What is "troland"

n. A unit (symbol Td) of conventional retinal illuminance, intended as a means of correcting photometric measurements of luminance values impinge on the human eye by scale them by the effective pupil size.

The troland (symbol Td), named after Leonard T. Troland, is a unit of conventional retinal illuminance. It is meant as a method for correcting photometric measurements of luminance values impinging on the human eye by scaling them by the effective pupil size. It is equal to retinal illuminance produced by a surface whose luminance is one nit when the apparent area of the entrance pupil of the eye is 1 square millimeter.

The troland typically refers to the ordinary or photopic troland, which is defined in terms of the photopic luminance:

T = L × p
,

where L is the photopic luminance in cd m and p is pupil area in mm.

A scotopic troland is also sometimes defined:

T' = L' × p
,

where L′ is the scotopic luminance in cd m and p is pupil area in mm.

Although named "retinal luminance", Trolands should not be confused with actual photons flux incident on a retina Lr. Conversion between the two is dependent of the specific eye optical parameters and not done by a simple units conversion factor.