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By WILFRID B. WHALLEY Adjunct Professor of Electrical Engineering Brooklyn Polytechnic Institute The development of the vidicon pickup tube. IN THE February issue of RADIO - ELECTRONIC ENGINEERING, the development of the image orthicon was discussed. This is the most used type of pickup tube for both outdoor and studio entertainment television at the present time. While the image orthicon has high sensitivity, good resolution and reasonable contrast range, it is admittedly quite complex and requires correspondingly complicated associated apparatus. Due to its many internal parts, and the careful processing required, the tube is quite expensive. Occasionally also, there can be added spurious patterns due to variations in the secondary emissivity over the surface of the first dynode, so that the number of secondary electrons varies with the angle of arrival of electrons returning from the mosaic. Therefore, a great deal of effort has gone into the development of lower -cost and simpler pickup devices. To satisfy the requirements for industrial operation, a pickup tube should preferably contain only a light -sensitive target and an electron gun. Transfer Methods Until recently, all pickup tubes commercially available have made use of photoemission. These include the iconoscope, the image iconoscope, the orthicon and the image orthicon. In all four types, the electrical signal depends GLASS ENVELOPE PHOTOCONDUCTIVE TARGET I THICKNESS EXAGGERATED) SCANNNG SCAM 10 -30V T VIDEO SIGNAL OUT GLASS FACE Or TUBE TRANSPARENT ...ESA" COATING ON CLASS FORMS CONDUCTIVE SIGNAL PLATE INCIDENT LIGHT I.OVAR METAL RING IGNAL LEAD CONNECTION) upon the light energy releasing electrons from a suitably treated surface. However, many experimenters had tried other light -to- electric transfer methods, such as thermoelectricity, photovoltaic cells and photoconductivity. The thermoelectric effect is well known in the use of thermocouples. It could be of value in the infrared rather than the visible light region. Some experiments were performed with films of germanium made of vapor deposition on mica, but the reported sensitivities were low. Photovoltaic cells made, for example, with cuprous oxide on copper having a translucent film of silver to make contact to the oxide surface have shown useful variations in potential with light intensity. Such construction is not appropriate for pickup tubes since both surfaces are good conductors. Also, the changes in voltage are very small compared to the voltage of an electron scanning beam. Photoconductivity shows up in some materials as a variation in resistance with light intensity. Among others, H. Iama, H. Miller and J. W. Strange demonstrated tubes using photoconductive surfaces nearly twenty years ago. Photoconductivity It was expected that photoconductivity could provide higher sensitivity than photoemission, since the photons could interact more completely during Fig. 1. Details of the target con - struction in the experimental vidicon. Fig. 2. Basic operating principle of vidicon camera tube. Cou rteay of RCA Review SIGN KATE M1111111011111111111111 SC. N.oraA.0 . á°á,0A CA.KITC. penetration into the volume of the material. Photoconductive cells had shown high sensitivity in light intensity meters. The first experiments for television use were made with selenium films mounted on graphite, but these showed very long recovery times. Resistance would decrease with light, but recovery was too slow to take care of the scene motion. Selenium was difficult to process, due to the high rate of evaporation at the usual tube bake -out temperatures. To be suitable, a material must have sufficient "dark" resistivity so that thin layers can be comparable to the effective resistance of the scanning electron beam of a pickup tube. The layer should not be thicker than required to absorb the light completely; this entails resistivities of more than 10" ohm cm. Also, the material must recover its dark resistivity quick enough to avoid "smearing" in rapidly moving objects. Then the decrease in resistance, due to light, can produce useful changes in output voltage. Early targets were made with aluminum oxide and zirconium oxide treated with caesium vapor. Later materials consisted of various sulphides, oxides and selenides. The Vidicon Figure 2 illustrates the basic principle of a vidicon