If you’ve ever looked into LED lighting, it’s almost certain you’ve crossed paths with the term “lumen.” Lumens, quite simply, are the standard unit of measurement used to describe how well a light source will illuminate objects. Because output is typically one of the major factors people use to evaluate LED lights, many manufacturers prominently display this figure on product literature and boast high lumen numbers.
What these manufacturers may fail to tell you, however, is that those big numbers are actually the raw lumen output rather than the effective lumen output. What’s the difference? Why does it matter? Keep reading and we’ll explain.
The raw lumen output of a light is actually a theoretical value rather than an actual measure of useful light output. Manufacturers calculate the number of raw lumens by taking the number of LEDs in a light and multiplying that by the maximum output rating for those LEDs.
For example, if a light uses 10 LEDs that have a maximum output rating of 100 lumens, the raw lumen output would be 1,000 lumens (10 x 100 = 1,000). No photometric testing is necessary to come up with this number — it’s just simple math. The reason that raw lumens should not be relied upon for evaluating LED lights is that the metric doesn’t take into account real world factors that can decrease the light output as much as 75 percent.
What causes these decreases in light output? Well, there are a couple of major contributing factors. First, there are thermal losses. LEDs produce less light the hotter they get. And, as LEDs are powered for longer and longer periods of time, they typically heat up. In fact, it is not uncommon for LEDs to reach temperatures of over 212-degrees F (100-degrees C). So, it stands to reason that if you measure the light output of an LED when you initially light it up (when it is cooler) verus after it has been on for 30 minutes (when it is hot), you’re going to see a decrease in the light output.
Remember how the raw lumen calculation relies on the maximum output rating of the component LEDs? Some LED manufacturers calculate maximum output ratings by measuring the light output of the component LED after 25 milliseconds (equivalent in duration to the burst of a flash bulb). We guess that most of you reading this article probably use your lights longer than 25 milliseconds at a time...which means that your light output is going to be less than the raw lumen value. How much less? That will depend upon the thermal management of the light, but the loss is typically in the neighborhood of 10 to 25 percent.