It’s interesting what you see when you exert black light on everyday objects. We strongly suggest that you do not do this in a hotel room, but if you turn on any UV light, for example, a circuit board, you might see what [Sam Ettinger] made, which led him to build these discrete seven segment fluorescent PCB displays.
It turns out that at least some FR-4 PCBs fluoresce under UV light, giving off a ghostly blue-green glow. Seeing the possibilities, [Sam] designed a PCB with cutouts in the copper and solder mask in the form of a traditional seven-segment display. The back of the PCB features pads for UV LEDs and current limiting resistors, which shine through the board and cause the segments to glow. The through slits between the segments prevent light from one segment from spreading into the next; while [Sam] left the slots empty, they could easily be filled with solder. The fluorescent property of FR-4, and therefore the brightness and tint of the segments, seems to vary depending on the thickness of the board and the PCB manufacturer, but it seems that most PCBs will perform very well.
We’d say the first obvious improvement might be to cover the back of the screen in black epoxy, reduce stray light, and improve contrast. But they look pretty good the way they are. We can also see how is displayed with other shapes, like icons and simple symbols. Or maybe even alphanumeric characters – say, haven’t we seen something like this before?