# Photometry

## Photometric Efficacy

Photometry is the science of radiometry applied in the context of human vision. All of the rules and results of radiometry are the same, but are modulated by luminous efficacy curves. These curves describe the sensitivity of the human eye to different color (wavelength) stimulus. The following form compute blackbody output in the same manner as the radiometry blackbody calculator, but results are modulated by either the photopic or scotopic response $V(\lambda)$, and the output is in lumens instead of watts. Chart output is in lumens/cm^2-sr-micron for luminance $L_v$, or lumens/cm^2-micron for luminous exitance $M_v$: $$L_v(\lambda, T) = \varepsilon L_e(\lambda, T) V(\lambda)$$ $$M_v(\lambda, T) = \pi L_v(\lambda, T)$$

#### Photopic Response

Photopic response applies in brightly lit conditions, where cones (color-sensitive cells) provide the signals to the retinal ganglion cells, color vision is in effect, and peak response occurs at 555nm (green) at 683 lumens/watt.

#### Scotopic Response

Scotopic response applies in dimly lit conditions, where rods (cells that detect overall light intensity) provide the signals to the retinal ganglion cells, night vision is in effect, and peak response occurs at 507nm (blue) at 1700 lumens/watt.