From: Dave Stampe-Psy+Eng <[email-masked]>
Date: Wed, 23 Oct 91 20:16:58 -0400

Dave Wargo ([email-masked]) replies:

>>
>>I would be interested to hear descriptions of any homebrew eyephone
>>projects that have been completed.  What diplays are you using?
>>Monochrome or color?	Binocular or monocular?  Field of view?
>>Optics?  Are you monitoring the user's head positon?  Weight?
>>I'm trying to get an idea of the level of sophistication that
>>is currently being used, for use in future postings.
>>
>>Please post to the list, as I'm sure others care too!
>>
>>- Dave Stampe

>I am in the process of trying to put somthing together. I have tryed at
>first to use a couple of lcd tvs from radio shack and they just would not
>work for what I wanted to do.
>
>Right now we are trying a couple of sharp displays. I think the real trick
>is going to be the optics. This is not a trival task and I would be
>interested in hearing what can be done short of spending $800.00 for a pair
>of LEEP optics.
>
>Please pass along what you find out.
>
>Do you think that this could be the begining of a mail list for
>eyephone-list?
>
>Thanks
>
>Dave Wargo
>

What model of the Sharp displays are you using?  I recently did a (non-VR)
project using the 5" color NTSC/RGB displays (forget the model).  What did
you pay?  I can't seem to find smaller ones for less than $800 (maybe it's
my supplier though...)	and the really decent 5" models are over $1000.
From what I've seem recently, a lot of the newer pocket TV LCD display
have crummy resolution-- it's cheaper for them to digitally preprocess
the image than to use a decent LCD panel, I guess.

I guess the LEEP optics cost *is* dropping-- last time I talked to Eric
Howlett (designer of LEEP systems) they were still $3500 each.	Guess
he decided the big markup on 6 cheap plastic lenses and a case wasn't
helping sales (B_{) or maybe the production volume went up.
From what I know of the LEEP optics, though, it would be difficult to
add LCD panels, as the display's active areas must actually TOUCH at
the center to get a full display.  The monchrome LEEP units that I worked
with actually had part of the PCB cut away on each display, and bridged
with 40 or 50 wires, and the right panel was flipped over.  The original
NASA design used one 640x240 LCD panel to drive both eyes (not a bad
idea, if you can find one that has the right dimensions).

As for optics, I think that we should try to stick with single-lens
systems.  More lenses will either add distortion and chromatic abberation
or will reduce the field of view.  The LEEP system actually USES the
distortion and chromatic effects, and compensates for them at the
camera side.

The way to get a big FOV is to bring the displays close to the eye.  But
then the user can't focus properly, so a converging lens must be added.
I've checked out these types:

- Biconvex: Forget it , the distortion is TOO GREAT (unless you want it...)
- Planoconvex: Less distortion.  Fits nicely against glasses.
- Meniscus: Least distortion, but can interact with glasses.
- Aspherical prescription lenses: These have NO distortion over a fairly
  large FOV.  I think +10 or +12 Diopter is the strongest available
  with a decent FOV.  There is some interaction with glasses.  If you
  can get them, try out cataract surgery lenses.  Prescription lenses
  are special, as the back surface is ground to work properly with the
  eye "rolling" behind them.

A rig that Telepresence Systems in Vancouver put together abuot 3 years ago
(no they're not currently doing much on VR, concentrating instead on
3D video for telerobotics) used a pair of +10D glasses and color LCD
panels from handheld TVs.  There was a lot of other problems with the
system, but the optics worked (if you didn't need strong glasses).
Some adjustment was provided in the spacing between the displays and
the glasses, for about +/- 4 diopters for those who needed prescripton
eyewear.

It is probably better to mount the lenses on a frame than to use glasses,
because of the convenience factor.  Also, if the displays (mounted on
the headband) move with relation to the glasses on the head, it
causes a LOT of display shifting.

Some of the wholesalers for prescription eyewear stock preground plastic
lens blanks and may be willing to cut them to the desired shape.  I got
a set of +10D blanks cut at Imperial Optical in Toronto for $35 each
(or was it for the set? Lost the invoice).

Distortion may not be too great a consideration-- UNLESS you're using 2
displays for stereo!  Then the distortions cause a reverse-spherical
distortion in depth except at the center of the display, and loss of stereo
depth toward the display corners, due to vertical misalignment.  Look at
a sheet of grid paper thru the lenses you propose to use to judge the
amount of distortion.  To reduce the effect of distortion, set up the
displays and the 3D software for a 1 m convergence distance, as this
will minimuze disparity between the displays for a stereo range of
40 cm to infinity.

Another consideration with stereo displays is how to make the 2 displays
overlap in the visual field.  The average distance between a person's pupils
is about 6 cm, so unless you have small displays you can't just hang them
in front of the eyes-- they'll be too far apart.  One way to fix this is to
tilt the displays away from each other and use prisms to "bend" the images
back in front of the eyes.  This can also be achieved by offseting the lenses
(Brewster stereoscope).  Problem is, this causes a WEIRD warping of the
picture (vertical lines are bent), more color problems and blurring, and
greater distortion at the display edges.

