A plastic shutter level is formed by a cap assembly which fits into the bottom of a vertical stile of the shutter. One of the walls of the housing has an output window extending in the longitudinal direction of the housing for the sensing beam and an input window parallel to the output window for the sensing beam reflected from a workpiece or its support. The storage unit is disposed in the liquid container to store a liquid usable for image formation. The inputoutput function is defined between a positive maximum point and a negative maximum point.