To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 22:58 2003-07-13 -0700, Bob May wrote:

>The only reason why the test is done in a dim or darkened room is to make
>the shadows stand out better for the camera or the eye as the eye will be a
>bit more open.
>Don't worry about the room being completely dark as that is really too dark
>for really doing work.  Just a heavy shade over the windows in the daytime
>is plenty of darkness.

Consider the contrast sensitivity of the eye, which we are depending on to
do zone gray matching in the Foucault test.  The test is already tough in
that respect because, as W. J. Smith says in "Modern Optical
Engineering", p.109, "When the areas are separated ... contrast
sensitivity drops markedly."

On that same page, he says bright fields help the sensitivity, and
"dark surrounded fields" hurt.  So use a bright test lamp, a
white mask, and do the test in ordinary room lighting.

The page before, he tells how to do a better job of matching by making a
series of readings. "In half the readings, the brightness of the
variable area is raised until an apparent match is obtained; in the other
half of the readings, the brightness is lowered to obtain the apparent
match."  In Foucault, of course, this means approaching the
longitudinal reading from below and above.  "The average is then much
more accurate than either set."

         -- Jim Burrows
         -- mailto://burrjaw{at}earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)

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