You searched for: “isolumes
isolume (s) (noun), isolumes (pl)
1. A line on a chart or a map that connects points of equal shining intensity: Astronomy students were utilizing the presentation of the isolumes in order to analyze the various degrees of brightness of the planets and the stars.
2. An intense or strong effulgence of nature; such as, from the sun: Some sea animals follow isolumes up and down water columns.

As sunlight decreases in the late afternoon, an isolume moves toward the sea surface, and the migrating community follows such radiance with a precision seldom seen in other natural populations.

Daily or seasonal changes in an isolume, or lucid intensity, seems to be the most likely stimulus for vertical migrations of some sea creatures.