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