DIY HUD using a cheap LED projector?

[Laggger164]

EDIT: Read the second post, I found the information needed myself, finally.

 

So I am just asking whether it would be possible and or practical to use the transparent LCD screen from a projector to make a DIY HUD that could cover a large part of your vision on a helmet.

 

Would it require some adjustments to get both eyes to focus on it?

 

Cheers engineers!

Edited May 30, 2017 by Laggger164

[Laggger164]

So I found out that unless I want to put a metric crapton of backlight behind an LCD screen, I will not be able to use that as a HUD.

 

So LCDs are not an option. But what about small OLED displays? I know that 4D systems were selling some back in the day, but were discontinued due to quality issues. But from what I know, every OLED screen is transparent. It just has a reflective material on the back.

 

So I am asking now is if I was to buy something like this: https://www.adafruit.com/product/1431

Took it apart and removed the reflective foil at the back. Would it be suitable to use as a HUD display for darker-moderately bright conditions? Would it even be possible to take the foil away?

[rhyseyness]

There is a reason HUDs are only on planes and are expensive af.

It's not really something you can DIY effectively.

There's so many optical effects and stuff to deal with.

Ideally you'd use a waveguide and the innards of a projector (DMD and LED).

However, this is massively expensive and impractical.

It's not as simple as shining an image onto some glass.

Sorry to shoot you down, but hope this helps nonetheless.

[Laggger164]

1 hour ago, rhyseyness said:

There is a reason HUDs are only on planes and are expensive af.

It's not really something you can DIY effectively.

There's so many optical effects and stuff to deal with.

Ideally you'd use a waveguide and the innards of a projector (DMD and LED).

However, this is massively expensive and impractical.

It's not as simple as shining an image onto some glass.

Sorry to shoot you down, but hope this helps nonetheless.

How about a transparent OLED display?

Or is it possible to tear apart a cheap oled display for arduinos and make that transparent? I know the resolution won't be that great, but for simple text should be fine.

[rhyseyness]

34 minutes ago, Laggger164 said:

How about a transparent OLED display?

Or is it possible to tear apart a cheap oled display for arduinos and make that transparent? I know the resolution won't be that great, but for simple text should be fine.

Never tried, so wouldn't know.

If it's cheap, might be worth a go

I've worked extensively on HUD's though, and I know they aren't simple/cheap

[Laggger164]

3 hours ago, rhyseyness said:

Never tried, so wouldn't know.

If it's cheap, might be worth a go

I've worked extensively on HUD's though, and I know they aren't simple/cheap

I know, but I am trying to make something that displays simple text, maybe very jagged icons, maybe I will just put them on a side, top or bottom of the helmet's visor shield and just look at those. I dunno, this is just an idea

[rhyseyness]

On 2017-5-31 at 3:36 PM, Laggger164 said:

I know, but I am trying to make something that displays simple text, maybe very jagged icons, maybe I will just put them on a side, top or bottom of the helmet's visor shield and just look at those. I dunno, this is just an idea

Definitely worth having a go!

Don't wanna crush your dreams

I'd love to see what you can come up with!

Shame no-one here seems to know enough about displays

[Mira Yurizaki]

On 5/31/2017 at 1:54 AM, rhyseyness said:

There is a reason HUDs are only on planes and are expensive af.

It's not really something you can DIY effectively.

There's so many optical effects and stuff to deal with.

Ideally you'd use a waveguide and the innards of a projector (DMD and LED).

However, this is massively expensive and impractical.

It's not as simple as shining an image onto some glass.

Sorry to shoot you down, but hope this helps nonetheless.

If you were trying to aim for aircraft grade HUDs, then sure. You have to do infinite focusing and something on the glass itself so it reflects the display and not be distracting. But if those aren't your requirements, then it really is simple as shining an image onto glass. And even then, attaining infinite focus may not be that hard.

 

There's a few items on Amazon for add-in HUDs on cars. Sure they're not as nice as factory HUDs, but they're there.

[Gay4Jesus]

I'm actually working on a motorcycle helmet mounted HUD at the moment. I'm not an expert by any means, but I can share what I'm doing and what I've learned.

