The photopic sensitivity functions describe the relationship between electro-magnetic radiation and human vision that is mediated by cone photoreceptors, and is the basis for the unit of "lumen". The "Standard Observer" or "the" photopic sensitivity function (also called vlambda or ybar2) comes from the CIE and dates to 1931. This data represents the sensitivity of the 2 degree visual field associated with foveal vision. The two degree field can be estimated by the circle filled by a US penny held at (an adult's) arm's length.
In 1964, the CIE published the "Supplementary Observer" sensitivity function which corresponds to the 10 degree visual field that is described as para-foveal. The ten degree field can be estimated by the circle filled by a four inch disc (such as the top to a coffee can) held at (an adult's) arm's length.
These CIE sensitivity functions are considered valid when the visual system is mediating photopic luminances, defined as occurring at or above 3.4 cd/m2. At lower luminances, the sensitivity of the visual system changes and the visual system is described as being in a state of "mesopic" adaptation.
The following diagram shows the different visual sensitivity functions: the two degree field as a white line, the ten degree field as a gray line and the scotopic sensitivity function as a black line.
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