"RECEIVES HIGH INTENSITY OPTICAL PULSES FROM OPTICOM EMITTER; PULSES ARE TRANSFORMED BY THE DETECTOR INTO ELECTRICAL SIGNALS WHICH ARE TRANSMITTED TO THE PHRASE SELECTOR; DETECTOR ARE BI-DIRECTIONAL; UTILIZES SOLID STATE CIRCUITRY; RECEPTION RANGE IS ADJUSTABLE FROM 1800 FT TO A FEW HUNDRED FT WHEN USED WITH THE END ITEM; HEIGHT 10.250 IN.,LENGTH 13.750 IN."