This is a window that increases the frequency of light that travels through it.
The window would be made of a sheet of nonlinear crystal. An extremely bright pump laser would be fired into the sheet from the edge at a critical angle and bounce between the two faces of the sheet due to total internal
reflection. The laser light would be trapped within the sheet and thus invisible to someone looking at the sheet.
Light that passes through the sheet, through the magic of nonlinear optics, would be upconverted via sum frequency generation. That is, the frequency of the incoming light would be added to the frequency of the pump laser, to make a new higher frequency. The pump laser light would be chosen based on the frequency desired to be made visible.
For infrared upconversion, it would be interesting to see people glow according to their body temperature.
The efficiency of the upconversion would be low because: a) the pump beam and incoming light are not colinear, and b) the incoming light is not coherent (and thus not phase-matched to the laser light). But if the pump beam were bright enough, the upconverted light might still be visible.
This could be used as a camera add-on to take nice infrared photos.
If filters were used on both sides of the window, it might also be possible to create a one-way window.