[ryandamm]

This is probably what's called a 'video assist' in the film industry: when the film is being pulled through the gate, it's shielded from the light by a spinning mirror. In a fully-analog camera, that mirror reflects the light up to a ground glass target in the eyepiece, so the cinematographer sees the light that isn't recorded onto the film.

With a video assist, that light is simply captured by a sensor (yes, exactly like in a video camera) and presented to the user on a video display. Some use a beam-splitter to deliver the analog view to the cameraperson while still sending some photons to the video tap.

Note, in both cases, the light that hits the film itself is never visible to anyone until after development -- the camera crew gets to see the photons that are rejected, which can do weird things when you go to unusual shutter speeds (the image gets darker in the viewfinder as you increase the exposure)... though I believe there were some 16mm Bolexes that simply used a beamsplitter, so the viewer saw the same scene as the film stock -- but don't quote me on that.