Author Topic: attenuator to put in front of thermal camera lens for high temprature work?  (Read 229 times)

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Offline coppercone2

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Is it possible to put some kind of optical attenuator infront of a thermal camera to rescale the temperature and then calibrate it to a known process in a useful way?

What I wanted to do is use a torch and indicator pencils to try to see when aluminum is preheated enough to start welding, or for metal bending, etc. I know numbers will be baloney but you can look for what you know. I can visually correlate the color those range modes show with the correct temperature doing side by side mode with a indicator pencil (if you don't know, its a special wax-like pencil that melts at the correct temperature, you strike it like a match when you think its good until it melts while sweeping a torch)

I want to do this on a seek thermal camera. I expect a finicky solution but is it possible to ballpark this thing with a clip on lens and some kind of stack of attenuator sheets that I can put in front of the camera one by one? I have no problem building a thing in front of it with a 3d printer that I can load up lens elements on the go.. but I am not sure where to start here. I was imagining like a hopper you drop em into while its angled slightly up so they slide into a tube in front of the lens.

Could I do something to ZnSe windows with coatings or whatever that are 'dirty' and resistive, then make a few to find the right combo? I thought to make a hopper infront of the camera I can put them into while I find the correct setting. Also, for lower temperatures, you can use a soldering iron to get an idea of whats going on. I thought maybe.. clear coat?

https://markal.com/collections/temperature-indicating-sticks

Then I can label the attenuators and put the appropriate ones for the process im interested in.

Also, is there a danger to looking at a torch from a few feet away with the thermal imager (acetylene). Candles don't seem to do anything bad.

It does not need to be super duper, it seems like it should be able to ballpark grease melting at least?

Then when you figure it out you can put a disk infront of the camera that rotates into place like a revolver cylinder so the right optics are switched in for the correct temperature range. Also, no thorlabs or other expenisve things.
« Last Edit: Today at 09:47:08 am by coppercone2 »
 

Offline Ultrapurple

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Try drilling a range of different size holes in a piece of thin metal and placing them in front of the lens, selecting the size of hole that gives you the best result. You'd be looking at holes starting at about half the diameter of the lens.

A hole will act as an attenuator - a sort of external iris - by preventing so much energy reaching the lens. You'll need to put the metal very close to the lens (almost touching) to avoid too much vignetting (dark edges to the image). If the holes are too small you'll end up restricting the field of view; hopefully you'll find a 'sweet spot' that works for you. Ideally the metal would be dark on the lens side and shiny on the other side (to reflect heat better and stop the metal heating up, which would affect the image by resucing the contast, at the very least).

Professional cameras with high temperature ranges often have a movable metal leaf with a 'calibrated hole' that goes between the lens and the sensor. It's better to put it in that area but it's not practical with an existing camera.

I have a FLIR camera that has a calibrated hole that can be switched in and out as described. If you're interested I can take some photos when I have some spare time.
« Last Edit: Today at 10:44:57 am by Ultrapurple »
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Online Fraser

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This thread may be of interest  :)

https://www.eevblog.com/forum/thermal-imaging/potential-attenuator-lens-for-our-cameras-znse-and-ge-available-tr-23-and-30/

I have also experimented with placing a common photographic glass Haze filter in front of a thermal camera as an attenuator and it worked  :-+ I will look for my posting showing the results of that test and post a link for you.

Fraser
« Last Edit: Today at 01:25:55 pm by Fraser »
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Online Fraser

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Online Fraser

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I managed to buy an official FLIR temperature range doubler a while ago. It is the FLIR solution to imaging temperatures that would normally exceed the measurement range of the Exx, Bxxx and Txxx series cameras.
Normally a very expensive accessory and it permits calibrated measurements to be taken.

https://www.eevblog.com/forum/thermal-imaging/flir-2000celcius-temperature-range-doubling-adapter-exx-txxx-t197993t199235/

As you will see from the discussion in the thread. Others suggested cheaper alternatives such as an iris or pin hole attenuator.

