It is the ratio of luminous flux to power, measured in lumens per watt in the International System of Units (SI).

By definition, light outside the visible spectrum cannot be seen by the standard human vision system, and therefore does not contribute to, and indeed can subtract from, luminous efficacy.

This is a standardized function representing photopic vision, which models the response of the eye's cone cells, that are active under typical daylight conditions.

(Mesopic vision describes the transition zone in dim conditions, between photopic and scotopic, where both cones and rods are active.)

[note 2] Scotopic luminous efficacy of radiation reaches a maximum of 1700 lm/W for monochromatic light at a wavelength of 507 nm.

The luminous efficacy of the source is a measure of the efficiency of the device with the output adjusted to account for the spectral response curve (the luminosity function).

Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous [efficacy] is 95 lumens per watt.