What is the difference between absolute and relative laser power measurements?
Monday, May 29, 2023
It’s not uncommon to hear about “relative” and “absolute” measurements when talking about laser power measurements. Those are simply two different ways to approach and define the meaning of your readings. So, let’s sort this out.
Absolute power measurement
First, an absolute power measurement needs to come down to the basics of the known physics, so what actually is a watt? Once this question is answered, then, by a very rigorous process, you can determine what the actual value of a watt should be according to its definition. That being said, you still have to understand that even with a very strict protocol, the “True value’’ is always known only within a certain uncertainty.
From that point then, absolute laser power measurement involves using a device whose measurements are linked to that true value. So, naturally, it needs to be calibrated. In North America, devices that can perform such measurements are typically calibrated to NIST standards, which ensure that the measurements are traceable. By traceable, we mean that we can guarantee a specific uncertainty to the end user by using an unbroken measurement chain linked to the original ‘’True value’’.
So, when your device displays 1 watt, that absolute numerical value means you are really measuring 1 W ± uncertainty. This uncertainty is largely impacted by how many layers of calibration there are between your device and the original NIST reference. The first layer, which would be calibrated directly with the NIST standard, is what we call a gold standard. An instrument that is calibrated relative to a gold standard will be called a silver standard. There is no limit to how far you can go there, but the further you go, the less accurate it is. As a popular saying, it’s kind of like making a copy of a copy of a copy, etc. Eventually, the absolute measurement itself would be worthless. On that matter, take note that, at Gentec-EO, all our calibrations are done relative to a gold standard. For more details, check out here on how we do it.
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Relative power measurement
Second, when we talk about relative power measurements, it generally involves uncalibrated equipment and a lack of traceability. When using a single uncalibrated detector for relative power measurements, the detector is first used to measure the power output of the laser under a set of specific conditions, which is then considered the reference value.
Subsequent measurements of the laser power are then compared to this reference value to determine relative changes in power output. This method is often used to track changes in laser power over time or to compare the power output of two lasers. It can also be used when a traceable reference value is simply not possible.
Despite not being related to a true value, for relative measurements to be worth something, you need at least a device that has good repeatability, so even though it might not be related to a NIST value, repeated uses of a power meter in the same environments will give you the same results.
Comparing both methods
Ultimately, relative and absolute power measurements are similar as they both need a reference value to be compared to. The main difference is that an exclusively relative measurement does not provide any information regarding how close or far the actual readings are from the ‘’True value’’.
On the other hand, a relative measurement can tell you that a specific laser has twice the power of another one, but without telling you what that power truly is. As a matter of fact, without traceability or calibration, there is no way to know what the absolute numbers on your device really mean.
Therefore, when in need of accurate measurements, remember to use a high-quality detector and perform regular calibrations to minimize uncertainty and limit drift over time.
