The luminosity function is the average visual sensitivity of the human eye to diverse wavelengths. It is standardized by the International Commission on Illumination (CIE) and can be used to convert radiant energy into luminous.

The older term for candela is candle, such as foot-candle and candlepower. Prior to 1948, luminous intensity was generally measured by the brightness of the flame from a "standard candle." These terms are defined as follows:

• Candlepower: This is used by the English and is the light produced by a spermaceti (wax from a sperm whale) candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. (The Germans, Austrians and Scandinavians used the Hefner lamp with 1.128 hefnerkerze equal to 1.019 candela.)
• Foot-candle: This is the light level on a surface one foot from a standard candle. One foot-candle is equal to one lumen per square foot.

The frequency chosen in the optical spectrum that is visible to the human eye is near the color green. According to the CIE, the human eye is most sensitive to the green frequency.

To some professionals the CGPM specifications are unclear. The EM-field power level of 1.46 x 10 ^-3 W is too small. The frequency of 540 THz resembles a wavelength of about 556 nanometers (nm), which is in the middle of the visible-light spectrum.