# Towards a Multi-Perspective Time of Flight Laser Ranging Device Based on Mirrors and Prisms
## 2 Materials and Methods
### Direct and Redirected Time of Flight Measurement  
The oath of light from a laser ToF ranging device may be direct or redirected before reaching the target surface. A direct measurement is common in laser rangefinders, where only one dimensional ranging is required. Redirected measurements are common in lidar scanners, where the FOV of a single device is redirected in multiple directions. The distance between the device and the reflecting surface that is used for redirection of light in this configuration is constant and never changes. Redirection may be carried out in one or two dimensions. The former results in 2D measurements of the surroundings, while the later generates a point cloud on the surface of observed objects. Both approaches have been thoroughly explored and are standard in industrial environments. Even though redirection using mirrors is the most common, beam steering using prisms can be used instead, such as those detailed in [20; 21].  
The comparison of how a direct or redirected laser ToF ranging works is depicted in Figure 1. Figure 1 shows direct operation, Figure 1 shows how light FOV can be redirected by using a reflective surface within the devices dead zone, which is commonly used in industrial lidar scanners, and lastly, Figure 1 shows the operation where the reflecting surface lies beyond the device's dead zone. Boxes marked Rx and Tx represent light transmitters (lasers) and receivers (photodiodes), respectively. The system's dead zone ranges from the transmitter/receiver to the dashed line, and partially dead zone extends up to the solid line. While operation between the first two configurations should not be critically different, the third one may pose problems, as any imperfections on the reflecting surface may cause reflections that will interfere with the ranging device's operation.  
This article addresses a similar approach to redirecting the FOV as described previously, with the major difference that the device used to redirect the FOV is stationary and immovable, with respect to the ranging device. The idea is to transmit a laser beam along the length of the robot's joint and redirect it perpendicularly outwards when it hits the redirection device. The light, reflected from a target, then travels essentially the same path back to reach the receiver. Multiple optical paths are established along the robot's segment, leading to a multi-perspective view of the surroundings. A multi-perspective view might provide a significant advantage in monitoring the robot's surroundings, enabling enhanced safety features. The principle of operation is depicted in Figure 2, where orange bands represent individual light paths.  
Figure 1: Diagrams of different areas of operation for light-based ToF distance measuring devices are shown for **(a)** direct method, **(b)** redirected method, where reflecting surface is entirely within the ranging device’s dead zone, and **(c)** for when the reflecting surface is located beyond the ranging device’s dead zone. Boxes marked Rx and Tx represent light transmitters (lasers) and receivers (photodiodes), respectively. The system’s dead zone ranges from the transmitter/receiver to the dashed line, and the partially dead zone extends up to the solid line.  
Figure 2: The principle of operation of a robot proximity monitoring system based on one centralized ranging device per robot’s segment and individual fields of view redirected by mirrors. The robot segment lengths are usually from 0.2 m to 0.8 m, and the detection range from the robot’s segment to the potential obstacle should be in compliance with the SSM mode of operation, as specified in ISO/TS 15066:2016. In a real application, the field of view redirection optics would be located as close to the robot’s surface as possible.