Apparatus for making incoming infrared rays visible includes an infrared optical system which serves to focus the rays through an optical grating onto the surface of a deformable liquid film deposited on a plane face of a transparent prism, the prism being so disposed in the path of a Schlieren-optical system that a beam of visible light produced by a source enters the liquid film from the baCk, is totally internally reflected at its surface adjacent the grating onto a concave mirrored surface of the prism which returns the beam to the liquid where it is again totally internally reflected by the liquid surface and then imaged on the bar-and-slot system of the schlieren-optics. In the absence of incoming infrared rays, the surface of the liquid film remains undeformed and the beam of visible light is turned back into its source. Upon deformation of the liquid surface by incoming infrared rays, visible light returning from the prism passes through the slots of the bar-and-slot system to establish an observable image of the deformation.