Power Meters FAQ's

Pyroelectric Laser Energy Sensors



All PE sensors are less accurate at a low percentage of full scale. Therefore it is always recommended to measure energies on the lowest range available (e.g. measure 1.8mJ on the 2mJ scale not the 20mJ scale). In order to get highest accuracy from your sensor, especially at low percentage of full scale, it should be zeroed against the meter the first time it is used with this meter. This is especially true for the PE-C series that can be used down to 3% of full scale if zeroed but can have an error of 2% at 10% of scale if not. The sensors are factory zeroed against the Vega/Nova II so need not be rezeroed if used with these types. If used with the Juno, Pulsar, USBI or LaserStar, they should be rezeroed. If zeroed with one type and then later used with a different type, they should be rezeroed the first time used with the different type.


Ophir pyroelectric sensors have a positive temperature coefficient of 0.2% per degC which means that if the sensor heats up 10 degrees, the reading will be 2% high. The newer PE-C series of pyroelectric sensors have a temperature sensor inside and this dependence is compensated for in the software. The PD10 and PD10-pJ sensors use photodiode detectors so their temperature coefficient is the same as the PD300 sensors as listed on the PD300 pages of the Ophir catalog.


All Ophir pyroelectric sensors can measure average power with Ophir Power and Energy Meters. The instrument measures the number of pulses each second and multiplies the energy reading by the pulse rate. If the pulse rate is constant, then the accuracy of power measurement will be the same as the energy accuracy since the pulse rate measurement is very accurate.


Basically, as the pulse rate increases, the sensor has less and less time for its output signal from a given pulse to drop back to zero before the next pulse; this is a function of both the thermal and electrical time constants of the sensor. We have a number of “tricks” in the design to enable the sensor to work at much higher pulse rates than it otherwise would, but eventually a maximum pulse rate is reached above which the sensor’s response is no longer able to keep up; as it approaches that maximum pulse rate, the “additional error with frequency” begins to show up.

So, in theory, that additional error is negligible through most of the frequency range of the sensor, and becomes relevant from about 70-80% of the sensor’s maximum pulse rate and up.

In practice, this is not always the whole story; there are sometimes acoustic resonance effects in the crystal that cause “peaks” and “dips” in this frequency-related uncertainty, so the maximum specified “additional error with frequency” is not always necessarily at the maximum frequency. When we specify “additional error with frequency” as maximum +/- 1.5% to 25KHz, what we guarantee is no more than +/-1.5% at any point - not necessarily the end points.



PE sensors are Pyro Electric and they require both a heating and cooling cycle to generate a voltage to produce the signal read by the display. With a CW laser the sensor is never able to go through the cooling cycle, so it does not generate the voltage needed to produce the signal read by the display. If you have a CW laser, it is better to use a Thermal or Photo Diode sensor depending on your laser’s wavelength. Please contact your local Ophir representative for assistance in selecting the correct sensor for your application.

Using Your Sensor


Our energy detectors measure the total energy deposited within a time window defined by the pulse width setting selected via the Power and Energy Meters. There is no minimum pulse width limitation since we are measuring the energy deposited, not power or peak power.


Above the maximum rep rate of the sensor, the reading starts dropping until presumably at some rep rate it does not respond at all. This is because the maximum rep rate of the sensor is a function of the electrical (i.e. “RC”) and thermal time constants of the sensor, and when you go beyond those, the sensor is physically unable to respond fast enough. This is not to be confused with going beyond the maximum rated pulse rate of the meter. The meter will sample the data at the meter’s maximum rate, and when the meter is ready for the next pulse it will sample the next one that comes in; the sensors, on the other hand, have a physical limitation on how fast they can respond to pulses altogether.


Before using the pyroelectric sensor for power or energy measurement, check that your laser power, energy and energy density do not exceed the head ratings. Use the laser damage test slide that has been sent with your sensor at the laser energy you want to measure to make sure it does not damage. 
Please check the included data sheet or check on the website for the same information; 


With the pyroelectric head, you have been supplied a test slide with the same coating as on your pyroelectric detector. You can also obtain this slide from your dealer. You should use this slide to test the damage threshold with your laser pulses. If the slide is damaged, then either enlarge your beam or lower the laser energy until damage is no longer seen.



