Despite the standard incandescent light bulb becoming all but extinct, the association of brightness to wattage is still an ingrained part of our selection process for finding the right lighting product. As LEDs become the standard in lighting, it’s important to understand the difference between watts and lumens, and how to determine what’s a sufficient amount of lumens for creating optimal atmosphere in a space.
So, what are lumens? The lumen is the international scientific standard measurement for the amount of light produced by a light source, or its actual measured brightness. Watts on the other hand, measure the amount of energy required to operate a light source.
We’ve been been incorrectly associating energy consumption with light output for decades. Conversion charts are a helpful guideline for the consumer who may be changing out their old incandescent light bulbs for new LED light bulbs.
The challenge is there’s no simple mathematical conversion for watts to lumens, as in converting kilograms to pounds or miles to kilometers. Conversion charts illustrate a general correspondence between lumens and watts, but don’t tell the whole story, and can be easily misunderstood when trying to evaluate the adequacy of a light source’s lumen count.
Adapting these charts for anything beyond a bulb-for-bulb swap is like trying to find the equivalency of an apple to an orange. The only real information you can gather is that they’re both pieces of fruit. The incandescent wattage on a conversion chart does not equate to the delivered lumens from a complete luminaire.
Consider this chart. Chances are you associate “40W” with familiar applications, such as a bedside table lamp. This association may mislead you into thinking a 40W / 450 lumen equivalent will be inadequate for your project.
The light emitted from a complete luminaire with dedicated LEDs, however, will behave differently than a single bulb. LEDs are inherently directional and emit almost all of their light output in a desired direction, rather than dispersing it in all directions as an uncovered bulb does. Therefore, a reliable and accurate equivalence between watts and lumens delivered by a fixture cannot be determined by these charts.