The present disclosure relates devices and a method for monitoring a display image (2) in a transparent optical projection display. They disclose a display image monitor (1) comprising a fiber optic face plate (4) and an image sensor (5). The fiber optic face plate (4) comprises a first surface (4a) with optic fiber ends and a second surface (4b) with corresponding optic fiber ends and transfers light from the first surface (4a) to the second surface (4b) through the optic fibers, wherein the first surface abuts against a part (2a) of the display image (2) at a focused image plane (2b) and thus the light of the part of the display image enters the optic fiber ends and wherein the second surface is arranged to direct the part of the display image to the image sensor, thereby transferring the part of the display image to the image sensor.