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: Can I use a sensor from the PD300 family to measure average power of pulsed lasers?
A: In general yes, but several technical issues need to be kept in mind (most of which are results of the fast physical response time of these sensors):

• The pulse rate should be more than about 30Hz, otherwise the reading is unstable. At higher pulse frequencies, the sensor will respond as if the beam were CW. 
• It is possible for a pulsed beam to have average power within the sensor spec and yet have the energy of the pulses themselves be high enough to cause a momentary saturation of the sensor. It is important to be sure that pulse energy is also within sensor spec (the parameter "Max pulse energy" is included in all specs for the PD300 family, for just this reason).
• The beam diameter should be no less than about 1mm .
• The average power and power density restriction in the spec should not be exceeded

Note: At the maximum pulse energy limit given in the spec, the reading will be saturated by about 5%, i.e. the reading will be about 5% lower than it should be. At 1/3 the maximum, the saturation will be about 1%.

Q: Can a lost PD300 filter be replaced?
A: Technically it could be replaced, but it is not just a matter of replacing the filter. Since the PD300 is a "calibrated" sensor it requires that the filter also be "calibrated". Especially since the PD300 response varies with wavelength, it requires that both the PD300 and the filter be calibrated over the entire spectral range with a monochromator. Because of the cost to calibrate the replacement filter with the PD300 sensor, we recommend purchasing a new PD300 sensor when a replacement filter is needed.

Q: The total error vs wavelength when using a PD-300 head is listed as ±x%. Is that ±x% of the reading or of the total range?
A: It's ±x% of the reading from full scale down to 5% of full scale. Below 5% of full scale one should switch to next range down for the best accurate linear results.

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.