Poorly designed metrics hinder the development of efficient lighting technologies, writes Adrian Rawlinson, managing director of Marl International
Under the stricter requirements of Part L of the UK Building Regulations which came into effect in October 2010, lighting should produce at least 45lm/W measured at the circuit.
It is a measure that a well-designed solid-state LED lighting system can meet quite easily, but care needs to be taken to evaluate the light output of the system properly and this means having effective measures of the energy used and the useful light emitted. While the regulation lays down a clear approach to measuring the power used by a lighting system, there is, unfortunately, still no universally agreed standard method of measuring light output.
The apparent efficiency of a lighting system depends on the point at which you measure the power used. By specifying the mains circuit as the point of reference, the new regulations ensure that all technologies are compared consistently with each other and with incandescent lights which are driven direct from a mains supply: fluorescent lights are driven through an inverter and LEDs need a rectified, usually low-voltage supply.
The regulations ensure that designers look at the efficiency of the drive circuit as well as that of the LED. Older drivers could have an efficiency of as little as 50%, so that an LED producing nominally 70lm/W would only deliver 35lm/W measured at the wall socket.
Best in-class drivers from manufacturers like Mean Well, Lumotech and IST provide 95% efficiency, improving the ‘true’ output of the LED to 65lm/W. For high-power LEDs of 60W or more it is worth using power-factor corrected drivers.
While this means that the input power to a lighting system is evaluated consistently, there is still no standard method of measuring light output power across the US, Europe and Asia. Outside the lighting industry, few understand measures such as candela, lumens and mean spherical candle power. Buyers instead relate light output to the wattage of the incandescent bulb needed to produce ‘equivalent’ light output.
LEDs have different characteristics from other forms of light, since they are directional, as opposed to spherical. This is another way that LEDs are potentially more efficient but it also makes it hard to provide genuinely equivalent measures of light output.
The increasing use of integrating spheres to measure the total radiant power or luminous flux of light sources is a step in the right direction. The inside surface of an integrating sphere is coated with a diffusely reflecting material which guarantees complete integration and homogenisation of the emitted radiation.
The integrated light is then measured at the detector port. Independent tests can be carried out by accredited test houses like 42 Partners Limited.
Models can be downloaded from suppliers’ websites into lighting simulation tools like the freely downloadable Relux. Architects can then produce accurate simulations of the appearance of any given lighting installation in their design.
Although warm white LEDs are popular, their efficiency is rather lower than the less attractive ‘cool white’ devices. Allowing for the effect of the optics, temperature and the drive circuit, 70lm/W is realistic for a cool white LED while 45-50lm/W is a good for warm white.
LED lighting manufacturers are working on different approaches to address this issue. One is to use multi-die LED arrays with mixed colours to warm up the colour output.
For high CRI (colour rendering index) application, adding a red LED to a cool white array can make a significant difference. As the light is dimmed, the red can be allowed to intrude more.
Quantum dot technology has great potential to produce efficient warm white LEDs, though currently it is very expensive. It uses a film impregnated into the lens glass in a vacuum, which shifts the wavelength of the light emerging towards the red end of the spectrum.
Quantum dots emit light at a very specific frequency, which allows glass to be designed to maximise transmission in the visible part of the spectrum and minimise wavelengths that the human eye cannot perceive. Products based on this technology are being released, mainly in the US, but unfortunately since they use materials that are not RoHS compliant, they cannot currently be sold in Europe.
Energy consumption is important: it is estimated that 22% of US demand is for lighting, and an eight-fold improvement in efficiency is possible by switching to solid state lighting.
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Adrian Rawlinson, www.leds.co.uk