Abstract: An exposure indicator which is attachable to a hinged glass mask cover frame, the indicator comprising a light emitting diode having one end lead connected in series through a light sensitive cell; a sensitivity-controlling resistance and switch to the appropriate pole of a source of electrical potential, such as a battery, the other pole of the battery being connected to the other end lead of the diode; the switch being arranged so that it is closed when the glass mask frame is closed against the blanket frame on which there is loaded a light-sensitive printing plate, and onto which an activating light is adapted to be directed, whereupon the light-sensitive photo cell is activated (resistance decreased) so that current is allowed to flow from the battery, through the switch and the light-activated silicon controlled rectifier, causing the diode to glow, thus indicating that the material has been exposed, and thereby preventing a second exposure due to the inattentiveness or forgetfulness of the operator.