Power Meters FAQ's

Laser Power/Energy Sensors

Calibration

Our thermal sensor specification states "Power Accuracy +/- 3% at calibration wavelength". To what exactly does the 3% refer?

05/26/15

The Power Accuracy of +/-3% refers to the absolute uncertainty of the measured value. For example, for a 2 Watt reading, the actual "true" value would be between 1.94 W to 2.06 W (with reference to NIST, to which all our calibration is traceable). This assumes the reading is from about 5% of full scale up to full scale. It should be noted that our accuracy specification is in general based on 2 sigma standard deviation.

Repeatability of the measurement (assuming the laser itself is perfectly stable) is limited in the best case by the power noise level of the sensor, and is typically better than  +/- 1%  depending on the thermal stability of the environment. Stability at higher powers from the middle to the top of the range of the sensor head is usually better than the low end. This is due to small temperature variations having less of an effect as they are proportionally a lower percentage of the total power. For more information, refer to our Web tutorial at: https://www.ophiropt.com/laser-measurement-instruments/laser-power-energy-meters/tutorial/calibration-procedure

What is the standard recalibration interval for Ophir Power and Energy measurement sensors?

05/26/15

With normal usage we recommend calibrating every 12 months. To accommodate shelf time and shipping time new manufactured product comes with a calibration sticker that shows a recalibration period of 18 months from manufacturing. However this does not negate the recommended 12 month recalibration interval should you receive the product with more than 12 months remaining on the new manufactured calibration sticker.

If I have a sensor calibrated for a particular wavelength and a curve of the relative sensitivity at different wavelengths, how do I correct for a reading at a different wavelength?

05/26/15

If the calibrated wavelength is W1 and I want to measure at wavelength W2 then I look at the relative sensitivities on the curve at W1 and W2 and calculate as follows: Sensitivity at W1: s1 Sensitivity at W2: s2 When instrument is set to W1 and I measure W2, then multiply reading at W2 by s1/s2 to get correct reading at W2.

How is the power meter sensor calibrated for wavelengths other than the specific wavelengths used for calibration?

05/26/15

All absorbers used in power/energy measurement are not entirely flat spectrally, that is, they vary in absorption with wavelength. For this reason, Ophir measuring sensors are usually calibrated at more than one wavelength. If the absorption changes only slightly with wavelength, then we define wavelength regions such as <800nm, >800nm and give a calibration within these regions. In that case, the error in measurement between the wavelength the device was calibrated for and the measurement wavelength is assumed to be within the primary wavelength calibration error.

How can we be sure of the calibration of our high power sensors when we calibrate at a fraction of full power?

05/26/15

An explanation of how we can accurately calibrate at a fraction of the maximum power is given in our catalog introduction and on our website. In addition, in order to be sure of the calibration at higher powers, we have to know if the linearity of our sensors is within specification. For this purpose we have a 1500W sensor calibrated at various powers at a standards lab. Using a beam splitter and a 15,000 Watt laser we periodically check the linearity or our highest power sensors against this secondary standard.

What is the expected difference in reading between two correctly calibrated Ophir Sensors?

03/12/20

Customers often measure the same laser with 2 different Ophir sensors, both of which are specified to be within calibration. Let’s say that both of the sensors are specified to have a calibration uncertainty of ±3%. Do I expect the difference in reading between them to be less than 3%? On the first thought, this is what one might expect. However this is not necessarily so.
 
First of all, when we specify a calibration accuracy of ±3%, we are talking about a 2 sigma uncertainty, i.e. the readings of various sensors will be within a bell curve with 95% of all sensors reading within 3% of absolute correct calibration and 5% reading outside this accuracy. Thus there is a small chance that the meter will not be reading within 3% of absolute accuracy.
 
A more important reason is that the two sensors’ calibration error may be in two different directions and thus show a larger discrepancy between them than 3%. Say one sensor has been calibrated and reads 2.5% above absolute calibration and the other 2.5% lower than absolute calibration. Both of the sensors are within the specified ±3% absolute calibration but they will still read 5% differently from each other.
 
If we do statistical analysis, the analysis will show that there is in fact a probability of >16% that two correctly calibrated sensors will differ in reading from each other by more than 3% and a probability of over 6% that the sensors will differ in reading between each other by more than 4%.