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154
BRITISH KINEMATOGRAPHY
Vol. 17, NO. 5
that the television scanning lines are not meeting and that a closed shutter angle of less than 73 is necessary to reduce the banding. The proper adjustment of the shutter under these conditions is usually to an angle less than 73° but greater than 72". This precise adjustment is made by removing a few thousandths of an inch from the trailing edge of the shutter and making exposure tests following each adjustment until no banding can be detected in the picture frame, as seen in Fig. 13.
The exposure time of each film frame must be accurately determined and must include the exact number of television scanning lines in two television fields or one television frame, which is 525 lines less those blanked during retrace time.
Electronic Shutter.
Several attempts have been made to utilize an " electronic shutter," that is, to blank the picture information during the pull-down of the film, and to include just sufficient lines to obtain the frame combine in the picture. Theoretically, this appears to be the ideal method of obtaining a perfect picture utilizing the exact number of television lines to complete the spliced film-frame. Unfortunately, this was not the case ; with the electronic shutter in the earlier
Television cycle in degrees |~287^
Television
cycle field and
frame
Film cycle frame and pull-down
Film and h287^
2870'
K287
1
1
i 2
i 3
i 4
i 5
l
l 7
i 8
T
9
i 10
— i — II
'I2
3J4
5
6
7
8
9 | 10
ILJI2
I3|l4
15
16
17
18
I9J20
2l|22
Frame
I
Frame 2
3
4
5
' 6
7
8
9
shutter cycle in degrees
|<2 87^
73°
2880°
7^<
Fig. 12. Shutter action of television recording cameras.
experimental stage there was always noticeable banding, regardless of blanking adjustment. However, at present there are cameras operating with electronic shutters that are very satisfactory. One of the advantages of the mechanical shutter over the electronic shutter is the vignetting effect obtained at the combine in the film of the two television fields. While it is difficult and timeconsuming to adjust a mechanical shutter to reduce banding, it is, to date, the accepted method by which satisfactory results are obtained.
Cathode-ray Tube for Photography.
The cathode-ray tube that has been universally accepted for Kinescope photography is a 5-inch, flat-face tube using electro-magnetic deflection and electrostatic focusing. This tube features a new electron gun having superior modulation characteristics and is adapted for high beam current.
The phosphor used in the tube is one of a new series of materials recently developed in the P-ll group, and will withstand the high beam current better than previously developed phosphors. It is blue in colour, peaking at approximately 4600A in its spectral-energy characteristics, and has an extremely short phosphorescence decay.
The decay time of the phosphor from its peak brightness to the 10 per cent. o\' maximum point is about 0.3 millisecond. The persistence of the radiation