Disclosed is a completely portable, hand-held digital ultrasonic instrument about the size and shape of an ordinary pen. The housing is contoured such that it can be easily grasped in the manner of a writing pen. An activation button is located on the interior dorsal surface in close approximation to the index fingertip of the user. A liquid crystal display provides a digital readout of the corneal thickness on the barrel of the instrument housing. The instrument incorporates a 10 MHz or 20 MHz solid phase piezoelectric transducer, a microprocessor, a gate array, a hybrid analog circuit, a liquid crystal display, batteries and a removable battery cover. The 20 MHz transducer is attached to a swan neck connector which is attached to the instrument housing. When a 10 MHz transducer is used, it is attached to a truncated conical connector which is also attached to the instrument housing. In addition, a pinjack connector is avai ' able to link the unit to other electronic media such as a microcomputer, personal computer, or printer.