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attenuator to put in front of thermal camera lens for high temprature work?
Posted by coppercone2 on 30 Oct, 2020 09:35 -
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. -
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. -
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 -
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 -
Here you go.... adjustable aperture Irises
Pick your size
https://www.ebay.co.uk/itm/Microscope-Condenser-Camera-Adapter-Aperture-Diaphragm-Adjustable-Optics-Iris/254728325878?_trkparms=aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D225076%26meid%3D7acf66ea4e3e450a9a9fafb78886e68a%26pid%3D100005%26rk%3D1%26rkt%3D12%26mehot%3Dco%26sd%3D133552146116%26itm%3D254728325878%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv5PairwiseWebWithDarwoV3BBEV2b%26brand%3DUnbranded&_trksid=p2047675.c100005.m1851
Fraser
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A search for “Iris Diaphragm” brings up good results. These units come in many sizes to suit your needs. Plenty to select from on eBay.
https://www.ebay.co.uk/itm/1-12mm-Amplifying-Diameter-Metal-Zoom-Optical-Iris-Diaphragm-Aperture-Condenser/323311033123?_trkparms=ispr%3D1&hash=item4b46d6db23:g:dQAAAOSwVvRbKw6c&amdata=enc%3AAQAFAAACcBaobrjLl8XobRIiIML1V4Imu%252Fn%252BzU5L90Z278x5ickk5v8gVt3hEWLVg%252F253w6XCefw04aNvZ2nUyJaMJw56wXpxdADI%252FGZe%252FRmxzlPD0bn7o6QpbqJOgTn292LCvRiIJsl3Otts5%252BU70LbrZidxXJBY1ujaq6B49dzK7nZRGsNnFHPvDw0XV4d668w6ObZigvMpSitCc9xspg41ouYRGRmEIIjKHViexe7BmiiGqeVz37bwpaTr%252BBLY8PnQ6iUP5NH4gAGztBV2pbu3WkPh6xzpYE0c78R4Pn8LzC4cHT8JHZ%252FHufm0kBDm10qvga5lT15DqyBuL13Hbz7d84tEVKwBzcdLy%252BOjURTClO5d0HIhy5dPiigQ%252BqTJ%252BdhU7%252BmPiui%252B8gkNc2GpqvW%252F9wVxblihOmlK4qP3U3yEiOGG3cuAshxkwY8%252BvnsiStgkGaHLwqrvCQDzUu1mz1yeK%252BT0hX%252Fz6bkrTUx3UYhPzY4UWYmTfqTICDNociWHWkkvQYajLVFVzLjxiHi0uvQu5P0Bb7zvA8EDA5AiQTKUtGCtCdCPeFAILmyZ6epqSaxG0K4XC1%252BZun2Q99aGF5xoMhl03Bffnvl72qiFZdWZEG3hsm1Yu7vPM4KkY1V37tUR87rXgYiQu2UZVGRuv1TWRd3Ls%252Fdo5K1eGtq%252Bp2KztPPwJXqCqC2bw310rzvFtRASFkS2vuwZsZKWMFa8HHBDVww5ttUWzQonCfP2nH%252FNVf230HKG5c8rBLze9v%252BqeXpP2UZtJxeE9aP3tcs9Ebp5N9CbM0FxDSeF2%252F9y7jeTxkJzXr5m01BKHgD3cmWawwL990p7w%253D%253D%7Ccksum%3A3233110331232e969809168b48c8b3ef9419ea4e17bc%7Campid%3APL_CLK%7Cclp%3A2334524
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 -
An interesting paper...... they used a FLIR T640 on its 1500C range.
https://repository.tudelft.nl/islandora/object/uuid:2e14d054-2d10-4096-a73c-ca606c98d493/datastream/OBJ/download
Fraser -
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 -
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 -
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 -
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 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? -
Often a hybrid mixture of the two using special coatings
Fraser -
wow thats fast
I forgot an iris lowers the amount of light entering a camera, I actually have a big square iris (not optimal) that I can try it with (it moves metal plates around on o-ring friction)
I could setup the camera far away on a cable and use digital zoom to protect it from the torch (the seek seems to have this). It will be interesting to see what effect the molten flux has on the emmisivity (fluoride salts). I tried candle wax and it looks like lava. I have a cable so I can orient it properly on a tripod. And yea you would need to move the torch away for a second to let the camera ignore it.
