22-12-2012, 03:31 PM
Vidicion Video Camera Tubes
[attachment=44970]
Definition of video camera tube:
An electron-beam tube used in a television camera to convert an optical image into a corresponding charge-density electric image and to scan the resulting electric image in a predetermined sequence to provide an equivalent electric signal. Also known as pickup tube; television camera tube.
Introduction to Video camera tube
In older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device (CCD) for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s. The most commercially successful of these tubes were various types of cathode ray tubes or "CRTs".
Any vacuum tube which operates using a focused beam of electrons ("cathode rays") is known as a cathode ray tube. However, in the popular lexicon "CRT" usually refers to the "picture tube" in a television or computer monitor. The proper term for this type of display tube is kinescope, only one of many types of cathode ray tubes. Others include the tubes used in oscilloscopes, radar
displays, and the camera pickup tubes described in this article. (The word "kinescope" has also become the popular name for a film recording made by focusing a motion picture camera onto the face of a kinescope cathode ray tube, a common practice before the advent of video tape recording.)
Video camera tubes typically had a certain maximum brightness tolerance. If that limit were exceeded, such as by pointing the camera at the sun, sun-reflecting shiny surfaces, or extremely bright point light sources, the tube detecting surface would instantly "burn out" and be rendered insensitive on part or all of the screen. The only remedy was replacing the video tube.
Basic Principle
When minute details of a picture are taken into account, any picture appears to be composed of small elementary areas of light or shade, which are known as picture elements. The elements thus contain the visual image of the scene. The purpose of a TV pick-up tube is to sense each element independently and develop a signal in electrical form proportional to the brightness of each element. The light from the scene is focused on a photosensitive surface known as the image plate, and the optical image thus formed with a lens system represents light intensity variations of the scene. The photoelectric properties of the image plate then convert different light intensities into corresponding electrical variations. In addition to this photoelectric conversion whereby the optical information is transduced to electrical charge distribution on the photosensitive image plate, it is necessary to pick-up this information as fast as possible. Since simultaneous pick-up is not possible, scanning by an electron beam is resorted to. The electron beam moves across the image plate line by line, and field by field to provide signal variations in a successive order. This scanning process divides the image into its basic picture elements. Though the entire image plate is photoelectric; its construction isolates the picture elements so that each discrete small area can produce its own signal variations.
Photoelectric Effects
The two photoelectric effects used for converting variations of light intensity into electrical variations are (i) photoemission and (ii) photoconductivity. 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 is
made 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. The number of
electrons which can overcome the potential barrier and get emitted, depends on the light
intensity. Alkali metals are used as photocathode because they have very low work-function.
Cesium-silver or bismuth-silver-cesium oxides are preferred as photoemissive surfaces because
they are sensitive to incandescent light and have spectral response very close to the human
eye.
The second method of producing an electrical image is by photoconduction, where the
conductivity or resistivity of the photosensitive surface varies in proportion to the intensity of
light focused on it. In general the semiconductor metals including sel nium, tellurium and lead
with their oxides have this property known as photoconductivity. The variations in 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.
Image Storage Principle
Television cameras developed during the initial stages of development were of the non-storage
type, where the signal output from the camera for the light on each picture element is produced
only at the instant it is scanned. Most of the illumination is wasted. Since the effect of light on the image plate cannot be stored, any instantaneous pick-up has low sensitivity. Image disector
and flying-spot camera are examples of non-storage type of tubes. These are no longer in use
and will not be discussed. High camera sensitivity is necessary to televise scenes at low light
levels and to achieve this, storage type tubes have been developed. In storage type camera
tubes the effect of illumination on every picture element is allowed to accumulate between the
times it is scanned in successive frames. With light storage tubes the amount of photoelectric
signal an be increased 10,000 times approximately compared with the earlier non-storage
type.
Storage Action
Though light from the scene falls continuously on the target, each element of the photocoating
is scanned at intervals equal to the frame time. This results in storage action and the net
change in resistance, at any point or element on the photoconductive layer, depends on the
time, which elapses between two successive scannings and the intensity of incident light. Since
storage time for all points on the target plate is same, the net change in resistance of all
elementary areas is proportional to light intensity variations in the scene being televised.
Signal Current
As the beam scans the target plate, it encounters different positive potentials on the side of the
photolayer that faces the gun. Sufficient number of electrons from the beam are then deposited
on the photolayer surface to reduce the potential of each element towards the zero cathode
potential. The remaining electrons, not deposited on the target, return back and are not utilized
in the vidicon. However, the sudden change in potential on each element while the beam scans, causes a current flow in the signal electrode circuit producing a varying voltage across
the load resistance RL. Obviously, the amplitude of current and the consequent output voltage
across RL are directly proportional to the light intensity variations on the scene. Note that,
since, a large current would cause a higher voltage drop across RL, the output voltage is most
negative for white areas. The video output voltage, that thus develops across the load resistance
(50 K-ohms) is adequate and does not need any image or signal multiplication as in an image
orthicon. The output signal is further amplified by conventional amplifiers before it leaves the
camera unit. This makes the vidicon a much simpler picture tube.
