Debris on any PAR sensor lens will partially block the optical path and will lead to low readings. Dust and other organic deposits are best removed using water, or window cleaner. Never use an abrasive cleaner on the lens. Salt deposits can also accumulate on the sensor lens over time, due to evaporation from sea spray, sprinkler irrigation water, or wave splash. Salt deposits should be dissolved with vinegar and then removed using soft cloth or cotton swabs.