Ophir's Photodiode PD300 and PD300-1W sensors offer automatic background subtraction so the measurement is not sensitive to room light. With "filter out" (i.e. the external filter removed for low light measurements), 2 separate detector elements are visible. The beam to be measured is incident only on the outer of the 2 detectors, but background light reaches both detectors. The instrument will show the power measured by the outer detector minus that measured by the inner detector.This patented method cancels out 95% - 98% of background light under normal room conditions, even if it is constantly changing.