I didn’t know that one either, I am just somewhat familiar with joules, so I looked it up to see if that was kilojoules, and it was. It looks to be US/Canada that still uses Calories/kcal from what I read. Niagra Falls is the furthest I’ve been out of the US, so don’t assume that’s 100% factual from me, that’s just the one source I read. 😉

Lumens is already a metric (SI) unit, but it’s used in the US as well. It’s when we get into things like lux vs footcandles we go back to metric vs imperial again.

Measuring light gets all funky and I am always forgetting what all the terms mean, so I always have to look them up. That article goes into lumens, lux, candela but the tl;dr is :

• The amount of light emitted is measured in lumens.

• A surface’s luminous flux, measured in lux, is the measure of its brightness.

• The visual intensity of a light source is quantified in terms of candela.