Watching Jerry's videos when comparing plastics made it clear that the unit has to be on a horizontal position for extended periods of time. Additionally, it needs to be connected to a computer at all times to send the data, and it needs an external source of power. 

Therefore, it was important that the unit's front surface be at the same height as the LED's , in case they have a plastic sample that extends a little over the perimeter of the LED's cylinder.  Since the unit would not work without external communication and power, the ports need to be easily reachable at all times.