 

You can't just stick a transparent LCD in front of your face unless you are OK with placing it several feet away, and even that wouldn't give you that cool, "image in an image" HUD look. A transparent LCD is not actually particularly beneficial because the problem of focusing on something a few inches from your eyes is much more difficult to deal with than getting a transparent image.

 

To explain why, I'm gonna have to explain some biology and optical theory, so bear with me. If you already know this, feel free to skip ahead.

 

The light coming off of an screen (OLED, LCD, whatever) can be treated a bit like a lightbulb; light shoots straight away from it in all directions. This is usually depicted as a point with "rays" shooting off of it in all directions, kinda like a sun drawn by a 1st grader. The rays are said to "diverge," meaning they spread out more and more the further you go from the point. If the viewer is close to the screen, a larger portion of those rays makes it into their eyes, and the angle between the rays is large. If the viewer is far away, they only receive a small portion of the rays, and the angle between them is small. If the viewer is far enough away, the angle between the rays is so small you can ignore it; effectively, the rays of light coming into their eye are parallel to one another.

 

That's kinda hard to visualize, so here's a diagram. You can also draw a dot on a piece of paper, then draw 2 line segments of equal length, one close to the dot and one far from the dot. Draw lines from the ends of the line segments to the dot. Notice that the angle between the lines is much smaller for the far segment is than it is for the near segment.

 

Why does that matter? In order to see an image clearly, the lens in your eye needs to focus that image onto the back of your retina. Images are focused when the diverging rays coming off an object are bent (or "refracted") so that they converge on a single point. Your eye does this by using little muscles to actually squish the lens in your eye into a different shape, changing the focal length (more on focal length later) in a process called "accommodation." When relaxed, the human eye is focused for distant objects, and the closer the object is, the harder those little muscles have to work. That's why staring at your phone screen for long periods of time makes your eyes tired, BTW: your eye muscles get fatigued. Here's a good diagram explaining that.

 

So unfortunately, just mounting a screen a few inches from your face would be too uncomfortable to wear for long periods without discomfort. That's not the only problem though; it would also be impossible to see anything else while focusing on the screen. Since the screen is only a few inches away and the road in front of you is far away, focusing on the screen would mean that the background would be out of focus. Focusing on the background would make the screen a big blurry mess. Not only would it be extremely uncomfortable to use, but it wouldn't give you the cool floating HUD effect and it would be pretty unsafe to use while driving.

 

If you want to see what this would be like, hold your phone a few inches in front of one eye while looking at a tree or something far away with the other. Try to keep the tree in focus while also making out the screen. It's impossible, and you'll probably get a headache or throw up if you try for any length of time.

 

So how to do things like Google Glass and military HUD's work? Optics, that's how. You need to trick your eye into thinking the screen is very distant, instead of a few inches from your face. Remember how the rays coming from distant objects are more or less parallel?  Well, you can take the diverging light coming from a point a few inches away and use a lens to refract those rays so they are parallel to one another. That's called "collimation," and as far as your eye is concerned, your screen is now floating in space out near the horizon instead of a few inches from your eyeballs.

 

The easiest way to do this (although not the most compact) is to use a single element convex lens like a magnifying glass placed a distance from the screen equal to the focal length of the lens. Focal length (f) is defined as the distance from the center of the lens at which parallel (collimated) light entering a lens will be focused. It also works the other way, meaning that light entering the lens from a point source will be collimated if the point source is 1f from the lens. Here's a diagram.

 

If you get a lens with a focal length of 5" and place it 5" away from the screen, the screen will appear larger and will be focused on infinity. That means you will be able to keep the screen AND the background in focus simultaneously. Unfortunately, you will only be able to see the background with one eye, since the other is looking directly through the lens. Everything beyond the screen will be heavily distorted and out of focus (imagine trying to walk around looking through a magnifying glass). Depending on what you're doing, this could be fine: if you're using it for gaming or watching movies or just for shits and grins, binocular vision may not be important. VR goggles are essentially just two of these stuck together.. You may also be able to mount it to your helmet in such a way that you can glance at it when you need to, but it doesn't significantly block your vision the rest of the time. If you want the full "image in an image" HUD experience though, you're going to have to go a step further.

