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. Itís 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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