Most UV-A lamps on the market are set to provide 5,000 µW/cm2 at a distance of 15 in / 38 cm. What’s so special about those numbers?
hysics of Light
Light intensity follows an inverse-square law. Say there’s a bright lamp that’s one meter away from you. If you move out to 2 meters, the light from the lamp won’t be half as bright, it’ll be one quarter as bright. Going farther out to 3 meters drops it down to 1/9th the original intensity. It works the same way as you move closer too. The same lamp will be 4 times as bright 50 cm away, and 16 times as bright at only 25 cm. What this means for us is that the distance to the lamp is just as important as the amount of light it puts out.
To compare the intensity of UV-A lamps in fluorescent inspection, our industry has set a standard distance of 15 in / 38 cm as a benchmark. Using a standard distance, the relative performance of different lamps can be compared directly.
UV-A irradiance is required to make penetrant and magnetic particle materials fluoresce (that’s what makes them visible). The more UV-A available, the brighter the fluorescence will be. But there are limits. ASTM E2297 and E1316 define UV-A light as wavelengths between 320-400 nm and define visible light as wavelengths between 400-760 nm. But in nature, the light spectrum is continuous. There is no natural difference between what we call UV-A and what we call visible light – those are artificial divisions. Human vision, in the same way, is continuous. There’s no hard-line cutoff of what the eye can perceive, and light perception changes as we age. Younger eyes can perceive light down to 390 nm (even though we technically define that as UV-A), and if there is no other light source, even shorter wavelengths down to 380 nm can be seen as deep violet light.