Light therapy dose is calculated with this formula:
Power Density x Time = Dose
Fortunately, most recent studies use standardised units to describe their protocol:
Power Density in mW/cm² (milliwatts per centimeter squared)
Time in s (seconds)
Dose in J/cm² (Joules per centimeter squared)
For light therapy at home, power density is therefore the main thing you need to know – if you don’t know it, you won’t be able to know how long to apply your device for to achieve a certain dose. It is simply a measure of how strong the light intensity is (or how many photons are in an area of space).
With angled output LEDs, the light is spreading out as it moves, covering a wider and wider area. This means relative light intensity at any given point gets weaker as distance from source increases. Differences in beam angles on LEDs also affects the power density. For example a 3w/10° LED will project light power density further than a 3w/120° LED, which will project weaker light over a larger area.
Light therapy studies tend to use power densities of ~10mW/cm² up to a max ~200mW/cm².
Dose is simply telling you how long that power density was applied for. Higher light intensity means less application time is required:
5mW/cm² applied for 200 seconds gives 1J/cm².
20mW/cm² applied for 50 seconds gives 1J/cm².
100mW/cm² applied for 10 seconds gives 1J/cm².
These units of mW/cm² and seconds give a result in mJ/cm² – just multiply that by 0.001 to get in J/cm². The full formula, taking into account standard units is therefore:
Dose = Power Density x Time x 0.001