Ghost images of distant objects are sometimes visible in twin-glazed window units. This is often caused by a pressure differential between the enclosed airspace and the surrounding atmosphere, which distorts the window. The ghost images result from internal reflections where the panes are not parallel. Even if the window is distortion-free for a particular atmospheric pressure and temperature, variations in either of these parameters will introduce distortion. Expressions are derived that relate the magnitude of the ghost separation to the structural parameters of the window and variations in atmospheric pressure and temperature of the enclosed airspace. Ghost separations of half a degree may often be expected as a result of environmental changes.
W. Swindell, "Effect of Environmental Changes on the Ghosting of Distant Objects in Twin-Glazed Windows," Appl. Opt. 11, 2033-2036 (1972)