Transcript TOPIC TELEVISION CAMERA TUBES

ECE 5th SEMESTER
Subject
Consumer Electronics
(CE)
TOPIC
TELEVISION CAMERA
TUBES
OUTLINE OF LECTURE
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CAMERA TUBES
BASIC PRINCIPLE
TYPES OF CAMERA TUBES
TELEVISION CAMERA TUBES
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An electron tube use a light-sensitive material/receptor
that converts an optical image into an electrical television
video signal.
The tube is used in a television camera to generate a train
of electrical pulses representing the light intensities
present in an optical image focused on the tube.
Each point of this image is interrogated in its proper turn
by the beam in tube , and an electrical signal
corresponding to the amount of light at that point of the
optical image is generated by the tube. This signal
represents the video or picture portion of a television
signal.
BASIC PRINCIPLE
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Photoconductive effect
Photoemission effect
Photoemission principle
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Certain metals emit electrons when light falls on their
surface.
These emitted electrons are called photoelectrons
and the emitting surface a photocathode.
Light consists of small bundles of energy called
photons.
When light incident on a photocathode ,the photons
give away their energy to the outer valence electrons
to allow them to overcome the potential energy
barrier at the surface.
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The number of electrons which can overcome the
potential barrier and get emitted ,depend on the
light.
Alkali metal are used as photocathode because they
have very low work function.
Cesium silver / bismuth silver cesium oxides are
preferred as cathode surface because they are
sensitive to incandescent light and have spectral
response very close to the human eye.
Photoconductive principle
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In some metal , when light photon fall on it then its
conductivity / resistivity change.
Semiconductor metals like selenium, tellurium and
lead with their oxide have this property known as
photoconductivity.
The variation of resistance at each point across the
surface of the material is utilized to develop a varying
signal by scanning it uniformly with an electron
beam.
Types of Camera Tubes
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Image Orthicon tube
Vidicon
Plumbicon
Image Orthicon tube
Image Orthicon tube
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Principle : Photoemissive
It has three section:
Image section
Scanning section
Electron gun –cum-multiplier section
Image section
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Glass plate : coated with Silver-antimony serve as
photocathode
Lens assembly : light from the scene to be televised
is focused on the photocathode surface by lens
system
Photocathode surface is semitransparent and light
rays penetrate it to reach its inner surface where
electron emission takes place .
Target plate : when electron emit from cathode ,it
can not store charge being a conductor .
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For this reason the electron image is made to move
towards the target plate located at short distance from
it.
The target plate is made of a very thin sheet of glass
and can store charge received by it. This is maintained
at about 400v more +ve w.r.t photocathode
When electron move from cathode to target plate
there is chance that it may repel each other ,this can
result in distortion of the information available as
charge image.
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To prevent this divergence effect an axial magnetic field
generated by focus is employed
This magnetic field impart helical motion to electrons
and focus them on target plate .
The image side of target plate has coating of cesium
that emit secondary electron.these secondary electron is
collected by wire mesh which is located at front of target
plate .
The +ve charge appeared on target plate is proportional
to light intensity distribution on photocathode
Scanning section
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Fine electron beam
Focus ,alignment coil
Deflection coil
1. Horizontal deflection
2. Vertical deflection
Electron gun assembly and multiplier
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Electron gun
Multistage electron multiplier
Vidicon
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Principle : Photoconductive
It has three section:
Target section
Scanning section
Electron gun
Target section
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Glass plate
Lens assembly
Scanning section
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Fine electron beam
Focus ,alignment coil
Deflection coil
1. Horizontal deflection
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2. Vertical deflection
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Electron gun assembly
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Electron gun
Principle
of operation
Application of vidicon
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Close circuit TV system
Earlier type of vidicon were used only
where there was no fast movement ,
because of inherent lag
Plumbicon Camera Tube
 lower
image lag.
 It
has fast response and produce high quality
pictures at low light level.
 It
has small size and light weight and has low
power operating characteristics .
 It
is similar to vidicon tube except small change in
target plate
Target plate
Glass face plate
n-type layer
sno2
p type layer PbO
Scanning beam
Light
Intrinsic layer
Plan of a colour television camera showing
generation of colour signals and Y matrix for
obtaining the luminance (brightness) signal.
Figure shows a simple block schematic of a
colour TV camera.
It essentially consists of three camera tubes in
which each tube receives selectively filtered
primary colours.
Each camera tube develops a signal voltage
proportional to the respective colour intensity
received by it.
Light from the scene is processed by the
objective lens system.
The image formed by the lens is split into three
images by means of glass prisms.
These prisms are designed as diachroic mirrors.
A diachroic mirror passes one wavelength and
rejects other wavelengths (colours of light).
Thus red, green, and blue colour images are
formed.
The rays from each of the light splitters also
pass through colour filters called trimming
filters.
These filters provide highly precise primary
colour images which are converted into video
signals by image-orthicon or vidicon
camera tubes.
Thus the three colour signals are generated.
These are called Red (R), Green (G) and Blue
(B) signals.