Thermal camera / IR Thermometer simple DIY Calibration Check source by Fraser

[Fraser]

A little thermal calibration check source project for those who own thermal imaging cameras.

Some readers may have heard of a thermal camera calibration reference source, commonly called a Black Body calibration reference. For those who have not, let me explain what one is in simplified terms.:

Materials will emit thermal energy at differing efficiencies depending upon the thermal emissivity of their surface. That is to say, bright shiny surfaces are usually very low emissivity and dark dull surfaces are often high emissivity. Note that the colour of a coating on a surface does not necessarily indicate its emissivity as the actual chemical makeup of the coating is the significant factor at thermal wavelengths.

Emissivity is covered here:

https://en.wikipedia.org/wiki/Emissivity

Raytek provide details of various materials emissivity here:

Non metals:

http://www.raytek.com/Raytek/en-r0/IREducation/EmissivityNonMetals.htm

Metals:

http://www.raytek.com/Raytek/en-r0/IREducation/EmissivityTableMetals.htm

A Black Body Calibration source uses a coating on its radiation plate that is as close to perfect emissivity (1.0) as the manufacturer can provide. However, a perfect 1.0 Emissivity surface is very hard to produce so often a lesser, but known, Emissivity material is used on the radiator. 0.98 Emissivity is a common specification.

Knowing a materials emissivity is the important factor for any calibration checks. The radiator must have the same emissivity over its operating temperature range and it should not be subject to age related changes in emissivity. SOOT is an excellent coating that provides excellent emissivity. It is however a fragile coating that would easily degrade with time and moisture effects. Various high emissivity paints are produced for these Black Body radiators. They are stable and predictable.

On a thermal camera, there is usually an Emissivity setting that may be changed by the user in line with the target object that is being monitored. If a surface is known to be rubber, such as a car tyre, the emissivity should be set at 0.95 for accurate temperature measurement. The camera then calculates the true temperature of the rubber using the 0.95 (or 95%) Emissivity value (that which the camera 'see's Vs true surface temperature.)

That is enough about Emissivity for now. I only explained it in simple terms but such knowledge is necessary for what follows.


Black Body calibration sources or calibration check sources (there is a difference!) are specialist equipment and so are produced in small quantities. What this means for hobbyists is that they are very expensive and beyond many hobby budgets. It would also be true to say that they do not offer value for money to many as they are infrequent use items, unlike the thermal camera.

I own several professional Black body calibration check sources that I use when servicing and repairing thermal cameras. They were not cheap but fortunately affordable for me via the second hand market place. I had to import one unit from the USA as they are quite rare on the secondary market. A reasonable quality Black Body check source is around £800 to £1000 in the UK and they are not anything wonderful inside the case either ! Basically a PID driving a heated plate with a fan blowing over it for cooling. The clever part is usually the radiating plate that the camera looks at.

As already stated, its all about the emissivity of the surface and its predictable nature over a range of temperatures. This is not rocket science though and many budget Black Body calibration check sources just use a decent high emissivity matt black paint. A good emissivity paint can be found through experimentation but I bought a can of VHT high temperature paint for use on stoves, car exhaust manifolds etc. It should have decent emissivity. I may, or may not need it for this little project.

OK, to the project, what am I presenting here ?

Well to be clear, I have not built this unit yet and so have no test data for it. I am presenting it as a cheap and simple project that some may wish to make. It is a very cheap and easy to build Ambient Temperature Black Body Calibration Check Source.
Yep, I said Ambient temperature. This is a novel design that is passive and does not require a heater, cooler or fan !  Such a Black Body is available commercially at high cost. There is very little to it though. Basically a high emissivity metal plate to which an accurate thermometer is attached to measure the surface temperature. The metal plate has some mass and collects the Ambient heat energy and radiates it efficiently via the painted surface.

Such a check source is available from WAHL for $584   

http://www.palmerwahl.com/product_home.php?itm=5014

Granted its thermometer may be very accurate, but you can get accurate thermometers quite cheaply these days. The Black body radiator looks impressive with its annular rings, but such are not essential.

Why use this check source ? Well firstly, it is a very good idea to check your non contact thermal measurement equipment to ensure that it is within normal operating specification and not in some way damaged or compromised. A loss of accuracy will be noticed if regular tests are carried out and logged. It is also a useful ambient temperature reference to have on a job so that the reflected temperature can be established. The fact that the unit uses the ambient temperature means that there are no control loops to fail or need for calibration. In this case, simplicity is best.

