Image dissector
Encyclopedia
An image dissector, also called a dissector tube, is a video camera tube
in which photocathode
emissions create an "electron
image" which is then scanned to produce an electrical signal representing the visual image. The term may apply specifically to a dissector tube employing magnetic field
s to keep the electron image in focus, and an electron multiplier
to scan the electrons.
Among the first to design such a device were German
inventors Max Dieckmann and Rudolf Hell
, who had titled their 1925 patent application Lichtelektrische Bildzerlegerröhre für Fernseher (Photoelectric Image Dissector Tube for Television), and American television
pioneer Philo Farnsworth
.
material, such as caesium oxide
, which emits negatively-charged "photoelectrons" proportional to the intensity of the light striking the material. Electrostatic deflecting plates or magnetic field
s then periodically
manipulate the resulting electron image horizontally and vertically before an electron multiplier, or a small aperture leading to a positively-charged detector or "anode
" in the case of the earliest dissector tubes. The electron multiplier or aperture permits only those electrons emanating from a very small area of the electron image, representing a similarly small area of the visual image. The entire image is scanned several times per second to produce an electrical signal suitably representative of a moving visual image.
by electric
or magnetic
fields
.
Farnsworth submitted a patent application for the image dissector on January 7, 1927. On September 7 of that year, the image dissector successfully transmitted its first image, a simple straight line, at Farnsworth's laboratory at 202 Green Street in San Francisco. By September 3, 1928, Farnsworth had developed the system sufficiently to hold a demonstration for the press, the first such successful demonstration of a fully electronic television system.
In 1929 Farnsworth eliminated a motor generator from the system, so it then had no mechanical parts. Further developments that year included improvements in image clarity and an increase in the number of lines of resolution, such that it exceeded that of the mechanical television systems. Also in 1929, Farnsworth transmitted the first live human images with his system, including a three and a half-inch image of his wife Elma ("Pem") with her eyes closed (possibly due to the bright lighting required).
Since the electrons emitted within an image dissector are collected by the electron multiplier or anode only during the very brief time an area of the "electron image" is exposed, the bulk of the electrons are lost. Thus the earliest image dissectors were very inefficient, and extremely bright illumination was required for it to be used effectively. Farnsworth addressed this problem with the invention of an "electron multiplier" (not to confused with contemporary electron multiplier
s), a device that increased the number of electrons in a circuit by generating "secondary emissions" of electrons from a pair of opposed surfaces, thus amplifying the electrical signal.
Farnsworth applied for a patent for his "electron multiplier" on March 3, 1930 and demonstrated its application in 1931. Farnsworth continued to improve the device, which would come to be called a "multipactor," such that it reportedly could amplify a signal to the 60th power or better, and showed great promise in other fields of electronics. A significant problem with the multipactor, however, was that it wore out at an unsatisfactorily rapid rate.
On August 25, 1934, Farnsworth gave the world's first public demonstration of a complete, all-electronic television system, which included his image dissector, at the Franklin Institute
in Philadelphia, Pennsylvania
.
In April 1933, Farnsworth submitted a patent application entitled Image Dissector, but which actually detailed a cathode ray tube
(CRT) camera tube.
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 for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s...
in which photocathode
Photocathode
A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound...
emissions create an "electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
image" which is then scanned to produce an electrical signal representing the visual image. The term may apply specifically to a dissector tube employing magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s to keep the electron image in focus, and an electron multiplier
Electron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons...
to scan the electrons.
Among the first to design such a device were German
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
inventors Max Dieckmann and Rudolf Hell
Rudolf Hell
Rudolf Hell was a German inventor. He was born in Eggmühl, Germany.From 1919 to 1923 he studied electrical engineering in Munich....
, who had titled their 1925 patent application Lichtelektrische Bildzerlegerröhre für Fernseher (Photoelectric Image Dissector Tube for Television), and American television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
pioneer Philo Farnsworth
Philo Farnsworth
Philo Taylor Farnsworth was an American inventor and television pioneer. Although he made many contributions that were crucial to the early development of all-electronic television, he is perhaps best known for inventing the first fully functional all-electronic image pickup device , the "image...
.
Operation
An image dissector focuses a visual image onto a layer of photosensitivePhotoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
material, such as caesium oxide
Caesium oxide
Caesium oxide or cesium oxide describes inorganic compounds composed of caesium and oxygen. The following binary oxides of caesium are known: Cs11O3, Cs4O, Cs7O, and Cs2O. Both the oxide and suboxides are brightly coloured...
