Power Meters FAQ's

Laser Power/Energy Sensors

General Specifications


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), The energy density is E/(.785 * D2). The sensor finder will automatically calculate the power and energy density.


It depends on the sensor and on the range the sensor is set to for the measurement. The easiest way to figure out the resolution is to look at the display. For example, a 30 W sensor has three places after the decimal in the 5 W range and two in the 30 W range. So the resolution is 1 mW for the 5 W range and 10 mW in the 30 W range.


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 https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/tutorial/calibration-procedure and https://www.ophiropt.com/laser--measurement/knowledge-center?search=calibration&=SEARCH


We have had customers in the past who measured electron beams with our thermal sensors (with the BB absorber), and it seems the absorber absorbs close to 100% of the radiation. So it can be done. Presumably this is in a vacuum, so that has to be taken into consideration.


Generally, our sensors are calibrated (traceable to NIST) to within ±3% accuracy 2 sigma which means that 95% of the sensors are accurate within ±3%. However, if your application requires very high accuracy, we also offer something called “double calibration” which can bring the error down to ±2%.


Each sensor from Ophir includes a performance specification datasheet but not a user manual. This datasheet lists operational parameters for the sensor, including items such as spectral range, power range, aperture size, accuracy specification, etc. Copies of these datasheets for current sensors are available on the website by clicking on the “Specifications” tab of the sensor's product page.
As for the user manual, a hard copy of the user manual is included with every meter sold. The manual also has a section explaining the operation of the various types of sensors; thermopile, photodiode, and pyroelectric. Copies of the meter user manuals are available on the web site at: https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/services/manuals.
If for any reason you are not able to find the particular information needed or are not able to retrieve the manuals or information from the website, just send us an email request to service@us.ophiropt.com and we’ll get the information for you.


The damage threshold refers to the power or energy density at any point in the beam. So if we have a gausssian beam, the damage threshold refers to the power in the center of the beam. For a top hat beam, the damage threshold will be at any point in the beam. The sensor finder allows you to choose between a top hat and gausssian beam when calculating the damage threshold.
Some beams are not smoothly top hat or Gaussian and may have hot spots. Furthermore the damage threshold is not always an exact level. So the user is recommended to choose a sensor that does not exceed more than 50% of the specified damage threshold.


This depends on whether you are using a thermal sensor or a photodiode sensor. With our most sensitive thermal sensor, model RM9, one can measure down to about 500nW. With our photodiode sensor heads we have a several types, silicon, InGaAs and Germanium. Each has a spec on minimum power, which can be as low as 10pW


In order to get a meaningful power reading on a power meter, the signal being measured must be considerably larger than the noise. In fact there is a measure of this, the signal to noise ratio or SNR that is often used. If the SNR is 1 then the noise is the same size as the signal and this signal is barely distinguishable. There are some that will quote this number as the minimum measurable, but most agree that the signal must be considerably larger than the noise to be measurable. One criterion used by many is 10:1 SNR as the minimum useable measurement. However, when the noise referred to is 1 standard deviation or 1 Sigma, then a certain amount of the time the noise is 2 times this or even 3 times so 10:1 SNR 1 Sigma is still not a very precise measurement. For these above reasons, Ophir has taken a particularly strict definition of minimum measurable power and that is 20 times the 3 Sigma noise value. This is 6 times as strict as the usual 10:1 1 Sigma value often given or implied. The Ophir value means that most of the time, the noise does not exceed more than 2% of the signal.


For pyroelectric energy sensor heads there is no limit on how short the pulse is, as they are integrating devices. As long as one does not exceed the damage threshold expressed in terms of energy density then they will accurately integrate pulses as short as femtoseconds. With thermopile sensors they similarly can be used as integrating devices to measure energy, although one can only measure single pulses every few seconds as they have a much slower response time than pyroelectrics. With repetitive short pulses one can measure the average power with a thermopile with no restriction on how short the pulses are, as long as the maximum energy density is not exceeded. The spec for damage threshold varies on type of absorbing surface of each sensor head type. Consult our damage threshold charts or use the Sensor Finder for detailed information.