During the past decade, infrared thermometry has seen remarkable success in measuring human body temperature, as evidenced by the many variations of the infrared ear thermometer.  Ear thermometers measure temperature by detecting the amount of energy emitted in the infrared region of the electromagnetic spectrum.  All objects having a temperature higher than absolute zero (-273°C) emit infrared energy.  In fact, as shown below, the majority of body heat loss in a room temperature environment is by emitted infrared energy.

There is a direct correlation between the amount of infrared energy emitted and an object's temperature.  The graph below shows the amount of infrared energy emitted by objects at a number of different temperatures ranging from 3,000 to 5,000°C.  When the temperature of an object increases, it emits a greater amount of infrared energy.  By measuring the amount of infrared energy emitted by an object, its temperature can be calculated.

Thermal imaging cameras, like the OptoTherm InfraSight camera shown on the right, operate in a similar manner to ear thermometers.  However, whereas ear thermometers can only measure the temperature at a single spot, thermal imagers can measure the temperature at tens of thousands of spots that together create a thermal image, as shown below.  Like ear thermometers, thermal imagers are passive devices and emit no harmful radiation.

There are two primary detector technologies used in thermal imaging cameras for human fever screening; medium and long wave.  Medium wave infrared detectors are sensitive to electromagnetic energy that is 3 to 5 microns (0.003 to 0.005 mm) in wavelength.  Long wave infrared detectors are sensitive to electromagnetic energy that is 7 to 15 microns in wavelength.  As shown in the graph above, there is a peak of emitted energy for each specific temperature and the peak increases in wavelength at lower temperatures.  The peak of emitted energy for objects at human body temperatures is approximately 9.5 microns in wavelength.  For this reason, ThermoScreen was developed using long wave detector technology to take advantage of the increased sensitivity these detectors provide when measuring the temperature of human bodies.

How Fever Screening Works

ThermoScreen operates in real-time at 30 frames-per-second by sampling the infrared energy emitted within its field-of-view.  It processes the sampled data using a computer and then creates and displays thermal images on an LCD monitor.  A color palette, as shown to the right of the thermal image on the right, is applied to the thermal images so that each pixel is displayed in the color that represents that pixel's specific temperature.  Thermal images are displayed in real-time and can be analyzed instantly to assess the skin temperature of individuals being screened.  Human skin provides an excellent target for thermal imaging because skin emits infrared energy very efficiently.  This property of human skin helps thermal imagers provide accurate and consistent temperature measurements.

As individuals pass through the field-of-view of the camera, their thermal image is analyzed and areas of skin with temperatures exceeding the fever threshold temperature are displayed in bright colors.  Setting the appropriate value for the Fever Threshold is necessary to properly detect febrile individuals.  The specific value of the Fever Threshold depends on several factors, such as site environmental conditions, and the procedure for properly setting the value will be explained in detail in the following section.

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