A common criticism of compact, battery-powered devices is that they do not deliver sufficient total optical power (“watt”), particularly when compared to large, AC-powered panels.
What determines local irradiance, however, is power density (mW/cm²) rather than total emitted power. For a given power density, total required power scales directly with emitting area: larger panels necessarily require more total optical watts to achieve the same local irradiance. Irradiance from panels is also distance-dependent. At typical treatment distances (on the order of tens of centimeters), panels exhibit a nonlinear falloff with distance that is steeper than near-field behavior but generally shallower than a pure inverse-square relationship. As a practical reference:
- Moving from 20 → 30 cm often reduces irradiance by ~20–35%.
- Moving from 30 → 60 cm often reduces irradiance by ~40–60%
By contrast, devices intended for near-contact or localized application operate entirely in the near field, where geometric losses are minimized and distance can be controlled more tightly. This allows a smaller device to achieve comparable local irradiance with substantially lower total power, provided the emitting area is appropriately matched to the intended use.
The primary design challenge in such systems is not peak output but sustaining the desired irradiance over a useful area while managing heat within a compact, thermally constrained package.