More tangents
With a projector in bits, and before I revisit the original mounting plans, I decided to see if it would be possible to use an alternative display on the VR4 headset. I might as well, while it’s in bits.
I found a broken Vuzix VR920 on ebay, it was reported that ‘one of the eye’s didn’t work’ – as it turned out, one of the backlight power wires had broken off, with a brief bit of solder work (not the first on that unit, by the looks of things!) it was fully functional again – but this didn’t deter me from continuing my tear down (demolishing) job.
I wanted to see how easily is would be to mount the transmissive Kopin AMLCD’s from this unit into better optics. I had seen a previous tear down of some Vuzix eyewear and it showed two distinct display units. Sadly, this model, or perhaps just this version of this model is basically an unmodified Kopin OEM Module.
In the right hand picture you can see my solder fix 🙂
First thing I had to do was remove all of the nasty brown sticky tape, which hold everything together. I shit you not.
Here are the kopin optics. The clear lens on the right hand side is a hollow pyramid shaped thing.
For reference, here are the VR4 optics:
And the Kopin VGA Display – note that the backlight is integrated nicely, powered by the black / red connector. Also note the non-scary signal ribbon for the display! If only the pico projector was as nice as that. So from a mount-away-from-the-driver point of view, this unit is quite straight forward.
Kopin VGA Display From Vuzix VR920
And here is that display plonked infront of the VR4 optics
Kopin VGA with VR4 Optics
The brainiacs amongst you would have guessed that outcome, I suppose! that’s correct, the Kopin display, being 11mm in diameter, take up only 1/3 of the viewing area that the normal 33mm displays do!
So, what happens if we just magnify that display ? – we can either replace the center lens of the VR4 eyepiece, or add another magnification lens. The results are pretty much the same:
Kopin VGA with VR4 Optics + extra Lens
The image gets bigger, but the exit pupil is vastly reduced. this is what happens when you magnify things. This means that, when perfectly aligned, everything is hunky-dory, but moving the eyepiece, or your line of sight, even a little bit, causes the image to distort horribly.
I was at this time, completely new to all of this, I know nothing of lenses, focus, virtual images or exit pupils. I emailed Tone from VRtifacts, who, by now, must be getting really sick of my schoolboy questions!
His reply made sense:
I have since, found a document which using the power of maths and Science!(tm) describes a method of using small LCOS displays with a modified Erfle style eyepiece, whilst maintaining the FOV, eye relief and, all importantly, the exit pupil size.
I am unsure if I am allowed to repeat any of the document here, so I won’t, suffice to say, I had to pay for it.
I got it from here – you can follow that link for the abstract. Or, simply search for “Head-mounted display with LCOS using diffractive optical element” using your preferred on-line search engine.
The bottom line is, it may be possible to use a smaller display with my VR4 optics, but i’m not clever enough.
I could just rebuild using my tried and tested rear-projection method now, but, I have a few more things I want to try out first. Front projection, for a start.
Hi
I found your side, after mailing with Tone asking him a way to replace the displays in a VR4. I think your idee is a way to do this proper. What material you are using for the screan? Do you discribe the screan anywhere? I only read at the end of -6- you are looking the find a good one.
Thank you Andy
Hi, I never did decide upon a screen material, I tried a variety of high-end rear projection materials and some of the lighter ones did yield good results (after reducing the brightness considerably with linear filters). Nothing near as good as I would have liked, though. I have various glass materials to try out too. I gained some very good info through searching ‘DIY Depth of Field’ – since these devices use a high quality rear projection screen, some also utilise vibration to further reduce surface blemishes – though I would hope not to require this.
Since these things do appear to be solvable, and if not I can always switch to front projection (different headset, or perhaps just different mount, but VR4 optics).
My project has stalled a little as I need to find a way of extending the cable in the pico projector. As a proof of concept the weight was tolerable, but to be used for anything more I have to be able to move the electronics away from the light engine.
Hi,
Do you have a supplier of the linear filters to reduce the brightness?
Thank you Andy
The ones I picked on ebay (A few pence each) were unbranded… just said ‘filter’ on the box.
Hi, when a display is 22mm diagonal, you need only magnify 1,5 times to get 33mm.
Do you think the exit pupil is also to smal by magnifying this low?
Thanks Andy
That’s about the same size as the ForthDD LCOS display VR use in their VR1280… so it must work. I’ve tried a VR1280 and it was incredibly good visuals; though didn’t feel like 60degreesFOV; perhaps due to widescreen content ruining the vertical FOV
Hi
Do you think about using a emagine Z800 as back projection devive, placing the VR4 optics in front?
Thanks Andy
Indeed I have… also the same displays are in the Headplay, supplied at the moment my Dell in the US for a mere $150! (but would need two)
How works the Z800 and the Headplay?
Tahnka Andy
Oh, I havn’t tried. yet. Too much £$€.
Hi
What kind of problems you found with the screans exactly?
What a “good” does?
What a “bad” is doing?
Is it not plane enough, have inner strukture seen with the optic or what else?
Thanks Andy
Hello,
I have nearly the same problem like you..
There is two kopin vga in my head mounted display (922k), and one of the two display,the ribbon cable is out of the optic..
I tried to repair without success ..
Where can I buy only one VGA 922k display? (No answer from kopin.com)
Or how can I fix it?
Thanks.