The Science behind Thermography
Thermography has been used for many years in military, industrial and medical applications. Infrared Thermography uses sensors that detect Infrared Radiation as part of the electromagnetic spectrum. That spectrum includes visible light that humans can see with their eyes.
The infrared spectrum runs from .8 to 100 microns and is below visible light. Most IR devices used for telethermometry operate in the 8-14 micron (m) range. All of the Infrared Cameras and IR Thermometers today use uncooled microbolometers. They all have common feature and considerations for accuracy.
All Infrared detectors quantum detectors in a focal plan array to detect heat signatures. The larger the array, the better the resolution. The focal plan array is sized to provide micrometer detectors that give resolutions measured in pixels. An Infrared camera with a 320 x 240 resolution has 76,800-pixel detectors. A 640 x 480 camera has 307,200 pixels. An IR Thermometer may have an array of 10×10 with about 100 pixels.
Why this is important – The more pixels, the more temperature sensing points in the IR image. The more sensing points, the greater the accuracy.
Infrared detectors measure infrared radiation based on thermal emittance of the surface being scanned. A perfect thermal emitter is measured as 1.0. Human skin is measured as 0.98 and almost a perfect emitter. IR thermometers are preset at 0.98 emittance. IR cameras typically have an adjustable thermal emittance to measure infrared radiation on different surface. The lower the thermal emittance, the greater the chance of an inaccurate reading.
Why this is important – Different infrared equipment have different abilities to adjust the emittance of the target. IR thermometers cannot be adjusted. A industrial spot radiometer, such as those at one of the big box stores, may have a different preset emissive value that a medical device and this can affect accuracy.
Forehead screening is common for IR Thermometers. The FDA guidelines recommend oral targets (digital thermometer only) or tympanic membrane (ear drum) as targets. They do not specifically mention forehead for scanning locations. Most IR Thermometers use the forehead, and some can use the ear canal for readings. IR Cameras, which have increased resolution, use the nasolacrimal duct (eye tear ducts) as a measurement target. This was the recommendation from the SARS research 17 years ago.
Why this is important – Each IR device has strengths and weaknesses for reliability, speed and recordablity for long term data retention. If an IR camera is used as the primary screening tool, then a secondary screening with a digital thermometer is recommended.
The focal distance is important to get accurate temperatures. Lenses are used on all infrared equipment to give a Field of View (FOV) and Instantaneous Field of View (IFOV). IR Cameras use their larger arrays to give a larger Field of View to measure temperatures. IR Cameras come with different lenses and adjustable lenses so they can better utilize distance to measure temperatures.
An important consideration to improve accuracy with IR temperature detection is something called the cavity effect. When you have a concave surface (such as the inner canthus), the thermal emitted energy reflects off the other surfaces of the cavity and increase the temperature accuracy. The forehead is convex and while it makes a good radiator, the temperature reading can vary more for the body core temperature.
Why this is important – IR thermometers have a very short focal distance so they must be placed .5 to 2.5 inches away from the object being measured to be fairly accurate. That distance can vary with IR Thermometers. As distances increase, the spot size of the target decreases, but the pixels size increases. Increased pixel size means that the temperature reading of a particular spot (or target) will be averaged with temperatures from areas that surround the spot. The higher the pixel resolution, the potential for better temperature readings so long as the spot size is considered. The average size of the tear duct in the corner of the eyes is about 2-3 mm. That is a small spot size. It takes good resolution camera to be able to get an accurate reading from 3 feet away.
The process speed of the IR device is the speed that allows the Infrared image to be reset for accurate measurement. Some equipment has a slow speed such as 9 Hz so it takes longer to perform the scan. Faster processor speeds, such as 50 or 60 Hz would allow for faster imaging.
Why this is important – The faster the processor speed, the faster the scanning speed and quicker you can perform your EBT scanning.
Temperature sensitivity and drift (tendency to wander of the correct temperature calibrations) are other factors to be considered when selecting your infrared equipment. IR devices can have some drift in their temperature readings over time (so they have internal systems that check that drift to make adjustments and assure that the device is accurate).
It is a good idea to periodically check those measurements with an external device called a Black Body that creates specific temperatures that can be used to test the device accuracy. That would be included in part of your monthly maintenance contract.
Why this is important – The FDA recommendations suggest a ±9° F (± 0.5° C) sensitivity for your infrared equipment. IR Thermometers typically have a 0.5 – 0.9° F range of sensitivity. Infrared cameras can have up to a 0.1 ° F sensitivity for better accuracy and selection of better temperature targets for measurements
Price and process design are the last factor that you will want to consider. The current crisis has created a backlog of infrared equipment availability that can delay a permanent solution, so a temporary solution may have to be considered until the equipment is available to install in your facility.