Dosing in photobiomodulation is determined by the amount of energy received from the device, measured in joules per square centimeter, and operates within an optimal window: too little dosing produces no results, while too much dosing may negate benefits or cause deleterious effects.

For a pad at 25 milliwatts per square centimeter applied directly to skin for 1 second, the dose is 25 millijoules per square centimeter; for 10 seconds the dose is 250 millijoules per square centimeter; for 60 seconds the dose is 1,500 millijoules per square centimeter; and for 10 minutes (600 seconds) the dose is 15,000 millijoules per square centimeter or 15 joules per square centimeter.

Approximately 90% of light is lost after the first centimeter of tissue depth due to reflection and scatter; only 10% of light penetrates past the first centimeter, and only 10% of that 10% (or 1% of original) penetrates to the third centimeter of depth.

Even if light cannot directly reach a target area (e.g., a broken arm in a cast), biological benefits can still occur through secondary mechanisms: light absorbed elsewhere triggers vasodilation, increased blood flow, increased oxygen delivery, increased lymphatic flow, gene transcription, angiogenesis (new capillary formation), and stem cell differentiation, which collectively benefit tissues beyond the directly illuminated area.

To calculate approximate irradiance at the device surface, add the power of all bulbs together and divide by the total surface area in square centimeters. This calculation is relatively straightforward for rectangular devices but more complex for circular or irregularly shaped devices.

Wavelength is measured in nanometers and determines the color of visible light; 660 nanometers produces red light while 450 nanometers produces blue light, and wavelength is the greatest variable in determining penetration depth into tissue.

Near-infrared wavelengths of 810 nanometers penetrate deeper into the body than 660 nanometer red light, even if the 660 nanometer light is given more power output and longer exposure time.

Photons are small packets of light created when an atom is excited and its electrons release excess unstable energy in the form of light; each atom releases a specific wavelength corresponding to a specific color, and when multiple wavelengths are combined, the perceived color is white (as with sunlight and household bulbs).

Power is measured in watts or milliwatts; a milliwatt is one-thousandth of a watt. Companies often tout total watts by summing the wattage of all bulbs on a device, but this total number is not meaningful without considering the area over which that power is distributed.

Irradiance is the amount of power per specific area (typically per square centimeter) and is a more appropriate measure than total watts for comparing photobiomodulation devices. If two panels of equal size and power per bulb are compared but one has twice as many bulbs, that panel will have approximately twice the irradiance.

In a worked example, a panel with 100 bulbs at 2 watts each (200 watts total) and dimensions of 45 cm × 25 cm (1,125 cm²) yields an irradiance of approximately 0.178 watts or 178 milliwatts per square centimeter at the device surface.

Full-body panels typically provide 178 milliwatts per square centimeter at the device surface, while pads or helmets provide much lower irradiance of 10-30 milliwatts per square centimeter.

Distance from the light source is a critical variable: light power decreases substantially as distance increases because photons are deflected and scattered by molecules in the air. A 178 milliwatt surface irradiance drops to approximately 89 milliwatts at 6 inches away and further drops to approximately 45 milliwatts at 12 inches away.

A pad or helmet at 25 milliwatts per square centimeter placed directly on the skin (zero distance) loses very little power through air scatter and remains nearly as powerful as a 178 milliwatt per square centimeter panel positioned 6-24 inches away, despite the apparent 7-fold difference in surface irradiance.

Joules measure total energy delivered as the product of power (in watts) and time (in seconds): one joule equals one watt multiplied by one second. This metric is critical because photon absorption into tissue requires time.

For a panel at 178 milliwatts per square centimeter surface irradiance positioned 6 inches away (delivering 89 milliwatts per square centimeter to skin), the dose is 890 millijoules per square centimeter at 10 seconds, 5,340 millijoules per square centimeter (5.34 joules) at 60 seconds, and 53,400 millijoules per square centimeter (53 joules) at 10 minutes (600 seconds).

Hundreds of scientific studies have established that there is an optimal amount of energy (dose) that cells need in order to create a biological change, specifically to increase ATP production; the optimal window is approximately 3-50 joules per square centimeter.

Penetration depth is critically important because wavelength determines how far light penetrates: blue wavelengths penetrate the least, red wavelengths penetrate a moderate depth, and near-infrared wavelengths penetrate the deepest (approximately 4 centimeters). The "sweet spot" must match the wavelength to the target tissue depth to achieve optimal biological effect.

Manufacturers generally recommend photobiomodulation doses of 5-10 joules per square centimeter, which fall comfortably within the 3-50 joule optimal window; following manufacturer recommendations closely ensures users remain within effective dosing ranges.

When light hits skin, reflection and scatter occur: some light bounces off the skin surface while much of the light that enters the skin quickly scatters in multiple directions. Surface scatter is beneficial for skin but is counterproductive for reaching deeper tissues like muscle, bone, and brain.