Afternoon Dave, Maximo,
Criticism is very much encouraged - I don't expect to sell many of these first products into the top-end market, but am very much interested in what you'd like to see in future products.
I'll start with the easy answers, get onto some more interesting ones, then what the future may hold:
MOT - no problem. The film is a tint (not opaque) and therefore exempt from the usual positioning restrictions. Don't restrict your view of the road ahead though, as that is a construction and use offence. (yes, you can have an MOT pass vehicle that's still illegal to use on-road) So long as it just hides some of the bonnet/road that you'd be interested in only when parking that's fine.
Drilling the dash - not necessary. I did it on my MX5 as replacement dashboards are 'free to collector' and take all of 30 minutes to swap, but you can just as easily tuck the wire between the screen and back of the dash and it'll come out in the same location behind the dash. (the benefits of asking one or two others to install these units is that they 'know their cars' and all the little tricks for hiding wiring etc easily) Most cars can take the 3 mm thick wire between screen/dash or a-pillar trim/dash.
Films on the screen - necessary I'm afraid. There are quite a few different techniques for getting a display up on the windscreen without a "special film on the glass" but the one thing that they have in common is a "special film" inside the glass! It isn't practical to replace the windscreen in an after market application so we've got to live with either ghosting (double/triple/quadruple images) or apply a film of some description. What type of light and film do we use though?
You can use polarised light and a 'clear' polarising film. In the same way that the sun reflecting off a wet road or the sea is mostly polarised the same way (hence polarised sunglasses), reflections off the glass are also polarised the same way, so if you add a 'clear' polarising film to the glass you can allow direct reflections (reflected ray) but cancel out most secondary reflections (refracted ray) as the way that they are polarised is different. Science here:
http://en.wikipedia.org/wiki/Brewster's_angle
The downside of this is that you're ditching the light you don't want (you actually ditch most of the light) and you now need a massively brighter initial light source to begin with. (5-10x brighter) As I understand it most OEMs do it this way, with a hulking great light source, running through a diffuser, then projecting through an LCD (which itself blocks most of the light as it works by darkening the areas you don't want to see, rather than lighting the areas you do want to see) to create the image and polarise the light, with a bunch of mirrors to shape the light before hitting a special windscreen with polarising filter in the HUD area.
Here's a little info on the E60 (I will remove this if BMW object) system - note the size of the projection unit here, and that's an OEM packaged solution!
http://www.speedview.co/useful/E60BMW/
One 'advantage' touted by BMW is that they virtual image appears at the end of the bonnet. The idea is that you eye takes less time to adjust from infinity focus to 2.2 metres and back to infinity than it does to adjust from infinity to 50 cm and back to infinity. This is absolutely true, especially as you get older and approach the need for reading glasses. An unfortunate side effect is that the display "draws your eye" toward the end of the bonnet. (it is shiny, bright, and the pattern changes compared to the road, and humans are programmed to respond to this) This they don't mention. The SpeedView has a similar effect - you move from 50 cm to about a metre, which is most of the benefit for those of reading glasses age but is still too close to you to draw your eye's attention too much.
Frickin' laser beams are another approach. You coat the windscreen with (transparent) phosphors (like an old CRT television/monitor) and then hit them with a laser to 'excite' them and make them glow (emit visible light) for a while. This creates a display 'on' the glass but is still in the experimental stage.
SpeedView uses a lower cost approach. I use non-polarised light and one-way mirror-tint film. The light is mostly reflected off the film and towards the driver. (you keep most of the light) The small amount of light that does pass through the film (or any that arrives from outside) is mostly reflected away. The primary advantage of this is that your light source doesn't need to be anywhere near as bright, but it does mean that you need to use a darker film. (approx 10% light transmission from outside)
I use a direct image rather mask. Instead of everything being light then blocking out the bits you don't want, I have everything dark then light just the bits that you want. This again reduces the lighting/power/heat dissipation requirement. I don't use mirrors and lenses to shape the image either - what you see is what you get - as for basic speed information with nice large digits a little distortion really doesn't hurt the same way it does when you're trying to display the whole of ESPN or Sky Sports on the windscreen as the OEMs prefer.
Together this greatly reduces the cost and footprint of the unit to the point that its practical to offer as an after market item.
Size and style of the digits - we do have options here. Quite a lot of thought goes into the style of these. In the SpeedView, they're styled (fairly skinny segments) and sized (15 mm digit height) so that one LED per segment (21 total) gives adequate and even brightness. A 30 mm high display with the same styling would need 4 LEDs per segment, and if made with slightly fatter segments we'd be talking 6 LEDs per segment.
Peak current draw (startup) is 350 mA on the SpeedView with the 21 digit LEDs and 6 additional LEDs at maximum brightness, and that's about 4 watts - actually quite s lot of heat for electronics to dissipate. Typical draw is much less - say 2 watts in bright daylight. Pop up some fat 30 mm digits and you're looking at 24 W peak, typically 12 W in bright daylight, and that's not quite a hot thing to be sat on your dash/would likely need an alloy backplate and heatsink. All doable, but its a level up in cost again.
13-segment displays are also an option - here's a 7 and a 13 for comparison. Again doable, but up a level in cost.
One thing that increasing the size of the display can be used for is reducing the impact of ghosting. The ghosting is only in one direction, and although 'in theory' it goes on infinitely, its really only the second image we have to worry about:
Now, if we design our display with this in mind - fat 'verticals' with thin 'horizontals' - and make it so that the top of the first reflection touches the bottom of the second reflection, we can improve the image quality without the film. See these two images, particularly the 13 segment display:
This will be sensitive to windscreen rake/windscreen angle, but
it may be possible to make a display where we deliberately take advantage of the ghosting to create the final image, or at least minimise its impact.
What do folks think to this?
) and the feedback; I've been deliberately hands-off to see what your thoughts were first 
Which Road Angel do you use on the road?
. Also, I am pretty thick with this stufff but if your one doesn't use GPS at all, then how do you make it dead accurate, factoring in different size wheels etc? I am genuinely interested in this so please don't think this is a troll post.
