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: What is the accuracy of the PD300 sensor at 350-360 nm?
A: It works there but accuracy is very poor ~20% so it is not specified below 360nm. The PD300-UV is recommended for these shorter wavelengths.

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.

Q: Can I use a PD300 photodiode sensor to measure power of a scanned beam?
A: For measuring power of a scanned beam we recommend using the BC20, and not the PD300. Since a scanned beam will spend only a fraction of the time of each scan on the detector, the average power measured by the detector will correspondingly be only a fraction of the actual power of the beam. The BC20 is specially designed for such applications by having a peak-hold circuit integrated in its electronics.

Q: Can I use a PD300 sensor inside a thermal chamber, in which the temperature cycles beyond the recommended operating temperature range?
A: It should be okay, as long as:

• the wavelength is not near the long wavelength limit where the PD300 has a large temperature dependence;
• there is no condensation on the window of the detector which could interfere with the beam and affect the reading.

We suggest the customer does an experiment with a stable laser source (such as a pointer laser) shining in through a window onto the detector while the unit is temperature cycled to see if the reading changes. The final measurement should be back at the original temperature so as to make sure the laser hasn’t changed.

Q: Why do Photodiode-based power sensors like the PD300 have a specified “Max pulse energy”?
A: Although these sensors measure average power (of both CW and repetitively pulsed beams), not pulse energy, it is possible for a pulsed beam to have average power within the sensor’s rated limits 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 – not just the average power. This is explained in detail in this White Paper.