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Wavelength Correction. Does it Matter?

Snecho

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Hey all.

So I've was having an interesting conversation with another member about the effects of different wavelengths regarding the accuracy of the readings when measuring a laser's optical power.

I found it very odd because I know LaserBees for example, are rated for broadband coverage of the spectrum (300nm-1500nm), and I've never heard it mentioned before.

And yet when looking at the Sanwu Tracer LPM, it offers a "wavelength correction feature." So supposedly, you set the LPM to what laser you are measuring to "improve accuracy of the reading." For example if you are measuring a 445nm blue laser, you set the LPM to 445.

So basically I was wondering if this is justified feature?
Is wavelength correction for accuracy really a thing? Should it be considered for other LPMs?
 

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Cyparagon

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This depends on the topology and the quality. For an optical meter, the response is not uniform for each wavelength. A correction NEEDS to be applied for optical meters to read properly.

For thermal meters, IF the surface coating is uniformly black to all colors, a correction factor would be... redundant. However, if they've got a shitty coating, and have yet somehow managed to characterize the absorbance of that shitty coating, it would indeed improve accuracy.
 

CurtisOliver

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Cyp above has answered your question. Laserbee use thermopile sensors. Don’t know enough about the Sanwu one.
 

paul1598419

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Normally you only see this type of correction with LPMs that use optical sensors. Those are typically low power meters. They are quite accurate for measuring low power HeNe lasers.
 

Encap

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This depends on the topology and the quality. For an optical meter, the response is not uniform for each wavelength. A correction NEEDS to be applied for optical meters to read properly.

For thermal meters, IF the surface coating is uniformly black to all colors, a correction factor would be... redundant. However, if they've got a shitty coating, and have yet somehow managed to characterize the absorbance of that shitty coating, it would indeed improve accuracy.
Cyp above has answered your question. Laserbee use thermopile sensors. Don’t know enough about the Sanwu one.
Normally you only see this type of correction with LPMs that use optical sensors. Those are typically low power meters. They are quite accurate for measuring low power HeNe lasers.

Exactly, right.

The OPs question should be directed to Sanwu about the Sanwu LPM.
Hard to fathom how anyone could imagine that Sanwu would include an unjustified, useless, and meaningless feature in their LPM.
Obviously Sanwu thinks it matters in their product and provides the feature to overcome/correct for some of the products deficiencies and limitations.

There are hundreds of different LPM and sensor combinations made by dozens of manufacturers for use in various applications using different technologies and with different levels of accuracy and different characteristics.
Every LPM and sensor is different due to many factors and thus the need for a manufacturer to individually characterize and calibrate every LPM they make, individually.

For an example of some of the real world factors involved and which are end-user adjustable in one makers product, see following video that explains how Ophir addresses the subject concerning one of their "LPM" products which can use any of the Ophir thermal, pyro-electric, and photodiode sensors it manufactures.

 
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Snecho

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This depends on the topology and the quality. For an optical meter, the response is not uniform for each wavelength. A correction NEEDS to be applied for optical meters to read properly.

For thermal meters, IF the surface coating is uniformly black to all colors, a correction factor would be... redundant. However, if they've got a shitty coating, and have yet somehow managed to characterize the absorbance of that shitty coating, it would indeed improve accuracy.
Ok, yes, I think I'm starting to understand. Basically a well made, reputable thermopile does not need any kind of correction.
Cyp above has answered your question. Laserbee use thermopile sensors. Don’t know enough about the Sanwu one.
Normally you only see this type of correction with LPMs that use optical sensors. Those are typically low power meters. They are quite accurate for measuring low power HeNe lasers.
Yes sorry, I hope it didn't seem like I was just brushing away what Cyp said. Just the interesting conversation I was having with another member I was hoping would spill out over here about the subject.

Here is the response from Sanwu when asked why is the correction there when the Tracer has a thermopile head in case anyone is curious:

"It is an additional feature to make the reading slightly more accurate on the testing wavelenth, it's not necessary but we add it anyway."

So kind of an odd and counterintuitive statement from them 🤷‍♂️
 

hwang21

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@Cyparagon, do you happen to have any examples of what the absorbance of a thermopile sensor looks like across the general 300nm-1500nm band? I imagine it to be relatively even in a broad sense, but I would be hesitant to believe that the thermopile material is completely uniformly black across that entire range. I'm sure that with enough resolution/"zooming in" there must be peaks/dips in absorbance across the wavelength range, no matter what material you use. Just a matter of whether these peaks/dips correspond to mW of variance? uW? nW?
 

Cyparagon

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As I said before, that depends entirely on the coating used. It has precisely nothing to do with the sensor.

Carbon black (which is essentially soot) is a cheap, common colorant, so something like this might be a worst-case scenario.

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