Understanding Irradiance
Understanding Irradiance: What do the numbers mean?
When comparing red light therapy devices, you’ll often see numbers such as 30, 50 or 100 mW/cm².
These describe irradiance: the optical power reaching a surface per unit of area. It’s a useful specification, but understanding how it was measured matters just as much as the number itself.
What Does mW/cm² Mean?
Irradiance is commonly expressed in milliwatts per square centimetre:
• mW means milliwatts, a unit of power.
• cm² means square centimetres, a unit of area.
A reading of 50 mW/cm² means that 50 milliwatts of optical power are reaching each square centimetre at the measured location.
Think of irradiance as the rate at which light energy arrives at a surface. Session time determines how long that exposure continues.
Irradiance, Wavelength and Wattage Are Different
These specifications answer different questions.
Wavelength, measured in nanometres (nm), describes the light being emitted.
Irradiance, measured in mW/cm², describes the optical power arriving per unit of area.
Electrical wattage describes how much electrical power a device uses. Some of that energy becomes heat or powers other components, so electrical wattage cannot be treated as light output.
LED count describes how many emitters a device contains. It does not tell you the irradiance at your skin.
Why Distance Matters
An irradiance reading belongs to a particular measurement distance.
With many panels, moving further away allows light to spread across a larger area, generally reducing irradiance while changing coverage and uniformity.
However, a panel contains multiple emitters whose beams overlap. You cannot reliably assume that doubling the distance will reduce its irradiance to one quarter.
For a meaningful comparison, look for readings taken at the same distance and with comparable measurement methods.
A reading taken directly against a device cannot be compared fairly with one taken 30 cm away.
From Irradiance to Energy Exposure
Irradiance describes power per area. Energy exposure describes how much light energy arrives per area over time, commonly expressed in joules per square centimetre, or J/cm².
For constant irradiance:
Energy exposure (J/cm²) = irradiance (mW/cm²) × time (seconds) ÷ 1,000
For example:
50 mW/cm² × 300 seconds ÷ 1,000 = 15 J/cm²
In this hypothetical example, five minutes delivers 15 joules per square centimetre at the measured surface.
This illustrates the calculation, not a recommended treatment dose.
Surface Exposure Is Not the Same as Absorbed Dose
The calculation above describes light arriving at the surface.
It does not tell you how much energy reaches or is absorbed by deeper tissue. Reflection, scattering and absorption affect what happens after light reaches the body.
For pulsed light, the calculation also requires time-averaged irradiance. Using the peak output as though it were continuous can overestimate exposure.
Does Higher Irradiance Mean Better Results?
A higher reading means more optical power per area under the stated measurement conditions. It does not automatically establish greater effectiveness.
Photobiomodulation research describes responses that depend on both the exposure and the biological context. Increasing exposure beyond an appropriate range can reduce the intended benefit.
Two protocols can also deliver the same calculated J/cm² using different irradiances and session times without necessarily producing the same biological response.
There is no single irradiance number that is ideal for every purpose, body area and device.
Why Measurement Methods Matter
Reliable measurements require equipment suited to the wavelengths being tested, appropriate calibration and a clearly described setup.
A meter’s response can vary with wavelength. A device emitting both red and near-infrared light therefore needs a measurement method that accounts for both.
Visible brightness is not a substitute for irradiance measurement. Neither are photographs, phone-camera images or lux readings, which describe brightness weighted to human vision.
A meter reading is most useful when accompanied by information explaining how it was obtained.
One Reading May Not Describe the Whole Treatment Area
Output can vary across a panel’s illuminated area.
A reading at the brightest central point may be higher than readings near the edges. An average across a defined area answers a different question from a single maximum reading.
When comparing devices, check whether the figure represents:
• One measurement point.
• The maximum reading.
• An average across a specified area.
Also check whether it includes red light, near-infrared light or their combined output.
*Light output isn’t always evenly spread. These example beam maps show how irradiance varies across an illuminated area. They do not represent measurements of EternaGlow devices.
Illustration adapted from Serrage et al. (2019), Figure 3, Journal of Biophotonics (CC BY 4.0). Colours show irradiance across each beam, rather than the visible colour of the light. These patterns relate to the study’s LEDs, not EternaGlow device testing.
View the original scientific figure and study.
What to Look for When Comparing Devices
A useful irradiance specification should identify:
• The measurement distance.
• The wavelengths and operating mode tested.
• The brightness setting.
• Whether the reading is a maximum or an average.
• The area or measurement points assessed.
• The measuring equipment and calibration information.
• Whether testing was performed by the manufacturer, seller or an independent laboratory.
Independent testing can add confidence, but the testing method and conditions still matter.
If information is missing, it is reasonable to ask for it. An incomplete specification does not prove that a device is ineffective; it limits how confidently you can compare its output.
Using Irradiance Information Wisely
Irradiance helps explain a device’s light output. It is one part of a broader picture that includes wavelength, coverage, exposure time and evidence relevant to the intended application.
Use it to understand specifications and ask better questions. Follow your device’s instructions rather than choosing session times from another product’s advertised output.
Sources and Further Reading
The Dark Art of Light Measurement: Accurate Radiometry for Low-Level Light Therapy
Research on measurement methods and reporting light parameters.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4851696/
How to Report Photomedicine Dose and Beam Parameters
Guidance on the information needed to describe a light treatment.
https://pubmed.ncbi.nlm.nih.gov/22107486/
Parameter Reproducibility in Photobiomodulation
Discussion of why clearly reported treatment parameters matter.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4799704/
Biphasic Dose Response in Low-Level Light Therapy
An explanation of why increasing exposure does not always increase benefit.
https://pmc.ncbi.nlm.nih.gov/articles/PMC3315174/
This page provides general education. The numerical example is hypothetical and does not represent a tested EternaGlow product or a personalised treatment recommendation.