Focus/fixation tolerance

[T3D John O. Merritt <JOMERRITT@xxxxxxxxxxx>]

TOLERANCE LEVELS FOR MISMTACH BETWEEN OCULAR FOCUS AND OCULAR FIXATION

I would like to serve as a collection/collation point
for any references or anecdotes related to the variation
among individuals for mismatch between ocular focus and
ocular fixation  (visual accommodation and convergence).

The Boeing guide for the design of photointerpretation equipment
suggests a liberal limit of 0.75 diopters mismatch between
the observer's focus distance (usually taken to be the screen
in a stereo display) and the observer's fixation distance,
which could be either on the screen or in front of or behind the
screen.

It would be nice to have a family of curves showing how long
it takes for various percentages of the population to become
aware of discomfort when viewing various degrees of discrepancy
between where the eyes are focused and where they are fixated.

This same cumulative plot could be used to present data on
how long it takes for various degrees of vertical or rotational
misalignment to create an awareness of discomfort.

In addition to perceived discomfort, there wouild also be data
on adverse after-effects such as headache, difficulty in normal
stereo vision (fusing or focusing), diplopia, etc.

I will try to nail down this issue of focus/fixation mismatch
by reviewing any suggested references, and making up a table 
form of annotated bibliography.  This would include rules of
thumb and anecdotal information as well as carefully controlled
experimental data.

The design of the stereo display parameters in video systems
used for relatively long continuous periods of time, such as
undersea teleoperators, telesurgery, hazardous materials handling,
remote inspection, etc., depends on solid answers to this focus/
fixation mismatch issue.          

Too little caution will result  in rejected and cursed 3D display 
systems, whereas too much caution results in very shallow depth
viewing volumes, or overly conservative camera separation (with
resulting limited depth acuity).
\
I also plan to query the crew-systems design information group
at WPAFB (CSERIAC).  I'll publish the results as email to the
TECH3D@xxxxxx group.

John Merritt
Fax  413 268 7692
Tel  888 268 3888


------------------------------