Obviously the 50's was before the LED was invented. Did they use light bulbs? Also, they would also have had to use CdS photo-resistors instead of photo-trans

Obviously the 50's was before the LED was invented. Did they use light bulbs? Also, they would also have had to use CdS photo-resistors instead of photo-transistors. Probably required a few vacuum tubes for the interface too. Seems like a lot of effort vs. a simple potentiometer.