Talking of light, many eons ago, the organisation I was working for got one of the first Intel Hypercubes that were allowed out of the US (export restrictions). The debugging software was f*ng awful, but it had red and green lights that indicated communications and processing. These came to be invaluable in debugging and optimising thre codes we were writing (parallel Fortran with a McFarland comp[iler - the one with the infamous "this error should never occur" compile message).