Measuring human visual response to light is not straightforward because the eye is a highly complex organ.
An internationally recognised system of measurements was first established in 1924 by an international commission called CIE (Commission Internationale de l’Eclairage).
The photopic curve shows that, in bright light, the strongest response of the human eye is to the colour green with less sensitivity towards the spectral extremes, red and violet.
A second set of measurements of the typical responsiveness of the human eye to wavelengths across the visible spectrum at low levels of light, where determining colour differences is difficult, resulted in data compiled into the scotopic curve.
Having defined the eye’s spectral response, CIE sought a standard light source to serve as a yardstick for luminous intensity. The first source was a specific type of candle, giving rise to the terms footcandle and candlepower. In an effort to improve repeatability, the standard was redefined in 1948 as the amount of light emitted from a given quantity of melting platinum.