early improvement over the Nipkow disk was the lens disk. It was much more efficient
in that the lens could pass many times more light compared to the the small apertures
of the original Nipkow. There were as many lenses as there were lines in the picture
and the focal center of each lens was at the at the same
point as the aperture in the standard Nipkow. As it rotated, each lens caused
a line to be scanned below the previous one. Beyond 60 lines with lenses of a
reasonable size, the lens disk became excessively large and costly. The lenses
were also difficult to locate exactly in their proper locations.
As photo cell sensitivity and light sources improved, scanning hole size or lens
diameters could be reduced. This allowed the disk diameter to be reduced. Reducing
the disk size resulted in a more compact scanning assembly. As an example, Western
Television of Chicago produced a televisor with a 17 inch aperture disk and a
flat plate neon lamp. It provided a 45 line, 1 1/8 by 1 1/2 inch picture. The
following year (1931) they developed their new 45 line model with an 8 inch diameter
lens disk. It provided a much larger and brighter picture, measuring 4 by 3 7/8
inches. The improvement was due to the use of both the lens disk and a more efficient
light source, the crater arc neon lamp. This new model included an 8 tube receiver,
which the previous model did not and the complete receiver occupied only about
half the table space as before. From this time on, there was a major effort by
those involved in the mechanical television business, to develop small, low cost
scanners that would produce large, bright pictures, with improved resolution.
Meanwhile, there was constant pressure to increase the number of lines in the
image. However, this tended to increase the size of the scanning assembly as well
the signal bandwidth requirements. Commercial television started with 24 line
pictures, mostly because of the bandwidth limitations of transmitters operating
in or just above the Broadcast Band. As the transmissions were allowed to move
up in frequency, more lines could be added to the image, thereby improving image
resolution. With pictures up to 60 lines, scanning disks with holes, lenses or
mirrors could provide acceptable pictures with disks of reasonable size. Beyond
that, the holes, lenses or mirrors became too small or the disk too large and
the requirement for manufacturing precision too great for these scanning disks
to see further use in receivers.
the Nipkow disk pretty much had had its day. This was the end of the "low
definition" era of television and the beginning of the "High Definition"
era, which continues to this day. With 60 lines (or less), television images were
limited to close-ups of actors or scenes with large recognizable objects. At 120
lines and more, television takes on movie like qualities, with "long shots"
as well as close-ups, where the sets and scenery are adding to the impact of the
actors. In the beginning of high definition television (HDTV), there were some
innovative ideas tried to make it happen. Better and sooner. Early efforts tended
to develop small mechanical scanning assemblies that fell into two general categories;
vibratory scanners and rotating mirror scanners.
above is a 60 line, 18"diameter lens disk model. The disk is an original
one from the early 1930s. The rest of this receiver was built by Peter Yanczer
for Jack Davis of the AWA. You can see this set at the AWA Museum Annex.