Dakota Richline
“Every one of us is, in the cosmic perspective, precious. If a human disagrees with you, let him live. In a hundred billion galaxies, you will not find another.”
― Carl Sagan, Cosmos

Opinion: Lumens should be an SI base unit

Disclaimer: I’m only an armchair metrologist. Experts, have mercy

In learning the definition of a candela (watts per steradian of visible light), I realized that lumens are simpler and easier to understand despite being a derived unit. They’re also widely used in the real world, unlike candela.

Sphere cutaway showing one steradian

[3] Understanding the steradian is necessary to derive the value of the commonly-used lumen, which doesn’t require steradians at all!

The original definition of a candela is the luminous intensity of a whale fat candle. If the entire point of a candle is to radiate in every direction, shouldn’t the base unit measure the total flux (light) the candle produces? Using lumen as a base unit is also simpler because its definition would equal the power of a light source weighted for human vision, and there’s no need to understand solid angle unless you need to use the (very niche) candela.

LED, and laser

One common use for candelas: explaining the difference in behavior between an LED and a laser that have the same number of watts or lumens [1] [2]

Car headlight, and candle

Why on Earth is the directional light source rated in lumens, while the isotropic light source is rated in candela? [4] [5]


[1] “3 led circuit” by spatulated is licensed with CC BY 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by/2.0/

[2] “High Power Green Laser, Dark Background (1)” by FastLizard4 is licensed with CC BY-SA 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-sa/2.0/

[3] Andy Anderson, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

[4] “GTR LED HEADLIGHT SIDMARKER” by Specialized Performance is licensed with CC BY-NC 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc/2.0/

[5] “Candle” by Smabs Sputzer (1956-2017) is licensed with CC BY 2.0. To view a copy of this license, visit https://creativecommons.org/licenses/by/2.0/

Last modified: 10/26/2021