# Towards a Multi-Perspective Time of Flight Laser Ranging Device Based on Mirrors and Prisms
## 2 Materials and Methods
### Laser ToF Ranging Device Device  
Many commercial ToF-based ranging devices are available on the market as either off the shelf products or as development kits. Those offerings, however, do not offer much insight into what is going on with the measurement. For that reason, we opted to use a custom direct time of flight ranging device, which not only allows us to measure the distance to the target, but also to indirectly observe the amplitude of the return signal. This section discusses the laser ToF ranging device, its construction, and principles of operation.  
The laser ToF ranging device that was used has a 905 nm laser diode as a light source and an avalanche photodiode (APD) connected to a transimpedance amplifier (TIA), with the gain of 25 kV/A on the receiving end. The receiver APD is also masked with an optical low pass filter to eliminate much of the ambient illumination. The model used is the Optolite IR filter that cuts off light with wavelengths below approximately 750 nm. For outdoor use, a band pass filter would be more appropriate but is unnecessary for indoor use, as modern lights produce little to no light in the IR spectrum [22]. Both the transmitters and the receiver are equipped with focusing optics to achieve a narrow FOV of 5\({}^{\circ}\). Time of flight measurement is taken care of by a pair of TDC7200 time to digital converters (TDC). This model has a resolution of 55 ps (8.8 mm) and a standard deviation of 35 ps (5.2 mm). This is improved by using a running average of eight samples. Both TDCs start the measurement on the positive front of the signal to emit a light pulse but their stop signals are different. One TDC stops the measurement when the current through the laser diode, as measured on a shunt resistor, exceeds the threshold. The other TDC measures both the time of detecting the reflected pulse and its width. This is achieved by passing the signal from the TIA through a comparator with a selected threshold voltage, and then passing the resulting waveform through a pulse generator for both the falling and rising front from the first comparator. The described setup eliminates measurement error that comes from the temperature dependent delay between triggering the transmitted pulse and the laser actually turning on. The system can select between three different illumination channels, all of which trigger the same stop signal for the reference TDC. Ranging is controlled with an external microcontroller. A simplified schematic is shown in Figure 3. The dashed line encircles components of each of the three equal transmit channels and the dotted line encircles components on the laser ranging device's printed circuit board (PCB), which comprise the analog front end. Throughout the experiment, only one channel 1, which is located 2 cm from the receiver, was used. The remaining two channels will allow the same ranging device to be used in later research.  
Figure 3: Block diagram of laser ToF ranging device. Dotted line encircles the crucial components that are mounted on the same PCB. The dashed line encircles the components of one of three transmit channels, constructed in the same way.  
The microcontroller, which controlled the ranging, was an ATmega328P on an Arduino Nano development board. It was used for generating appropriate transmit pulses and controlling TDCs, which includes initialization, arming, and reading measurement data. The later was sent over to the PC with minimal additional processing carried out on the microcontroller.