The power density of light from any LED or laser therapy device can be tested with a ‘solar power meter’ – a product that is usually sensitive to light in the 400nm – 1100nm range – giving a reading in mW/cm² or W/m² (100W/m² = 10mW/cm²).
With a solar power meter and a ruler, you can measure your light power density by distance.
You can test any LED or laser to find out the power density at a given point. Full spectrum lights such as incandescents & heat lamps cannot be tested this way unfortunately because much of the output is not in the relevant range for light therapy, so the readings will be inflated. Lasers and LEDs give accurate readings because they only output wavelengths +/-20 of their stated wavelength. ‘Solar’ power meters are obviously intended for measuring sunlight, so not perfectly calibrated for measuring single wavelength LED light – the readings will be a ballpark figure but accurate enough. More accurate (and expensive) LED light meters do exist.