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The standard Emitron camera tube was developed in the UK during the early 1930’s &, except for the period of World War 2, was in regular use in the BBC Television Service from November 1936 until the mid fifties.
The tube consisted of a spherical glass bulb approximately 19cms in diameter, near the centre of which the target was mounted. The optical image passed through a flat window in the bulb. A side tube, 4cms in diameter & attached to the bulb, contained an electron gun with the axis passing through the centre of the target & made an angle of about 35 deg with the normal. The target was not transparent. The gun arrangement resulted in an awkward tube shape which was not well suited for mounting in a camera.
The inherent insensitivity of the tube together with the size of the target, 12.5cms x 10cms & its distance behind the glass window imposed limitations in the design of the external optical system & in practice resulted in a restricted depth of field.
The oblique scanning of the target produced a ‘keystone’ picture which had to corrected manually for both frame and line signals. This was known as ‘tilt and bend’ & the operators needed a certain degree of skill to straighten the waveforms.
The tube produced spurious signals and these were corrected by a small light illuminating the target – electronic flashing.
The first versions of this camera had no proper means of focusing them and the operator had to look through a small porthole to see the image on the target. It was not very long before an optical viewfinder, ganged with the main lens, was fitted. This gave a color picture but upside down and laterally inverted. There was an advantage to this viewfinder – the depth of field could be made shallower than the camera lens so the operator could notice degradation of focus before the viewers did.
The camera tubes were completely stable and had a very low effective exposure time & have very sharp images of rapidly moving objects. The color response approximated that of the human eye and there was very little geometric distortion of the images despite the oblique scanning. There were some disadvantages – strong spurious shading signals, limited sensitivity, oblique scanning and an awkward shape for mounting in a camera. Another problem was that it was very easy to snap the gun off the bulb while handling the tube and this resulted in people being awarded the OBE – Order of the Broken Emitron.
The picture output from the 4 valve amplifier in the camera was 405 lines interlaced with an aspect ratio 5:4.
From Television Engineering, Volume One by S W Amos & D C Birkenshaw, published 1953 by Iliffe & Sons, London. Additional note by Butch Calderwood ACS
from Australian Cinematographer - Issue 14