Optical compressor
A compressor in which the signal lights a lamp and a photocell beside it turns the gain down — slowly, smoothly and a little unpredictably.
The audio drives a bulb or an electroluminescent panel, and a light-dependent resistor next to it reacts to the glow. The cell is sluggish: it responds in milliseconds rather than microseconds and releases in two stages, quickly at first and then slowly, depending on how long it has been lit. The release is therefore program-dependent without anyone touching a knob, which is why vocals and bass sound so natural through one.
The archetype is the Teletronix LA-2A from the early 1960s, designed by Jim Lawrence, with its T4 cell and effectively two controls. It is still in production. Cells age and vary from unit to unit, so no two LA-2As sound quite the same — a quirk that digital emulations go to considerable lengths to reproduce.