OK so you’re researching lights and have a massive range of options. So many things to consider, lies to detect, hidden whatnots and all the rest. You may have noticed that some manufacturers have a lumen value, others have a raw lumen and effective lumen value. First to note, if they don’t state if it is raw or effective, you can be sure it is raw. So, here’s the gist of what you’re looking at.
| Raw Lumens | Effective Lumens | |
|---|---|---|
| What it measures | Light output from the LED in lab conditions | Actual light output from the finished lamp |
| Measured by | LED manufacturer (e.g. Cree, Luxeon) | Lamp manufacturer after full assembly |
| Temperature used | Nominal (typically 25°C) | Real operating temp (typically 70°C) |
| Accounts for optics/reflectors? | No | Yes |
| Accounts for thermal loss? | No | Yes |
| Typical difference | Higher figure | ~25% lower than raw |
| Usefulness when buying lights | Limited | The one that actually matters |
What Are Lumens?
Lumens or luminous flux is the theoretical amount of light produced by an LED in ideal or laboratory conditions. This value is calculated by the LED manufacturer (LG, Luxeon etc.) and is affected or changed by the amount of input current, phosphor dome and other factors. The manufacturers produce hundreds of different types of LEDs, all for specific applications. Some need high output/size ratio (driving lights) while others are focusing on reducing energy consumption, like for warehouse lighting. A very important part of the lamp is that the designer chooses an LED that is relevant to the application.
- Related reading: Lumens vs Lux
To understand the application of luminous flux or lumens we then have to further split it up into two categories: raw lumens and effective lumens.
What Are Raw Lumens?
What the manufacturer states as the lumens is Raw Lumens. For example, a Cree 10W LED has a manufacturer raw lumen figure of 1,050 lumens. If you have eight of these in one lamp, the raw lumens for that lamp is 8 x 1,050 which equals 8,400 raw lumens.
This figure doesn’t take into account the system into which the LED is put or what current it is running at. Manufacturers also often state this value at a nominal temperature i.e. 25°C, which may have no bearing on the actual temperature of how it is being used. For instance, a driving light may be operating at 70°C, which will dramatically reduce the raw lumen figures. How we control the light using a reflector or secondary optics also affects how much light comes out the front of the lamp. While the raw lumens remains unaffected, we can’t claim this value for the system or lamp we are producing as it is misleading. This raw, luminous flux is emitted in a very wide arc and is totally uncontrolled i.e. it is just a ball of light, so doesn’t give any indication of the actual performance of the finished product.
What Are Effective Lumens?
Effective lumens is the actual measured lumen figure that takes into account real losses. Thermal, optical and assembly losses all impact on the output of the lamp. Typical losses can equate to 25% of raw lumens. Using the example above, 8,400 raw lumens less 25% equals 6,300 effective lumens.
Effective lumens is a measurement after the LED has been built into a system, or in our case, a lamp. This takes into account these other factors and specifies at what temperature it is relevant at i.e. 70°C. This is the ‘junction’ temperature that is measured on the PC board, not the housing. One way to improve the number is using open form reflectors as they tend to improve the raw-effective ratio, but that is another discussion.
Why Does Driving Them Harder Matter?
For most LEDs the harder you drive them the brighter they are but the less efficient they become. This can be a bit confusing but we can explain it like this. If you drive a 5W LED at 5W, it will produce, say, 100 lumens/watt. If you drive it at 2W it may produce 120 lumens/watt but will still not be as bright as the 5W running at a lower efficiency. Generally speaking for a driving light we are wanting maximum punch for a sensible size package and energy efficiency is not really a consideration.
So Which Figure Should You Care About?
So when you’re considering buying some lights the most relevant measurement is likely to be the effective lumens, not raw lumens, as this takes into account all the other factors that drop the lumen value. Therefore the only true measurement of light output is effective lumens as this takes into account the whole system rather than a theoretical value from the LED manufacturer. As a final point of explanation, if you had a black lens cover over your lights, the raw lumen value would still be the same as claimed by the manufacturer, but the effective lumens would be zero.
However this is still not the whole story and needs to be balanced with the lux values (find out more on our blog: lux vs lumens) as lumens still doesn’t tell you how the light is controlled. Having 10,000 lumens of uncontrolled light may give you a massive ball of close up light, but won’t necessarily go more than 50 metres down the road!
