Originally Posted By dyezak:
I know you don't apply the emissivity value like that...if you did my $20 handheld laser thermometer would measure my room temp (68F) brass sitting here next to me somewhere between 2F and 41F.  Funny thing...this $20 laser is telling me my 68F brass is 68F (the brass has been sitting in my 68F house for weeks).

Even if there were significant deviation at elevated temperature, you just need calibration.  For instance if my laser reads 400F when the brass is 750F that's fine, as long as it does it every single time in a repeatable way.  You lose some resolution capability, but really, you don't need incredible levels of detail anyway.

I know that emissivity certainly affects IR laser temperature readings...but I also know as a fact the impact isn't anywhere near what you describe.

ETA - I just came across an engineering study where they explain how emissivity impacts ir temp measurement equipment.  There's tons of formulas in there, but the one on application of emissivity states:
Q = esT(4) where:
Q = radiation intensity
e = emissivity of material
s = Stefan-Boltzmann constant
T = absolute temperature  
In their test example they were using a piece of stainless steel, emissivity of 0.12, baked at 175F and their standard meter measured it at 84F.  That same meter measured the sample as 1600F when it was really 2000F.

There's a catch to emissivity however, it depends on the specific wavelength of light naturally emitted/reflected by the subject material itself.  And as you see the example above as the material starts to turn/radiate red itself from heat this color shift also impacts the light reflectivity.  The closer to the IR spectrum the material's color is (i.e. the redder the material is naturally) the less emissivity impacts the reading.  And if you match the IR Laser's wavelength to that of the material being measured, emissivity is effectively a non-factor.  That formula looks like:

lmax = b / T where:
lmax = peak wavelength
of radiant energy
b = 2897 micron / °K
T = temperature (Kelvin)

When they matched the wavelength of the material itself in the 2000F test case (0.12 emissivity), they got a reading of 1950F.  97.5% accurate.  That's with no calibration for emissivity.  Once calibrated for a 0.12 they got 1998F.

Now, I have no way of measuring the reflected wavelength of light off of brass casings without starting a whole new project...BUT, with my $20 chinese laser and a piece of brass sitting right here I can infer the gold color of the brass shifts the reflected light closer to the red/IR spectrum and gives me pretty freaking good readings at room temp.

BUTx2 - the deviation factor is (in the simplest terms) a sine wave, which means it may be imperceptible at room temp, but at 750F it might be pretty evident.

BUTx3 - that can be easily tested by using a second temp measuring device and calibrating my reading (which won't be off by 728F like stated previously...).

Conclusion - It's something for me to take into account with calibration but won't prevent the implementation of the idea.  Thank you for pointing out why my readings *will* be off when I build my control circuit.

Originally Posted By NDT3:
That is awesome. Thanks for posting that. Really appreciate it.

Originally Posted By AZ_Sky:
@NDT3
Just a note: the holes and plate spacing on mine is for a 6.5CM case, you may need to adjust the holes and spacers for whatever case you are using...