The new PE-C sensors use a different pin on the D15 connector for the voltage output from the sensor than the previous sensors. All other meters can accept the voltage on either of two pins, so they work with either the previous sensors or the PE-C series. But the Nova does not have this additional input. Therefore, in order for a Nova to work with the PE-C series, an adapter, Ophir P/N 7Z08272 has been made available. The adapter plugs between the D15 socket of the Nova and the D15 plug from the PE-C sensor. If you want to use the Nova RS232 PC adapter, this can be plugged in as well onto the PE-C adapter and used at the same time. Note that the Nova does not support all the new features of the PE-C family such as user threshold and 5 different pulse width settings but will support all the features that were available on the previous PE line.


The SH-to-BNC Adapter is meant only for power sensors, i.e. thermal or PD300 type sensors. For seeing an analog representation of energy measured by a pyro sensor, including Pyro-C sensors (other than using the AN OUT from the meter), we have the PE Scope Adapter. It is different that the SH-BNC adapter used for power sensors. With the pyro scope adapter connected between the pyro sensor and the meter, you can look at the output of every single pulse on an oscilloscope at up to the maximum pulse rate of the sensor, even if that is beyond the maximum pulse rate of the meter. Unlike "dumb" sensors, here you look at a square output after signal processing where the voltage is approximately proportional to pulse energy. (The temporal shape of the pulse form on the scope, however, is not related to that of the laser pulse; it is a function of the sensor's electronics).


In general, pyroelectric sensors physically respond only to pulsed sources (the pyroelectric crystal itself responds to the acoustic pulse – essentially a heat pulse – resulting from absorbing a light pulse). The heat from the laser itself is presumably not pulsed, so it shouldn’t directly affect the reading. Note that pyroelectric detectors have a temperature coefficient of sensitivity, on the order of 0.2%/degC. The “PE-C” type sensors (Ophir’s current line of pyroelectric energy sensors) have built in temperature compensation that eliminates most of this variation, but if your laser significantly heats the nearby sensor, then the compensation may not be perfect.


The old pyro sensors and the newer PE-C sensors are almost identical; the differences between them are as follows:

  1. More compact
  2. User Threshold – minimum energy threshold (below which the sensor will not trigger) can be selected according to users' needs
  3. Measures longer pulses (up to 20ms depending on model)
  4. Has up to 5 pulse width settings as opposed to only 2 pulse width settings

Smaller size and therefore:

  • May need a heat sink (P/N 7Z08267) in order to stand up to higher average powers
  • May need a mechanical size adapter (P/N 7Z08273) if it must fit into an existing mechanical jig designed for the older models

Meters and Software Support:
StarLite, Juno, Vega, & Nova II fully support the Pyro-C series. Laserstar, Pulsar, USBI, Quasar, and Nova / Orion with adapter* partially support the Pyro-C series:

  • Only 2 of the 5 pulse width settings are available
  • Lowest measureable energy cannot be selected (no User Threshold).

StarLab software supports both Pyro-C and older pyro series.

*Note: The PE-C series will only operate with Nova / Orion meters with an additional adapter Ophir P/N 7Z08272 (see details in Ophir website).

Wavelength Setting Names:
If you have your own software for communicating with the sensor, it may be important to note that for some models, the names of the wavelength settings are a bit different between the old pyro and the new PE-C, even though they mean exactly the same thing.

For example, with diffuser OUT, the settings in the PE50BB-DIF-V2 are called “<.8u” (i.e. visible, represented by a calibration point at 532nm that covers the full visible range), and “106” (i.e. 1064nm), while in the PE50BB-DIF-C these same settings are called “532” (i.e. 532nm, the calibration point for the visible) and “1064”.


When logging energy measurements on a PC with the StarLab software from a Pyro sensor via either a Nova-II, or Vega, or a USB enabled StarLite meter, the timestamp for each Energy pulse measured in the log is provided entirely by the clock on the PC which has millisecond resolution. (Note: Because a timestamp provided by a multitasking Windows PC is not from a true real time system, there could be instances where the timestamp is not well synced with the actual energy pulse measurement in the log, depending on how ‘burdened’ the computer was at any particular moment.)

When logging energy measurements on a PC with the StarLab software from a Pyro sensor via either a StarBright, Juno or Pulsar, each of these meters provides a precise microsecond resolution timestamp from their on-board clock.

This timestamp is synced to the Energy measurement and the data is written together in the log. The precise on-board clock in the StarBright, Juno or the Pulsar is used here to determine the time differences between measurements - rather than the PC clock which is used here just to set the initial baseline time of the log. This is the best method to log Energy if timing of pulses is critical.

