## 4 Environmental Approach Using Air and Thermometric Sensors  
### Proposed Sensor System and Recognition Method  
For the second approach, we aimed at developing a privacy-aware sensor system. Figure 6a depicts the overall configuration of our second proposed system. We selected five environmental sensors that measure air and temperature distributions. A single-board computer (SBC), Raspberry Pi 4 Model B, was used to save measurement signals to a flash memory device. This SBC is a commonly and widely used prototyping device available for scientific research and industrial application development [217]. Measurement signals obtained from two independent sensor modules are sent to an IoT cloud service called Ambient (AmbientData Inc.; Tokyo, Japan) for storage and visualization. The data-receiving interval of this free service is 1 min. Figure 6b depicts details of the sensor module structure. Each sensor module comprises an SBC, which is used for sensor control, data storage, and wireless communication with an Ambient server, along with three sensor boards: a thermopile sensor board (D6T-44L-06; Omron Corp.; Kyoto, Japan), a CO\({}_{2}\) sensor board (K30; S \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Experiment** & \(A_{tra}\) & \(A_{val}\) & \(L_{tra}\) & \(L_{val}\) \\ \hline A & 100.0 & 100.0 & 34.1 & 35.1 \\ B & 100.0 & 100.0 & 69.7 & 78.6 \\ C & 99.3 & 99.7 & 55.0 & 53.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Results of accuracy and loss obtained from the respective experiments (%). Figure 6: Overall system architecture of the proposed privacy-aware environmental sensor system. (**a**) System configuration. (**b**) Sensor boards and SBC in the sensor module. (**c**) Images of IoT cloud service Ambient for storage and visualization. Sweden), and an air pressure, humidity, and temperature combined sensor board (BME280; Robert Bosch GmbH; Gerlingen, Germany). The SBC and these three sensor boards are wired by serial cables with serial communication protocols managed using a universal asynchronous receiver transmitter (UART) and an inter-integrated circuit (I2C). Figure 6c depicts images of data visualization results on browsers of a website provided by Ambient. We checked the sensor operating status periodically through the website during examination for data collection. Table 11 presents some major specifications of a thermopile sensor, D6T, which measures the temperature distribution of objects, especially for humans [218], and their surroundings. As a contactless sensor, the D6T sensor measures surface temperatures using thermocouple elements that receive radiant heat energy from objects and surroundings. Table 12 presents some major specifications of a CO\({}_{2}\) sensor: K30. The operating principle of this sensor is a non-dispersive infrared (NDIR) technology based on microelectromechanical systems (MEMS) for a small, inexpensive, and lightweight gas sensor. We calibrated this sensor in our earlier study aimed at in-situ atmospheric measurements [219]. Table 13 presents some major specifications of an integrated sensor; BME280, which comprises an air pressure sensor, a humidity sensor, and a temperature sensor as a single sensor package. The measurement ranges for air pressure, temperature, humidity are, respectively, 300-1100 hPa, \(-\)40-85 \({}^{\circ}\)C, and 0-100% with 0.18 hPa, 0.01 \({}^{\circ}\)C, and 0.008% resolutions. \begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Specifications** \\ \hline Number of elements & 16 (4 \(\times\) 4) \\ Horizontal view angle & 44.2\({}^{\circ}\) \\ Vertical view angle & 45.7\({}^{\circ}\) \\ Object temperature detection range & 5–50\({}^{\circ}\) \\ Ambient temperature detection range & 5–45\({}^{\circ}\) \\ Object temperature output accuracy & \(\pm\)1.5\({}^{\circ}\) (maximum) \\ Temperature resolution & 0.06 \({}^{\circ}\)C \\ Operating temperature & 0–50 \({}^{\circ}\)C \\ Operating humidity & 20–95\% \\ Sensor board dimensions & L11.6 \(\times\) W12.0 \(\times\) H10.7 mm \\ \hline \hline \end{tabular}
\end{table}