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Pharmaceutical & medical


COVID-19 fever screening


COVID-19 has created a worldwide demand for infrared cameras that are able to screen humans for a fever. But how can you be sure the camera you choose can actually do the job it is supposed to do, asks Micro-Epsilon’s Glenn Wedgbrow


that are able to screen for a fever condition in humans. Many businesses have, or are considering, the installation of appropriate fever screening systems to protect their most important assets, their people. Now that staff and employees of some companies are allowed to return to work after protective measures have been lifted, it is vital for these companies to ensure that the health of returning workers is not put at risk by workers who may have become infected with a virus outside the company. A key part of any automated fever screening system is an infrared (thermal imaging) camera. This should be easy to integrate into a fever screening system, which can then be used in real time at point-of-entry into establishments such as factories, offices, public buildings, warehouses, supermarkets, airports and schools, to prevent people with elevated body temperatures entering. The basis for the effectiveness of thermal imaging cameras as a fever screening tool lies in the correlation of the outside skin surface temperature


V


iral epidemics such as the current coronavirus (COVID-19) have created a global demand for infrared cameras


with the internal or core body temperature. There are two main approaches to fever screening: crowd-based and individual. In crowd-based fever screening, the IR camera monitors a crowd of people at once or sequentially. Assuming that the majority of the measured maximum head temperature values are coming from healthy individuals, the exceptions with an elevated body temperature can be detected. In individual fever screening, which is primarily used at security gates and controlled entrances, the IR camera is used to measure an individual person, one at a time. The Medial Canthus (tear duct) provides the strongest correlation between outside skin temperature and core body temperature and is measured more precisely from a close distance. The tear duct is located in both eyes in the corner next to the nose, where the upper and lower eyelids meet. This “hot spot” is ideally suited as a measurement point. This measuring range has a diameter of approx. 3-4mm. With numerous IR camera suppliers out there to choose from, it is important to select a camera that is fit-for-purpose, reliable and accurate for your specific application. Most IR cameras with a spectral response of 8- 14 μm are specified with an accuracy of ± 2°C or two per cent of the measurement reading, whichever is greater based on deployments in industrial environments in a wide variety of ambient conditions from 0°C to +50°C. Many IR cameras are being promoted today with accuracies of ± 0.5°C or better. However, these accuracies cannot be achieved without the use of a black body reference source. The source needs to be stable, have high emissivity and should be positioned in close proximity to the person to be scanned (normally, the black body is wall-mounted and placed directly behind the person to be scanned). By


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doing this, camera uncertainties resulting from device adjustment, ambient temperature drift and short term stability can be reduced and a system accuracy of ± 0.5°C (with a confidence interval of 95 per cent or better) can be achieved. The real ability of an IR camera-based screening system to detect humans with fever lies more in its ability to discern which external face temperatures, as measured at the tear duct, are significantly higher than those in a given population exposed to similar ambient temperature conditions. In order to achieve the best possible measurement results, the following factors should be considered before deployment of the IR camera:


1. Use a camera with a spatial resolution of 382 x 288 pixels or better and with a NETD of 80 mK or better.


2. Combine the camera with an external black body reference source. The accuracy of the measurement can be increased to ± 0.5°C.


3. Select the right optics. Check the Field of View (FOV) and Measurement Field of View (MFOV) which defines the smallest spot size that can be measured accurately.


4. Set the emissivity for temperature measurement on human tissue to 0.98.


5. Set the temperature span for maximum contrast on face temperatures (typically from 23°C to 40°C) and apply colour isotherms to highlight the hottest temperature on the human face, which makes it easier to see a person with temperatures outside a normal range.


6. The span and isotherm settings depend on ambient temperature variations, so optimal setting is important. For differential thermography methods, measure tear duct temperatures of test subjects and set alarms for 1…2°C above that average temperature, ensuring you adjust if ambient temperature changes.


7. Camera software should be set to alarm visually or audibly when a temperature inside the area tool exceeds a set threshold. This can be combined with camera snapshots. You should set your own alarm threshold based on the degree of sensitivity to false negatives and positives (and advice from medical professionals).


8. Eyewear and sunglasses are opaque in the 8-14 μm infrared spectral range. They should therefore be removed before the individual screening. Contact lenses do not need to be removed as these do not cover the tear duct.


INFLUENCES OF EXTERNAL SKIN TEMPERATURE It is important to recognise that temperatures made on the outside of the body – even at the tear duct – will not match the core body


June 2020 Instrumentation Monthly


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