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.

The underside

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.

Kopin LensKopin Lens

For reference, here are the VR4 optics:

Virtual Research VR4 OpticsVirtual Research 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

Kopin VGA Display From Vuzix VR920

And here is that display plonked infront of the VR4 optics

Kopin VGA with 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

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:

The main issue with small displays such as the Kopins is that you can’t get both a wide FOV and a large exit pupil. Exit pupil refers to how tightly aligned your eyeball needs to be with the lens and how close your cornea needs to be to the lens. In order to achieve a comfortable exit pupil (like on the Virtual Research VRs) you need an objective lens of about 25mm or more, so that you can look into the lens and see an aligned image even if you’re not looking directly into the center of the lens. Lens systems which magnify small things like the Kopin display are very intolerant of any error in alignment of the eyeball to the lens, and furthermore larger diameter lenses which have sufficient magnification are near impossible to make without introducing unacceptable distortion and chromatic aberration.

You could achieve a wide FOV with the Kopins by using a very small objective lens, perhaps 5-10mm dia., but you will need to bring it very close to the cornea – no eyeglasses permitted, and possible discomfort due to eyelash interference. Because the eyeball does not pivot around the cornea, instead pivoting about some internal point within the eye, as you pivot your eyes to take advantage of this wide FOV, the critical alignment of your cornea to the small lens is knocked out of whack.

In short, to achieve wide FOV with any sort of affordable optics, the lens diameter needs to be fairly large. You can sketch this out on a piece of paper if you like. From a side-view, draw an eyeball with cornea. Then place an edge on sketch of the lens at a distance from the cornea that is reasonable, perhaps 3/4″ to allow for eyeglasses. Then draw two lines emanating from the center of the cornea to the top and bottom edge of the lens. The angle between these two lines is your FOV. Try again with smaller and larger lenses, and/or different distances to the eye. In reality, it isn’t practical to use the edge zones of a lens. The usable area is a much smaller diameter.

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.

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