To make an Ambient Temperature Black Body calibration check source you will need the following parts:

1. A black painted Aluminium project case of reasonable proportions to act as the Radiator and housing for the thermometer module.
2. An accurate battery powered LCD thermometer module with external probe sensor.
3. Possibly some VHT or similar Black paint, depending upon the paint used on the project case by the OEM.
4. Rubber feet for the project case to isolate it from the surface on which it sits..
5. Tools to cut a hole for the thermometer module in one end plate of the project case.
6. A known accurate thermometer against which to check the thermometer module.


To build the Check Source. Cut a rectangular hole in one of the project case end plates and insert the thermometer into it. The thermometer will latch into place using its integral claws. Attach the external sensor probe to the front face of the project case that will face the camera when in use. I recommend that the sensor is positioned in the middle of the panel. It may be held in place with glue or a lump of high density foam pressing against it and the rear pane. God thermal contact with the front panel metal is preferable. The thermometer module is self powered via two LR44 cells and its end plate my be attached to the Front panel using the provided screws. I suggest orientating it so that the display may be read when stood in front of the unit looking down on its thermometer display. The rear panel may then be fitted, followed by the bottom end plate. Affix some rubber feet to the bottom of the unit to thermally isolate it from the surface on which it will be sitting.

That is it, nice and simple. Battery changing is carried out by unscrewing the thermometer end plate so leave enough slack probe cable to permit this.

The next step is to check the accuracy of the thermometer module against one of known good accuracy. We are using a cheap Chinese Thermometer module so quality can vary. I bought 5 and all read within 0.1C of eachother and are accurate to plus or minus 1C in their specifications. Mine are well within that specification when checked against a known accurate thermometer.

Now to the emissivity question......

What is the emissivity of the paint that has been applied to the project case ?
In truth, I have no idea yet !  What I do know is that if it is not good enough, I will paint the front panel with VHT paint and test again.

To discover the Emissivity of a surface I suggest the following approach.

1. Point the IR thermometer and/or thermal camera at the project case front panel ensuring that the minimum focus distance is respected.

2. Do NOT stand behind the camera as your own body heat MAY reflect off of the project case surface. Some paint finishes can act like a mirror at thermal wavelengths.

3. Read the Ambient temperature on the Check source top and note it down somewhere.

4. Adjust the Emissivity setting on the IR thermometer or thermal camera until the measured temperature is the same as that displayed on the check source. You will then have effectively established the emissivity of the check source radiating surface.

5. If the emissivity figure of the check source surface is poor, say 0.75, it would be worth the effort to paint the radiating surface with a higher emissivity matt black paint. Various paints may be used as already stated. Even Humbrol matt black enamel is pretty good emissivity. I personally would want an emissivity of 0.96 or higher for my check source.

6. If you paint the check source, retest its emissivity.

7. Once the emissivity is known and satisfactory, produce a label that states the Emissivity of the radiator surface so tat IR Thermometers and Thermal Cameras may be configured correctly. Some IR thermometers have the emissivity locked to 0.96 as that is common in the real world. This is not a significant problem if the check source has the same or slightly better emissivity. The reading error will not be great at ambient temperatures.

Well that is it. The Ambient Temperature Calibration Check source mat now be used.

Fraser

Component sources I used:

Thermometer module (I bought 5 for tests)

http://www.ebay.co.uk/itm/361569717305?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Project case. Nice solid quality item.

http://www.ebay.co.uk/itm/122087616108?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[Fraser]

A better picture of the $584 Wahl Ambient Temperature Black Body Check source.

My BoM comes to approx £13.00 without VHT paint but including two LR44 cells and four rubber feet.

The VHT paint costs £7.95 but is useful for many thermal imaging related tasks.

Fraser

[frenky]

Hmm would it not be better to have temperature sensor inside of the box? Just wondering... 

[Fraser]

I did not make it clear. The sensor does sit inside the case, but is in contact with the back face of the front panel.

Fraser

[mzzj]

Nice, but I have to complain that IR-termometer errors are quite often "gain"-related.

BUT. There is 2 cheap, accurate and reasonably easy methods to check your thermometer:

1. Ice bath, preferably made of snow, ice shawings or finely crushed ice.
Hollow ice cube will do in a pinch for thermal imagers, it has already emissivity enhancing cavity! 

2. hacked old 4L /1gal pot. This gets more complicated since you have to drill 40 to 50mm hole on the side of the pot and sodder/weld/glue ~200mm long capped tube to that hole.
Paint inside of the tube with black exhaust header paint. D=40mm L=200mm cavity will enhance your header paint effective emissivity to around 0.995 or so.
Fill with water and bring to boil, compensate for altitude/atmospheric pressure and you have 100.0Cel reference point.

[Fraser]

Hi mzzj,

Thanks for the comments.

