Red Light Panel Distance and Minutes: The J/cm² Maths

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Energy density (often called "dose") in J/cm² equals irradiance in mW/cm² multiplied by time in seconds, divided by 1,000. Because irradiance drops as you move away from a panel, the same number of minutes delivers less energy at 30 cm than at 15 cm, so you can only work out minutes once you know the irradiance at the exact distance you use.

The formula

J/cm² = mW/cm² × seconds ÷ 1000

And the reverse, to find time:

minutes = (J/cm² × 1000) ÷ (mW/cm² × 60)

Why ÷ 1000? Irradiance is in milliwatts (thousandths of a watt) per cm². One watt for one second is one joule. So mW × s gives millijoules, and ÷ 1000 turns that into joules.

Worked example

Say a panel delivers 40 mW/cm² at the spot where you sit, and you sit there for 10 minutes.

  1. 10 minutes = 600 seconds.
  2. 40 × 600 = 24,000 mJ/cm².
  3. 24,000 ÷ 1000 = 24 J/cm².

Reverse: to reach 10 J/cm² at 25 mW/cm²:

  1. 10 × 1000 = 10,000.
  2. 25 × 60 = 1,500.
  3. 10,000 ÷ 1,500 = 6.7 minutes (about 6 minutes 40 seconds).

These numbers show the arithmetic only. They are not recommendations.

Irradiance falls with distance

Move away from a panel and the light spreads over a larger area, so each cm² gets less. How fast it falls depends on the panel:

  • Small panels behave more like a single point. Irradiance drops steeply, close to the inverse-square law (double the distance, about a quarter of the light).
  • Large panels are wide sources. Close up, light from many LEDs overlaps, so irradiance falls more slowly at first.

Published examples (each brand's own figures, seen October 2026):

Device Distance → irradiance Who measured
Mychondria MyLight MOVE 2.0 (small) 5 cm: 58.4 · 10 cm: 39.3 · 20.3 cm: 15.0 · 30.5 cm: 7.1 mW/cm² Brand, spectrometer
Red Light Rising Target Light 670 20 cm: 55.96 · 30 cm: 33.40 mW/cm² Independent measurement
Lichtblock Forte (large) 15 cm: 74 · 90 cm: 54 mW/cm² Brand, spectrometer

Same minutes, different distance

Using the small-device figures above, 10 minutes (600 s) at each distance:

Distance Irradiance Energy density in 10 min
5 cm 58.4 mW/cm² 35.0 J/cm²
10 cm 39.3 mW/cm² 23.6 J/cm²
20.3 cm 15.0 mW/cm² 9.0 J/cm²
30.5 cm 7.1 mW/cm² 4.3 J/cm²

Moving from 5 cm to 30 cm cuts the energy by about 8 times for the same 10 minutes. That is why a minutes figure without a distance means little.

What you need to use the formula

  • Irradiance at your distance. Measured, not calculated, and ideally with a spectrometer. A centre reading is higher than the average across the panel.
  • The measuring instrument. Solar power meters can read high on red and near-infrared LEDs. See solar meter vs spectrometer.
  • Which channels were on. Red only, near-infrared only and both together give different numbers.

What about Rubrawell panels?

We don't publish irradiance for Rubrawell panels yet. When our own measurements are done, we will publish values at 15, 30 and 45 cm for each size, so you can plug them into the calculator. Until then, follow the distance and time in your panel's manual. Manuals for this kind of panel commonly give 15–45 cm. We don't give health-based timing advice.

Sources

  • Mychondria MyLight MOVE 2.0 distance figures: mychondria brand site, as published, seen Oct 2026.
  • Red Light Rising Target Light 670 figures: redlightrising, as published, seen Oct 2026.
  • Lichtblock Forte figures: lichtblock brand site, as published, seen Oct 2026.
  • Formula: standard unit conversion (1 W = 1 J/s).

Last updated: 16 October 2026. The Rubrawell team.

Questions

How do I calculate J/cm² for a red light panel?
Multiply the irradiance at your distance (mW/cm²) by the time in seconds, then divide by 1,000. For example, 40 mW/cm² for 600 seconds is 24 J/cm².
How do I work out minutes from a target J/cm²?
Minutes = (J/cm² × 1000) ÷ (mW/cm² × 60). At 25 mW/cm², 10 J/cm² takes about 6.7 minutes.
Does distance change the result?
Yes, a lot. Irradiance falls as you move away, so the same minutes deliver less energy. In one published example, energy in 10 minutes dropped about 8 times between 5 cm and 30 cm.
Can I use the irradiance number on the box?
Only if you know the distance and how it was measured. Figures from solar meters, at zero distance, or at the centre point can be much higher than what reaches you.
How many J/cm² should I aim for?
We don't give dose recommendations. This page explains the maths only. Follow the routine in your panel's manual.
What irradiance do Rubrawell panels deliver?
We haven't published a figure yet. We will publish our own measured values at 15, 30 and 45 cm per size.
Is the inverse-square law accurate for panels?
For small devices at a distance, roughly. For large panels close up, irradiance usually falls more slowly because light from many LEDs overlaps.