attenuator to put in front of thermal camera lens for high temprature work?

[coppercone2]

so this polystyrene idea lines up with my home made q-dope idea (some guy on youtube has it), it looks like if I buy the right solvent I can make polystyrene sheets and also q-dope. (ethyl acetate and styrofoam make a polystyrene goo that is castable over coils).

I was about to ask if there are plastics that work.

[Fraser]

Look at ABS Styrene modelling sheet. It comes in many thicknesses and is relatively uniform for imaging through it. A 0.5mm sheet plus the addition of some thinner sheets to make up the correct thickness fir the task maybe ? Look for “plasticard”.
Cheap and easy to work with. All the best for your experimentation  It is nice to see someone wanting to carry out experiments in thermal imaging 

https://www.newmodellersshop.co.uk/plasticard.htm

https://www.stationroadbaseboards.co.uk/z_faq-plasticard.htm

[AkiTaiyo]

A fine mesh will work well, provided the mesh size is quite small, otherwise you can get some odd moire effects.  Nice thing about mesh is that its usually fairly flat across all wavelengths.

For calibrating, you can just apply a suitable emissivity adjustment (same can be done for all windows/filters).  For example if the mesh/window etc has a 50% opacity, just apply this to the surface emissivity of the taget.  Eg, target emissivity of 0.8 with a window of 50%, set emissivity to 0.4.
If you can't work out the opacity of your filter, then try to do a calibration at the highest temperature you can get to with your camera, ie read the temperature of a target, place the filter in front, then adjust the emissivity until it reads correctly again. 
This may not work with all cameras though, depending on how the brightness is calculated.  Some cameras use a table (precalculated brightness taking into account all optical/electrical factors in the instrument) which may be limited to the working range of the camera.  Other instrument that use a mathematical model may by highly innacurate outside the working range.

Depending on how hot you want to measure, you could try calibrating a standard silicon based camera (eg phone camera).  You ideally need to lock down as many parameters as possible (aperture, shutter speed, gain, etc) then use a NIR bandpass filter infront of the camera, though you can do it without a filter provided theres no ambient change in the scene.  To calibrate it you might need to find a couple of hot sources of known temperatures though.  Dynamic range on most 'phone' style cameras though is only 8-bit so you really dont get much dynamic range, especially given the energy from the high temperatures.  Some 'machine vision' cameras work at 12 or 14 bit.
Another fun thing you can do with an optical camera is use the Bayer filter to provide ratiometric measurements, though for this you need access down to the subpixel level.  Once again though, the limited dynamic range is a real problem.

[Ultrapurple]

Using the Bayer filter on a visible light camera to provide thermal radiometric values is intriguing - care to say more?

(Oh, and I've probably tripped myself up over radiometric vs ratiometric - I know they're different but I can never remember which to use where).

[AkiTaiyo]

From my understanding, with ratiometric you use two wavelengths and the difference/ratio between the two brightnesses give you the temperature, free from issues such as emisivity and window transmission and smoke (though non-greyness is a factor instead).

[Fraser]

Ratiometric thermal measurement is a pretty specialised area of physics and chemistry. I think Common old Radiometric radiance measurement and conversion to temperature Units is more than enough for most of us to cope with 

Fraser

[Bill W]

As has been stated, a LWIR thermal camera is not ideal for high temperature measurement as can be seen from the standard Black Body Plank curves provided in the Stemmer Imaging brochure. It is possible to image a high temperature target with a LWIR camera but measurement calibration and associated accuracy can become ‘interesting’! You are measuring on the slope of the Plank curve.

Even so, it is a log scale graph so 800°C is 3x the radiant energy of 500°C.

I have some doubts over that graph, 25°C is not half way between 20 and 80 !

My relative radiance values, for a whole camera system, come out as:
°C           'R'
0               16
25             24
60             40
100           65
200         158
400         479
600         977
800        1655 
1000      2514 

Pixel response (voltage) is linear in 'R' value, but you need to base it to the flag radiance (say 25°C) as a false zero.
So a 60°C object is (40-24) = 16 and a 100°C object is (65-24) = 41 and a 1000°C is (2514-24) = 2490.

As such you can see that to put 1000°C into the normal range of a basic camera at 100°C you need to reduce energy to less than 1% (16/2490) if you cannot otherwise alter the camera settings.
The accuracy problem that Fraser notes is in accounting for the other 99%, making assumptions as to the radiance of that is the main problem.