You can buy a nicely made variable IRIS on ebay very cheaply. Look for “microscope Iris” As they are used on the below stage light source.

Fraser
« Last Edit: Today at 11:57:28 am by Fraser »
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Online Fraser

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« Last Edit: Today at 01:27:22 pm by Fraser »
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Online Fraser

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One other comment regarding the proposed use of the ‘attenuator’....... if working with Aluminium you are imaging a reflective material that often has low emissivity. As such the gas torch flame energy will be reflected towards the thermal camera and the low Emissivity of shiny metal will provide a very low reading of its surface temperature unless the cameras Emissivity setting matches that of the metals surface.

Emissivity of Aluminium:

Rough : 0.07
Anodised : 0.77
Polished : approx 0.05

This table gives greater detail and includes Emissivity at different temperatures......

https://www.omega.co.uk/literature/transactions/volume1/emissivitya.html


The poor Emmisivity of shiny Aluminium is a form of measurement ‘attenuation’ in itself.
Imagine having your camera configured for a target Emissivity of 1.0 when it is in fact only 0.07 ....... the readings provided will be 0.07 of the actual surface temperature.

You do still have the issue of an acetylene torch flame in the field of view though ! I personally would not expose a thermal cameras microbolometer to an acetylene torch flame at close range. Such is asking for trouble and the correct type of thermal camera with a high temperature range attenuator is recommended.

Fraser
« Last Edit: Today at 02:00:39 pm by Fraser »
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Online Fraser

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« Last Edit: Today at 01:52:58 pm by Fraser »
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Online Fraser

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Another interesting paper that used a FLIR A655sc camera that was fitted with what they describe as a “Neutral Density filter”.

https://www.sciencedirect.com/science/article/pii/S0264127519305167
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Online Fraser

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Thermal camera Neutral Density Attenuators.......

https://www.edmundoptics.co.uk/f/infrared-ir-neutral-density-nd-filters/13765/

Not cheap, but the 12.5mm diameter that you could use is not horrifically expensive either  :-+ Using such a science grade attenuator permits meaningful temperature measurements to be made  :-+

Fraser
« Last Edit: Today at 02:37:33 pm by Fraser »
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Online Fraser

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An added benefit of using a proper germanium attenuator is that it also acts a a splatter guard for the cameras lens. Better to damage a replaceable attenuator than the cameras lens.

Fraser
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Online Fraser

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I just remembered, it is worth knowing a bit about how optical attenuators work as this can be important.

There are reflective attenuators that prevent energy passing through their special coatings and substrate. These do not suffer badly from self heating. Then there are absorbing type attenuators that effectively intercept the energy and dissipate it within their structure. These suffer badly from self heating. Forced air cooling is sometimes used to cool such energy absorbing attenuators. Many attenuators are a hybrid with both reflection and absorption properties. The ratio of these properties can effect suitability for a particular task.

There is also wavelength filtering that can offer attenuation of areas of the EM spectrum that may overload a thermal sensor or need to be attenuated to reduce dynamic range in a spectrum before it is presented to the sensor.

Fraser
« Last Edit: Today at 03:46:44 pm by Fraser »
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Offline SilverSolder

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I just remembered, it is worth knowing a bit about how optical attenuators work as this can be important.

There are reflective attenuators that prevent energy passing through their special coatings and substrate. These do not suffer badly from self heating. Then there are absorbing type attenuators that effectively intercept the energy and dissipate it within their structure. These suffer badly from self heating. Forced air cooling is sometimes used to cool such energy absorbing attenuators. Many attenuators are a hybrid with both reflection and absorption properties. The ratio of these properties can effect suitability for a particular task.

There is also wavelength filtering that can offer attenuation of areas of the EM spectrum that may overload a thermal sensor or need to be attenuated to reduce dynamic range in a spectrum before it is presented to the sensor.

Fraser

I guess a filter can also be either the reflective or the absorbing type?
 

Online Fraser

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Often a hybrid mixture of the two using special coatings

Fraser
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