Movies for TV - Early Television Foundation

106 FUNDAMENTALS
is the lenses used. Only too often this is a standard home-movie
type, of limited resolving power. As soon as a first-class, high
definition model replaces it, the picture quality improves im-
mensely. The lens has to illuminate a very small area on the
mosaic, but it should be uniformly clear and well lit with critical
focus at all parts on the mosaic.
A brief description of the film camera may be of interest here.
Figure 4-5 shows the RCA projector and camera, and Figure 4-9
illustrates the GE film camera. The similarity is quite noticeable.
In each case, an iconoscope tube is used for the light conversion
unit. Since the film picture can be focussed directly on to the
mosaic by the projection lens, no camera lenses are required.
Thus, the effect of the widest possible aperture is obtained. The
iconoscope is not an image multiplier tube; therefore, the sensi-
tivity is less than that of the image orthicon, but because the light
available is virtually unlimited a stronger lamp can always be
inserted in the lamp-house this does not matter, and the tube
is capable of excellent definition.
Unfortunately
the film camera tube suffers from a defect
which results in the appearance of unwanted shadows and whitish
areas on the picture. This is known technically as shading. The
production of these spurious signals is caused by dark spot signals
in the camera tube. These are extra electrons emitted by the
mosaic which have random emission and tend to fog the picture
in the same way that unwanted light on a photographic plate will
fog it.
Readers who have seen a television film camera will have
noticed that there are two small lamps mounted inside the camera
housing. These are for the purpose of providing edge and bias
lighting. The edge lighting is used to balance flare at the edge of
the mosaic where the picture closes. The flare is caused by the
scanning system and the sudden change in electron emission due
to the scanning beam overrunning the picture. Flare usually shows
up as a bright band at the right edge of the picture and is reduced
by shining a strong light on the mosaic just off the side of the