 

You need a way to combine the light coming from the background with the collimated light from the screen so you can see them both simultaneously with the same eye. A semi-transparent mirror positioned at an angle relative to the lens works great for this (#32 in this diagram). This is usually called a "collector," or a "combiner" in the context of optical imaging. This is how most optical gunsights and aircraft HUD's work. The downside is that it only allows SOME of the background light through, making the image darker. This may or may not matter to you, but if it does, there are all sorts of exotic and no doubt grotesquely expensive coatings designed to get around that particular issue. Regular old acrylic one way mirrors are a much cheaper option, or you could try to source some teleprompter glass. I haven't played with that, but I hear it works great.

 

You may notice in that last diagram that there are actually two mirrors: the collector and another normal mirror below it. There are two reasons for this: packaging (these things can get pretty large depending on the focal length of the lenses and the screen size) and flipping the image. If you can figure out a way to reverse the image on your screen, this extra mirror is not needed. Otherwise, the image will be reversed with only one mirror. You may also notice that almost all commercial HUD's use a combination of concave mirrors, multi-element optics and lots of other fancy shit. This stuff improves the image quality by removing optical problems caused by simple spherical lenses and can also shrink the device, but the cost and complexity increases massively every time you add a new optical element. I'm not an optics expert, so I opted (ha) to use a single lens and deal with the minor optical aberrations. It's not important for what I'm doing, since I'm not using my HUD to aim the autocannon on a helicopter gunship.

 

My goal was to display Google Maps or a similar navigation app on a HUD so I could get directions on my motorcycle without having to look down. I suppose I could have just used headphones, but where's the fun in that? The current iteration uses a smartwatch (a Moto 360 V1, to be specific) as the screen, and a small fresnel lens with a focal length of roughly 110mm as the collimating lens. My first version used a convex glass lens with a 50mm focal length, but it had too much magnification to be useful in this situation: I would have to position the collector very close to my eye in order to see the entire screen, which would be sketchy as hell in the event of a crash. I developed another version using a second, concave lens to reduce the effective magnification, but it ended up only marginally smaller than the much simpler single lens fresnel setup. My collector is a piece of one way acrylic mirror from Tap plastics, and the mirror is... well, a mirror. I had to use a mirror, because as far as I can tell, there is no way to flip the screen on the Android Wear OS. The casing is 3d printed from PLA, and I tested the second prototype this past weekend by just hot-gluing the damn thing to my helmet. Stupid looking, but functional.

 

Overall, it worked surprisingly well for a janky homebuilt contraption. I was able to get to where I was going without getting lost, and it wasn't overly distracting while I was riding. It also stayed on my head and didn't get blown around too badly at freeway speeds, although I would like to make it more aerodynamic. I did uncover some problems though:

1. I positioned it fairly low in my field of view so it wouldn't be distracting. Unfortunately this had the effect of placing the image over the road a few feet in front of me instead of on infinity. It made focusing on the image somewhat difficult, and my eyes ended up a bit tired after an hour or so of using it. I'm going to adjust the mirror to fix this on the next iteration.

2. The backlight on the watch is nowhere near bright enough in direct sunlight. It was difficult to see everywhere except under overpasses.

3. The Google Maps color scheme sucks for a HUD. The contrast is very low and the background is white, meaning you're always looking at the road through a glare. The light colors make the brightness issue much worse. Ideally, the background would be black and the roads would be a contrasting color like bright green or white. I am currently looking for a fix for that, but the Wear app library is pretty limited.

4. There is no easy way to change the brightness while driving. It's actually too bright at night, and the only way to change it at the moment is to remove the watch from the housing and manually change the brightness.

 

I'm going to stick with the smartwatch setup for now, but it's obviously not ideal. I am looking into screencasting an HUD app (of which there are several for Android, just not for Wear) from my phone onto one of those nifty pico-projectors, and using that instead of a screen. That should fix the brightness issue, and using full-fat Android would give me a ton more options for graphical interfaces. If anyone knows a good GPS navigation suite that will run on a Raspberry Pi or something, I'd be interested in that too.

 

You mentioned you wanted something that would " could cover a large part of your vision on a helmet," but that is quite a bit more difficult than a simple monocular setup. In order to cover a wide area of your vision, the collector either has to be very large or very close to your eye.

