The formula is:
Dose (J/cm²) = Power Density (mW/cm²) × Time (sec) ÷ 1,000
If we want to find out the time required, we rearrange to get:
Time (sec) = Dose × 1,000 ÷ Power Density
For example, the half body panel we sell has ~62mW/cm² irradiance at 10 inches. If we want to get 3 joules, we will work out:
Time (sec) = 3 × 1,000 ÷ 62 = 48 seconds
If we pulse it at 30hz, we need to multiply 48s by 2 as the irradiance is halved, so we will use it for 96s.
Life hack: Irradiance is often overestimated by 30-40%, so we can multiply the duration by 1.5 to get an even more accurate and generous dose to account for this margin of error.
In this case 96s x 1.5 = 144s or about 2.5 minutes.
This is why knowing your real irradiance is so important. Only when you have a rough idea of it, can you use your panel properly. To add to the complexity, irradiance also changes by distance from the panel and it is not a linear change.
Based on my experience, 99% of panels out there state a blatantly false or totally inaccurate irradiance. It is quite easy to spot these by looking at their irradiance at different distances which clearly make no sense. Which is also why I insisted on verifying my panels irradiance myself with metres as well as a spectrometer report.
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