I believe these Ambient temperature calibration check sources are really designed to be a quick and portable product that allows the user of a thermal camera to confirm that their camera measurement function tallies with a known surface temperature with known emissivity. It is about as simple an arrangement as can be made for such a task and thankfully its simplicity makes it pretty reliable as a check source. It is a single temperature device in that it is passive so the radiated temperature will be quite a narrow range unless you live in Africa or the Arctic that is 

It should be noted that this simple passive design is not suitable for calibration of a thermal camera. Such calibration requires active Black Body sources with selectable accurate temperature output. This simple unit is just a confidence check that also informs the user of teh ambient temperature so that it may be entered into the camera if desired.

Errors in measurements at temperatures far away from ambient cannot be detected with such a simple passive radiator.

Fraser

[-jeffB]

Very nice write-up, as usual!

One question: you mention "ensuring that the minimum focus distance is respected". It's not clear to me how this matters if your goal is to present a uniform surface -- in fact, "blurring the image" would if anything make it more uniform at small scales.

I used this technique (stop down lens, set to closest possible focus, point at sky) to look for debris on the sensor of DSLR cameras. The sky provided a pretty uniform field, but throwing it out of focus ensured that passing planes or birds wouldn't confuse the issue. It seems to me that throwing a thermal camera out of focus would tend to reduce the effect of small nonuniformities on the test surface. Your thoughts?

[Fraser]

Hi Jeffb,

Sorry to cause confusion.

I was referring to the focus characteristics of some IR thermometers such as those from Raytek. I own and use Raytek GP,TX and Mi Industrial IR thermometers and incorrect distance can lead to the sensor seeing beyond the intended target area and so possibly introducing errors. For common IR thermometers the same issue must be considered as their FOV (D:S) can be quite large.

Please see attached documents for detail of my Raytek IR Thermometer heads. Note how wide the FOV becomes with the GP head at a certain distance from the head.

Fraser

[frenky]

Fraser do think that box with grooves is better than one without?
(I will coat it with matte black high temp paint.)

Perhaps this groves prevent thermal reflections from sorrounding objects?

[Fraser]

Grooves are an enhancement to reduce reflection issues, if such exist.

I would have bought the box you have referenced had I seen it. I have yet to build the unit so may get that case instead. Where is it sold please ?

The standard, as supplied, paint may be OK in this application. Just test it to see.

Fraser

[frenky]

Tnx.
Link to the box:  http://s.aliexpress.com/qENnYnmI

[Bill W]

Some thoughts.

One classical thermodynamics black body is the cavity.  It does not need to be emissivity 1 inside as the multiple reflections ensure that the radiation is indeed 'black' regardless. There are cavity black bodies that are white ceramic http://www.omega.co.uk/pptst/BB-4A.html

I would therefore cut a hole in the box to look into it, but probably still paint it thermally black (got a couple of cans of very good paint, but the brand escapes me now, but it came off eBay).

You can usually tell how good surface emissivity is by trying to see the reflection of a soldering iron or even strip lights.  I did this to compare some heat sink blocks to use as thermal targets.  As Fraser says just because it looks black does not mean 99.9% emissivity. 

When measuring anything with low emissivty rarely works when people simply to dial in the emissivity and expect it to give the right reading.  This will never work as the instrument has to make assumptions.  Say you dial in 60% emissivity, what you are saying is that the other 40% of the apparent temperature of the object is due to reflections.  How is the instrument ever going to know the average temperature of the random reflection of the world you have just asked it to measure ?  It won't and simply substitutes the local ambient temperature of the instrument.


Bill

[Fraser]

Thanks for the comments Bill.

As further comment on this simple design......

I did not go for a very simple cavity as I thought it would need to be appropriately dimensioned and engineered. Most of my Black Body sources are flat plate radiators, though one does have 3" long tube attached to it, and another has a cowled around the plate. The Cavity type Black Bodies I have are all small diameter and the length dimension is much larger than the diameter, plus they have a cone shaped end to the cavity to prevent on-axis reflections. The cavity just seemed a bit too complex for a simple Ambient temperature source. Now that Bill has mentioned it, maybe cutting a hole in the front face and viewing the inside of the rear panel through that hole would be a better idea as it may reduce off-axis reflections. Hmm food for thought and experimentation. You can actually make up test boxes from cardboard. A cereal box may come to mind but they are normally reflective on the outside due to the glossy printing technique.

Thermal reflections can be a real PITA. Common contamination sources of a temperature measurement with a thermal camera are the users own body heat reflecting off of a surface that is within the measurement area of the camera, and the camera seeing its own reflection. Why does a camera seeing its own reflection matter ? Well the camera heats or cools it sensor array, depending upon design. As such it can see its own sensor temperature through its lens. This is on-axis with the camera view and so often falls within the measurement area of the camera. In a worst case scenarios, the camera will be looking at a surface that is, say 12C, yet will measure it as 32C as it is measuring the reflection of its own microbolometer temperature. In truth the reflection will not be perfect and a lower temperature will actually be measured...... still incorrect though.

