Device for projecting ink droplets onto a medium

A device for projecting ink droplets onto a medium so as to form dots defining one or more patterns, said dots being selected from an array of dots, the device comprising a perforated first plate which in use is adapted to be positioned substantially parallel to and spaced by a small distance from the medium, a second plate attached and substantially parallel to the first plate and defining therewith a chamber for containing ink to be deposited on the medium, and means for projecting from a selected hole in said perforated first plate a small quantity of ink by the application of voltage to at least one plate in conjunction with the passage of electromagnetic radiation through the second plate, wherein the improvement comprises:

• said second plate comprises a layer of photoconductive material whose electrical resistivity is reduced by exposure to electromagnetic radiation and a layer of electrically conductive material which is transparent to said radiation disposed in current conductive relation to the side of said photoconductive layer opposite the ink chamber side and
• an electrically conductive member disposed on the ink chamber side of said photoconductive layer, said device being adapted to have a preselected voltage applied between said electrically conductive layer and member, the value of said voltage being chosen such that exposure of a region of the photoconductive layer registered with a selected hole in said perforated first plate to electromagnetic radiation of predetermined intensity will permit sufficient current flow between the electrically conductive layer and member through said region of photoconductive material to expel an ink droplet from the selected hole as a result of local thermal expansion of the ink.