EEVblog #622 - How To See-through Objects With A Thermal Camera

[Taucher]

Find a large antistatic bag and cut it. IR will go through it like if it was transparent instead of dark silver/grey.
An old large xray totally exposed film should be transparent to IR as well.

Or even better, run the unit with the stock cover, open it and take a quick snapshot with the FLIR.  Then you have all the time in the world to analyze it with the FLIR software. Air flow and heat dissipation will be more realistic.

That would need to be done really quick as thermal equilibrium is astonishingly fast-changing and the extra air-movement would cool in addition - but I guess it could be solved with the PE wrapping below the case to be taken off (perfectionist solution) *G*

[Fraser]

For info, FLIR produce 'cheaper' disposable lens protectors that use Polyolefin heat shrink plastic as the window. FLIR don't advertise the material used as they still charge serious money for these lens protectors ! I detailed the material in my E4 Useful information thread in November 2013:

https://www.eevblog.com/forum/reviews/flir-e4-the-useful-information-thread/msg332125/#msg332125

The Polyolefin plastic is chosen because of its excellent transmision characteristics, and the good news is that it is easily available cheaply as detailed in my posting.
 
The material is more controllable and robust than thinner 'cling' type plastic films and may be stretched over a frame and shrunk tight with a hair dryer.

If working with equipment that could suffer catastrophic capacitor failure it would also be a good idea to protect the camera face/lens from flying fluids with such material.

Aurora

[nitro2k01]

The followup video is up!

[zapta]

Dave, can you compensate for the reduced temperature reading through the plastic wrap by changing the emission setting of the FLIR?

[Taucher]

Dave, can you compensate for the reduced temperature reading through the plastic wrap by changing the emission setting of the FLIR?

technically it is possible to change the emissivity setting inside the camera to adjust for errors.

[speedyant]

I know the temperature of the "thing" is 20 degree, try all the "clean wrap" and see what happen, make a kind of calibration.

[Taucher]

I know the temperature of the "thing" is 20 degree, try all the "clean wrap" and see what happen, make a kind of calibration.

actually the real method should be very similar:
- take reference measurement at thermally stable surface/object body with thermocouple
- measure the value with the thermal camera
- calculate correction-factor
- change correction factor in camera according to match the real world - keep in mind the already set up factor in camera
- iterate or fiddle around with value in case it does not match until happy with result
- re-verify that both measurements are equal (crosscheck both in case of deviations over time)

[miguelvp]

So, what happens if you blow hot air on the antistatic bag, will it show airflow?
Seems to retain the heat long enough for that.

[Fraser]

For those interested in IR transparent materials there is plenty of information on the NET detailing how different materials behave at the various wavelengths used in thermal imaging. I attach a simple table of materials that may be used in lenses and a link to another version that clearly shows the three different wavelengths used in the industry, Short, Medium and Long wave. The E4 is a long Wave thermal camera. Some material, such as silicon are not well suited to Long wave lens structures yet work fine at shortwave 

http://www.infraredtraininginstitute.com/infrared-transparent-materials/

There are similar tables for thin plastic but I have not tracked them down yet. I will add them if/when I do.

Dave's experiments have been good at demonstrating how materials behave in these areas of the EM spectrum.

Something that is also worthy of experimentation is emissivity. This can have a really dramatic effect on any temperature readings. As an example, a nice shiny aluminium heat sink can be running at +80 Degrees Celcius yet the thermal camera can misread it due to emissivity error and show it at ambient ! The emissivity setting in the camera should be set to the value of the target item to achieve the most accurate measurement. Life is never that simple though....how do you know the true emissivity of a target surface ? That's a whole other story that I will not go into here. Some people may think that to solve the emissivity problem, just paint the target matt black..... well they are on the right path but readers should be aware that where paint in concerned, colour often makes no difference at thermal wavelengths in terms of emissivity. Silver paint and paint loaded with metals may behave differently (worse) to a nice matt paint but basically you will still not have a perfect emissivity in either case. To achieve 'Black body' levels of emissivity (1.00) a very special paint coating is needed that radiates the heat far more efficiently than any generic paint. It may also be interesting to know that the soot from a candle is a very good coating where good emissivity is required. Also a common material used as a quick and dirty emissivity enhancer is everyday standard PVC insulation tape. Colour is NOT critical  A shiny metal surface with low emissivity may be easily measured with a TIC once a piece of PVC insulation tape is applied. From memory the emissivity of PVC insulation tape is 0.96 to 0.97.

Readers should also be aware of the issues created by reflections. If a thermal camera is brought into close proximity of an object that has reflective thermal properties, it is possible to actually skew the measurements as the camera actually starts to see itself reflected in the target and the E4 has a core running at around +30 degrees C that emits through the lens and may be reflected back at the camera. To see an extreme case of this, point the E4 at a mirror or glass and you will see a selfie  The lens will look warmer than the case....you are seeing the reflected core temperature.
Cryo-cooled cameras project a cold image from the 70K core through the lens that has the opposite effect on readings. These effects are most apparent when working in a close-up configuration with variable target materials. There are plenty of traps for the newbie thermographer 

All this is drifting off topic I know, but I just wanted to highlight that there are some very interesting experiments that may be carried out using a thermal camera. Hours of fun on a wet day if you are bored 

Oh and nit picker time..... the Ex series cameras do not use a Germanium lens as stated in the videos. Chalcogenide glass is the material used in these and many other 'budget' thermal cameras as it is far cheaper than Germanium in terms of production costs. Germanium crystals are cut to shape with a single diamond tool, Chalcogenide glass may be moulded.

http://www.lightpath.com/infrared-optics/thermal-imaging-assemblies.html

[EEVblog]

Dave, can you compensate for the reduced temperature reading through the plastic wrap by changing the emission setting of the FLIR?

