There’s a sneaky secret in outdoor lighting that the bulb manufacturers don’t want you to know: the way “lumens” is used for light bulbs is effectively a meaningless term.

To explain that, let’s talk about candy bars for a moment. If one candy bar weighs 300 grams, and another one weighs only 200 grams, then we can be sure the 300 gram one is heavier. Labels can’t lie about the weight, and “grams” has a specific real-world meaning.

This is because a “gram” is one of the International System of Units, which also includes things like “second” for time and “ampere” for electrical current. This way, we can all agree how long a second is, and that there are 60 of them in an hour, etc. No funny business.

But imagine if “grams” wasn’t a standard. One candy maker could say their candy bars were 600 grams, but you wouldn’t really know what that means. Would a 600 gram “Chocolate Monkey Bar” be more or less than a 400 gram “Bubblelicious Yummy Snack”? You couldn’t tell, and it wouldn’t be fair.

Money LightbulbWell when it comes to light bulb packaging, “lumens” might as well be a made up word too. One brand’s “300 lumens” might be less bright than another brand’s “200 lumens”, and both depend on the shape of the bulb, the nature of the lens, and even the fixture it the bulb gets put into.

Lumens does have a fixed specific meaning (in fact it’s a “derived SI term”), but in real world use there’s no way to know how bright a fixture of a certain amount of lumens will actually appear in practice. So it may as well be a made up word.

For outdoor lights in particular it’s a tricky thing, since what we really care about is how bright a given surface or area ends up appearing to a viewer, not how much light is sent out in all directions from a bulb. A directional lamp (such as a spot light) will appear much brighter to your eye than one that casts a general area effect (such as a post light).

Even things like lux (lumens per meter) and foot candles (lumens per foot) can be slippery characters when it comes to package claims too. These terms should be a much more accurate way of expressing how bright a light will appear to you. But alas the reality is the output from most bulbs is not “measured” but just “calculated” (which really means “guessed at”).

All these words can still often be useful when comparing the brightness of one bulb to the next, but really only when comparing two bulbs of the same kind, from the same manufacturer, in the same light fixture, shining on the same surface. And even then, it’s not like you can count on anything other than “higher means brighter”. You can’t assume that 200 lumens will appear twice as bright as 100 lumens, only that 200 will look brighter than 100. Probably.

So the next time you are shopping for light bulbs, be skeptical of brightness claims. That no-name bulb with “500 estimated lumens” might not be such a bright idea after all.