First of all, what is a Black Body?
While it’s quite easy to understand that a fever screening system actually detects elevated body temperature or, fever, the term black body is somewhat rare to most people. A black body is an important part of a more comprehensive fever screening system, It consists of a heated object with a predetermined temperature value, for example, 37.3 Celcius, and is the basis for accurate calibration of infrared thermography devices. A blackbody is an ideal radiator that emits thermal energy (visible and infrared energy) that follows Planck’s Law of radiation that correlates the energy intensity for a given wavelength and temperature. It completely absorbs and emits all radiation, extremely non-reflective, and has an emissivity value of 1.
These blackbodies consist of a temperature-controlled cylindrical thermal cavity which has a small aperture at one end. The cavity surfaces are covered with pigmented paint which gives a very high emissivity of 0.995 at the aperture. Because there no material exists that could make a true perfect blackbody, in practice a blackbody calibration source always has a slightly lower emissivity than 1, however, by careful design, it is possible to get very close. The emissivity setting of the sensor should be adjusted to match the emissivity of the blackbody, and the sensor should be mounted close enough that the measured spot diameter is no larger than half the aperture diameter.
Why should we use a Black Body?
In order to achieve the best and accurate Fever Screening results over a long period of time, a black body must be introduced in the fever screening setup. Unlike thermal measurement in engineering or industrial purpose, human body measurement (for fever detection) require less than ±0.5C in order to be useful. This requirement is actually stated in the IEC 80601-2-59 standard, “Medical electrical equipment -Part 2-59: Particular requirements for the basic safety and essential performance of screening thermographs for human febrile temperature screening” which is a critical component as an infrared radiation reference source. Without it, it’s almost impossible to achieve lower than ±0.5C, especially in 3-6 meters measurement range.
If such accuracy cannot be achieved, the practice is not more than taking visual thermal images with no purpose in detecting mild or high fever, an important symptom of Covid19 infection. Most industrial thermal imager, even though has much higher resolution (because the targeted application requirement) has only ±2C. The black body can work as the temperature calibrator and it can absorb all the electromagnetic radiation and no reflection of them, thus it can be used for the calibration on the IR camera. Through over observation, there are misleading information and practices that use top branded thermal imager designed for industrial use, but adopted at a fever detection system. Although the industrial infrared imager may have higher IR resolutions, generally does not has low accuracy and not integrated with black body features.
How does it work?
By comparing the preset temperature on the black body with the temperature measured by the IR thermal camera, the environment inference on the IR camera can be lowered and the accuracy of the IR camera will be improved. The larger the thermal capacity of the black body is, the effective radiation is bigger, then the temperature will be more easily to be stable in a different environment and thus the thermal balance can be kept. A connection is established between the IR camera and the black body, which will be instructed in the software part. The black body has to be switched on 20-30 minutes before starting the temperature measurement. The black body has to be mounted on tripod or ceiling mounted, close to the area which will be measured and it cannot be blocked by people who will be measured. Thus, the black body is suggested to be displayed on the left or right part of the IR camera view.