What are Lumens?
We are often asked: What are lumens?
The technical definition: a measure of how much visible light is emitted from a light source.
The layman’s definition: how bright a light is.
The lumens-rating of say an LED light basically tells you how much light is being emitted. If there are more lumens, the light is brighter.
You may also see lumens referred to as ‘light ouput’.
Read our short guide below for examples, definitions and more.
by David Lewis, Chief Strategy Officer | first published 5 February 2019 | revised 28 February 2024
What are Lumens? – An Example
Let’s Compare LED Battens
Imagine two different 5-foot 45W LED battens:
- 5-foot 45W LED batten giving out 6,000 lumens, and
- 5-foot 45W LED batten giving out 4,500 lumens.
Both battens draw the same amount of power (45W), but the first one gives out more light than the second.
The first batten is able to deliver more lumens (6,000) because the LED chips inside are more efficient.
There are many advantages to LED lighting, but not all LED luminaires are created equal. If you are considering upgrading to LED lighting, always ask your installation company how efficient their lights are. Inefficient LED lights will cost you money every month in terms of higher electricity bills.
Lumens and Luminous Flux Definitions
SI Units
Technically, a lumen is an ‘SI unit’ like metres, Celsius, kilograms, and so on. Whereas a metre is a measure of distance, a lumen is a measure of ‘luminous flux’.
When you see either ‘luminous flux’ or ‘lumens’ on an LED light data-sheet, you need to check whether these terms refer to the LED chip or the overall fitting.
For example, the LED chips in a light may have a luminous flux of 4,300 lumens, but the overall fitting might have a luminous flux of only 3,700 lumens.
Where do the missing lumens go?
It is important to focus on the 3,700 lumens figure, as that is the actual amount of light coming out of the fitting.
Where are the missing 600 lumens? Most of these are lost as the light travels through the diffuser that sits between the LED chips and the observer.
Lumen output will drop off very slowly over time. See our separate guide on how long LED lights last.
Lumens per Watt
LED Efficiency
Another way of answering the question “what are lumens?”, with a focus on efficiency, is to consider ‘Lumens per Watt’.
The simple definition of Lumens per Watt is: a ratio which tells you how efficient an LED light is. It is a very important ratio to keep an eye on if you want to save money.
Consider two LED 600x600mm panel lights (the ones that fit into office ceiling grids):
- LED A is rated at 26W and gives out 3,380 lumens. This means it only needs 26W of power to deliver the 3,380 lumens. LED A has, therefore, a Lumens per Watt rating of 130 (3,380 divided by 26).
- LED B is rated at a higher 40W, but gives out the same 3,380 lumens. LED B’s Lumens per Watt rating is therefore 84.5.
False Economy
The question to ask then is: if I need 3,380 lumens to light up my work space adequately, why would I install a 40W LED – which consumes a lot of power – when I can have those same 3,380 lumens for only 26W of power? The 26W LED will reduce my electricity bill by 35% compared to the 40W version.
40W LED panel lights are often sold because they are cheaper than more efficient LED panel lights with a higher Lumens per Watt rating. These 40W LED panel lights are, however, only cheaper at the point of sale. They end up being much more expensive when annual electricity savings are taken into account.
For the best overall cost of ownership, ask for LED lights with a high Lumens per Watt rating.
What are lumens and what they are not
Lumens, Lux & Watts
The question “what are lumens?” should not to be confused with “what is lux?”. Please read out separate guide for a definition of lux.
Lumens are also sometimes mixed up with Watts. Watts simply state the power rating of a say an LED light. Watts are about electricity flowing through an LED light over time – not about how much light is emitted.
Candelas & Colour Temperature
You may also come across the term ‘candela’. This is relevant to lumens, but it’s not the same thing. The definition of a candela is a measure of ‘luminous intensity’ across a particular beam angle. Once you get into candelas, unless you’re a physics graduate, you’re likely to get confused. Just stick to lumens.
Don’t confuse lumens with colour temperature. If you see ‘4000K’, or ‘5000K’, that’s colour temperature, not lumens.
Conclusion
Understanding what lumens are can be difficult to get your head around at first, but basically lumens are about how bright a light is. The higher the lumen rating, the brighter the light.
If you’re purchasing LEDs for your organisation, get a full audit carried out first by an expert. Calculate how bright various work spaces need to be. Someone working in a warehouse needs different lumens levels than someone writing reports at a desk.
When it comes to choosing the right LED types and characteristics, don’t make the mistake of buying the cheapest luminaires with the lowest lumens per Watt ratings. It will cost you dearly over time.
