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

Features:

1. The horizontal axis is wavelength in nanometers (nm).

2. The visible light band has fiducial range 0.4--0.7 μm---i.e., the range everyone remembers to say.

3. The vertical axis is actually luminous flux per unit power.

   Luminous flux is the quantity of psychophysical sensitivity of the human eye with units lumens.

   Power has units of watts.

4. 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.

5. Actually, humans can see a bit beyond the fiducial range 0.4--0.7 μm of the visible light band.

   The extreme human visible range ≅ 310--1050 nm. To see beyond the fiducial range 0.4--0.7 μm requires bright sources, ideal conditions, and for the ultraviolet part to have young eyes.

   There is no revelation though---ultraviolet probably just looks violet and infrared just looks deep red (PP-19).

   It may not be safe to look for very long at intense sources beyond the he fiducial range 0.4--0.7 μm of the visible light band---but I don't know.