T3D Visual acuity resolved

[john bercovitz <bercov@xxxxxxxxxxx>]

OK, I get points for the punnish subject line, don't I?  

As I said before, I think the resolution of the seeming
discrepancy is that those quoting 2' for visual acuity
are basing it on the standard eye chart's letter for 20/20
vision while those quoting 1' for visual acuity are basing
their statements on laboratory tests.  Just to make sure I
wasn't lying to you, I went to our little on-site medical
center yesterday and took a look at the eye chart.  First
I looked at it from 20 feet, read the 20/15 line, and then 
went up closer to make a measurement.  I measured the height
of a letter on the 20/20 lines as being at or a hair under 
9 mm.  At 20 feet, this would subtend 5'.  The letter E is
composed of three black and two white horizontal lines.  So
that works out to 2' per cycle.

The interesting thing was that these letters had serifs. 
So they really aren't as easy to see as just 3 black and 
2 white lines would be.  That would mean that you have to 
be able to see better than 2' to be able to make them out,
I think.

What I'm working up to here is that 1' is really more 
correct than 2' for an absolute visual acuity.  The next
thing that occurs to me is that the eye isn't perfect so it 
has a spread function of some sort for light so to get the
best acuity, you might want to slim down the white lines
relative to the black lines: don't make them equal width
or thickness, however you want to call it.  That way when
the white light bleeds out of its bounds in the eye, it 
will still leave lots of black between the white lines.
You need that black for the contrast.  I think contrast is
the key.  We need very little of it to make out an image
but we do need to have some.

John B


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