It's exactly what it sounds like: a pico projector slapped on the back of a smartphone. Stupid-looking? Yes. Stupid idea? Definitely not.
The projector-phone like this LG Expo from AT&T  is the sum total of about 100 years of share-happy technology. Its forebears, the movie projector and the overhead projector, made the act of sharing cutting-edge long before social graphs and semantic Web tools. Those devices then turned into digital projectors, that sweet-ass $1,200 gizmo that--admit it--you've considered stealing from your company conference room to install in your living room.
So why does one of these things need to be on the back of a phone? We'll get to that later. First, how did it get there?
The pico (or handheld) projector  first showed up  about five years ago, a miniaturized version of that bigger digital projector you covet at work. But unlike its bigger brother, the pico projector uses an LED (or several LEDs) as its light source, not a lamp, meaning that it's less bright. And since LEDs are anorexic when it comes to power usage, they're much better suited to battery power than full-blown alternatives.
Big digital projectors and pico projectors use pretty much the same mechanism to create their images: It involves a light, a semi-transparent image-substrate that light shines through, and a bunch of mirrors and lenses to get that image onto your wall sharply. The simplest (and therefore most common) way of doing this uses a small color LCD screen (like the one on your laptop) as the image-substrate. More expensive projectors use something called a "digital light valve," or DLP, which is just complicated enough  that I can't easily summarize it in one readable sentence. Let's just say there are several hundred thousand microscopic mirrors, each of which can be independently manipulated to change the color of a pixel. The rest of the magic is handled by a hamster with a degree from Hogwart's. The core component, called a DMD, or "digital micromirror device," looks like this. (Yep, that's what a few hundred thousand mini-mirrors looks like.)
It's good that LCD and DLP projectors showed up when they did, because their forerunners were big, stupid, messy devices called "eidophors," or "image bearer" in Greek. Eidophors like the one pictured below were used by NASA's Mission Control back in the early Tang-and-capsule days. But they were so unwieldy and expensive that companies like Paramount Pictures couldn't figure out a way to make them profitable for movies in the postwar years, even though they were 80 times brighter than other projectors of the day and could produce images 60 feet wide.
Eidophors worked like movie projectors, except instead of film, they used a slowly-rotating mirrored disk covered in thick, viscous oil. The device shot a beam of electrons at the oil, causing it to change the way it transmitted light. Areas that weren't hit by the beam let colored light pass through and onto mirrors, which sent it outward. Later, these contraptions became refined enough for commercial success, and GE sold them as Talaria brand projectors (below) with fancy-pants Xenon arc lamps providing the light.
Xenon arc lamps are the same lights used in movie projectors , and are perhaps most notable for the insane amounts of heat they produce. So much heat, in fact, that if a spool of film slows down too much during projection, it will instantly begin to melt under the heat of the lamp. Heat is a spectral issue even in the diminutive pico projectors, even though they use LEDs; most of them will turn themselves off after an hour or so to prevent heat damage to their little LCDs, which are built on small silicon semiconductor chips. Liquid crystal on silicon, or LCoS, work sorta like the DLP projectors pictured above; individual pixels are turned off or on to either block or transmit light that comes from the LED lamps. Check out this graphic, from HowStuffWorks, and their explanation:
Says HowStuffWorks : "The semiconductor has a reflective, pixilated surface. The lamp shines light through a polarizing filter and onto the device, and the liquid crystals act like gates or valves, controlling the amount of light that reaches the reflective surface. The more voltage a particular pixel's crystal receives, the more light the crystal allows to pass. It takes several layers of different materials to do this."
So what the hell is the point of making a projector so small? And why does it need to be grafted to a device like a phone or a digital camera ? One word: maps.
In a paper called "Projector Phone: A Study of Using Mobile Phones with Integrated Projector for Interaction with Maps," researchers at the University of Munich in Germany and Lancaster University in the U.K. argue that the future of mobile mapping is going to blow away our tiny little smartphone screens. "Applications that require interaction with large amounts of information will benefit from the large projection and its high resolution," the paper says. It goes on to describe an experiment: How could people use projector-phones to make maps more useful?
The answer: Extend your "augmented reality" mapping to the nearest wall. After giving experimental groups a series of map-related tasks (pictured above), the paper found that "results clearly show that the higher resolution and display size improved the task completion time, reduced the time needed for scrolling, leads to a lower error rate and a very positive user feedback." The researchers determined that users could use a phone's touchscreen as an input device (ie, keyboard) while the projector did all the displaying on a nearby wall. "The results show that participants preferred to use both displays when considering simplicity, comfortableness, enjoyment and perceived speed," the paper says. You can read the rest of it below.
The LG Expo just came out  on AT&T for $200 after rebate. The projector add-on is $180, and will be available within weeks, according to LG.
More Undead Tech: