Laser power meter sensor calibration questions

[sbk]

I just acquired a nice and brand new, never used Coherent Fieldmaster, and its LM10THD 10W sensor head off eBay.

Calibration wavelength of the head was made at 10,6um (CO2) from the certificate, and I was wondering if these types of thermopile heads are really flat along the whole spectrum, giving me accurate results at visible wavelengths as well? Cal uncertainity is done at +-2%.

And if yes, why manufacturers does calibrate their heads to different wavelengths?

As it seems it measures less power compared to a Coherent Lasercheck, around 20% less. A laser which repeatedly peaks at >970mW on the Lasercheck only gives 750mW on the Fieldmaster Even with an additionnal IR filter.
Well my Lasercheck may be out of cal since the years I use it, but its results seemed me consistant until now...

[mixedgas]

Calibration within 500 nanometers of bandspread is usually quite close. Ie I would not hesitate to use a meter calibrated for Red at blue at near IR or even 1000 nanometers away. However CO2 is very different when it hits the adsorber and has a tendency to reflect more, compared to visible light.
This explains your error nicely.

Steve

[sbk]

Thanks Steve.

However I did further researchs about this and found this from Coherent site :

Coherent uses three primary coatings to capture the incident radiation on our thermal sensors. The specifications for each
sensor list which coating is used. Typical wavelength ranges and response curves for these coatings are shown in the chart
below. Each sensor contains a spectral curve generated from reflectance measurements taken with spectrometers. The
reflectance data are converted into a wavelength compensation look-up table that is loaded into the sensor. This data is
accessed by selecting a wavelength of operation in the meters.

So this reflectance curve should be into the EEPROM of my sensor, and the readings adjusted if I set the Fieldmaster on the right wavelength, correct?

They have basically three coatings for their sensors, mine is HTD and it share the same curves as the visible coating, according to this graphics



Also I measured a few other lasers : one 1W 445 from RGBLS, at 1.05W, this is exactly what the manu told me before shipping...
Same on a 12-diodes (150mW Opnext) 640nm from RGBLS : outputting 1.79W, so close to what it's supposed to be. This is conforting me a bit more than this sensor is very precise for visible wavelengths as well

On the other hand, the Lasercheck gives very incoherent results at 445nm, this 1W was peaking at 750mW max, and the more I measured it, the more the power reading was decreasing... while being rock stable at 1.05W when I measured it with the Fieldmaster.

This should be useful to many peoples here, as the majority of the affordable sensors on eBay (OEM with "x"mV/W, etc) are calibrated at 10.6um...