tube, the first commercial tube to use a photoconductive target. The electrons moving from the cathode to the photolayer cause a varying current through the load resistor, as the resistance of portions of the photoconductive layer is changed by the focused light from the scene. Figure 1 shows the construction of the target in detail. The inside surface of the tube face is coated with a transparent conducting metallic film extending to the embedded kovar metal ring. This ring provides the external signal terminal. The photoconductive film, which is only about two ten -thousandths of an inch in thickness, is deposited upon the glass. Figure 3 shows the essential parts of a complete tube. Located some distance behind the target is a fine mesh wire screen which is used to maintain a uniform electric field over the inner face of the target. An electron gun similar to that used in other pickup tubes, but with a very small limiting aperture. is positioned at the other end of the tube. Deflection of the electron beam is accomplished by magnetic fields from a yoke similar in design to that used with the image orthicon. The tube and deflection yoke assembly is immersed in an axial d.c. magnetic field produced by the focusing coil. As in the orthicon and image orthicon, the axial field (Continued on page 27) 22 R A D I O - E L E C T R O N I C E N G I N E E R I N G APRIL, 1953
O N FREQUENCY CONTROL FOR MILITARY LICATION MIL CRYSTAL UNIT BLILEY CRYSTAL HOLDER FREQUENCY RANGE MEG RATING SROERATURE GE (Centigrade) FREQUENCY TOLERANCE OVER OPERATING RANGE CR-15 CR-16 CR-18 CR-19 CR-23 CR-24 CR-27 CR-28 CR-29 CR-30 CR-32 CR-33 CR-35 CR-36 CR-37 CR-42 CR-44 CR-45 CR-46 CR-47 AR23W BH6A BH6A BH7A BH6A BH6A AR2 AR23W BH6A BH6A BH6A BH6A BH9A BH9A BH6A BH6A BH6A BH6A 080 -0.19999 --40° to +70° 0.080 -0.19999 -40 0.8- 15.0 90° -55° to +90° -55° to +90° 15.0-510 0.8 -15.0 +70° to +80° 0.080 - 0.19999 -55° to +90e +80° to +90° 0.800 -15.0. 0.455 0.2-0.500 -40° to +70° -40° to +70° 0.2 - 0.500 +70° to +80° + .01% + .01% + .005% (- .005% + .005% + .005% + .002% + .002% + . + .002% + .005% + .002% o + .02% + .003% + .002% .± .02% + .01% ± .002% BULLETIN NO. 43 CONTAINS A QUICK REFERENCE INDEX FOR MILITARY TYPE CRYSTAL UNITS -- -SENT UPON REQUEST 111 c CRYSTALS BLILEY ELECTRIC COMPANY UNION STATION BUILDING, ERIE, PA. APRIL, 1953 R A D I - E L E C T R O N I C E N G I E E R I N G
Page 1 and 2: T EVIS i N APRIL 1953 IN THIS ISSUE
Page 3 and 4: YOU PRACTICE COMMUNICATIONS with Ki
Page 5 and 6: I NOW...from D.T.I.s MILLION DOLLAR
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Page 25 and 26: is done, the meter on the Microvolt
Page 27 and 28: I voltage regulation', '.'. as well
Page 29 and 30: DECAY CONSTANT IN VIBRATING SYSTEMS
Page 31 and 32: I I (5000), and Curie point* occurr
Page 33 and 34: COAXIAL CAVITY TUNING ELEMENT Fig.
Page 35 and 36: Make your U H F circuits as simple
Page 37 and 38: (// Capacitors shown 21 /7 times ac
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Page 43 and 44: i Gpoßt i. NOW YOU CAN 4 pvRAVE ti
Page 45 and 46: known as the 5894 -A, because the c
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Page 49 and 50: Converter Calibration (Continued fr
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Page 69 and 70: 7. Solder to the ends of the Fa " s
Page 71 and 72: THE semi -automatic or "bug" key is
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Page 79 and 80: CHANNELS Pertinent frequencies and
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Page 83 and 84: conicals are usually stacked two or
Page 85 and 86: THE U T SPEAKER ENCLOSURE Design de
Page 87 and 88: a drop of 6 db -per- octave above 5
Page 89 and 90: Fig. 1. (Left) Front panel view of
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Fig. 3. Bottom view of the 30-watt
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A LOWR[SISTA NCE OHMMETER _ By J. P
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Sete/t SHORT -WAVE Compiled by KENN
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Indicates frequency of energized ci
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Fringe Antennas (Continued from pag
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Receiver Change s (Continued from p
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:nrh BF "BASIC ELECTRONIC TEST INST
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