Rear windshield projection

How would someone go about projecting onto the rear windshield of one’s car, so that the driver could still see behind him (within reason), and the car behind him, or cars passing, could see what was projected? Need a special type of projector, or maybe a special treatment for the glass, or what?

Any help is, as always, greatly appreciated.

I imagine it would be done in much the same way as the “heads-up” displays in fighter planes are done. Special Glass, Special Lenses, Special Projectors and a Special Wallet to pay for it all.

Just google on “Heads Up Displays”


      • Some cars already have this from the factory. Along the top edge of the rear windshield there is a spoiler that is partially translucent. The spoilers I recall seeing appeared to be factory-issue, they were integrated with the overall design theme of the car; and the rest of the car was nondescript. The glass itself was ordinary. The name of the car is printed on the underside of that spoiler so that the reflection is visible by anyone behind it, but not to the driver. I can’t remember the car, but it’s a hatchback body with a longer manufacturer name- not Ford, Chevy or Dodge. Mitsubishi -seems- like it, but the cars are a few years old now so I am having trouble finding anything on Google about it–everything I turn up is add-on accessories for the current model year.

It’s pretty tricky. If the windshield were concave, it could be set up to reflect a small bright LCD panel. Since it’s only reflected by the outside surface of the windshield, it would be totally invisible to the driver. But with a convex windshield the reflected image would be smaller than the projector panel, so you’d need a very large panel. Also, most rear windshields are tilted forward so the projector panel needs to be above the windshield.

You could use a regular projector to project the image onto the windshield. Instead of reflecting off the windshield surface, it would get scattered by dust and irregularities on the windshield surface. (Or more likely, you would etch or coat the windshield to provide the right amount of scattering.) Unfortunately the image will be visible to the driver, but maybe it won’t be too distracting - at least during the day, the outside view would be much brighter than the projected image. And you can optimize the surface to provide maximum scattering towards certain directions (i.e. towards other cars and not towards the driver).

But if I were designing such a system, I’d give up on projectors altogether and use light sources attached to the windshield. LEDs are pretty small. Lay them out on the windshield in, say, a 1/2" pitch grid pattern and you can still see through the gaps. The back side of each LED can be painted black so no light leaks towards the driver.

DougC – I know this isn’t the car you’re talking about, but the old Honda CRX’s had something like this – below the rear windshied where the deck lid started going vertical was a piece of glass that didn’t look at all like a piece of glass. From inside the car, though, it was essential for proper rear-view mirror function!

Toyota MR2: did a simple trick of reflecting the wing over the back window…the wing was dark, the letters were white and flipped…and the rear glass reflected the whole thing - white letters on a dark background reflected by the rear window.

Simple daylight reflection.

Some years ago, somebody marketed a wand with a line of LEDs along its length. The idea was that you would program a message into the wand, and when you waved it back and forth, the lights would blink in sequence so that the message would appear to be standing in mid-air.

This was marketed as a toy, but at least one commentator observed that it would be most useful for communicating a message such as “BACK OFF” to a tailgater. (Although many people might not use the work BACK!)

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Now that I think of it, my car’s speedometer cluster works with reflectivity. The glowing needles and computer display and some of the other LCD stuff is in the cluster where it normally would be on any old car, behind the highly-reflective black piece of lexan or plexiglass or whatever it’s made of. The numbers and lines and other gauge graphics, though, are on the underside of the dashboard; You’d have to stoop down under the steering wheel and look up under the dash to see them. But they glow really nicely when the car’s on, and they reflect off the lexan. The overall effect is pretty neat, making the indicator gauges look like some 3-dimensional hologram. It’s the nicest, most legible gauge cluster I’ve seen since back when I drove Hondas.
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Back on topic: it’d probably work for a rear-windshield, too.

Side note: my last Bonneville had a heads up display, always in the off position. I’m a gadget guy, but that thing was so dang distracting it was unusable for me. It was pretty neat, though, and a cool toy.

Guide Corporation developed a CHMSL (center high mounted stop lamp) based on the heads-up speedometer. The active part of the glass is invisible until the lamp image is projected from underneath. I don’t think they tried anything bigger than that. I suppose you’re picturing a message, such as, “Don’t tailgate, driver chewing tobacco.” or “I may be slow, but I’m ahead of you.”