. Author manuscript; available in PMC: 2022 Mar 1.

Published in final edited form as: J Am Acad Dermatol. 2021 Feb 25;84(5):1219–1231. doi: 10.1016/j.jaad.2021.02.048

Table 1:

Definition of light parameters.

Terminology	Definition	Standard unit
Wavelength	Distance between two peaks of a wavefunction.	Meters
Irradiance (power density)	The power (energy/second) of light delivered to a unit of surface area.	Watts/meter²
Fluence (dose)	The amount of energy delivered to a unit of surface area over a given time. Fluence = irradiance x time.	Joules/meter²
Duration of exposure	The amount of time that the skin is exposed to light.	Seconds
Pulse width	The length of time that a pulsed device delivers light (i.e., “flashes” in a repetitive mode).	Seconds
Beamwidth	The diameter of the beam. The edge of the beam is defined by having an irradiance of at least 1/e² of maximum irradiance.	Meters
Duty cycle	Percentage of time that the signal is active during one cycle.	Percentage of time
Luminance	The intensity of light at the source weighed by human perception of VL.	Lux
Photon flux	The number of photons delivered per unit of surface area at a given time.	Photons/meters²/seconds
Coherence	Light waves have an identical wavelength and constant phase.	N/A
Collimation	Light waves are parallel.	N/A