Accurate screening of elevated temperatures without the use of a blackbody
Industry, businesses, and government agencies are increasingly using FLIR thermal imaging cameras to screen for elevated skin temperature – a possible sign of infection – especially in high-traffic areas such as airports, factories, hospitals, and public spaces. To guarantee optimal measurements, the U.S. Food and Drug Administration (FDA) and International Organization for Standards (ISO) has recommended pairing each thermal camera with a blackbody. Although FLIR cameras are designed to be used without blackbodies, they still are the perfect tool for screening individuals quickly as they move through high-traffic areas. Here’s why.
What is a blackbody and why would you need it?
A blackbody is an important contributor to accurate thermography. In theory, a blackbody is an object with a known emissivity of 1, which means that it perfectly absorbs and radiates all thermal energy. In practice, blackbody simulators based on the theoretical model have an emissivity ranging from 0.9 to 0.99. This is good enough to provide the information needed to correctly calibrate a thermal camera.
Blackbody simulators – which for simplicity’s sake we will refer to as “blackbodies” – are typically used as an optical reference source to obtain more accurate thermal measurements. The blackbody provides a reference temperature point in the field of view of the camera, reducing the potential for drift or detection errors during measurement.
In its recommendations for the use of thermal imaging cameras for skin temperature screenings, the FDA suggests positioning the blackbody beside the person being screened so that camera always remains calibrated.
Accurate measurements without a blackbody
Certain FLIR thermal cameras do not require the physical presence of a blackbody during screening in order to consistently and accurately measure temperatures. This is because:
- FLIR cameras are calibrated during production using multiple high-end blackbodies.
- FLIR engineers its lenses and camera bodies with multiple temperature sensors to compensate for a possible calibration shift (ambient drift) that typically occurs with aging electronic components.
- FLIR cameras include a shutter between the camera sensor and the lens which is used as a reference when the camera performs a nonuniformity correction, or NUC. During this process, the shutter presents a uniform temperature source to each detector element in the camera array. While imaging the source, the camera updates the offset correction coefficients, resulting in a more uniform image after the process is complete.
This proprietary mix of technologies ensure that the thermal camera remains calibrated over time. They do not require a blackbody reference in the field of view to meet the ambient drift and stability specification of less than 0.2°C (0.36°F) within a specified time frame.
IEC standards also refer to acceptable alternatives to the use of a black body:
IEC 80601-2-59:2017, 201.101.4 A SCREENING THERMOGRAPHY may use SELF-CORRECTIONS to maintain the drift with acceptable limits. An automated service test mode may be utilized to perform these measurements to allow for the substitution of the CALIBRATION SOURCE for a FACE.
This 14-hour period of in-room measurement with a FLIR camera shows that the camera remains within a 0.15°C (0.27°F) range for 90% of the time, and within a 0.2°C (0.36°F) range for 99% of the time.
Challenges of blackbody calibration
Thermal cameras that do not offer this mix of calibration technologies (high-end blackbody calibration, lens temperature sensors, shutter for NUC) cannot guarantee accurate measurements when used as a standalone system. For example, cameras that are not calibrated for temperature measurement or are typically used for night vision applications will require the addition of a blackbody.
Although the combination of a thermal camera and a blackbody provide a viable solution for elevated temperature measurement, the application of a blackbody comes with a set of challenges.
- The set up for a blackbody can be complex and usually requires an additional piece of hardware. In addition, integrating a blackbody into any system makes mounting, powering, and ultimately maintaining it more complex. Such an addition also introduces another potential point of failure into the overall solution.
- Proper focus on the blackbody is essential to getting accurate measurements. For a blackbody to be effective, it must be mounted in the same plane as the person being screened. A blackbody that is significantly closer to or farther from the camera than the person being screened will be out of focus and not function as an accurate reference source.
- Environmental conditions can also affect blackbody measurement; these include room temperature differences or fluctuations in an individual's temperature due to natural environmental changes.
Several ways to obtain accurate measurements
Accurate skin temperature screening may help authorities and businesses to control the spread of COVID-19. As we have shown, how one achieves that accuracy depends on how the thermal camera itself is engineered. Many thermal cameras on the market today were not calibrated during production or do not offer the required drift compensation and real-time calibration technologies. The skin temperature screening station must therefore include a physical blackbody in order to achieve reliable, continuously calibrated results.
In contrast, most FLIR thermal cameras models are calibrated during production with multiple blackbodies and are engineered to maintain calibration during use. They do not require a blackbody to provide accurate measurements. By avoiding the need for a complex blackbody set-up, FLIR cameras minimize the risk of performing wrong measurements due to failed system set-up or unfavorable measurement conditions.
You can learn more about obtaining accurate skin temperature measurements and blackbodies in The Complete Guidebook on Thermal Screening for Elevated Skin Temperature.