Figure 1: Applications for Infrared technology
So Many Choices, So Little Time!
For the purpose of detecting EBT, the target temperature range is 37 °C (98.6 °F) with an upper range of 38 °C (100.4 °F), identified by the CDC as the level of a fever that requires isolation. The instrument you choose for these measurements must be accurate enough to give you a correct reading. There are many non-contact thermometers available on the market. Some have an accuracy of ±0.3 °C (±0.5 °F) while others are as wide as ±2 °C (±3.5 °F). With this latter example, if you’re not paying attention to the accuracy, you could get a reading of 98.6 °F and, due to the inaccuracy of the instrument, the true temperature could be as high as 98.6 °F + 3.5 °F = 102.1 °F! In that situation, the person being monitored would likely exhibit other symptoms; the point being that you are not getting valid measurements because you’re not using the right tool for the job. In the case of the more accurate non-contact thermometer, a reading of 98.6 °F might be as low as 98.6 °F – 0.5 °F = 98.1 °F or could be as high as 98.6 °F + 0.5 °F = 99.1 °F. A much more acceptable level of confidence, wouldn’t you agree?
What happens when the measurement of an individual is 100.0 °F? Taking the inherent error of the instrument into account, the temperature could actually be 100.0 °F + 0.5 °F = 100.5 °F. And that holds true as long as the instrument has maintained its expected accuracy, according to the manufacturer of the thermometer. Instruments change value over time due to many factors, including aging of the electronic components, handling and care of the instrument, environmental effects, etc. How do you make sure the instrument continues to read correctly? That is the reason to have the thermometer calibrated on a recurring basis. A calibration laboratory has the means to compare your thermometer against more accurate instruments in a controlled environment in order to determine the amount of error in your thermometer. If the error exceeds the manufacturer’s stated accuracy, the lab will not only make adjustments to the instrument to restore the correct readings but will also report the error to you on a calibration report, which you can then review against the temperatures you have taken to determine if the outcome would have resulted in anyone having an EBT at or above the CDC threshold of 100.4 °F. That measurement on the day the person was checked would then be called a ‘False Negative’ because it was thought at the time that the person did not have a fever when they really did have one. This is also why it is important to have a Contact Traceability list: so that anyone exposed to this person can be contacted/alerted to potential COVID-19 risk without revealing the person’s identity (following HIPAA laws). I realize this sounds like a lot of work and perhaps you aren’t maintaining a Contact Traceability list… yet. I defer to the previous statement about unintentional transmission. Infectious disease doesn’t care if we are diligent or lazy about eliminating every path/route; if we give it an inch, it will take a mile!
Thermal Imagers and Line Scanners
In order to efficiently monitor the temperature of multiple people at once, a non-contact thermometer will not do. This is where Thermal Imagers and Line Scanners provide a solution, such as the Fotric 226B or Ametek Land Viralert3, available at Transcat.com. This method of monitoring typically requires a queue to be formed, which can be as simple as selecting a hallway that naturally confines people to a defined area as they are passing through, or may require barriers to form a defined space where the monitoring will occur. These instruments usually have wider accuracies than do thermometers. So, while they are more efficient for monitoring larger masses of people, they are not always as accurate as a thermometer. That is where the concept of relative measurement vs absolute measurement becomes important to understand. If you want to know the exact temperature of each individual passing through the queue, this is not the method to use. But, if you want to detect the individuals who are running hotter than the others passing through the queue, this use of technology is brilliant! The relative temperature of individuals who have a fever will show on the imager/scanner as a different color than those whose temperature is normal. Those few potentially febrile individuals can then be pulled aside for a secondary, more accurate temperature check using a thermometer.
Calibration of Thermal Imagers and Line Scanners
The calibration of these instruments is still very important because, even for relative measurements, you need to ensure the difference in temperature is within a degree of accuracy that is reliable. But there is more to the calibration on these instruments than there is for a thermometer. With an IR thermometer, there is a single focal point that senses the temperature of the skin. During calibration, a thermometer is checked at multiple temperature values across its designed measuring range to ensure it is maintaining its accuracy no matter what temperature you are reading. The same holds true for a thermal imager or line scanner with regard to checking multiple temperature values across the measuring range (see fig. 2). But, with these devices pixel calibration across the image is just as important. Again, a thermometer has a single focal point for temperature measurement while a thermal imager (which is a camera that uses the infrared spectrum) measures multiple points at once across the image.