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What Do Irradiance (mW/cm²), Dose and Session Length Mean? How to Read the Specs
September 27, 2026 2 min read
Short answer: Irradiance (mW/cm²) is the light power reaching your skin. Dose (J/cm²) is the total energy in a session: irradiance (mW/cm²) × time (seconds) ÷ 1,000. To convert W/m² to mW/cm², divide by 10. A device listing that gives irradiance without a measuring distance tells you only part of the story.
Last reviewed: September 2026. Numbers come from product listings. Worked examples are arithmetic, not treatment recommendations. Educational only, not medical advice. See our disclaimer.
The formulas
- Dose (J/cm²) = irradiance (mW/cm²) × time (seconds) ÷ 1,000.
- mW/cm² = W/m² ÷ 10. So 803 W/m² is 80.3 mW/cm².
- Irradiance falls as you move away from the LEDs, so ask what distance the number was measured at.
Irradiance listed by products we carry
| Product | Listed output | Session timer | Arithmetic example |
|---|---|---|---|
| Solbasium Optix Knee Pad | 20 mW/cm² | 15 minutes | 20 × 900 ÷ 1,000 = 18 J/cm² |
| LS Pro Deep Pad (DSD120) | 803 W/m² (80.3 mW/cm²) | Per manual | 80.3 × 600 ÷ 1,000 = 48.2 J/cm² for 10 min |
| Solbasium Bright | 100 mW/cm² | 10 minutes | 100 × 600 ÷ 1,000 = 60 J/cm² |
| LS Pro MXP Power Bed PRO | 120 mW/cm² | 10 or 20 minutes | 120 × 600 ÷ 1,000 = 72 J/cm² for 10 min |
| Merican M4N Plus | 353 to 613 W/m², up to 115 mW/cm² | Per manual | Ask which figure applies at treatment distance |
| Body Balance System ApolloGLOW / ApolloARC | 150 mW/cm² | Per manual | 150 × 60 ÷ 1,000 = 9 J/cm² per minute |
These are not recommended doses. Every device has its own instructions, and irradiance at your skin depends on distance and design. Where a listing does not state irradiance (for example the LS Pro Classic bed), ask the manufacturer for the spec sheet.
Why “more” is not always better
Photobiomodulation research often describes a range of doses where results are seen, with too little or too much less effective. That is one reason to follow the manufacturer's timer and distance rather than extending sessions.
Checklist for comparing devices
- Is irradiance listed, in which units and at what distance?
- Are the wavelengths stated? See 660 vs. 850 nm.
- What is the built-in session time? See how often and how long.
- How much area does it cover?
Frequently asked questions
How do I calculate red light dose?
Multiply irradiance in mW/cm² by session time in seconds and divide by 1,000 to get J/cm².
How do I convert W/m² to mW/cm²?
Divide W/m² by 10. For example, 803 W/m² equals 80.3 mW/cm².
Read the Red Light Therapy Guide or see Every Red Light Therapy Device We Carry.