There are many applications in electronic products that are expected to make our lives more comfortable and safer. More and more people believe that thermal imaging cameras have such a promise, especially since the Covid-19 pandemic.

The rapid global spread of the virus has brought unprecedented challenges to public health, food systems and the world of work, and has led to the death of a large number of people worldwide. It reduces work and puts millions of people’s livelihoods at risk. In the face of this crisis, finding a way to quickly measure body temperature from a distance—including the ability to scan a group of people at the same time without interfering with their activities—has become very important for managing the pandemic. This is because elevated temperatures are one of the common symptoms of viral infections, therefore, the ability to effectively identify people who are feverish helps limit the spread of the virus that causes Covid-19.

Thermal imaging sensors are at the core of the technology that gives us this ability.

But what is a thermal imaging sensor and how does a thermal imaging camera allow us to measure core body temperature from a distance? A thermal imaging sensor is a microelectromechanical system (MEMS) chip that includes a set of detectors sensitive to incident long-wave infrared electromagnetic radiation (LWIR) with a wavelength range of 8 to 14 microns. All objects and all living things above 0 Kelvin radiate in this spectral range, and the intensity of this radiation represents their surface temperature.

Unlike visible light with a wavelength between 400 and 700 nm, LWIR radiation is invisible to the human eye. However, the detectors forming the thermal imager array can respond to incident infrared heat by changing their characteristics in a measurable manner, for example by changing their resistance or by generating a voltage output due to the Seebeck effect. These changes are amplified and digitized by the readout circuit, and the generated digital code is finally converted into a temperature value.

Therefore, each thermal detector measures the surface temperature of an object in its own instantaneous field of view-the part of the world where it can sense the thermal radiation emitted. The ordered two-dimensional detector array then produces a two-dimensional pixel array-similar to a grayscale visual image, but in this case, each pixel represents how hot that part of the scene is, not how bright it is.

In addition to simple analogies, referring to grayscale images is useful because people can easily intuitively think that thermal images can also be processed and analyzed by more or less standard visual image processing techniques. It can also be displayed as an informative heat map, where for each temperature value, there is a specific associated color.

Figure 1 shows examples of visual images, grayscale thermal images, and color thermal images-all three images belong to the same scene.

Figure 1: Visual camera image and thermal camera image

These thermal images are captured by a special camera module using the LWIR thermal imaging chip, as shown in Figure 2.

Figure 2: Meridian Innovation 80 × 62 resolution long-wave infrared thermal imaging chip and module

As we said before, thermal imaging cameras measure the surface temperature of objects or subjects in the field of view. The question now is whether and how we use the information in thermal images to determine the core body temperature of human subjects.

Before answering this question, let us recall that traditionally, core body temperature is measured by mercury or a digital thermometer (Figure 3) and must be in physical contact with a hidden area of ​​the body that is very close to or the same as the body temperature. Viscera. Typical contact sites are the rectum (rectal thermometer, which is the most accurate contact thermometer), sublingual (oral thermometer, reading about 0.3°C lower than rectal temperature) or axilla (armpit thermometer, about 0.6 lower than rectal temperature reading) °C).

The practical limitations of contact thermometers have been overcome by the development of different types of non-contact infrared induction thermometers that can be distinguished by their physical dimensions and the way they must be positioned to perform the measurement—for example, tympanic thermometers. A thermometer located in the distal ear canal; a temporal artery thermometer that slides over the superficial temporal artery on the side of the forehead; or a forehead thermometer (Figure 4) that is placed directly in front of the forehead.

Figure 3: Boots Contact Thermometer (L) (Source: Meridian Innovation)

Figure 4: Braun Forehead Thermometer (R) (Source: Meridian Innovation)

These thermometers have the following three factors in common:

On the other hand, a thermal imaging camera is built around a multi-pixel thermal imaging sensor, as described earlier. The design and manufacturing process used to form the detector not only allows for miniaturization, which facilitates packaging more detectors per chip (for example, 5,000 pixels or more), but also results in more sensitive detectors with shorter response times, This allows people to capture not only static scenes, but also hot video streams at a frame rate of 1 to 30 frames per second.

Combined with a dedicated lens that usually has a field of view of at least 40°, this technology allows people to capture complex dynamic thermal scenes, including multiple people at the same time, from a distance of 50 cm to several meters.

All of these seem to be able to quickly measure the body temperature of a group of individuals at the same time, from a distance without stopping them-but is it really enough? Obviously not.

First, the detector can only sense the temperature of the skin surface. Similar to a forehead thermometer, the core body temperature must be calculated based on some biophysical models of the human body. The question that now arises is which pixel to use as a representative of the skin temperature of interest. This is an extraordinary thing, because the thermal imager uses multiple pixels to resolve a human face, and the temperature reading may vary by more than 1°C.

In addition, factors such as lens defects; the angle of view between the camera and the subject; the presence of a mask, face-shaded hairstyles, glasses, etc. will cause inaccurate temperature readings and are more complicated than simple forehead or in-ear infrared thermometers.

This is where the key role of thermal image processing and analysis comes into play. Considering the "too much" information available in thermal images, most of the various inaccuracies have system characteristics and can be compensated. However, this makes the design of a thermal imaging camera for fever detection a rather complicated task. It requires a detailed understanding of the characteristics of thermal imaging sensors, a good grasp of thermal imaging and some biophysics, and of course, a good knowledge of images and data. analyze.

Typical temperature scanning systems include vision and thermal imaging sensors. Analyze the visual image stream for face detection and mask detection, and map the generated region of interest to the thermal data stream for temperature analysis.

The result of this processing is shown in Figure 5, where three people are identified in a given visual and thermal frame pair, and the estimated core body temperature of each person is marked on the visual frame. However, when personal privacy must be protected and therefore visual image sensors cannot be used on the system, fever scanning becomes significantly more difficult. In this case, the entire process of object detection and subsequent core body temperature estimation must be based entirely on thermal video streams.

Figure 5: Meridian Innovation multi-person fever detection camera solution (Source: Meridian Innovation)

For more details on how it works, please refer to the "Fever Detection Research Instructions" written by Dr. Stanislav Markov of Meridian Innovation at erp.meridianinno.com/documents/On_Fever_Detection_english.pdf.

As mentioned above, the necessity of timely delivery of thermal imaging cameras for detecting high temperatures and the complexity of building good solutions have led to joint efforts to provide a large number of reference designs for OEMs and ODMs. Examples of such reference designs are provided by Arrow Electronics and are described in more detail at www.arrowopenlab.com/HkOpenLab/solutions/2021E001.pdf.

The core of the reference design is the new thermal imager from Meridian Innovation (meridianinno.com). The imager is mass-produced by CMOS/MEMS technology and is a hybrid of traditional microbolometer and bulk thermopile in terms of pixel design. In addition to the thermal imager, the reference design also includes an environmental sensor and a time-of-flight sensor to estimate the distance to the object. It benefits from AI-enhanced application software, which improves the accuracy of thermal readings and core body temperature calculations, as well as thermal image quality for visualization. The choice of cost-effective components and software development kits helps to solve the most important obstacles-including the calculation of core body temperature-reduces the barriers to adopting the design and helps meet the need for thermal imaging cameras to detect fever .

Thermal imaging sensors are now easily available, and mass-market products are also affordable. Accessible reference hardware designs and software solutions promote the adoption of this technology and enable a rapid development cycle of thermal imaging cameras, including cameras for automatic fever detection. Such products can help control the pandemic and show a fascinating story in which innovative technologies promote a safer and better life.

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