As opposed to Pyro Energy measurements, when logging Power measurements on a PC via StarLab with either Photodiode or Thermopile sensors, where fast measurements are not required anyway, the log timestamp is provided entirely by the millisecond resolution clock on the PC when connected to any of our meters.


If the energy is just a bit over-range, up to 10% above the top of the scale, the meter will give a correct reading of energy and frequency, together with an “Over” warning. If the energy is way above the top of the scale, though, the reading will very likely be nonsensical, but without the “Over” warning. With Ophir’s Pyro-C energy sensors, there is never a “saturation” message on the meter - the output from the sensor can never actually reach saturation. Of course, being “way over” the top of a measurement scale – and not noticing - is not a likely scenario. Common sense is often the best defense.


In theory, if a beam is completely parallel and fits within the aperture of a sensor, then it should make no difference at all what the distance is; it will be the same number of photons (ignoring absorption by the air, which is negligible except in the UV below 250nm). If, nevertheless, you do see such a distance dependence, there could be one of the following effects happening:

  • If you are using a thermal type power sensor, you might actually be measuring heat from the laser itself; when very close to the laser, the thermal sensor might be “feeling” the laser’s own heat. That would not, however, continue to have an effect at more than a few cm distance unless the light source is weak and the heat source is strong.
  • Beam geometry – The beam may not be parallel and may be diverging. Often, the lower intensity wings of the beam have greater divergence rate than the main portion of the beam. These may be missing the sensor's aperture as the distance increases. To check that you'd need to use a profiler, or perhaps a BeamTrack PPS (Power/Position/Size) sensor.
  • If you are measuring pulse energies with a diffuser-based pyroelectric sensor: Some users find that when they start with the sensor right up close to the laser and move it away, the readings drop sharply (typically by some 6%) over the first few cm. This is likely caused by multiple reflections between the diffuser and the laser device, which at the closest distance might be causing an incorrectly high reading. You should back off from the source by at least some 5cm, more if the beam is not too divergent.

Needless to say, it’s also important to be sure to have a steady setup; a sensor held by hand could easily be moved around involuntarily, which could cause partial or complete missing of the sensor’s aperture at increasing distance, particularly for an invisible beam.


Pyroelectric sensors are energy sensors, meaning they measure the energy of pulsed laser sources…not the power of steady-state CW sources. In order for pyroelectric sensors to output a measurement a single shot or repetitively pulsed laser source is required. If you are trying to verify that a pyroelectric sensor is operating and currently do not have a pulsed laser source to direct onto it, they will usually output various measurements when tapping on the mechanical housing indicating that they are operating OK.


The problem is most probably acoustic vibration. Pyroelectric sensors are sensitive to vibration as well as heat. On the most sensitive scales of sensitive sensors such as the PE9 and PE10, they may be very sensitive to vibration. The PE-C series of pyroelectric sensors have an adjustable threshold so you can set the threshold to a value above the noise level but below energies you want to measure and thus eliminate false triggering. You may also try putting a soft pliable material under the base of the sensor to damp out the vibrations.


The problem is most probably false triggering caused by acoustic vibration. If the pulse frequency as shown on the meter jumps around, then acoustic vibration is almost certainly the problem. Pyroelectric sensors are sensitive to vibration, and they in fact detect acoustic pulses through the same physical mechanism with which they detect laser pulses. On the more sensitive scales of sensitive sensors such as the PE9 and PE10, they may be very sensitive to vibration. You can see this by setting such a sensor to a low energy scale (e.g. 2 mJ) and clapping your hand once, just above the sensor's surface; you will get a reading.

The Ophir PE-C series sensors have a trigger level that can be set to above the level causing false triggering but below the level you wish to measure.  You may set the user adjustable threshold to above the noise level to eliminate the false triggering. An additional solution may be to put an acoustically absorbing material such as a thin piece of soft foam plastic under the base of the sensor to damp out any vibration; acoustic noise carries primarily through the base (rather than through the air). 


The catalog specification states the maximum power a sensor can be used with and without the heat sink. The purpose of the heat sink is to keep the sensor temperature below the maximum permitted at higher average powers. If you use the sensor for a short time only, on the order of 1-2 minutes at a time, you should be able to measure up to the higher power given in the spec even without the heat sink.