The perceived brightness of light diffusely reflected from a surface is not a linear function of the actual reflected light. Psychophysical experiments indicate that the perceived brightness increases approximately as the logarithm of the luminous flux. A doubling of the flux leads to an increase of about 1.5 units on the arbitrary brightness scale at left.

Luminous Efficacy

The curves represent the spectral luminous efficacy for human vision. The lumen is defined such that the peak of the photopic vision curve has a luminous efficacy of 683 lumens/watt. This value for the photopic peak makes the efficacy the same as the scotopic value at 555 nm.

The scotopic vision is primarily rod vision, and the photopic vision includes the cones.

The response curve of the eye along with the spectral power distribution of a luminous object determine the perceived color of the object.

The Photopic efficacy curve was extrapolated from testing done on 'Standard Observers'. This was done by taking a person with normal vision, and having them compare the brightness of monochromatic light at 555 nm, where the eye is most sensitive, with the brightness of another monochromatic source of differing wavelength. To achieve a balance, the brightness of the 555 nm source was reduced until the observer felt that the two sources were equal in brightness. The fraction by which the 555 nm source is reduced measures the observer's sensitivity to the second wavelength. This exercise is repeated through many wavelengths and many observers. The average of the results gives us the relative sensitivity of the eye at various wavelengths. In 1924, the International Commission on Illumination adopted the "relative sensitivity curve for the C.I.E. Standard Observer".

The Scotopic Efficacy curve was established in the same manner as the photopic curve. It's sensitivity is shifted however to peak at 507 nm, and decreases in proportionally the same manner as the photopic curve. This results in the scotopic curve reaching a relative value of zero sooner in the visible spectrum than the daylight curve. As a consequence of this fact, our nighttime vision does not see red!

The scotopic efficacy curve is assigned a value of unity at 507 nm, and is represented by the symbol V_λ. To determine spectral Luminous Efficacy, the scotopic efficacy value, V_λ , must be multiplied by 1700 lumens per Watt. This value was adjusted from 1754 to allow both curves to obtain the same value of 683 lumens/W at 555 nm. So, a source we see with our dark adapted vision at 507 nm produces 1700 lumens for every Watt radiated, and any other wavelength produces a fraction of that value based on the efficacy curve.