A sensor assembly for a printer arranged to displace printed material along a path, including a sensor housing and a platen assembly. The sensor housing includes top surfaces and resilient elements fixed to the top surfaces, and houses light emitting elements. The platen assembly includes transparent material and a platen housing with a plurality of steps, and is arranged to engage the sensor housing and displace in a first direction along the sensor housing toward an end of the sensor housing such that the transparent material is aligned with the light emitting elements in a direction orthogonal to the axis. As the platen assembly displaces in the first direction, the plurality of resilient elements is arranged to engage the plurality of steps to urge the piece of transparent material into contact with the sensor housing with progressively greater force such that the transparent material is sealed against the sensor housing.