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.
The troland typically refers to the ordinary or photopic troland, which is defined in terms of the photopic luminance:
where L is the photopic luminance in cd/m2 and p is pupil area in mm2
A scotopic troland is also sometimes defined: