I had one of these...

A Rockwell 61R Advanced Slide Rule, circa 1974. Rechargeable, even.

I was looking it up because I recalled that the numeric display was blue, and yet it predated blue LEDs by almost two decades. So what the heck was that? Turns out the technology was VFD, or "vacuum fluorescent display," which is not used much for more recent battery-powered devices because it is a huge energy hog.

OK, enough of the memory-lane stuff. Back to work.