Portuguese to English Tech/Engineering Electronics / Elect Eng street lighting illumination

This term appears in a residential project, in the part which analyses the proposed street lighting system. It is combined with another term thus: "campo de avaliação/linhas isográficas" and is the title of a line diagram which appears to show lines of equal illumination intensity in the street.

However, "isografia" (PT0 and "isography" (EN) seem to mean copying handwriting, which is not relevant to this situation.

Can anyone find a technical meaning of the term which relates to a study of street illumination?

Thanks!