, which emits negatively-charged "photoelectrons" proportional to the intensity of the light striking the material. Electrostatic deflecting plates or magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
s then periodically
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
manipulate the resulting electron image horizontally and vertically before an electron multiplier, or a small aperture leading to a positively-charged detector or "anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
" in the case of the earliest dissector tubes. The electron multiplier or aperture permits only those electrons emanating from a very small area of the electron image, representing a similarly small area of the visual image. The entire image is scanned several times per second to produce an electrical signal suitably representative of a moving visual image.
History
Dieckmann and Hell submitted their application to the German patent office in April 1925, and a patent was issued in October 1927. In 1951, Hell claimed that he had made a tube but could not get it to function, since at the time there was an insufficient knowledge of "electron optics", the manipulation of an electron beamCharged particle beam
A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same velocity . The kinetic energies of the particles are typically measured in keV or MeV, much larger than the energies of particles at ambient temperature...
by electric
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
or magnetic
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
fields
Field (physics)
In physics, a field is a physical quantity associated with each point of spacetime. A field can be classified as a scalar field, a vector field, a spinor field, or a tensor field according to whether the value of the field at each point is a scalar, a vector, a spinor or, more generally, a tensor,...
.
Farnsworth submitted a patent application for the image dissector on January 7, 1927. On September 7 of that year, the image dissector successfully transmitted its first image, a simple straight line, at Farnsworth's laboratory at 202 Green Street in San Francisco. By September 3, 1928, Farnsworth had developed the system sufficiently to hold a demonstration for the press, the first such successful demonstration of a fully electronic television system.
In 1929 Farnsworth eliminated a motor generator from the system, so it then had no mechanical parts. Further developments that year included improvements in image clarity and an increase in the number of lines of resolution, such that it exceeded that of the mechanical television systems. Also in 1929, Farnsworth transmitted the first live human images with his system, including a three and a half-inch image of his wife Elma ("Pem") with her eyes closed (possibly due to the bright lighting required).
Since the electrons emitted within an image dissector are collected by the electron multiplier or anode only during the very brief time an area of the "electron image" is exposed, the bulk of the electrons are lost. Thus the earliest image dissectors were very inefficient, and extremely bright illumination was required for it to be used effectively. Farnsworth addressed this problem with the invention of an "electron multiplier" (not to confused with contemporary electron multiplier
Electron multiplier
An electron multiplier is a vacuum-tube structure that multiplies incident charges. In a process called secondary emission, a single electron can, when bombarded on secondary emissive material, induce emission of roughly 1 to 3 electrons...
s), a device that increased the number of electrons in a circuit by generating "secondary emissions" of electrons from a pair of opposed surfaces, thus amplifying the electrical signal.
Farnsworth applied for a patent for his "electron multiplier" on March 3, 1930 and demonstrated its application in 1931. Farnsworth continued to improve the device, which would come to be called a "multipactor," such that it reportedly could amplify a signal to the 60th power or better, and showed great promise in other fields of electronics. A significant problem with the multipactor, however, was that it wore out at an unsatisfactorily rapid rate.
On August 25, 1934, Farnsworth gave the world's first public demonstration of a complete, all-electronic television system, which included his image dissector, at the Franklin Institute
Franklin Institute
The Franklin Institute is a museum in Philadelphia, Pennsylvania, and one of the oldest centers of science education and development in the United States, dating to 1824. The Institute also houses the Benjamin Franklin National Memorial.-History:On February 5, 1824, Samuel Vaughn Merrick and...
in Philadelphia, Pennsylvania
Philadelphia, Pennsylvania
Philadelphia is the largest city in the Commonwealth of Pennsylvania and the county seat of Philadelphia County, with which it is coterminous. The city is located in the Northeastern United States along the Delaware and Schuylkill rivers. It is the fifth-most-populous city in the United States,...
.
In April 1933, Farnsworth submitted a patent application entitled Image Dissector, but which actually detailed a cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
(CRT) camera tube.
External links
- The Farnovision - history of Philo Farnsworth and invention of the Image Dissector
- "Farnsworth's Image Dissector", IEEE Global History Network
See also
- IconoscopeIconoscopeThe Iconoscope was the name given to an early television camera tube in which a beam of high-velocity electrons scans a mosaic of photoemissive isolated granules...
- Image Orthicon tubeVideo camera tubeIn older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s...