Q: What does the PD300 "Background Subtraction" do?
A: 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.

Q: Do I need to recalibrate my instrument? How often must it be recalibrated?
A: Unless otherwise indicated, Ophir sensors and meters should be recalibrated within 18 months after initial purchase, and then once a year after that.