[attachment=44970]
Definition of video camera tube:
An electron-beam tube used in a television camera to convert an optical image into a corresponding charge-density electric image and to scan the resulting electric image in a predetermined sequence to provide an equivalent electric signal. Also known as pickup tube; television camera tube.
Introduction to Video camera tube
In older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device (CCD) for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s. The most commercially successful of these tubes were various types of cathode ray tubes or "CRTs".
Any vacuum tube which operates using a focused beam of electrons ("cathode rays") is known as a cathode ray tube. However, in the popular lexicon "CRT" usually refers to the "picture tube" in a television or computer monitor. The proper term for this type of display tube is kinescope, only one of many types of cathode ray tubes. Others include the tubes used in oscilloscopes, radar
displays, and the camera pickup tubes described in this article. (The word "kinescope" has also become the popular name for a film recording made by focusing a motion picture camera onto the face of a kinescope cathode ray tube, a common practice before the advent of video tape recording.)
Video camera tubes typically had a certain maximum brightness tolerance. If that limit were exceeded, such as by pointing the camera at the sun, sun-reflecting shiny surfaces, or extremely bright point light sources, the tube detecting surface would instantly "burn out" and be rendered insensitive on part or all of the screen. The only remedy was replacing the video tube.
Basic Principle
When minute details of a picture are taken into account, any picture appears to be composed of small elementary areas of light or shade, which are known as picture elements. The elements thus contain the visual image of the scene. The purpose of a TV pick-up tube is to sense each element independently and develop a signal in electrical form proportional to the brightness of each element. The light from the scene is focused on a photosensitive surface known as the image plate, and the optical image thus formed with a lens system represents light intensity variations of the scene. The photoelectric properties of the image plate then convert different light intensities into corresponding electrical variations. In addition to this photoelectric conversion whereby the optical information is transduced to electrical charge distribution on the photosensitive image plate, it is necessary to pick-up this information as fast as possible. Since simultaneous pick-up is not possible, scanning by an electron beam is resorted to. The electron beam moves across the image plate line by line, and field by field to provide signal variations in a successive order. This scanning process divides the image into its basic picture elements. Though the entire image plate is photoelectric; its construction isolates the picture elements so that each discrete small area can produce its own signal variations.
Photoelectric Effects
The two photoelectric effects used for converting variations of light intensity into electrical variations are (i) photoemission and (ii) photoconductivity. 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 is
made 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. The number of
electrons which can overcome the potential barrier and get emitted, depends on the light
intensity. Alkali metals are used as photocathode because they have very low work-function.
Cesium-silver or bismuth-silver-cesium oxides are preferred as photoemissive surfaces because
they are sensitive to incandescent light and have spectral response very close to the human
eye.
The second method of producing an electrical image is by photoconduction, where the
conductivity or resistivity of the photosensitive surface varies in proportion to the intensity of
light focused on it. In general the semiconductor metals including sel nium, tellurium and lead
with their oxides have this property known as photoconductivity. The variations in 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.
Image Storage Principle
Television cameras developed during the initial stages of development were of the non-storage
type, where the signal output from the camera for the light on each picture element is produced
only at the instant it is scanned. Most of the illumination is wasted. Since the effect of light on the image plate cannot be stored, any instantaneous pick-up has low sensitivity. Image disector
and flying-spot camera are examples of non-storage type of tubes. These are no longer in use
and will not be discussed. High camera sensitivity is necessary to televise scenes at low light
levels and to achieve this, storage type tubes have been developed. In storage type camera
tubes the effect of illumination on every picture element is allowed to accumulate between the
times it is scanned in successive frames. With light storage tubes the amount of photoelectric
signal an be increased 10,000 times approximately compared with the earlier non-storage
type.
Storage Action
Though light from the scene falls continuously on the target, each element of the photocoating
is scanned at intervals equal to the frame time. This results in storage action and the net
change in resistance, at any point or element on the photoconductive layer, depends on the
time, which elapses between two successive scannings and the intensity of incident light. Since
storage time for all points on the target plate is same, the net change in resistance of all
elementary areas is proportional to light intensity variations in the scene being televised.
Signal Current
As the beam scans the target plate, it encounters different positive potentials on the side of the
photolayer that faces the gun. Sufficient number of electrons from the beam are then deposited
on the photolayer surface to reduce the potential of each element towards the zero cathode
potential. The remaining electrons, not deposited on the target, return back and are not utilized
in the vidicon. However, the sudden change in potential on each element while the beam scans, causes a current flow in the signal electrode circuit producing a varying voltage across
the load resistance RL. Obviously, the amplitude of current and the consequent output voltage
across RL are directly proportional to the light intensity variations on the scene. Note that,
since, a large current would cause a higher voltage drop across RL, the output voltage is most
negative for white areas. The video output voltage, that thus develops across the load resistance
(50 K-ohms) is adequate and does not need any image or signal multiplication as in an image
orthicon. The output signal is further amplified by conventional amplifiers before it leaves the
camera unit. This makes the vidicon a much simpler picture tube.