Caption: A schematic diagram average human eye luminosity function: i.e., the average human eye of psychophysical sensitivity.

The vertical axis is actually luminous flux (the quantity of psychophysical sensitivity of the human eye with units lumens) per unit power (with units watts).

There are actually two luminosity functions:

1. Photopic vision: the luminosity function under well lit condtions.
2. Scotopic vision: luminosity function vision under poorly lit condtions.

Scotopic vision is rather insensitive to color---it's nearly black-and-white vision.

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