An active matrix liquid crystal display device comprising an array of picture elements (12) which include respective switching means (20), e.g. TFTs, and which are driven a row at a time by drive signals supplied via sets of row and column address conductors (14,16) further includes an array of active addressed sensing elements (18) each consisting of a photosensitive element (26), e.g. a thin film photoresistor or photodiode, charge storage means (25) and a switching device (24), e.g. a TFT, connected to respective row and column address conductors (14,16). The charge storage means are periodically charged through operation of their associated switching devices by said drive signals and are selectively discharged upon their associated photosensitive element being illuminated by, for example, a light pen (41). The states of the charge storage means of all the sensing elements are determined, one row at a time, by a detection circuit (40) connected to the column address conductors (16) during each display field period in synchronism with the driving of the rows of picture elements.