ARE THEY FAKING IT?
The great lumens per watt fraud
Lumens per watt. It is one of the simplest and clearest measures of the quality of a light fitting. Delivering more lumens for every watt of power consumed is a Good Thing. Good for the planet, good for your pocket, good even for your eyesight.
But before we go too far into this, let`s make sure we are all on the same page.
With regard to a light source…..
Lumens = the amount of light emitted by a source. The output.
Watts = the amount of electrical power consumed by a source. The input.
Question: “How much output (light - measured in lumens) do you get for every unit of input (electrical power – measured in watts)?”
Answer: “The more the better. There is no argument about it. 150 lumens per watt is better than 100 lumens per watt, just as 60 miles per gallon (if you drive a car) is better than 50 miles per gallon.”
In fact, having a high output per watt is so unequivocally a Good Thing that everyone wants to measure it, and everyone wants to claim that theirs is better than the other supplier’s. My high-bay / downlight / panel or weatherproof delivers more lumens per watt than yours.
One of the problems with apparently simple metrics is that everyone uses them, and a fair few abuse them. There are sometimes so many numbers flying around when lighting engineers, contractors and consultants get together that inevitably there are some dodgy ones in the mix.
Another problem is that the numbers keep changing. A few years ago, we were delighted with an LED panel that delivered 100 lumens per watt. Today, that would be a poor performance – 130 or 140 lumens per watt is now the threshold of acceptability.
So, when someone claims their panel delivers 175 lumens per watt, or their high-bay delivers 210, are they faking it, or did we just miss their LinkedIn (Facebook / Instagram / Twitter) post last month that announced yet another small step on mankind’s progress to sustainable living?
THE PHYSICS - simplified
The maximum number of lumens that can be created by 1 watt of electrical power is 683. However, this would be at a wavelength of 555 nm (bright green) – which is the point in the spectrum at which our eyes are the most sensitive. For the production of white light, the maximum theoretical efficiency is 250 – 350 lumens per watt, depending on the mix of wavelengths in the white light.
So, if anyone claims an efficiency in excess of 250 lm/W for a light fitting, be warned – they are almost certainly faking it.
However, plenty of people are claiming 200, 210 or even 220 lumens per watt. Are they faking it?
Lumens per watt (Lm/W).
This measure relates to bare LEDs. No driver, no reflector, no lens, no diffuser. Just the number of lumens being emitted per watt by the bare LED.
This is a bare LED. Bare LEDs can emit in excess of 250 lumens per watt in a laboratory, but not in a light fitting.
In relation to a light fitting, beware of anyone quoting performance in terms of Lm/W for two reasons:
1. Lm/W data are published by LED manufactures. They relate to bare LEDs, not light fittings. Light fittings very rarely employ bare LEDs (with no diffuser or lens), and they are never used without a driver, so the performance of a bare LED is a poor indicator of the performance of the fitting in which it is used. Diffusers, lenses and reflectors all reduce the amount of light coming out of a light fitting.
In addition, drivers consume a few watts themselves, so the watts that actually reach the LEDs are fewer than the total number of watts being consumed by the light fitting. Together, these facts mean that Lm/W is the wrong measure to use when comparing light fittings.
“LED watts” is sometimes used to describe the power that actually goes into the LEDs, as opposed to “circuit watts”, which is the electrical power going into a light fitting.
2. LED manufacturers measure the lumens per watt of their products when cold. As LEDs warm up (for example, in a lamp or inside a light fitting) they become less efficient, so this “cold-lumens” statistic can rarely be reproduced in real life. LED manufacturers publish their cold lumen data, and the engineers who design light fittings know how to interpret this data. Sadly, less scrupulous (or less knowledgeable) people sometimes seize these figures and wrongly attribute them to the light fitting in which the LEDs are used.
2.Cold-lumens are sometimes referred to as LED lumens. These two terms mean the same thing. The number of lumens produced by an LED, on its own, in laboratory conditions.
Luminaire lumens per circuit watt (l.Lm/c.W)
This measure relates to a light fitting.
LED (cold) lumens v luminaire lumens – an example
The LEDs in a 600x600 panel might be rated by the LED manufacturer to emit 50,000 lumens. However, the opal diffuser on the front of the fitting could absorb 10% of that, so the actual output from the light fitting would be, say, 45,000 luminaire lumens.
Q: “What happens to the 10% that the diffuser absorbs?”
A: “They turn into heat, so the diffuser gets warm and the inside of the panel gets a little warm too. This raises the temperature of the LEDs and their output then drops slightly, so the actual output of the panel might be just 44,500 lumens.”
That’s the difference between LED lumens and luminaire lumens.
LED watts v circuit watts – an example
The LEDs in a downlight might consume 10W of power. Those are the LED watts. The driver might consume another 2 watts, so the fitting should be rated as a 12W fitting – that’s “circuit watts”.
Q: “If the 10W that goes into the LEDs is mostly converted to light, what happens to the other 2W that is consumed by the driver?”
A: “It turns into heat – and in a really nasty cheap driver that buzzes some of the power is lost as noise too.”
That’s the difference between LED watts and circuit watts.
“Lumens per watt” is used by luminaire design engineers and LED manufactures, but when quoted in relation to a light fitting it is a sign that either:
A final word
When assessing a light fitting l.lm/c.W is a useful measure, but it isn’t everything.
It would be relatively easy to design a light fitting that delivered 175 or 200 luminaire lumens per circuit watt – but it might be horrible to live and work under. Lenses, reflectors and diffusers are all responsible for “wasting” a % of the lumens the LEDs generate, but their function is to put the light where it is needed, correctly distributed and without too much glare.