Wavelength, Irradiance and Dose as Research Variables

Why dose is a research problem

When a trial says light "worked", the result belongs to the exact light that was delivered, not to light in general. Researchers describe that light with several variables. Reviews of photobiomodulation (PBM) say that inconsistent dosimetry, methods and reporting across studies limit clinical translation. [1] This page explains the variables so you can read a study or product page critically. It deliberately gives no recommended values, because published trial settings are not instructions.

The main variables, in plain words

The "biphasic" idea, and why it complicates things

Reviews describe a biphasic dose-response, where a middle amount may work better than too little or too much. [1] This is a laboratory-derived idea. It does not mean a home device can be tuned to a "sweet spot" without trial data for that device, population and outcome. The same review points to uncertainty in how much light actually reaches deeper tissue. [1]

Reporting problems

• A review of low-power laser dosimetry noted that experimental and clinical results are hard to compare because a long list of physical and biological parameters should be checked and parts are omitted or inaccurately reported. It counted more than three dozen suggested parameters across authors. [4]
• The Cochrane review of light therapies for acne found that interventions differed greatly in wavelength, dose and comparators, which made most comparisons impossible. [5]
• The diabetic-foot-ulcer Cochrane review lists comparison of different output power, wavelength, power density and dose ranges as a question, but found trials small and at unclear or high risk of bias. [6]

What dose analyses in trials can and cannot tell you

Some reviews group trials by dose ranges recommended by a laser-therapy association. In knee osteoarthritis, the subgroup within recommended ranges showed a larger pain reduction than all trials combined. [7] This kind of subgroup finding generates hypotheses, and the authors include association officials, a disclosed potential conflict of interest. [7] A different review of tendinopathy found that recommended-dose trials reduced pain but with wide intervals. [8] Neither review tells a reader what setting to use on any device.

Questions to ask about any light claim

1. Which wavelength, and was it measured or only printed on the label?
2. Was irradiance measured at the skin, or is it a rated power?
3. Was the tested device the same as the one being sold?
4. What was compared against (sham light, no treatment, exercise)?
5. Was the outcome a symptom, a function or a lab marker?
6. Who funded the study?

More tools are in How to Evaluate a Health Device: A Verified-Specification Checklist and Light Device Specifications: Wavelength, Irradiance and Dose. The device-by-device view is on Device-Specific Evidence for Light Therapy Products.