IEE referred to projection displays as "In-Line" displays because the digits were displayed on a single plane instead of stacked as in a Nixie tube.
IEE projection display, exploded view.
The slides in a projection display are easy to replace; transparency prints and a fistful of RGB LED's allow the home experimenter to coax these devices into displaying nearly any symbol imaginable.
IEE projection display, cutaway diagram.
This module contains seven IEE 10000 projection displays mounted to a shared front screen.
Various different models of IEE projection displays for scale
comparision; from left to right: 340 series, 14928 series, 01100 series, 220H series, 00010 series, 10000 series. Note that IEE projection displays utilize composite part numbers and everything occurring after the first dash is encoded information about lamp voltage and character
sets.
