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

In the present ultra-speed optical systems with flint lenses, the fine-detail contrast ratio at 600 lines per picture height ranges between 2.0 and 3.0. The present goal is to raise this ratio to 10. The contrast ratio changes rapidly with line number and at 200 lines per picture height it ranges from 30 to 60. While contrast ratios of 2.0 and 3.0 at 600 lines do not satisfy development and commercial engineers, the appearance of 600-line detail at a normal viewing distance of 1^ screen widths is well above the point where it would be a serious commercial limitation at the present time. When the ogee lens is made of a single peice of glass, fine-detail contrast is limited by chromatic aberration. The effect of this can be reduced somewhat by making the ogee lens of low-dispersion crown glass instead of flint. A flint filter screen then has to be used, and unless it is finished with great care, its two surfaces can more than offset any improvement to be realized from the crown glass. Chromatic aberration effects can be further offset by a change of magnification. The use of a larger kinescope with shorter projection distance will make the chromatic effects smaller in relation to the size of the picture. It is estimated that the use of crown glass in place of flint, and of a 10-in. kinescope in place of the 7-in., can relatively reduce the area of the chromatic confusion disk a maximum of 80%. Another approach is to make the ogee lens of two different glasses and development work is proceeding along this line. This may involve manufacturing complications, and result in a more expensive lens. It is too early to predict the practical outcome of these approaches to the problem. Any increase in kinescope size involves an increase in projection angle. This is attended with some increase in vignetting, and also with a tendency toward reducing detail contrast and resolution at the corners and edges of the picture. The vignetting, however, tends to retard the loss in contrast and resolution at the expense of brightness in the outer portions of the image. In general, it can be said that, unless an increase in the size of the kinescope will yield an improvement such as suggested above in connection with the effect of chromatic aberration on fine-detail contrast, there is little or nothing to gain optically by choosing wide-angle projection. If the power in the kinescope beam could be increased in direct ratio to the raster area, the story would be quite different, but in present kinescopes this cannot be done. The conversion efficiency of the phosphor increases a moderate amount when the raster area is increased, but an increase in kinescope size usually results in reduced screen brightness for a given projected picture size when using an ultra-speed optical system of given focal length. The factors that influence fine-detail contrast also largely determine limiting resolving power. When flint lenses are used, the limiting resolution of the ultraspeed system is 1200 television lines per picture height, or better, in all parts of the picture. It exceeds 1800 lines in the center, and the television scanning lines are clearly reproduced throughout the picture. The present goal is a limiting resolution of 3600 lines. Proper location of parts is essential to good resolution and detail contrast in the image. All parts of the assembly are carefully made to insure accurate locations, and adequate adjustments are provided for locating the kinescope with reference to the optical parts. The center of curvature of the faceplate must be on the axis of the optical system, and installation and maintenance engineers are provided with means of testing to insure that the adjustment is properly made. Special installation and maintenance tests are also made to insure that the axis of the ogee lens is adjusted to pass through the center of curvature of the mirror. Sachtleben and Alice: Theater Television Optics 431