This
was one of the first electronic pick-up tubes developed by Zworykin in the late
20s for use in the all-electronic television system. It made use of electron
beam scanning of a photoemissive mosaic surface which had charge storage
properties. The schematic construction of the tube is shown in figure 1 below.
was one of the first electronic pick-up tubes developed by Zworykin in the late
20s for use in the all-electronic television system. It made use of electron
beam scanning of a photoemissive mosaic surface which had charge storage
properties. The schematic construction of the tube is shown in figure 1 below.
The
mosaic surface consists of a thin sheet of mica, one side of which is made
photosensitive by being embedded with minute separate globules of
caesium-silver compound. The other side is coated by a conducting metal film,
so that each tiny globule forms a small capacitor with the metal film, the mica
sheet forming the dielectric in between. The image is focused upon the mosaic
surface by means of the optical lens. Light falling in the mosaic globules
causes them to emit electrons most of which are drawn away by the collector
electrodes. This leaves a positive charge pattern on the mosaic proportional to
the light intensity distribution. Since the globules are discrete (separated
from each other), the charge distribution stores itself across the small
globules capacitors. A high velocity electron beam from the electron gun scans
the mosaic, impinging on each globule to restore the charge lost by
photoelectric emission. In this process, it sends a current pulse in the beam circuit
as each globule is capacitively coupled to the back electrode that gives out a
signal voltage across resistance R.
mosaic surface consists of a thin sheet of mica, one side of which is made
photosensitive by being embedded with minute separate globules of
caesium-silver compound. The other side is coated by a conducting metal film,
so that each tiny globule forms a small capacitor with the metal film, the mica
sheet forming the dielectric in between. The image is focused upon the mosaic
surface by means of the optical lens. Light falling in the mosaic globules
causes them to emit electrons most of which are drawn away by the collector
electrodes. This leaves a positive charge pattern on the mosaic proportional to
the light intensity distribution. Since the globules are discrete (separated
from each other), the charge distribution stores itself across the small
globules capacitors. A high velocity electron beam from the electron gun scans
the mosaic, impinging on each globule to restore the charge lost by
photoelectric emission. In this process, it sends a current pulse in the beam circuit
as each globule is capacitively coupled to the back electrode that gives out a
signal voltage across resistance R.
In
practice, the uneven secondary emission due to the high velocity scanning beam
and the charge leakage between adjacent globules tend to make the iconoscope
response nonlinear and limit its sensitivity. This led to the development of
low velocity scanning tubes like the orthicon and the image orthicon with
considerable improvement in sensitivity and characteristics. However, in the
early days of television, until the early 40s, the iconoscope was the most
widely used pick-up tubes for TV cameras.
practice, the uneven secondary emission due to the high velocity scanning beam
and the charge leakage between adjacent globules tend to make the iconoscope
response nonlinear and limit its sensitivity. This led to the development of
low velocity scanning tubes like the orthicon and the image orthicon with
considerable improvement in sensitivity and characteristics. However, in the
early days of television, until the early 40s, the iconoscope was the most
widely used pick-up tubes for TV cameras.
