As some will already be aware, I collect and repair thermal imaging cameras. An important part of testing a thermal camera is a check of its accuracy. This can be done in a casual usage scenario using a container of boiling water and another of iced water. Alternatively a container of water can be heated and monitored using a decent digital thermometer. Such accuracy tests rely on setting the emissivity correctly on the camera. Such can present a challenge if the emissivity of the target container is not known or the surface is unsuitable for such a test (shiny metal)
Most of us in the world of thermography have seen the shiny metal versus matt black painted bean can demonstration. Emissivity setting accuracy is key to accurate measurements or assessment of a camera's accuracy.
So what do professionals and labs use to check the accuracy of their equipment ?
Well firstly they have their cameras calibration checked every year. Note that I said checked and not calibrated. The reason is that most decent industrial & scientific thermal cameras hold their calibration well and do not need any changes to the calibration tables. The calibration check just confirms the accuracy of the camera is still within tolerance.
To ensure that a specific camera is still accurate before an important set of measurements are taken, it is common to have a thermal check source against which to do a quick confidence check. Depending upon the check source quality, they may also be used for annual checking of a camera's accuracy. The Check source is always more accurate in temperature than the +-2 Degrees C commonly found in thermal cameras.
The thermal check source is called a Black Body source. This is because it contains a heated plate or blind tube that is coated in special high emissivity black paint. This is not your common matt black enamel being used here; it is paint with greater than 0.98 emissivity and very close to 1.00. Most importantly it is a known emissivity that remains constant across a large temperature range. It is the predictability of the emissivity that is so important in such a source. As such the thermal plate or cavity presents the camera with a known emissivity against which it may be set in the emissivity menu.
I own three Black Body thermal sources against which to check my cameras. One is an original AGA/AGEMA large aperture source capable of temperatures from ambient to in excess of 200C. Another is a modern commercial flat plate Black Body that can deliver temperatures between 20C and 45C as it uses a Peltier element rather than a conventional heater. Finally, my latest acquisition, a Welch Allyn/Diatek M9600 IR thermometer and contact thermometer calibrator.
The M9600 is the topic of this thread so I will not go into further detail on the other two black body thermal sources here. I did cover the other black body sources in previous posts some time ago. Just search on “Black Body”. Do not do that search on Google unless you accept that you will get some pornographic hits !
The M9600 calibration check Black Body was designed for the medical industry as a confidence check tool. It can check the calibration of both non-contact and contact thermometers.
The unit contains a black body blind tube that is coated with the specialist paint already mentioned above. The IR thermometer is placed in front of the rube aperture, or even inside it, and the measured temperature is checked. For contact thermometers a hole is provided in the side wall of the black body chamber. The thermometer probe is inserted into the hole and allowed to warm up before checking its accuracy against the black body source. This probe tube is also used when checking the calibration of the black body source using a high accuracy thermometer probe. Whatever the probe reads will be the temperature in the black body cavity. The M9600 provides two temperatures for calibration checking. These are 36 Degrees C and 41 Degrees C. Not surprising considering it is designed for medical thermometer use.
The good news is that this check source is also usable for checking thermal camera measurement accuracy. We know that the M9600 contains a black body cavity so we can read the cavity temperature with the thermal camera. I should say at this point that this is not the optimum method of checking a thermal camera as you usually want the Black Body source to completely fill the cameras FOV. For my purposes however, the use of the small cavity source is fine. The M9600 also provides me with a probe thermometer check source which will be handy as well.
When viewed from the outside, this instrument is anything but impressive. It looks dowdy, simple and not very professional in its Vero Box ! Looks can be deceptive though. Hence why I bought it for only GBP50. These units cost closer to GBP1000 when new.
Once the top cover is removed we are presented with a small PCB onto which is mounted the Black Body cavity. The Black Body cavity is made from metal and is heated from the bottom by a Power MOSFET. The temperature is monitored by two thermal sensors (type not yet known).The control circuit for the Power MOSFET is all analogue with no microcontroller to be seen. The accurate setting of the two available temperatures is accomplished via multi turn potentiometers, of which there are four. Power and READY LEDS tell the user what is happening in the M9600. Nice and simple but done with high quality components as can be seen in the images.
The major components used are listed below:
IRF510 Power MOSFET
VN10K MOSFET
AD708JN high accuracy op-amp
LM10CN voltage reference and amplifiers
2N4124 transistor
2N4126 transistor
Temperature Sensors x2
Why the high cost for such a simple design ? Well when buying a Black Body you are paying for a relatively small production run device that uses high quality components and is designed to deliver high accuracy during its operational life. These are also medical related calibration checks and they always cost more !
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