The candela per square metre (symbol: cd/m) is the unit of luminance in the International System of Units (SI). The unit is based on the candela, the SI unit of luminous intensity, and the square metre, the SI unit of area.
The nit (symbol: nt) is a deprecated non-SI name also used for this unit (1 nt = 1 cd/m). The term nit is believed to come from the Latin word nitēre, "to shine".
Photopic vision is the vision of the eye under well-lit conditions (luminance levels from 10 to 10 cd/m). In humans and many other animals, photopic vision allows color perception, mediated by cone cells, and a significantly higher visual acuity and temporal resolution than available with scotopic vision.
The human eye uses three types of cones to sense light in three bands of color. The biological pigments of the cones have maximum absorption values at wavelengths of about 420 nm (blue), 534 nm (bluish-green), and 564 nm (yellowish-green). The color of the pure signal of the cones could be described as violet, blue-green, and scarlet red, respectively, but, in their wavelengths of maximum absorption other cones are activated as well. The sensitivity ranges of the cone cells overlap to provide vision throughout the visible spectrum. The maximum efficacy is 683 lm/W at a wavelength of 555 nm (green). By definition, light at a frequency of 5.4×10 hertz (λ = 555.17. . . nm) has a luminous efficacy of 683 lm/W.
Mesopic vision, sometimes also called twilight vision, is a combination of photopic and scotopic vision under low-light (but not necessarily dark) conditions. Mesopic levels range approximately from 0.01 to 3.0 cd/m in luminance. Most nighttime outdoor and street lighting conditions are in the mesopic range.
Human eyes respond to certain light levels differently. This is because under high light levels typical during daytime (photopic vision), the eye uses cones to process light. Under very low light levels, corresponding to moonless nights without artificial lighting (scotopic vision), the eye uses rods to process light. At many nighttime levels, a combination of both cones and rods supports vision. Photopic vision facilitates excellent color perception, whereas colors are barely perceptible under scotopic vision. Mesopic vision falls between these two extremes. In most nighttime environments, enough ambient light prevents true scotopic vision.