefore only helpful if the size of the illuminated area is also known.
Watt is the energy expenditure per period of time
In connection with the lux value, the watt specification is not the electrical power of the luminaire, but the radiant power of the light source.
Photometric equivalent of radiance
For the conversion between lux and watt, an important factor is still missing. This is called the photometric equivalent of radiance. The radiation equivalent is 683 lm/W at a wavelength of 555 nm (center of the visible light spectrum).
Lux to Watts Converter
After entering the lux or watt value and the illuminated area, you can calculate the missing value. Important: Here the wattage is not the electrical power but the radiant power of the luminaire. For the conversion into electrical power, a further step is necessary.
Convert Electrical Power to Lux
In practice, one often wants to convert between lux and the actual electrical power consumption of a luminaire or lighting system. For this purpose, the energy efficiency of the used lamp(s) must also be included in the calculation.
For an old incandescent lamp, this is just 5%. When converting between lux and watt, the watt specification of the calculator must therefore be multiplied by the efficiency.
Conclusion
The conversion between watts and lux can play an important role in the planning and calculation of lighting systems. By using the photometric equivalent of radiance, this calculation is possible in practice. It remains important to distinguish between the radiant power and the electrical power consumption, which can be included in this calculation via the efficiency factor.