Unfortunately the seek thermal pro only has 4 emmisivity settings??, it is not infinitely variable. It does have a difference because I tried lowering the emissivity and a cold matte area (5c) read as bellow freezing.
I need to wait for it to warm up next week because something is going to break if I try to setup tripods and lens and stuff in a cold shop, my dexterity and thinking ability becomes weak. If I like what I see with a few of these experiments maybe that germanium lens is worth it, would hate to have the torch pop and fling something at the lens from far away, this could be damage control and soot protection, plus I should probobly make some armor for the iphone display. I think I will make a box out of thin aluminum flashing with some reinforcements and a glass cover to put the phone in. -
what transmission % do you choose ? I.e. I have 330C maximum. What formula can I use to adjust it to another maximum temperature? Will it just double at 50%?
Thinking about the lens rather then the iris, seems easier to use.
And I totally forgot about welding glasses (you should use special blue glasses for aluminum, or a modified flip up helmet for sodium flare). So unless I hook this up to a imax projector I don' think it will be too useful because its too easy for the torch to mess up your vision temporarily. I can see migraines even if I got this technology working fine for welding.
Still want to do it however. -
It is not double.
Cameras are linear in response to radiant energy, not temperature. The effects of the Planck black body curve in the transmission band need to be used.
It is a fairly heavy duty bit of spreadsheet maths but generates a useful table at the end.
[not got mine handy at present]
Bill -
Will not work it's against physics
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There are reflective attenuators that prevent energy passing through their special coatings and substrate. These do not suffer badly from self heating.
Those are precisely interference filters and this is the solution used typically by camera manufacturers.
Max -
keep in mind this is for process control not metrology, it does not matter if i am just seeing a signature related to the true temperature instead of the true definition of temperature, its just so I can determine when something is ready to start forcing or laying a bead. As soon as you get into steel or copper its easy to look at it. Thankfully I can do it with the grease pencil and look at the camera to know what to look for in terms of color (it has the colorful gradient displays, I am sure you can pick one of those color schemes and adjust the range so that something vibrant/obvious/definable is seen when the temperature is correct). For instance I noticed looking at a candle flame one of those color schemes makes the hot gasses very easy to see, wheras with others its not really noticeable. It may be the case that the signature will not be accurate with another material for whatever reason, but I know what I am working with to a high degree.
However save for wearing some kind of augmented reality glasses under a welding helmet it seems hard to do.
It sounds like frazer is confident it would work, but I would appreciate a ballpark estimate of what one of those 50% pass attenuation germanium reflection window would give me. Probably gonna get a window anyway because it looks cool and I want to see what germanium glass looks like in real life. I have a metallic sample and its interesting.
Now having it accurate would also be interesting for high temperature chemical processes, but in most cases that uses a kiln. However it might be useful for manual control of induction heating reaction vessels. -
Can you see a sinusoidal 100 Mhz signal with a 1 Mhz scope
This is why sensors for the low-temperature spectral range will never measure high-temperature reliable who has energy in a totally different range, Therma energy spectral density is a function of the temperature -
Coppercone2,
This Patent may be of interest as it details thermal imaging of arc welding with a LWIR camera plus ND filter. Filter detail is also provided They use different thicknesses of Polystyrene material so this could be an option.
https://patentimages.storage.googleapis.com/42/2e/e2/cecc1014536e08/US9307156.pdf
Fraser -
An interesting brochure from Stemmer Imaging.......
https://www.stemmer-imaging.com/media/uploads/cameras/avt/87/87762-Allied_Vision_SWIR_and_LWIR_Camera_Brochure.pdf
Fraser -
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. FLIR provides options for their LWIR cameras to measure up to 2000C.
https://www.flir.co.uk/products/t129254_high-temperature-measurement-option/
Imaging and measuring can be two very different challenges.
Fraser