In the 1920's astronomer André Danjon devised a scale to assess the depth of an umbral lunar eclipse, as follows:
- L=4: Bright copper-red or orange umbra, with bluish rim.
- L=3: Brick-red umbra, with light grey or yellow rim.
- L=2: Deep red or rust-coloured umbra, with dark centre and brighter rim.
- L=1: Dark grey or brownish umbra. Difficult to see features.
- L=0: Very dark umbra. Moon itself difficult to see.