Power Meters FAQ's

Photodiode Laser Power Sensors



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.

Using Your Sensor


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.


Let’s look at an example. In the spec of the PD10-C, it gives the lowest measurable energy as 1nJ at wavelength 900nm. Let’s say the wavelength you work at is 633nm. Looking at the relative spectral sensitivity curve for this sensor (shown below), we see that the sensitivity at 633nm is only about 2/3 of what it is at 900nm (as seen in the attached spectral curve). That means that the lowest measurable energy at 633nm will be (1nJ / ⅔), or about 1.5nJ.


The PD300 sensors are not designed to measure with the fiber pushed up right against the detector surface. It may be reading lower in such a case due to saturation of the detector from the concentrated beam or higher due to back reflections off the detector and back again from the fiber tip. The optimal reading will be where the beam is expanded to a size of 2-5mm diameter. Therefore, you should back off the fiber to a distance where the beam has expanded somewhat. Do not back off too far, otherwise if the nominal beam size is larger than given above, you may lose some of the beam off the edges of the detector.


The PD300 series of photodiode-based sensors are calibrated with a full spectral curve using a scanning monochromator (plus a few laser "anchor points").


The wavelength ("Laser") setting tells the meter what wavelength is being used and hence what calibration factor to apply when a measurement is underway. It does not, however, physically limit the possibility of other wavelengths from entering. All light (within the sensor's specified range of course) entering the detector will be measured; the meter will apply the calibration factor meant for the selected wavelength, "thinking" that only that wavelength is present.


In other words, these sensors assume a monochromatic light source. Their relative spectral response is not flat and they are therefore not suited for broadband beams.


So, if you want to check one wavelength from a broadband source, you will need to use a wavelength filter that only passes that wavelength. Then you should set your meter to the appropriate wavelength to account for the detector's relative sensitivity.


Yes, the 5 default wavelengths are the discrete wavelengths that we have actually calibrated that sensor to NIST-Traceable standards. We have also run a spectrophotometer curve for that sensor and fitted that curve to the 5 discrete wavelengths that you see in the drop down menu. To set it to a wavelength that one needs (within its spectral bandwidth, i.e. 350-1100 nm for the PD300 and 200-1100 nm for the PD300_UV) one highlights one of drop down wavelengths in the drop down using the Down arrow key, and then you use the Right arrow selector to get into the menu for changing it. Then one uses the Up and Down and Right Arrow keys to select the wavelength that one wants to use it at. When done press SAVE and then SAVE again. Now it's stored in the E-PROM of the Smart Sensor connector and available for use. One can repeat the above procedure to store any 5 wavelengths that they use most often.
See the Video on how to change the sensor wavelength: http://www.youtube.com/watch?v=1qWXouQP18U


The PD300-1W also has an extra ND filter glass, one with more attenuation. This sensor works at up to 1W, and at this higher power the extra ND glass gets hot; as a result, its absorption can change (becoming less absorbing) so it is not specified for use at above 10W/cm2. Not because it will damage there, but because it will not perform correctly there . 

(It is worth noting that the PD300-3W has a different type of extra filter based on an evaporated reticle. This filter does not change its absorption when it gets hot, so can be used at up to 100W/cm2.)


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.


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%.


We don’t supply chillers, nor insist on specific models; the only important thing from our point of view is to simply keep to the requirements specified for the cooling water of the specific model of sensor, such as minimum flow rate at full power, water temperature range, and - more important than the actual water temperature - water temperature stability. The temperature of the water should not be changing by more than 1 deg/min (because changes in water temperature could cause heat flow in the sensor which would be detected as if it were laser power, and cause errors in the reading).

We also have a video on our site at https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/knowledge-center/water-cooled-sensors-youtube, which discusses various issues and tips about water cooling. There is a short discussion of coolant pressure requirements in our FAQ section at https://www.ophiropt.com/laser--measurement/knowledge-center/faq/2404


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.

Special Photodiode Sensors


The BC20 has a peak measurement and hold circuit which measures the peak power on the detector and holds it. Therefore when a beam is scanned over the detector, when the beam is on the detector it goes up to a peak which corresponds to the same power the detector would measure if the beam was stationary and therefore the BC20 reads the correct power whether the beam is scanned or not. In order for the BC20 to do this, the beam must be on the detector (of size 10x10mm) for at least ~13µs and therefore this limits the scanning speed on the detector to 30,000 inch/s.


Integrating Sphere Theory
Integrating spheres are used when we have divergent light sources. As shown in the illustration, an integrating sphere has its inner surface coated with a surface that highly reflects (typically 99%) in a scattering, nonspecular way. Thus when a divergent beam hits the walls of the integrating sphere, the light is reflected and scattered many times until the light hitting any place on the walls of the sphere has the same intensity. 

A detector placed in the sphere thus gets the same intensity as anywhere else and the power the detector detects is thus proportional to the total incident power independent of the beam divergence. (The detector is so arranged that it only sees scattered light and not the incident beam). An ideal integrating sphere has a surface with reflective properties are Lambertian. This means that light incident on the surface is scattered uniformly in all directions in the 2pi steradians solid angle above the surface. The surface used by Ophir closely approximates a Lambertian surface.

3A-IS Series
The 3A-IS series has two 50mm integrating spheres in series with a photodiode detector. The two series spheres scramble up the light very well thus giving output very independent of incident beam divergence angle. The two spheres in series also insure that the light hitting the detector is greatly reduced in intensity thus allowing use up to 3 Watts even though photodiodes saturate at about 1mW. There are two models, the 3A-IS with a silicon photodiode for 400 – 1100nm and the 3A-ISIRG with an InGaAs detector for 800 – 1700nm


The Ophir integrating sphere sensors, models 3A-IS and 3A-IS-IRG have a white diffuse reflecting coating on the inside of the integrating sphere. The sensitivity of the sensor is quite sensitive to the reflectivity of the coating. If the coating absorption goes up 1%, it can cause a 5% change in reading. Therefore, care must be taken not to soil or damage the white coating of the sensors. Also it may be a good idea to send the sensors for recalibration yearly.


For the most demanding accuracy requirements, a broadband source is used for the auxiliary lamp, and a spectrometer monitors the effect of self-absorption across the spectrum. For UVLEDs, in the limited spectral range of 350nm-400nm, using the auxiliary LED at 390nm is an efficient solution, and the error due to self-absorption is reduced from up to ±20% to up to ±5%.