The antibiotic zone reader instruments are acknowledged for their unique design and has the ability to measure diameter of the inhibited zone up to 0.1 mm within the range of 0 to 35 mm. A light from a particular source in the base passes up through the transparent and semi transparent portion of the agar.-agar, then to a reflecting mirror affirmed by an arm above the unit. The unit is suitable for operation on 230 V, 50 Hz, Single phase AC Supply.