New thermal detection technology serves as a powerful, automated and full-integrated solution to a wide variety of applications

The far-infrared sensing technology generally detects far-infrared heat radiation with a wavelength that ranges from 5µm to 15µm. By converting energy into temperature through an algorithm, the far-infrared sensor can contactlessly measure the temperature of objects, allowing it to be applied to a wide variety of day-to-day temperature sensing applications.

The exceptional features of PixArt’s Far Infrared series are shown in Figure 1. PixArt’s FIR (Far Infrared) SoC sensors not only detect heat radiation sources but can be customized to compensate for temperature variation according to customers’ target application to boost accuracy. With the highly integrated SoC, PixArt’s FIR product writes the calibrated temperature parameter value of the detected object into its memory, which can be directly read and converted to temperature for specific applications. Such design is conducive to simplifying our customers’ subsequent system polling process.

New thermal detection technology serves as a powerful, automated and full-integrated solution to a wide variety of applications

Figure 1. Features of PixArt’s FIR product series

The best way to get space, temperature, and thermal information

PixArt’s FIR array sensor (PAF9701C1) has a resolution of 64 pixels and can directly output calibrated and readable temperature values. Owing to its built-in algorithm, FIR sensor array is designed to identify the distribution of a thermal object in the detected space/area and outputs its thermal information as shown in Figure 2. When integrating PAF9701C1 with other CMOS imaging sensors, a low-resolution thermal image can be generated to provide additional information about the detected object, as demonstrated in Figure 3.

Figure 2. Thermal information (unit: oC) detected from PAF9701C1

Figure 3. A low-resolution thermal image obtainable by combining the output of PAF9701C1 with PixArt imaging sensors

The PAF9701C1 is packaged in an SMD-type to facilitate easy integration of subsequent system applications and low power consumption. The dimension of its CLCC package is merely 8mm x 8mm x 3.85mm. A photo of PAF9701C1 is shown in Figure 4.

Figure 4. Photo of PAF9701C1

With 64-pixels resolution and a built-in lens with FOV(V,H) of 60 degrees, the single equivalent FOV of PAF9701C1 is 7.5 degrees. This means that if the size of the detection area is 65 centimeters by 65 centimeters, the maximum detection distance can reach as far as 5 meters, as indicated in Figure 5. As we can see from Figure 6, the further the detection distance, the smaller the object becomes on the thermal image.

Figure 5. Detection Distance of PAF9701C1

Figure 6. The further the detection distance, the smaller the object shows on the thermal image

The PAF9701C1 product can support a wide sensing range, from -20°C to 380°C, making it suitable to be deployed in applications other than body temperature detection, such as thermal detection for kitchen equipment, home appliances, and more.

Comparing with similar types of products in the market, the power consumption rate of PAF9701C1 has reduced by more than 50%, allowing it to be used in battery-powered systems. Its built-in, self-calibrated algorithm supports immediate data processing to output readable temperature values via its I2C interface – all without the presence of a high-end MCU.

Combining PAF9701C1 thermal image with other PixArt sensors to maximize capabilities

Due to the COVID-19 pandemic, measuring and record forehead temperature has become an essential part of our daily lives. By combining PixArt’s FIR sensor (PAF9701C1), Global Shutter image sensor (PAG7920LT), and Low-Power Intelligent Object Detection SoC (PAG7681LS) in one single thermal detection device, the device will be able to quickly identify human beings, measure their forehead temperature, and record the temperature to making sure anyone who enters the indoor space will be automatically measured and recorded. An example of data output by the 3-in-1 device is shown in Figure 7.

Figure 7. An example of data output by a 3-in-1 thermal device

About PAG7920LT

PAG7920LT is an ultra-low-power CMOS monochrome image sensor that belongs to PixArt’s Global Shutter Image Sensor product line.

About PAG7681LS

The PAG7681LS SoC integrates PixArt’s proprietary algorithm to provide detection for human face and motion (even with their masks on) with ultra-low power design. It belongs to PixArt’s Low-Power Intelligent Object Detection product line.

Highly-integrated evaluation boards available from PixArt

PixArt provides a highly integrated evaluation board (check PAF9701C1’s product page for more information) that is easy to integrate and offers fast verification and system prototype. With 240×240 resolution and consisting of the PAF9701 (FOV(H,V) = 60 degrees), a PAG7920LT image sensor with lens set (FOV(H) = 72.5 degrees), and PAG7681LS SoC chip, this 3-in-1 evaluation board (as shown in Figure 8) comes with face detection, temperature result computation, and control setup capabilities to realize a comprehensive smart forehead sensing solution.

The system is powered by a 3.3V of power supply and its communication interface is I2C. With its default configuration, the recommended sensing distance of the evaluation board is between 50cm and 100cm. (Note: PixArt also provides a complete reference design guide and library for the evaluation board.)

a. PAF9701C1 FIR sensor with heat sink

b. FPC module of PAG7920LT Image Sensor

c. DSP chipset of PAG7681LS

d. External FLASH memory of PAG7681LS

e. 3.3V to 1.8V LDO

f. Reset button of PAG7681LS

g. 5-pin Reserved Host Control Interface

h. 30-pin Reserved Host Control Interface

Figure 8. Schematic Diagram of the evaluation board – Dimension: 43mm x 25mm

Connection method of PixArt’s 3-in-1 evaluation board explainedd

By controlling PAF9701C1 through PAG7681LS’s DSP, the architecture adopted by this evaluation board is particularly suitable for outputting thermal values.

Figure 9. System block architecture of the evaluation board

As shown in Figure 9, the circuit connection of the evaluation board allows PAG7681LS to obtain thermal data from PAF9701C1 and then directly output the forehead temperature value to the Host Processor. The process flow is shown in Figure 10. Note that the algorithm for forehead temperature calculation is embedded in PAG7681LS’s DSP.

Figure 10. Process flow of the evaluation board

While PAG7920LT and PAG7681LS are mainly responsible for face and mask recognition, the algorithm will link the face position data with the temperature distribution data detected by PAF9701C1 to compute the compensated temperature values. Under uniform background with the detecting human being 50cm away from the sensor, the evaluation board is able to achieve high accuracy of 37±0.5°C. Note that the accuracy may be affected by factors such as forehead size, hair interference, background situation, and sensing distance.

For customers who anticipate their 3-in-1 device to output both the thermal data AND the image data; or simply require more flexibility to tailor for specific applications, feel free to purchase the three sensors (PAG7920LT, PAG7681LS, PAF9701C1) from PixArt and establish the connection architecture your own way (for instance, allowing the Host Processor to directly control the three sensors). With the correct configuration and suitable connection method, this 3-in-1 solution can support applications such as indoor thermal detection, and living-creature (e.g. human body, pet, etc.) detection. With each purchased sensor, PixArt will provide its porting guide with detailed IC information, so that customers can customize their circuits, computations, and outputs.


With the competitive advantages of small form factor, low power consumption, and cost-effective architecture, PixArt’s FIR product series is suitable for a wide variety of applications, including body temperature monitoring for access control, room surveillance via thermal detection, and safety control for smart home appliances.

PixArt’s FIR products and the above-mentioned “3-in-1” solution make it easy to be designed into all sorts of applications and projects. With PixArt’s hardware support and strong built-in algorithm, we are here to deliver a great user experience as well as making your further design work feel almost like a breeze.

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Author Of this post: PixArt Imaging Inc.

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