isophote

noun

iso·​phote ˈī-sə-ˌfōt 

: a curve on a chart joining points of equal light intensity from a given source

isophotal

ˌī-sə-ˈfō-təl 

adjective

Word History

Etymology

International Scientific Vocabulary is- + -phote (from Greek phōt-, phōs light) — more at fancy

First Known Use

circa 1909, in the meaning defined above