On the topic of reflected temperature calculations within the camera. Most professional cameras have a reflected temperature value that may be set by the user. They also have a humidity and distance setting that helps the camera provide the most accurate reading possible within its limitations. Reflected temperatures can be a cause of measurement errors and this needs to be considered, especially when working on a PCB that has many very reflective surfaces. The solder mask, solder joints and component coatings to name just three ! Normally the temperatures being measured are high enough to not be too badly affected by ambient temperature reflections though. Sources of thermal contamination when working on a PCB are many. Ones that come to mind are......users own 37c body heat, incandescent light bulbs, sunshine through windows, soldering iron running in proximity to measurement area.

Remember, you can also have cold reflections ! Yes that is right, a Cooled thermal camera when aimed at a mirror, can see its own very cold sensor array and it appears very dark indeed. Such would ruin any close-up measurements on a surface that is reflective.

How do you solve this kind of thermal contamination problem ? First try to remove the thermal contamination sources from the equation. Remember that switching a lamp or soldering iron off does not remove the heat source until it has cooled to ambient temperature. Keep your body away from the test area if possible.... you are a large thermal radiator that cannot easily be cooled ! For really important measurements where accuracy is needed, the DUT needs to be modified to reduce any reflection issues and improve emissivity. To do this the DUT needs to be coated in a high emissivity, low reflection paint.  Water based and soluble paint would be ideal as it may be removed after testing in many cases.

I have high accuracy IR thermometers designed for small area measurements of around 10mm. These thermometers came with high emissivity tape and a black wax crayon. The surface to be measured is prepared using the tape or crayon as these improve emissivity and reduce reflections. I have not experimented with kids crayons though.

That is enough from me for now, but I hope readers can see how complex thermal camera temperature measurement can be. This is one of the reasons why a thermal camera is NOT always the best temperature measurement tool to use. They can be inaccurate for many reasons. It is also why an owner should not fret over the plus minus 2C specification. It does not sound like a good accuracy but there are so many variables involved that it is realistic and likely best case scenario ! Get the emissivity wrong and the accuracy can be severely impacted.

For really accurate surface temperature measurement, a contact probe technology is preferable, if feasible. I have a Fluke Hydra data bucket that can deploy and monitor many thermocouples within an equipment for more accurate thermal profiling than a thermal cameras. PT100 sensors are a good choice for accuracy.

Fraser

[frenky]

I have ordered about 40 DS18B20 sonsors from different stores so that I shall get different batches...
In the folowing weeks I'll be selecting the most precise ones from the crowd and use them for this passive calibration box. 

[joe-c]

...Say you dial in 60% emissivity, what you are saying is that the other 40% of the apparent temperature of the object is due to reflections...

that's why i see no sense in set the emissivity to lower than 0.8... normally i have it to 0.95 and don't change it.
if i see to an object with lower or higher emissivity i just round a little bit.
i mean... alu has some about <0.1 and the most cameras can set this value, but its just senseless to measure that...

some years ago i made some images...
Your eyes see: http://joe-c.de/media/thermo/fusion/vis_sonst2.jpg
Your Camera measure: http://joe-c.de/media/thermo/fusion/ir_sonst2.jpg

The German Wikipedia has a nice example too:
https://de.wikipedia.org/wiki/Emissionsgrad#/media/File:LesliesCube.png

for measurements on higher temperatures i suggest zinc spray... the data-sheet say it can go up to 600°C and Hi Temp spray to 1200°C
spray this on a flat iron and you have a low budged thermal radiator, the heat distribution depends on the thickness of the plate.

[Bill W]

This is the paint that I referred to above:

Krylon K04290000 Camouflage Black

Bill

[Fraser]

@Bill W,

Many thanks for the paint details Bill. I have read good reviews on Krylon paint for thermal coatings. I shall go look for some now.

There are 3M paints designed for thermal radiators but I could not find any retailers for any of the paints. The astronomy crowd also have a need for such paints as they are usually excellent light absorbers for the interior of optical blocks.

Fraser

[Fraser]

Best price in the UK including postage is from the following site:

http://www.patrolbase.co.uk/krylon-spray-paint/KrylonBlackSprayPaint#.V_u__PkrLIU

I purchased two can to make the postage more economic as there is no additional charge for the extra can. £18.93 inc postage for two cans.

On ebay there is a seller offering three cans for £29 but it is cheaper from Patrolbase.

I can see why this ultra low reflectivity paint is good for thermal black body applications. Great stuff.

Fraser