I remember hearing about these a long time ago, and wished they were available/affordable. It was a really clever invention - a single vertical column of LEDs, and a mechanically resonant mirror system which scanned across the LED column at a constant speed, so that if the LEDs were turned on and off at the right intervals, a virtual image would be formed. Well one showed up on haggle.com so I finally got to try it out. It was supposedly manufactured in 1989.
The LEDs are red. It looks similar to the Virtual Game Boy display, except that it's a monocular. If you view it with your dominant eye, you can indeed see the display and your environment at the same time; it looks as if it's floating in front of whatever else you might be looking at. There is a focus adjustment, but I found that my eye could focus on the image regardless how that was set; so it's really more like a depth adjustment. It should probably be set so that you can switch back and forth between focusing on the real world or the display with minimal effort. You'd want it set differently for reading and viewing at the same time than for say augmented-reality applications.
It seems to have a total resolution of 720x280; the docs say that you can write programs for this mode. By default the driver card emulates CGA, so 640x200 and 320x200 graphics modes are available in addition to 80x25 text with no ususual programming. I would imagine that some available dots go unused in these modes. There are no shading levels; each dot is either on or off. The connector is an 8-pin mini-DIN. I'm guessing it uses a synchronous serial connection to control the dots in real-time. There is a normal-looking CGA controller on the card, and a programmable logic device that probably implements the custom serial interface. The manual mentions that information is available about how to interface the display to custom systems, and says that it is a serial interface. So they would need power, ground, data, a clock, and some retrace signals - perhaps a vertical end-of-column signal and an end-of-scan signal for the whole screen. There are more than this many pins on the connector though. It's also possible that it is scanned like a CRT, with a row of LEDs and the mirror scanning vertically, but that would have necessitated a lot more LEDs and they probably wouldn't have done it, to save cost. Its maximum frame rate is 30, but it has memory to store the entire screen image, so perhaps it scans at a higher rate; it certainly didn't have any visible flicker AFAICT. It was patented in 1980, so presumably the design is public- domain now (it presumably has expired). It has been proposed to include this device in a pager, so that it would be possible to view full-size fax pages by holding it up to your eye. I would think we could build full-color displays like this by now; but I'm not sure if it is possible to create multiple colors of LEDs on a single semiconductor substrate.
The mirror bouncing back and forth causes some vibration, but not enough to be noticeable when wearing the headband.
The Linux console driver works with no changes. It looks like MCGA mode (320x200) might be supported in X but I haven't tried that yet. I had a hard time taking the screen-shot picture; trust me, it looks a lot more readable in real-life. For a CGA-resolution display, it looks way more crisp than any other CGA display I've ever seen. The third picture shows the cable connected to the ISA card to drive the display.
More info at http://www.ndirect.co.uk/~vr-systems/priveye1.htm, including a statement that this company sells them (still, or is this just an old web page?)
A wearable computer timeline at http://www-ics.ee.ic.ac.uk/surp99/article1/cml97/ mentions the Private Eye.