Troland

A troland is a proposed unit of luminous efficiency, a subjective measure of the perceived brightness of a light stimulus. It represents the retinal illuminance produced when viewing a surface with a luminance of one candela per square meter (cd/m²) through a pupil with an area of one square millimeter (mm²).

The troland is a useful concept in vision science and psychophysics because it allows researchers to control for variations in pupil size, which can significantly affect the amount of light reaching the retina and, consequently, perceived brightness. Because pupil size changes with light levels and other factors, expressing stimulus intensity in trolands provides a more accurate and consistent measure of the effective retinal illumination than relying solely on luminance values.

The unit is named after Leonard T. Troland, an American physicist and psychologist who made significant contributions to the study of vision and color perception.

The troland (Td) can be calculated using the following formula:

Td = L * A

Where:

• L is the luminance of the surface in cd/m²
• A is the area of the pupil in mm²

While theoretically useful, directly measuring pupil area during experiments can be complex. Therefore, variations on this basic formula or estimates are often employed. Furthermore, the simple troland calculation assumes a uniform light source and a perfectly round pupil, neither of which always hold true in real-world scenarios.