Troland

Physical unit
Unit name	Troland
Unit symbol	${\ displaystyle \ mathrm {td}}$
Physical quantity (s)	Retinal illuminance
dimension	${\ displaystyle {\ mathsf {J}}}$
In SI units	${\ displaystyle \ mathrm {1 \, td = 1 \; {\ frac {cd} {m ^ {2}}} \ cdot mm ^ {2}}}$
Named after	Leonard Troland
Derived from	Candela , square meter
See also: luminous intensity , luminance

The Troland (Trol, Td or td), also known as Luxon , is a unit of measurement for the so-called pupillary light intensity , which serves as a measure of the illuminance of the retina of the eye . With a luminance of 1 cd / m² and an opening area of the pupil of 1 mm², the pupil light intensity is 1 Troland.

The retina is illuminated by light falling through the viewing hole , the width of which can be changed. For this reason, the American experimental psychologist Leonard T. Troland (1889–1932) proposed in 1917 that the product of luminance and effective pupil area be given as a measure of the retinal brightness stimulus. This takes into account the respective pupil width adapted to the ambient brightness ( adaptation ). The Troland is not a legal entity in Germany.

definition

The prevailing illuminance of the retina depends on the luminance L of an optical stimulus and the opening width ( aperture ) A of the pupil of the observing eye. The pupil light intensity is defined as the product of these two quantities:

{\ displaystyle I _ {\ text {P}} = L \ cdot A _ {\ text {P}}}

If you give the luminance in candela per square meter (cd / m²) and the pupil area in square millimeters (mm²), you get the pupil light intensity in Troland. This is the same with a luminance and a pupil area of ${\ displaystyle L}$ ${\ displaystyle A _ {\ text {P}}}$ ${\ displaystyle L = 1 \, \ mathrm {cd / {m} ^ {2}}}$ ${\ displaystyle A _ {\ text {P}} = 1 \, {\ text {mm}} ^ {2}}$ ${\ displaystyle \ mathrm {1 \, \ mathrm {td = 1 \, cd / {m} ^ {2}} \ cdot {mm} ^ {2}} \ ,.}$

application

The pupil light intensity with the unit of measurement Troland - symbol: Trol or td - is used to indicate optical stimulus parameters in physiological examinations. It plays a role in researching visual stimulus thresholds , as it can be used to describe optical stimuli depending on the state of adaptation of the eye. In practice, the effective pupil area is kept as constant as possible. This can be achieved by fading in an artificial pupil whose aperture is smaller than the pupil of the eye.

Insofar as the luminance is defined for the area of photopic vision (daytime vision), the pupil light intensity can also only be valid for the calculation of the cone-oriented brightness impression . Different specifications are required for scotopic vision (night vision) or calculations of the rod-related brightness stimulus.

However, the pupil light intensity does not reflect the actual retinal illuminance. Because for this, in addition to the transparency of the refractive media of the dioptric apparatus , the beam path of the incident light must also be taken into account (see Stiles-Crawford effect ).

Individual evidence

^ François Cardarelli: Scientific Unit Conversion. A Practical Guide to Metrication , Springer Verlag, London, 1999, p. 354.

↑ Entry pupil light intensity in the lexicon of optics on Spektrum.de; accessed on October 9, 2019.

^ LT Troland: On the measurement of visual stimulation intensities. In: Journal of experimental Psychology. Volume 2, 1917, pp. 1-33 ( online ). The name Troland proposed was “ photon ”, but hardly anyone except himself used this term for it (cf. Klaus Hentschel: Lichtquanten . Springer 2017, ISBN 9783-6-625-5273-5 , p. 37. google book )
↑ DIN 5031 part 6 radiation physics in the optical field and lighting technology; Pupillary light intensity as a measure of the retinal illumination. Edition March 1982. p. 3.

[1] François Cardarelli: Scientific Unit Conversion. A Practical Guide to Metrication , Springer Verlag, London, 1999, p. 354.

[2] Entry pupil light intensity in the lexicon of optics on Spektrum.de; accessed on October 9, 2019.