## 1 Theoretical Background
Infrared (IR) sensors: These sensors usually measure the distance from an object by emitting infrared light signals and calculating the angle of reflection through triangulation [18]. When the infrared Light Emitting Diode (LED) emits a beam of light on an object, that light reflects in all directions. A proximity sensor positioned next to the infrared emitter acquires the reflected light waves. The proximity sensor then estimates its distance from the object under study from the angle of the received signal [19]. Figure 3 illustrates the performance of an IR sensor. In Figure 3, the infrared LED puts a beam of light on a particular point of a proximate object. Then, the light signal reflects off the object and reaches the proximity sensor at a certain angle. The proximity sensor estimates the distance mathematically using these data. 3. Light Detection and Ranging (LiDAR) sensors: The performance of LiDAR sensors is a combination of the ultrasonic and IR sensor processes. For measuring a distance of an object, the transmitter part of the LiDAR emits a laser beam on the target. Then, the receiver part of the LiDAR receives the reflected light signal from the target object. Finally, the distance of the object under study is calculated by multiplying the travel duration of the laser signal and the constant speed of light in the air [20]. This method has the highest accuracy (such as 0.005% of their final range [21]) among the techniques discussed above and can detect the distance of small objects [19]. However, these sensors are expensive (up to EUR 4700 [22]) and some of them can be harmful to bare eyes [23]. Even the datasheet for low-cost LiDAR sensors that use class 1 lasers advises users to avoid looking into the laser beam while operating the device [24]. 4. LED time-of-flight (ToF) sensors: These sensors fall under the broad LiDAR spectrum and use the time-of-flight methodology to measure distances. ToF sensors use short pulses of light and measure the time between emitted and received signals [17; 19; 25]. Figure 3: The triangulation procedure of IR sensor. Table 1 compares several widely recognized low-cost sensors based on the technologies mentioned above. The data are organized in the following columns: (1) sensor name; (2) sensor type; (3) reading frequency--sampling frequency speed of the sensor; (4) distance range--the indicated range by the sensor datasheet; (5) dimension; (6) limitations-- the limitation of the system such as sensitivity to objects with a complex surface (such as a sponge); (7) input voltage; and (8) price--VAT excluded. An analysis of Table 1 shows that despite the low price of the HC-SR04, it has a better reading range and faster data sampling than the GP2Y0A21 YK0F sensor. In addition, HC-SR04 is the only sensor in Table 1 whose calculation cannot affect ambient light. However, the datasheet for this sensor noted that the distance estimation is not accurate for objects with an area range smaller than 0.5 square meters or complex surfaces [26]. Further analysis of Table 1 illustrates that the Lite V3 sensor has the most extensive measuring range. It is also the most expensive sensor under review in this paper. On the datasheet, it mentioned that even though this device uses laser class 1, users should not look at the laser beam [24]. The sensors discussed in this section are among the most popular ones currently used in different industry programs [17; 35; 36; 37]. Therefore, it is valuable to introduce the latest trends in the distance measuring systems found in the literature. Terrestrial laser scanning (TLS) is a proposed approach for contactless distance measuring applications. Park et al. developed a new laser scanning measurement technology [38]. That research used a LiDAR system to assess the health of a structure. The sensor is placed within a distance of 350 m.  
Another recent breakthrough in sensor advancement is using an Unmanned Aerial System (UAS) equipped with a commercial-grade video camera to measure a structure's displacements.