The favorite test pattern of the B/W television era. The pattern itself was precisely etched into a CRT, thus did not require external lighting, focus or keystone adjustment. RCA considered its model to be a "camera" and designated it as TK-1. It allowed the television station technician or viewer at home a quick evaluation of key picture monitor (TV set) performance.
The converging line patterns gave a visual measure of resolution and focus. This particular graphic was obtained from either Umatic or VHS tape playback as horizontal resolution was clearly limited to 240 lines. Even in the 40's, broadcasters could achieve 320 line monochrome resolution. While we take vertical resolution for granted, the actual size of the scanning dot (electrical focus) is a critical parameter. Too large a dot reduces visual vertical resolution, too small a dot limits the available picture contrast. Properly adjusted, even the largest monochrome picture tubes never showed scanning line structure. Visable dots and scan lines are strictly a part of the new-fangled color TV era.
Circles gave a convenient indication that aspect ratio is correct. Old tube sets were notoriously bad in this respect. They frequently displayed both horizontal and vertical scan non-linearity which also would show on this chart. And to make matters worse, the adjustments for these parameters often interacted with picture size and position, so that optimum correction was a compromise at best.
The gray scales in the center ring was an aid to setting contrast and brightness controls (and these in turn would interact with focus adjustments!!) Broadcasters could be depended on to transmit test patterns for several hours prior to sign-on, which gave viewers at home ample time to diddle with TV set knobs, especially on weekends.
The modern SMPTE color test pattern allows a number of useful set adjustments, but unfortunately TV time has become too valuable for broadcasters to make it available for viewer use. Digital electronics generate a very stable picture so that most TV sets have eliminated user controls anyway.