A system close to the eye would be expensive and hard to make (lots of very complex optics and an extremely tiny, high resolution light engine), and would be inside the helmet with you. That could get very ugly in a crash (as in "removing shattered acrylic from your eyes" ugly). A large-collector system would be very difficult to integrate into a helmet and would probably blow your head around like crazy at high-ish speeds. It would be technically feasible for a hobbyist, but it would be difficult to make it practical.

 

There's also the issue of what to do with the thing. What do you need to display while you're driving that would take up that much space? Navigation, and maybe some basic vehicle info is really all you need, and that shouldn't be in the center of your vision anyway. A small display just above or below your field of view is a much easier project, and it will probably do everything you need it to anyway. Start small and work your way up if you really want Iron Man-vision while you're riding.

 

Anyhoo, a DIY HUD is extremely possible if you're interested.

[SlipperyPete]

On 6/13/2017 at 3:15 PM, Gay4Jesus said:

I'm actually working on a motorcycle helmet mounted HUD at the moment. I'm not an expert by any means, but I can share what I'm doing and what I've learned.

 

You can't just stick a transparent LCD in front of your face unless you are OK with placing it several feet away, and even that wouldn't give you that cool, "image in an image" HUD look. A transparent LCD is not actually particularly beneficial because the problem of focusing on something a few inches from your eyes is much more difficult to deal with than getting a transparent image.

 

To explain why, I'm gonna have to explain some biology and optical theory, so bear with me. If you already know this, feel free to skip ahead.

 

The light coming off of an screen (OLED, LCD, whatever) can be treated a bit like a lightbulb; light shoots straight away from it in all directions. This is usually depicted as a point with "rays" shooting off of it in all directions, kinda like a sun drawn by a 1st grader. The rays are said to "diverge," meaning they spread out more and more the further you go from the point. If the viewer is close to the screen, a larger portion of those rays makes it into their eyes, and the angle between the rays is large. If the viewer is far away, they only receive a small portion of the rays, and the angle between them is small. If the viewer is far enough away, the angle between the rays is so small you can ignore it; effectively, the rays of light coming into their eye are parallel to one another.

 

That's kinda hard to visualize, so here's a diagram. You can also draw a dot on a piece of paper, then draw 2 line segments of equal length, one close to the dot and one far from the dot. Draw lines from the ends of the line segments to the dot. Notice that the angle between the lines is much smaller for the far segment is than it is for the near segment.

 

Why does that matter? In order to see an image clearly, the lens in your eye needs to focus that image onto the back of your retina. Images are focused when the diverging rays coming off an object are bent (or "refracted") so that they converge on a single point. Your eye does this by using little muscles to actually squish the lens in your eye into a different shape, changing the focal length (more on focal length later) in a process called "accommodation." When relaxed, the human eye is focused for distant objects, and the closer the object is, the harder those little muscles have to work. That's why staring at your phone screen for long periods of time makes your eyes tired, BTW: your eye muscles get fatigued. Here's a good diagram explaining that.

 

So unfortunately, just mounting a screen a few inches from your face would be too uncomfortable to wear for long periods without discomfort. That's not the only problem though; it would also be impossible to see anything else while focusing on the screen. Since the screen is only a few inches away and the road in front of you is far away, focusing on the screen would mean that the background would be out of focus. Focusing on the background would make the screen a big blurry mess. Not only would it be extremely uncomfortable to use, but it wouldn't give you the cool floating HUD effect and it would be pretty unsafe to use while driving.

 

If you want to see what this would be like, hold your phone a few inches in front of one eye while looking at a tree or something far away with the other. Try to keep the tree in focus while also making out the screen. It's impossible, and you'll probably get a headache or throw up if you try for any length of time.

 

So how to do things like Google Glass and military HUD's work? Optics, that's how. You need to trick your eye into thinking the screen is very distant, instead of a few inches from your face. Remember how the rays coming from distant objects are more or less parallel?  Well, you can take the diverging light coming from a point a few inches away and use a lens to refract those rays so they are parallel to one another. That's called "collimation," and as far as your eye is concerned, your screen is now floating in space out near the horizon instead of a few inches from your eyeballs.

 

The easiest way to do this (although not the most compact) is to use a single element convex lens like a magnifying glass placed a distance from the screen equal to the focal length of the lens. Focal length (f) is defined as the distance from the center of the lens at which parallel (collimated) light entering a lens will be focused. It also works the other way, meaning that light entering the lens from a point source will be collimated if the point source is 1f from the lens. Here's a diagram.

 

If you get a lens with a focal length of 5" and place it 5" away from the screen, the screen will appear larger and will be focused on infinity. That means you will be able to keep the screen AND the background in focus simultaneously. Unfortunately, you will only be able to see the background with one eye, since the other is looking directly through the lens. Everything beyond the screen will be heavily distorted and out of focus (imagine trying to walk around looking through a magnifying glass). Depending on what you're doing, this could be fine: if you're using it for gaming or watching movies or just for shits and grins, binocular vision may not be important. VR goggles are essentially just two of these stuck together.. You may also be able to mount it to your helmet in such a way that you can glance at it when you need to, but it doesn't significantly block your vision the rest of the time. If you want the full "image in an image" HUD experience though, you're going to have to go a step further.

 

You need a way to combine the light coming from the background with the collimated light from the screen so you can see them both simultaneously with the same eye. A semi-transparent mirror positioned at an angle relative to the lens works great for this (#32 in this diagram). This is usually called a "collector," or a "combiner" in the context of optical imaging. This is how most optical gunsights and aircraft HUD's work. The downside is that it only allows SOME of the background light through, making the image darker. This may or may not matter to you, but if it does, there are all sorts of exotic and no doubt grotesquely expensive coatings designed to get around that particular issue. Regular old acrylic one way mirrors are a much cheaper option, or you could try to source some teleprompter glass. I haven't played with that, but I hear it works great.

 

You may notice in that last diagram that there are actually two mirrors: the collector and another normal mirror below it. There are two reasons for this: packaging (these things can get pretty large depending on the focal length of the lenses and the screen size) and flipping the image. If you can figure out a way to reverse the image on your screen, this extra mirror is not needed. Otherwise, the image will be reversed with only one mirror. You may also notice that almost all commercial HUD's use a combination of concave mirrors, multi-element optics and lots of other fancy shit. This stuff improves the image quality by removing optical problems caused by simple spherical lenses and can also shrink the device, but the cost and complexity increases massively every time you add a new optical element. I'm not an optics expert, so I opted (ha) to use a single lens and deal with the minor optical aberrations. It's not important for what I'm doing, since I'm not using my HUD to aim the autocannon on a helicopter gunship.

 

My goal was to display Google Maps or a similar navigation app on a HUD so I could get directions on my motorcycle without having to look down. I suppose I could have just used headphones, but where's the fun in that? The current iteration uses a smartwatch (a Moto 360 V1, to be specific) as the screen, and a small fresnel lens with a focal length of roughly 110mm as the collimating lens. My first version used a convex glass lens with a 50mm focal length, but it had too much magnification to be useful in this situation: I would have to position the collector very close to my eye in order to see the entire screen, which would be sketchy as hell in the event of a crash. I developed another version using a second, concave lens to reduce the effective magnification, but it ended up only marginally smaller than the much simpler single lens fresnel setup. My collector is a piece of one way acrylic mirror from Tap plastics, and the mirror is... well, a mirror. I had to use a mirror, because as far as I can tell, there is no way to flip the screen on the Android Wear OS. The casing is 3d printed from PLA, and I tested the second prototype this past weekend by just hot-gluing the damn thing to my helmet. Stupid looking, but functional.

 

Overall, it worked surprisingly well for a janky homebuilt contraption. I was able to get to where I was going without getting lost, and it wasn't overly distracting while I was riding. It also stayed on my head and didn't get blown around too badly at freeway speeds, although I would like to make it more aerodynamic. I did uncover some problems though:

1. I positioned it fairly low in my field of view so it wouldn't be distracting. Unfortunately this had the effect of placing the image over the road a few feet in front of me instead of on infinity. It made focusing on the image somewhat difficult, and my eyes ended up a bit tired after an hour or so of using it. I'm going to adjust the mirror to fix this on the next iteration.

2. The backlight on the watch is nowhere near bright enough in direct sunlight. It was difficult to see everywhere except under overpasses.

3. The Google Maps color scheme sucks for a HUD. The contrast is very low and the background is white, meaning you're always looking at the road through a glare. The light colors make the brightness issue much worse. Ideally, the background would be black and the roads would be a contrasting color like bright green or white. I am currently looking for a fix for that, but the Wear app library is pretty limited.

4. There is no easy way to change the brightness while driving. It's actually too bright at night, and the only way to change it at the moment is to remove the watch from the housing and manually change the brightness.

 

I'm going to stick with the smartwatch setup for now, but it's obviously not ideal. I am looking into screencasting an HUD app (of which there are several for Android, just not for Wear) from my phone onto one of those nifty pico-projectors, and using that instead of a screen. That should fix the brightness issue, and using full-fat Android would give me a ton more options for graphical interfaces. If anyone knows a good GPS navigation suite that will run on a Raspberry Pi or something, I'd be interested in that too.

 

You mentioned you wanted something that would " could cover a large part of your vision on a helmet," but that is quite a bit more difficult than a simple monocular setup. In order to cover a wide area of your vision, the collector either has to be very large or very close to your eye.

A system close to the eye would be expensive and hard to make (lots of very complex optics and an extremely tiny, high resolution light engine), and would be inside the helmet with you. That could get very ugly in a crash (as in "removing shattered acrylic from your eyes" ugly). A large-collector system would be very difficult to integrate into a helmet and would probably blow your head around like crazy at high-ish speeds. It would be technically feasible for a hobbyist, but it would be difficult to make it practical.

 

There's also the issue of what to do with the thing. What do you need to display while you're driving that would take up that much space? Navigation, and maybe some basic vehicle info is really all you need, and that shouldn't be in the center of your vision anyway. A small display just above or below your field of view is a much easier project, and it will probably do everything you need it to anyway. Start small and work your way up if you really want Iron Man-vision while you're riding.

 

Anyhoo, a DIY HUD is extremely possible if you're interested.

this sounds really awesome. can i get some pics or a link to a build log or something?

[Laggger164]

On 13. 6. 2017 at 9:15 PM, Gay4Jesus said:

I'm actually working on a motorcycle helmet mounted HUD at the moment. I'm not an expert by any means, but I can share what I'm doing and what I've learned.

 

You can't just stick a transparent LCD in front of your face unless you are OK with placing it several feet away, and even that wouldn't give you that cool, "image in an image" HUD look. A transparent LCD is not actually particularly beneficial because the problem of focusing on something a few inches from your eyes is much more difficult to deal with than getting a transparent image.

 

To explain why, I'm gonna have to explain some biology and optical theory, so bear with me. If you already know this, feel free to skip ahead.

 

The light coming off of an screen (OLED, LCD, whatever) can be treated a bit like a lightbulb; light shoots straight away from it in all directions. This is usually depicted as a point with "rays" shooting off of it in all directions, kinda like a sun drawn by a 1st grader. The rays are said to "diverge," meaning they spread out more and more the further you go from the point. If the viewer is close to the screen, a larger portion of those rays makes it into their eyes, and the angle between the rays is large. If the viewer is far away, they only receive a small portion of the rays, and the angle between them is small. If the viewer is far enough away, the angle between the rays is so small you can ignore it; effectively, the rays of light coming into their eye are parallel to one another.

 

That's kinda hard to visualize, so here's a diagram. You can also draw a dot on a piece of paper, then draw 2 line segments of equal length, one close to the dot and one far from the dot. Draw lines from the ends of the line segments to the dot. Notice that the angle between the lines is much smaller for the far segment is than it is for the near segment.

 

Why does that matter? In order to see an image clearly, the lens in your eye needs to focus that image onto the back of your retina. Images are focused when the diverging rays coming off an object are bent (or "refracted") so that they converge on a single point. Your eye does this by using little muscles to actually squish the lens in your eye into a different shape, changing the focal length (more on focal length later) in a process called "accommodation." When relaxed, the human eye is focused for distant objects, and the closer the object is, the harder those little muscles have to work. That's why staring at your phone screen for long periods of time makes your eyes tired, BTW: your eye muscles get fatigued. Here's a good diagram explaining that.