The BEST method is to mount the displays SIDEWAYS and use 45 degree mirrors
to put the image in front of the eyes.	The picture gets flipped
left-to-right, though, but this isn't a problem if you can define new
fonts on the computer.	If you're using CRT-type displays, just exchange
the leads to the horizontal deflection yoke in the display, and everything's
back to normal.  Interestingly, this limits you to 54 degrees of horizontal
FOV, because the display must be as far from the lens as it is wide (draw a
picture and you'll see).
 
If you're REALLY picky, use front-surface mirrors to prevent double
reflections of bright lines on dark backgrounds.  Front surface mirrors
are pretty heavy, though, as they are usually 3/8" thick.  Edmund Scientific
(Efton in Canada) stocks then in various sizes.  If you want lighter
weight or cheaper mirrors, just get standard mirror glass.

You might also try flipping the LCD panels over, but that is difficult
with color LCD modules, because the electronics are then on the wrong side.
Also, the vertical view angle of the panels then gets flipped, causing
contrast mismatches between the left and right eye displays.

Now, how to decide on lens powers, display distances, etc?  The formula for
lens diopter is 1/D = 1/F - 1/d; where D is the lens diopter required,
F is the lens-to-display spacing, and d is the desired "apparant" distance
(I recommend 0.5 to 1.0 meters for comfort).  All distances are in meters.
Be aware that distortion increases with lens power!  Nondistorting
prescription glasses have smaller FOV's with increasing power, too.

For FOV, use tan(FOV/2) = s/d/2, where s is the pertinant dimension of the
display, and d is the lens-to-display distance.  Most display sizes are given
diagonally, with hsize 4/5 * diagonal, and vsize = 3/5 diagonal.  A factor
of 1.1 or 1.2 increase in effective FOV _MAY_ happen because of interactions
between strong lenses and eye movements.

There are some subtle problems with LCD displays.  First, the vertical viewing
angle range of these things is less than +/- 15 degrees, so if you have a
vertical FOV of 30 degrees or greater, the top and bottom of the scene
will be washed out or darkened.  This is why the new color LEEP optics use
a diffuser plate: the viewing angle increases at the cost of resolution.

The other problem with LCD color displays is the visible RGB pixel bands.
These may be a problem with wide-angle displays, due to the high
magnification.	And, in some cases, they cut down the usable resolution.
If a display's specs say "288 pixels H resolution, RGB stripe" then you
KNOW that there are 96 red pixels, 96 blue... etc: so if you want pure white,
your horizontal resolution is 96 pixels!  (This type of display gives good
apparant resolution for small FOV's, but not for large ones...)

What do I recommend for FOV and pixel size?  Well...  I think that we
should try for AT LEAST 54 degrees horizontally and 40 degrees vertically
(about 2 1/2 by 3 1/2 feet at arm's length).  This is sort of a happy
medium between psychophysical effects and the limits of single-lens
technology.  Pixel size should be LESS THAN 1/3 degree plus a diffuser
(actually, 1/10 degree or less is best, but let's not kid ourselves).
Using 320x200 resolution with the 54x40 degree FOV, we get 0.2x0.16 degree
pixels... hmm, not bad.  BTW, these numbers based on the human contrast
sensitivity function, which peaks at 10 pixels per degree.  This means
that bigger pixels will be tend to be seen as _pixels_ rather than a
continuous image.  A diffuser acts as a spatial lowpass filter, blurring
the pixels together.

Myself, I am _considering_ using some 4" portable B&W TV's because of
their low cast, rlatively large screen size and high resolution.
Also, pixel blurring can be done by adjusting the focus.  The only problem
is the weight-- they would have to be counterweighted, so the inertial load
on the head is pretty big.  Also no color... Have to think about it some more.

DISCLAIMER:
Some of the above is based on work with a 35x25 degree system, some is
knowledge of the technology and experience with an (older) LEEP system
I'm not the most knowledgable person about optics, so maybe someone
with more experience can design a cheap 2-lens system with less distortion
and the full FOV needed.  Myself, I subscribe to the KISS philosopy, but
it would be nice to have a higher lens power so smaller displays can be
used.

--------------------------------------------------------------------------
| My life is Hardware,                    |                              | 
| my destiny is Software,                 |         Dave Stampe          |
| my CPU is Wetware...                    |                              | 
| Anybody got a SDB I can borrow?         | [email-masked] |
__________________________________________________________________________

---

From: James Helman <[email-masked]>
Date: Thu, 24 Oct 91 01:04:46 -0700

Dave Wargo writes:

      I think the real trick is going to be the optics. This is not a
      trival task and I would be interested in hearing what can be
      done short of spending $800.00 for a pair of LEEP optics.

Dave Stamp writes:

   I guess the LEEP optics cost *is* dropping-- last time I talked to
   Eric Howlett (designer of LEEP systems) they were still $3500 each.

LEEP has a rather strong quantity based pricing policy.  Clearly group
purchases are in order:

   1st unit: $3000
   2nd-3rd:  $1500 each
   4th-7th:   $750 each

Jim Helman					Lab:  (415) 723-9127
Stanford University				FAX:  (415) 591-8165
([email-masked]) 			Home: (415) 593-1233

"The power of the computer is locked behind a door with no knob."
							-B. Laurel