The Inverse Square Law for Red Light Panels, With Example Numbers
Published October 2026. Updated October 2026.
Quick answer
The inverse square law says a small point of light gets four times weaker when you double the distance. A red light panel is a large flat surface of many LEDs, so up close it falls off much more gently than that. Our stated FX300 numbers show it: 83.5 mW/cm² at 6 inches and 43.3 mW/cm² at 12 inches, not the 20.9 a point source would give. Use the published figure for your distance, not the law. This guide is arithmetic, not a claim about outcomes.
The law in one formula
For a point of light spreading in all directions, the same power is spread over a sphere that grows with distance. Area grows with the square of the radius, so intensity shrinks with the square of the distance. In formula form:
I2 = I1 x (d1 / d2)²
I1 is the reading at the first distance d1, and I2 is the reading at the second distance d2. Both distances must use the same unit.
Example numbers for arithmetic only
The table below uses an invented point source reading 100 at 10 inches. The numbers are example numbers for arithmetic only and do not describe any AWA device.
| Distance | Calculation | Reading | Share of the 10 inch reading |
|---|---|---|---|
| 5 inches | 100 x (10/5)² | 400 | 400 percent |
| 10 inches | 100 x (10/10)² | 100 | 100 percent |
| 20 inches | 100 x (10/20)² | 25 | 25 percent |
| 30 inches | 100 x (10/30)² | 11.1 | 11 percent |
| 40 inches | 100 x (10/40)² | 6.25 | 6 percent |
Notice how steep the curve is. Half the distance gives four times the reading. Four times the distance gives one sixteenth. If a panel followed this rule exactly, a small change in your position would matter a great deal.
Why a panel does not follow the law
The law applies cleanly to a point source, or to any source when you are far away compared with its size. A panel is neither. It is a flat grid of many LEDs. When you stand close, light reaches each spot from several LEDs at once, and moving back a little does not take you far from all of them. So the fall is slower than the law predicts at short range. Farther out, where the panel looks smaller and more like a single patch of light, the curve steepens and starts to look more like the law.
That is a description of how extended sources behave, not a claim about any one design. The proof is in the stated numbers.
Law prediction versus AWA stated numbers
This table starts from one published reading, applies the law, and compares it to the next published reading. All "stated" figures come from the AWA irradiance specs guide. The "law" column is our arithmetic.
| Panel | Start | Move to | Point source prediction (mW/cm²) | Stated (mW/cm²) |
|---|---|---|---|---|
| FX300 | 1 inch, 220 | 2 inches | 55.0 | 145 |
| FX300 | 1 inch, 220 | 3 inches | 24.4 | 116 |
| FX300 | 6 inches, 83.5 | 12 inches | 20.9 | 43.3 |
| FX500 | 2 inches, 220 | 4 inches | 55.0 | 149 |
| FX500 | 2 inches, 220 | 10 inches | 8.8 | 81.4 |
| FX500 | 6 inches, 94 | 12 inches | 23.5 | 73.8 |
| FX500 | 2 inches, 220 | 20 inches | 2.2 | 58.6 |
Stated figures are AWA published numbers. Prediction column is arithmetic using I2 = I1 x (d1/d2)² and does not describe our devices.
In every row the stated figure is well above the point source prediction. If you had used the law to plan a session at 12 inches from the FX500, you would have guessed 23.5 mW/cm² when the stated number is 73.8. That is a large error, and it would have pushed your arithmetic the wrong way.
Where the law is still useful
- As a reminder of direction. Light gets weaker as you move back, and that is true for any source.
- As a rough guide far away. When you are many times farther from the panel than the panel is wide, the fall moves closer to the law.
- For small, concentrated sources. A single LED or a small handheld head behaves more like a point than a large panel does.
It is not a substitute for a measured or stated number. If your distance is not on the spec sheet, use the nearest listed distance or the irradiance by distance chart.
A quick sanity check is to compare two published readings. If the second is much higher than the law predicts from the first, you know the source is extended, and you should lean on the spec sheet. If your own meter ever disagrees with a spec by a large margin, check that you measured from the same point, at the same distance, with the meter facing the panel squarely.
Using stated numbers to plan time
Once you have a real irradiance figure for your distance, the dose formula applies: J/cm² = mW/cm² x seconds / 1000. The dose formula guide and the dose calculator walk through it. Session time itself still comes from your manual.
Common mistakes
- Applying the inverse square law to a panel and getting a figure far below the stated one.
- Assuming that doubling distance halves the reading. It is neither half nor a quarter on a panel.
- Mixing units, such as inches for d1 and centimeters for d2.
- Using a reading at one distance as if it applied at every other distance.
- Taking predicted numbers for measurements. Only the published spec is a stated number.
Before you use any light device: read the label, follow the device manual, and use eye protection as directed. For medical questions, speak to a qualified health professional first.
Related guides in this series
- What irradiance means on a spec sheet
- AWA FX300 and FX500 irradiance specs
- Dose formula: irradiance, time and energy
- Time, distance and dose arithmetic
This post is part of our distance and placement series. For the overview, see the red light therapy distance guide.
Device pages
Read the full page and label for each device before you use it: AWA FX300 panel and AWA FX500 panel. You can also browse AWA panels.
Frequently asked questions
What is the inverse square law?
It says the intensity from a point source is proportional to one over the distance squared. Double the distance and the reading drops to a quarter.
Does it apply to red light panels?
Only roughly, and mostly far away. A panel is a large flat surface, so up close the stated numbers fall more gently than the law predicts.
How can I check this for myself?
Take two stated readings at different distances, apply I2 = I1 x (d1/d2)² to the first, and compare the answer to the second published number.
Should I use the law to plan my sessions?
No. Use the published irradiance at your actual distance and follow the manual for time and distance.
What if my distance is not on the spec sheet?
Use the nearest listed distance, or move to one that is listed, so you can calculate with a stated number.
Sources
- FDA: General Wellness policy for low risk devices. January 2026 guidance describing general wellness products as low risk products that promote a healthy lifestyle. It is a policy document, not a statement about any one device.
- FDA: Laser Products and Instruments. Explains laser hazard classes I to IV and labeling, and notes that LEDs are not subject to the federal laser product performance standard.
Regulatory note: select devices carry FDA 510(k) clearance for specific, limited uses. Most light devices are sold as general wellness products. Read each product page and label.
How we create this guide
AWA sells red light therapy devices. We write these guides to explain how devices work and how to choose and use one. This is general information, not medical advice. Read each product page and label. See our editorial standards.