 

So unfortunately, just mounting a screen a few inches from your face would be too uncomfortable to wear for long periods without discomfort. That's not the only problem though; it would also be impossible to see anything else while focusing on the screen. Since the screen is only a few inches away and the road in front of you is far away, focusing on the screen would mean that the background would be out of focus. Focusing on the background would make the screen a big blurry mess. Not only would it be extremely uncomfortable to use, but it wouldn't give you the cool floating HUD effect and it would be pretty unsafe to use while driving.

 

If you want to see what this would be like, hold your phone a few inches in front of one eye while looking at a tree or something far away with the other. Try to keep the tree in focus while also making out the screen. It's impossible, and you'll probably get a headache or throw up if you try for any length of time.

 

So how to do things like Google Glass and military HUD's work? Optics, that's how. You need to trick your eye into thinking the screen is very distant, instead of a few inches from your face. Remember how the rays coming from distant objects are more or less parallel?  Well, you can take the diverging light coming from a point a few inches away and use a lens to refract those rays so they are parallel to one another. That's called "collimation," and as far as your eye is concerned, your screen is now floating in space out near the horizon instead of a few inches from your eyeballs.

 

The easiest way to do this (although not the most compact) is to use a single element convex lens like a magnifying glass placed a distance from the screen equal to the focal length of the lens. Focal length (f) is defined as the distance from the center of the lens at which parallel (collimated) light entering a lens will be focused. It also works the other way, meaning that light entering the lens from a point source will be collimated if the point source is 1f from the lens. Here's a diagram.

 

If you get a lens with a focal length of 5" and place it 5" away from the screen, the screen will appear larger and will be focused on infinity. That means you will be able to keep the screen AND the background in focus simultaneously. Unfortunately, you will only be able to see the background with one eye, since the other is looking directly through the lens. Everything beyond the screen will be heavily distorted and out of focus (imagine trying to walk around looking through a magnifying glass). Depending on what you're doing, this could be fine: if you're using it for gaming or watching movies or just for shits and grins, binocular vision may not be important. VR goggles are essentially just two of these stuck together.. You may also be able to mount it to your helmet in such a way that you can glance at it when you need to, but it doesn't significantly block your vision the rest of the time. If you want the full "image in an image" HUD experience though, you're going to have to go a step further.

 

You need a way to combine the light coming from the background with the collimated light from the screen so you can see them both simultaneously with the same eye. A semi-transparent mirror positioned at an angle relative to the lens works great for this (#32 in this diagram). This is usually called a "collector," or a "combiner" in the context of optical imaging. This is how most optical gunsights and aircraft HUD's work. The downside is that it only allows SOME of the background light through, making the image darker. This may or may not matter to you, but if it does, there are all sorts of exotic and no doubt grotesquely expensive coatings designed to get around that particular issue. Regular old acrylic one way mirrors are a much cheaper option, or you could try to source some teleprompter glass. I haven't played with that, but I hear it works great.

 

You may notice in that last diagram that there are actually two mirrors: the collector and another normal mirror below it. There are two reasons for this: packaging (these things can get pretty large depending on the focal length of the lenses and the screen size) and flipping the image. If you can figure out a way to reverse the image on your screen, this extra mirror is not needed. Otherwise, the image will be reversed with only one mirror. You may also notice that almost all commercial HUD's use a combination of concave mirrors, multi-element optics and lots of other fancy shit. This stuff improves the image quality by removing optical problems caused by simple spherical lenses and can also shrink the device, but the cost and complexity increases massively every time you add a new optical element. I'm not an optics expert, so I opted (ha) to use a single lens and deal with the minor optical aberrations. It's not important for what I'm doing, since I'm not using my HUD to aim the autocannon on a helicopter gunship.

