How does this heads-up display work?

There is a new dashboard device called Navdy that is taking pre-orders now.

The device has a projector and a small transparent screen. I can’t figure out how projecting on a small screen about a foot away from you can look like something six feet in front of you.

Google Glass works by projecting the image directly onto your retina but that is not what’s going on with Navdy.

How do the optics of this device present a virtual image that is six feet in front of you?

so I won’t profess to this from a first-hand understanding…

but it’s a standard exercise in structuring the projective and combiner lenses to create a virtual image at “optical infinity”, the same distance you’re focused on when you look out to a distance (rather than a few centimeters or very few meters).

Stackexchange discussion

If I’m just restating what you’re asking, I’ll have to fall back to “it’s just optics”, in the sense that it’s just another application of standard optical design. I guess.

This banana works by projecting an image onto your retina. That’s really just a description of how your eyes work.

However, Google glass does so without creating an intervening ‘real’ image (i.e. it’s not projected onto a white screen for you to look at) - but that’s only a moving-image enhancement of the View Master.

  • it still sort of is. The optics of all of these devices are configured to create a virtual image at a comfortable or practical distance - not very different from the way that, say, a pair of binoculars creates a closer virtual image of a distant scene.

Let me ask this a slightly different way. How can you project an image onto a screen so that it creates a virtual image that is behind the screen? I understand how bananas and binoculars work. But in the Navdy the projection is onto a screen, and you are looking at the screen with no intervening optical device.

So my working theory is that the screen in this case is not a flat screen but is itself some sort of lens. If that’s so, then that’s as much of an answer as I need. But is that the case?

I haven’t seen the device, but I suspect that the description in the article isn’t correct. Heads-up displays classically don’t project onto a screen – they use a beamsplitter to give you a line of sight to a screen --there’s no projection. If you put a lens or lens system in front of that screen, you can make it act like something that is farther away than the screen really is. I suspect that’s what’s going on here – an interposed optical system provides a virtual image that appears to be six feet away from the viewer’s eyes. No projection used at all.

Of course, I might be wrong, and they’re doing something different. but that’s a credible, easily realized system with few optical parts.

It pretty much has to be a reflector or lens. A real image projected on a screen can’t be focused to appear focused anywhere other than the plane of the screen.

This might not be exactly how it’s done but the Pepper’s Ghost illusion shows an extremely classic way of projecting a virtual image onto a scene.

I think the “two meters ahead” is just marketing speak for “this small image overlaid on the road further away makes it seem like you have huge arrows painted on the road!”. If you look at their video at just after the 40-second mark, you see that the projector just beams it onto the screen, much like other car HUDs that use a semi-reflective sticker on your windshield.

It seems likely that it uses a reflector, not a matte screen (as with the Pepper’s Ghost illusion).

It probably uses a setup similar to that used in high-end flight simulators. See here specifically. The image is projected onto an intermediate screen, and then a complicated mirror is used to reflect that image to the eye.

It’s a bit hard to explain precisely how it works, but in essence, if you want to project something that looks far away, you want a highly collimated image (collimated = parallel light rays). If you imagine the triangle formed by a point on an object, and two points on opposite sides of your pupil, you can see that far objects will have a much skinnier triangle than close ones. The two long sides of the triangle get very close to parallel for very distant objects.

You may recall that a parabolic reflector takes light from a point source (say a light bulb) and converts to a beam of parallel rays (the beam from your spotlight). So, you want something along the same lines here (lenses and mirrors work the same way backwards as forwards), and probably with additional magnification.

There’s a very common misconception that the image you see in a mirror is at the same distance as the mirror itself. It’s not. If you’re standing two feet in front of your bathroom mirror, your reflection is four feet away from you. Yes, it’s further away than the mirror, behind where your bathroom wall is. This is not a problem.

Yep, though it should be noted that you only get a straight doubling for flat mirrors. Other shapes would have a different response. In this particular case, you wouldn’t get enough of a distance boost from a flat mirror, since the display piece is only a few inches from the screen.

I’m pretty sure you get the “distance boost” (effective focal length well beyond physical reflector distance) by the lens structures “upstream” of the reflector.

If you can bend the light beams into a convergence pattern identical to how they would converge from, say, two meters away, your eye will focus on them just as if they came from an object two meters away.

That’s what’s meant by “virtual image”.

It’s not just to make it look like a big screen further away, it’s to save your eyes from having to change focus between the screen and the road.

The product web site is at https://www.navdy.com/ (I am not shilling for the product but that site has the best images of the device). There are several shots of the device, including one exploded view.

The first image on their site (scroll down to the blonde lady in the green dress) shows a simulation that makes it look like there is a full image at the plane of the windshield.

The video referenced by Reply does’t give you the impression that there is a virtual image at a farther distance. It looks like you’re looking at a little bitty screen on the device.