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

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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  :scared: 

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...  :P

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.

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