Power/Energy Meters: Why Damage Specs are Derated for Larger Beams

For certain pyro sensors with a diffuser, such as the PE50BF-DIF-C P/N 7Z02941, there is a Note (b) that for 10mm beam size the damage threshold specification should be derated by 50%. To explain why the damage specification is derated for a larger beam size, please see the picture illustration below. This has to do with the smaller relative increase in spot size lowering the energy density less on a larger beam than the larger relative increase in spot size with a smaller beam lowering the energy density more.