Making of The Dark Crystal #13

The white contraption on the front of the camera is the Lightflex system used by cinemaotgrapher Oswald Morris.

Here’s an explanation of the device from American Cinematographer:

The on-camera accessory was invented in 1972 by Gerry Turpin, BSC. The Lightflex consists primarily of an oversized filter-hood faced with optical glass. Dimmer-controlled quartz lamps built into the hood reflect into the lens and overlay a controlled amount of light on the scene to be photographed at the time of exposure. The device can be used to adjust the gamma curve of the emulsion, and also extends its photometric range without affecting grain.