A unit of measurement for how much light something produces; the pathway inside a tube or vessel.
From Latin 'lumen' meaning 'light' or 'opening.' In anatomy, it describes the hollow space inside blood vessels or tubes. In physics, it became the scientific unit for measuring light output.
A 'lumen' is one of those words that scientists reused in completely different contexts! In medicine it's the hole in your artery, but in light measurement it comes from the same Latin word 'light.' When LED bulbs replaced incandescent ones, lumens became the new standard because they measure actual light output instead of power used.
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