It appears this is cost-per-lumen, not just cost-for-any-level-of-illumination. So it makes sense. The bottom of the page has this information: >Method: This p

It appears this is cost-per-lumen, not just cost-for-any-level-of-illumination. So it makes sense. The bottom of the page has this information: >Method: This project is a visualization of a 1990s paper[0] by William Nordhaus, wherein the economist assessed the amount of illumination that workers could purchase at various times in history. I’d recommend the paper even if you’re not a fan of academic writing; it’s approachable, and Nordhaus is the type of Nobel prize-winning overachiever who goes so far as recreating a Babylonian oil lantern to assess its illumination capacity. [0] https://www.nber.org/system/files/chapters/c6064/c6064.pdf https://www.nber.org/system/files/chapters/c6064/c6064.pdf