Q: How should I clean my sensor?
A: First, clean the absorber surface with a tissue, using Umicore #2 Substrate Cleaner, acetone or methanol. Then dry the surface with another tissue. Please note that a few absorbers (Pyro-BB, 10K-W, 15K-W, 16K-W and 30K-W) cannot be cleaned with this method. Instead, simply blow off the dust with clean air or nitrogen. Don't touch these absorbers. Also, HE sensors (such as the 30(150)A-HE-17) should not be cleaned with acetone.

Note: These suggestions are made without guarantee. The cleaning process may result in scratching or staining of the surface in some cases and may also change the calibration.

Q: Will the 3A-P sensor work at 10.6µm?
A: The 3A-P actually absorbs about 85% at 10.6µm and therefore it can be used to measure weak CO₂ lasers. Note the low power damage threshold, however, of 50W/cm².

Q: When an accuracy spec is given, what exactly is meant?
A: The Ophir specification on accuracy is in general 2 sigma standard deviation. This means, for instance, that if we list the accuracy as +/-3%, this means that 95% of the sensors will be within this accuracy and 99% will be within +/-4%. For further information on accuracy see Ophir Power/Energy Meter Calibration Procedure and Traceability/Error Analysis and our Knowledge Center.

Q:How do you calculate the power and energy density of a laser beam?
A: If the power is P and the diameter of the beam is D, then the power density is P /(.785 * D²). If it is a pulsed laser and the energy is E, the repetition rate is R and the diameter is D, then the power density is E*R/(.785 * D²) and the energy density is E/(.785 * D²). The sensor finder will automatically calculate the power and energy density.

Q: If according to the catalog specs or the sensor finder I am very close to the damage threshold but below it, should I choose such a sensor?
A: It is not recommended to choose a sensor if it is very close to the damage threshold if there is an alternative, since laser damage is not an exact figure and depends on many things. Use the Sensor Finder to find the best match where you are preferably below 50% of the damage threshold.

Q: Does the damage threshold depend on power level?
A: The damage threshold of thermal sensors does depend on the power level and not only the power density because the sensor disc itself gets hotter at high powers. For instance, the damage threshold of the Ophir broadband coating may be 50 KW/cm² at 10 Watts but only 10 KW/cm² at 300 W. The Ophir specifications for damage threshold are always given for the highest power of use of a particular sensor, something which is not done by most other manufacturers. This should be taken into account when comparing specifications. The Sensor Finder takes the power level into consideration when calculating damage threshold. For further information, check out our 'FAQ: Does damage threshold depend on power level?' video.

Q: Can a laser measurement depend on the distance from the laser to the sensor?
A: 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.