The second principle is
barrel vignetting (Fig 3.) where the pupil of the lens is cut off by the edges
of front or rear lens elements or perhaps by some mechanical obstruction at the
edges of the field. For good practical reasons, photographic lenses are
generally designed to have about 50% vignetting at the corners at full
aperture. The vignetting effect is considerably less or even absent at smaller
apertures.
In projection, these same effects are both operating and are
further complicated by the addition of the illuminating system. Here again, due to "cosine fourth" law and
vignetting, the standard lenses are incapable of transmitting more than about
50% in the corners of our special frame size. It’s possible to do somewhat
better, (about 65%), by adjusting the lamps so that it more strongly
illuminates the edges at the expense of the center. When it is considered that
the "cosine fourth" limit permits over 50% illumination in the corners, the
high order of optical inefficiency of the system would be evident.
Due to the structure of the arc itself, as well as the
optical system, there are marked differences in color over any given panel. These
color and illumination changes are affected strongly by very small shifts in
crater positioning as well as by fluctuations in voltage and current. While a
complete analytic treatment of the projection system is beyond this report, it
can be stated that no major breakthrough on this most important problem can be
expected as long as the hardware concept is applied to this problem. That is,
as long as lamphouse, projector, and lens are thought of as off-the-shelf items
purchased separately, progress will be essentially trivial. The problem
requires treatment as a whole optical system and will require more or
less radical departures in all three components, none of which will be made
until the problem is considered as a whole.
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