Troland
The troland, 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 unit was proposed in 1916 by Leonard T. Troland, who called it a photon.
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⋅m−2 and p is pupil area in mm2.
A scotopic troland is also sometimes defined:
where L′ is the scotopic luminance in cd⋅m−2 and p is pupil area in mm2.
Although named "retinal illuminance", trolands do not measure the actual photon flux incident on the retina; that quantity depends on the specific wavelengths of light that constitute the luminance used in the calculation.
Units conversion
Troland does not directly convert to other units, being a retinal luminance per unit area of a pupil.However Troland is linked to retinal illuminance in as follows. Assuming the corneal luminance L from an extended source, the pupil diameter p and the focal length of the eye F, the retinal luminance is:
Multiplying by the pupil area: Trolands = L ⋅ π ⋅ p2/4 = F2 ⋅ Lr ≈ 289 ⋅ Lr.
Alternatively, the retinal illuminance Lr = Trolands / 289 .
As provided by a more accurate optical calculations, the conversion factor is 278 rather than 289 as demonstrated by simplified considerations above.
Sometimes, retinal luminance is expressed in = . Assuming a Lambertian surface, 1 cd/m2 = π lm/m2 = π lux. That is, 1 = 289/π ≈ 92
Physical quantities
- luminance
- equivalent luminance
Unit system