Red light therapy: minutes by distance and power
This describes how to compute exposure time from published dose figures, not a treatment plan. Follow your device's instructions, protect your eyes, and consult a clinician for medical concerns.
"How many minutes?" has no single answer, because session time depends on two things your panel controls: its irradiance (power per area, in mW/cm²) and your distance from it. Fix a target dose in J/cm² and the minutes fall out of one formula.
The formula
Time (seconds) = target dose (J/cm²) ÷ irradiance (mW/cm²) × 1000
Because 1 J = 1000 mJ and irradiance is in milliwatts, the ×1000 converts to seconds. This is the same math behind calculating red light dose in J/cm², solved for time.
Minutes for a 6 J/cm² target
Session length at different panel irradiances, for a 6 J/cm² example target:
| Irradiance at your distance | Time for 6 J/cm² |
|---|---|
| 200 mW/cm² (very close, strong panel) | 0.5 min |
| 100 mW/cm² | 1 min |
| 50 mW/cm² | 2 min |
| 25 mW/cm² (farther back) | 4 min |
| 10 mW/cm² (across the room) | 10 min |
Pick your own target and the times scale linearly — a 3 J/cm² target halves every row; a 12 J/cm² target doubles it. The point is that the panel's real irradiance at your distance is the number that matters, not the wattage on the box.
Why distance dominates
Irradiance drops off roughly with the square of distance (the inverse-square law). Move from 6 inches to 12 inches and irradiance can fall to about a quarter — so the same dose now takes about four times as long. That's why two people with the same panel can need wildly different session times.
The dose-response is biphasic — past the helpful window, extra exposure can do less, not more. Compute a target and stop there rather than treating longer as strictly better.
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Gear this guide uses
Turning distance into minutes means knowing your panel's real output at that distance.
- A panel that publishes irradiance at a stated distance is one you can actually dose with.
- An irradiance meter lets you verify the figure at your real distance.
- Eye protection goggles for close-up use.
Lumen turns distance into minutes
Set a target and Lumen computes session time for your device and distance — flagging when a longer session would push you past sensible dose ranges, using biphasic-window guardrails from the Hamblin research. Free to download.
FAQ
How do I calculate red light therapy session time?
Time (seconds) = target dose (J/cm²) ÷ irradiance (mW/cm²) × 1000. A 6 J/cm² target at 100 mW/cm² is 60 seconds. Divide by 60 for minutes.
Does moving closer change the time?
A lot. Irradiance falls roughly with the square of distance, so doubling distance can quarter irradiance — about 4× the session length for the same dose.
What is a typical dose?
The literature gives ranges, commonly on the order of a few to around ten J/cm² at skin level. Treat figures as guardrails and follow your device's guidance.
Sources
- Huang, Chen, Carroll & Hamblin, "Biphasic dose response in low level light therapy" (PMID 22461763)
- Reviews of photobiomodulation dose-response (optimal-window ranges)
Photobiomodulation dosing is an active research area; figures are guardrails, not prescriptions.