It was proposed by André-Louis Danjon in 1921, when postulating that the brightness of a lunar eclipse was related to the solar cycle.
In addition, different parts of the Moon may have different L values, depending on their distance from the center of the Earth's umbra.
While the Earth's shadow blocks any direct light from striking the Moon during a lunar eclipse, some light is refracted through the Earth's atmosphere giving the Moon a red hue.
The amount of light refracted affects the brightness of the moon at mid-eclipse, and this depends on several factors.
It is also thought that the solar cycle has some effect on the darkness of lunar eclipses--indeed Danjon established the scale for this reason.