HMD Project – Part 10

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.