The light from the inside source of the base passes up through the transparent & semitransparent of the agar then to a glass prisam mounted at the magnified image of zone of inhibitions is clearly visible on the prisam. The unit is measuring the dia meter of finished zone in a petri dish, for determining the strength of antibiotic material.  This unit measures the dia meters of the inhibited zone within range of 0-35 mm with an accuracy of 0.2 mm.