LED Light Output – Why It’s Not All About Watts And Why You Need To Know About Lumens
When you buy an LED lamp (light bulb) or light fitting one consideration over rides all others, how much light is going to give out? That is after all the reason you’re buying the thing.
Back in the old days when we only had a single technology for each light source this was a simple matter of knowing the wattage of the lamp (this is the amount of power the lamp consumes).
All light “bulbs” were incandescent and all tubes were fluorescent. So if we knew the power consumed by say a candle lamp we could compare it with power consumed by a standard “100w bulb” and, because they work off the same technology, we can make a rough judgement about how much light we’d get. Same for a 2ft 18w florescent tube in comparison with an 8ft 125w tube. Everyone was happy and knew where they were.
The problems started when we began to mix up the technologies. All of a sudden we were trying to compare a “100w bulb” with a fluorescent product that did the same job, or a 60w incandescent with a 50w halogen. The reason of course is that light is not measured in wattage ( the wattage is the power the lamp consumes, very different to what we get out). Light is measured in lumens.
Okay, so that should be simple then, let’s just start telling everyone how many lumens our lamp produces and everyone will know where they are. Well, that will certainly help but there’s still a problem. Oh, what’s that? I here you ask. The problem is that you can have a lamp that throws out a lot of lumens in an inefficient way. Let me give you a quick example: You are sitting at home reading the newspaper or surfing the web, the lighting in the room is just right (neither too bright nor too dark). The critical light level in this scenario is how many lumens there are on the surface of what you are reading. This is very different from how many lumens are present at the front of lamp on the ceiling.
Where we have lamps (like the old fashioned “100w bulb”) that are non directional and may be covered by a shade or go into a light fitting with some sort of a directional device like a reflector or cowl this is less of a problem. But, when we look at lamps that have built in reflectors it becomes very important.
When we try to compare directional lamps we need to use other units of measurement, candella and lux. I talk about these in a separate post lumens per watt, candella and lux.