Calibration Factors – laser power/energy meter
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What are the various factors I see shown on
my power meter in calibrate mode, and should I ever adjust them and how? There are a number
of different factors in calibration mode, each of which has a different role. Some of
them are actually user adjustable – for example, in cases where you need to calibrate against
your own in-house master reference. I must stress that adjusting any internal calibration
factor voids the calibration that was done in the factory. This should only be done in
a case of a clear need; it should only be done with the greatest of care; and it should
only be performed by properly qualified personnel. If you’re not sure if you really need to be
adjusting these factors you probably should not be doing it. Having said that, I will clarify now the meaning
of these different factors. The shape of a given absorber type’s spectral curve is
known and is constant, since it’s a function of the material of which the absorber is made.
The relative response of a given absorber at a particular wavelength compared to the
response at a specified reference wavelength is one kind of factor. Normally 1064 nanometers
is the reference wavelength that we use for this purpose. The user does not
see this factor, by the way, but it’s in their. Let’s call that factor one. The height, up-and-down, of the overall spectral
response curve is a different factor. This represents the overall actual analog sensitivity
of the detector. This is what is normally adjusted to during the calibration process,
since it varies from unit to unit due to all sorts of production tolerances. That’s the
reason, by the way, that we need to calibrate every single unit that’s produced. Let’s call
this factor two. It’s one fixed number for each given individual sensor. As you probably know we use a speed-up algorithm
to speed up the response time of the sensor and give a reading after some one to three
seconds instead of the longer physical response time of the sensor which in some cases might
be as high is ten seconds for example. This speed-up is set during calibration and is
stored as a mathematical function inside the sensor’s EEPROM and is represented by yet
another factor. As I just mentioned we enable the users to
adjust the calibration of the sensor, with all due caution, in case they need to calibrate
against their own defined master reference. You can change the calibration for one individual
wavelength or laser setting or for all wavelengths together. Needless to say any changes in such
calibration factors performed with all due caution should be recorded for future reference,
since adjusting any internal calibration factor could be very significant. Now let’s see how this actually works. Here’s
the basic idea. For any given wavelength, the overall factor for the sensor – factor
too as we called it – (remember it’s invisible to the user) times the specific factor one
for that wavelength (its relative response compared to directorates wavelength 1064 nanometers)
gives the final overall factor for that wavelength, which will be displayed as simply “laser
factor.” Let’s see it on an actual instrument. We go
to “calibrate” in the main menu and set it to adjust laser. In other words, adjust
the calibration factor for just this laser wavelength, as opposed to all wavelengths
together. Laser factor 0.7953 over here means the final factor, normally factory set, for
the selected wavelength. In this case, near-infrared means 1064 nanometers. That includes factors
one and two that we just talked about. In other words, this represents both the overall
sensitivity of the detector and the relative sensitivity at a particular wavelength, both
taken into account. Factor 0.7953 over here is a user adjustable
field. If you go to it and press enter, a modify window will open and allow you to change
it. Saving that change will change laser factor accordingly. Note that as you modified the
factor, you see what the original number was and you see the effect on the measurement
reading over here. Note that for near-infrared, in this case, these two factors are actually
the same, because as I mentioned, 1064 nanometers is in fact the reference wavelength that we
use, so this factor represents only the up-down
adjustment of the curve. If you were to change the laser setting to CO2, you would see two
different numbers. There’s one other type of factor, as we said
earlier, and that’s related to the response time speed-up function. This, too, can be
user adjusted. If you go here to menu, response, and then following the onscreen instructions
to recalibrate the response time speed-up. Modifying this factor incorrectly could cause
a response time to slow down or to speed up but with an overshoot or undershoot. In general,
again, any modifications to factory set calibration factors should only be done with the greatest
of care, only if there’s a real need, and only by qualified personnel. [music]

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