Yes, there is a setting for emissivity if you want to experiment and find the correct value for a particular surface.

[babysitter]

I have a nice benchtop experiment that I showed our trainee to tell him about transparency and opacity outside the visible range - namely IR.

A GaAs Wafer in its plastic holder we got as a present, he had to irradiate it with a TV remote control from one side and look at the other side with his smartphone camera. Could have told him to look at the case of reflection too..Actually, different LEDs and heating/cooling might show the effect of its temperature-dependent optical properties of GaAs...

A Thermal imager is a instrument like any other, one needs to understand it to use it.

[Fraser]

More trivia

I have experimented with GaAs lenses and windows at Long Wave and they do perform OK. What is interesting though is the issue of anti-reflective coatings. Many materials including Germanium (yes Germanium) have poor transmission figures due to reflection off of the first surface interface. Special anti-reflective coatings are needed to achieve acceptable performance. This is why it is VERY important to treat a thermal camera lens with great care. Removing the anti-reflective coating can destroy lens performance and so ruin a cameras calibration. Anti-Reflective coatings can also be wavelength specific so a lens designed for Short Wave use may perform very poorly at Long Wave and so will be unsuitable fro use with the E4.

Aurora

[Wim_L]

Indeed. The reflection is caused by the very large refractive index difference at the interface between air and the lens. Optical glass has a fairly modest refractive index contrast, its refractive index is around 1.5 (versus about 1 for air or vacuum). Semiconductor lenses have a pretty high refractive index in the infrared, about 3 to 5. This can be beneficial sometimes (it allows for making lenses with a short focal length with less curvature than would be needed in a glass lens, so thinner and lighter lenses are possible), but does create enormous reflection losses if the lens isn't coated properly for the wavelengths it's used for.

[DJ]

Just watched Dave's update video, clears up all our questions nicely

As for lenses, the single element mods are nice for closeups,  wondering about options for telephoto - something along these lines but less expensive



http://www.nightoptics.com/no/product/TO-3X-AF.htm


eta: that 3x lens is ~$950usd on Amazon.com

[Fraser]

@DJ

Sadly the construction of a telescope for the E4 requires lenses that are not available cheaply. The close-up lenses are easy as CO2 lasers use them in cutting applications. For a telescope a larger lens diameter would likely be needed and longer focal lengths. I have found neither at reasonable cost.

I ended up buying used telescopes when the price was right (rare these days) I paid $150 each for my X3 Inframetrics telescopes and $400 for a superb quality FLIR X2 compound telescope. That was the only way I could find of getting a telescope with decent performance. If anyone knows where larger diameter and focal length lenses are available at reasonable cost, I would love to know. The X2 FLIR telescope uses only 2 Meniscus lenses but they need to have the right focal length for the application and Germanium is used due to its higher refractive index (greater bending of the EM beam) producing a compact design. A chunk of Germanium lens like that use in the FLIR telescope costs serious money 

I will take a quick picture of the Telescopes cross sectional diagram (supplied by FLIR) in case it is of interest.

I have also considered the construction of a reflector telescope but finding a surface mirrored concave mirror for the task has proved challenging.

Aurora

Update:

I have attached the cross sectional views of the FLIR PM series X0.5 and X2 auxiliary lenses. My Inframetics X3 telescopes contain four lens elements but are inverting and may not actually need all four lenses in our application (still under investigation)

[Zucca]

On the side... Thanks Dave to give me the idea on how to improve the calibration procedure on my Keithley 220:



Those unit needs to be on at least for one hour to get to the proper work temperature, only then you should start to calibrate by tuning the pots on the boards. The problem is to reach those ones you have to remove the cover... and bye bye heat.

[DJ]

I would think that some small access holes in the wrap would retain most of the heat whilst permitting access to the pots. Is the factory case solid (ventless)? A temporary IR wrap should mimic the original enclosure physically.  Note also a metal enclosure will sink and reradiate heat. Suppose one might look at the k-factor of thin plastic films vs whatever the original material was.

[Zucca]

I would think that some small access holes in the wrap would retain most of the heat whilst permitting access to the pots. Is the factory case solid (ventless)?

Yes you got it, little holes for the Wiha screw driver. The factory case is solid with no metal, just a piece of solid plastic.

[MindBender]

Cling wrap: Bloody brilliant Dave! As a software engineer, I'm not really involved in thermal design, but I have been using a Flir A40 camera for years during board bring-up, allowing me to cut the power before a precious prototype burns up at first power up.

However, if you want to see the airflow through your product to verify the thermal design, use your cling wrap setup as you did and spray a whiff of freeze spray at the fan inlet. Depending on how big of a whiff you're spraying, you should be able to see a black cloud traveling through the device.

[Crossphased]

I was just watching this video, and was wondering-

Why aren't Dave's hand and arm showing up as hot in the thermal pictures? His hand shows up as cool blue, when it should be hotter than the ~75 degree IC that was being measured. What was happening?

[IanB]

Why aren't Dave's hand and arm showing up as hot in the thermal pictures? His hand shows up as cool blue, when it should be hotter than the ~75 degree IC that was being measured. What was happening?

Degrees Celsius (shown as °C on the display).

If Dave's arm was hotter than 75°C he would be dead. The actual temperature is somewhere between 30 and 35°C.

[Brumby]

75ºC is 167ºF

Just sayin.....