When the unit is in stand-by state, hold the unit far away from metal and press the TEST button briefly to enter normal operation mode, place the unit with the front firmly and in a right angle against the coated surface to be measured .if the coating thickness is less than 1mm, the green LED will light up. if the coating thickness is more than 1mm and less than about 1.8mm, the yellow LED will light up. If the coating thickness is more than about 1. 8mm, the red LED will light up. To return to the stand-by state, press the TEST button when the red LED lights with the unit in normal operation mode.