 

My goal was to display Google Maps or a similar navigation app on a HUD so I could get directions on my motorcycle without having to look down. I suppose I could have just used headphones, but where's the fun in that? The current iteration uses a smartwatch (a Moto 360 V1, to be specific) as the screen, and a small fresnel lens with a focal length of roughly 110mm as the collimating lens. My first version used a convex glass lens with a 50mm focal length, but it had too much magnification to be useful in this situation: I would have to position the collector very close to my eye in order to see the entire screen, which would be sketchy as hell in the event of a crash. I developed another version using a second, concave lens to reduce the effective magnification, but it ended up only marginally smaller than the much simpler single lens fresnel setup. My collector is a piece of one way acrylic mirror from Tap plastics, and the mirror is... well, a mirror. I had to use a mirror, because as far as I can tell, there is no way to flip the screen on the Android Wear OS. The casing is 3d printed from PLA, and I tested the second prototype this past weekend by just hot-gluing the damn thing to my helmet. Stupid looking, but functional.

 

Overall, it worked surprisingly well for a janky homebuilt contraption. I was able to get to where I was going without getting lost, and it wasn't overly distracting while I was riding. It also stayed on my head and didn't get blown around too badly at freeway speeds, although I would like to make it more aerodynamic. I did uncover some problems though:

1. I positioned it fairly low in my field of view so it wouldn't be distracting. Unfortunately this had the effect of placing the image over the road a few feet in front of me instead of on infinity. It made focusing on the image somewhat difficult, and my eyes ended up a bit tired after an hour or so of using it. I'm going to adjust the mirror to fix this on the next iteration.

2. The backlight on the watch is nowhere near bright enough in direct sunlight. It was difficult to see everywhere except under overpasses.

3. The Google Maps color scheme sucks for a HUD. The contrast is very low and the background is white, meaning you're always looking at the road through a glare. The light colors make the brightness issue much worse. Ideally, the background would be black and the roads would be a contrasting color like bright green or white. I am currently looking for a fix for that, but the Wear app library is pretty limited.

4. There is no easy way to change the brightness while driving. It's actually too bright at night, and the only way to change it at the moment is to remove the watch from the housing and manually change the brightness.

 

I'm going to stick with the smartwatch setup for now, but it's obviously not ideal. I am looking into screencasting an HUD app (of which there are several for Android, just not for Wear) from my phone onto one of those nifty pico-projectors, and using that instead of a screen. That should fix the brightness issue, and using full-fat Android would give me a ton more options for graphical interfaces. If anyone knows a good GPS navigation suite that will run on a Raspberry Pi or something, I'd be interested in that too.

 

You mentioned you wanted something that would " could cover a large part of your vision on a helmet," but that is quite a bit more difficult than a simple monocular setup. In order to cover a wide area of your vision, the collector either has to be very large or very close to your eye.

A system close to the eye would be expensive and hard to make (lots of very complex optics and an extremely tiny, high resolution light engine), and would be inside the helmet with you. That could get very ugly in a crash (as in "removing shattered acrylic from your eyes" ugly). A large-collector system would be very difficult to integrate into a helmet and would probably blow your head around like crazy at high-ish speeds. It would be technically feasible for a hobbyist, but it would be difficult to make it practical.

 

There's also the issue of what to do with the thing. What do you need to display while you're driving that would take up that much space? Navigation, and maybe some basic vehicle info is really all you need, and that shouldn't be in the center of your vision anyway. A small display just above or below your field of view is a much easier project, and it will probably do everything you need it to anyway. Start small and work your way up if you really want Iron Man-vision while you're riding.

 

Anyhoo, a DIY HUD is extremely possible if you're interested.

How the hell do I make this a spoiler?

 

Also, you seem to have done quite some extensive research. You really clarified a lot for me!

If you could post some pictures of your project it would be a lot easier to visualise.

 

So... I was thinking then that I wouldn't need anything always in my FOV, maybe something on the top, bottom or sides to just quickly look there to see how it's doing, or even make a display that would get into my FOV when I need it and get out when I don't, but that would require a big helmet.

 

Maybe I could use a fresnel lens on a display inside of the helmet? Like, it would be right on the display or slightly in front of it. But that might not work so well with text and numbers unless I get a very small, very high resolution display, which are not that easy to get yet. Unless I wanna modify a smartwatch to do that.

 

I am definitely going to look at it more in the future and you really did help me a lot! Thank you!

[purplehorace]

Hey Lagger did you get anywhere with your HUD idea ?

I'm thinking of experimenting with something on a vehicle for reflecting off the front screen.