In the previous discussion about lumens (https://physbang.com/2025/12/30/what-are-lumens/) some numbers were quoted without any explanation. This post adds a bit more detail to those numbers. Firstly, the maximum theoretical output of a light source was stated to be 683 lumens-per-watt. Why that number? The answer is to be found in the definition of the candela, which … Continue reading What are lumens? (part 2)
What are lumens?
Modern light sources are rated in lumens or, more importantly, lumens per watt. The first figure expresses the brightness of the source whereas the second indicates its efficiency. But what, exactly, is a lumen? To answer that question it is useful to recall that the first man-made light sources relied on heat and were very … Continue reading What are lumens?
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