Ghostbusters have a handheld device that detects “psycho-kinetic energy” (PKE), called, appropriately, the P.K.E. Meter.
Early in the library scene, Spengler is holding the device up in front of his eyes when he explains to Stentz that, “It’s moving.” So theoretically, there is some way by which it helps locate the source of PKE concentration. When we see the front of the device, it’s a bunch of LEDs, and impossible to make any sense of what we see there. (In fact, the later scene seems to have the orientation of the device horizontally flipped.) The inscrutability of the interface is fine, diegetically, since it’s a custom device built by scientists for themselves, but as they try and hire new ghostbusters and train them, they’re going to want to think about ease-of-training.
You might think that that green display we see in this first scene is pointing in a cardinal direction, i.e. just behind Spengler, but when we see the same device later, right next to the Keymaster, those LEDs are spinning around a center that’s off the edge of the screen, so, again, the meaning of these lights is inscrutable.
