Journal of the Society of Motion Picture and Television Engineers (1950-1954)

The present Ultra-Speed System is capable of delivering a highlight level of 2 ft-c to the screen, when focal length is 15.515 in., projection distance is 62 ft, raster diagonal is 6 J in., magnification is 48 X and anode voltage is 80 kv on a 7-in. kinescope. The picture size is 15 X 20ft. Factors That Limit Performance Performance of optical systems of this type is limited in many respects, chief among which are image illumination uniformity, large-area contrast, finedetail contrast, resolving power, maximum projection angle and efficiency of light utilization. To some extent these factors are all interrelated, and tie in also with such factors as optical system size, weight and cost. In general, the projected images equal or exceed those projected from film, from the standpoint of flat illumination distribution. The flattest illumination results from making the mirror considerably larger in diameter than the minimum required to fill the ogee lens with light from the center of the raster. This rapidly increases size, weight and cost of the mirror, however, and it has been found practical to limit the mirror diameter to about 10% in excess of this minimum. Illumination at the sides of the picture exceeds 80% of the center at a magnification of 48 X • Large-area contrast depends upon design factors, quality factors and maintenance factors, because all of these influence the amount of stray light (light not directed to formation of the desired image) that emerges from the system to generally dilute contrast. Stray light also includes light returned to the kinescope screen by the system or by the projection screen. Design factors include surface coating of ogee lens (and radiation filter, if used), central blocking of the mirror, presence or absence of a radiation filter, finish of mechanical parts, etc. Quality factors include workmanship on all reflecting and refracting surfaces, quality of surface coatings and all optical materials, and quality of all surface finishes inside the system. The chief maintenance factor is summed up in one word, "cleanliness." The UltraSpeed System is capable of a large-area contrast of approximetely 100 to 1. This is measured by projecting a special uniformly illuminated test object of the same area as the kinescope raster, but with about 1% of its area black and opaque at the center. Light levels are measured within and without the image of this opaque area to determine the large-area contrast ratio. The 100-to-l ratio amply insures against "graying out" of small shadow areas in high-key pictures. Fine-detail contrast is influenced by factors that limit resolving power, as well as by general stray light which is negligible as just shown. For example, if the light that should go into the image of a grid of equally spaced parallel lines becomes uniformly distributed over the image of the grid, the contrast becomes unity and the lines are not resolved at all. Any optical loss of fine-detail contrast is primarily due to imperfections in the imagery of the optical system. This may be due to manufacturing imperfections in the optical parts, or their misadjustment, and may also be due to lack of refinement in the optical design. All these factors may contribute at the same time. It has proved practical and economical to finish the spherical mirrors so that their effect on fine-detail contrast is entirely negligible. The manufacturing imperfections most responsible for limiting fine-detail contrast are to be found in the ogee lens. These imperfections consist mainly of very small ripples or "zones" in the ogee surface of revolution, and also in failure of the surface to be purely one of revolution. The greatest care is taken in manufacturing to keep such errors at a minimum and methods are now being improved as rapidly as possible. 430 November 1951 Journal of the SMPTE Vol. 57