barcode printing using vb.net Simplified cutaway view of a vidicon camera tube. in Software

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29-8 Simplified cutaway view of a vidicon camera tube.
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A vidicon is sensitive, working well in dim light. But its response becomes sluggish when the level of illumination is low. This causes images to persist for a short while. You might have noticed this when using a VCR in a dimly lit environment. The motion smears and images blur.
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The image orthicon
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Another type of camera tube, also quite sensitive, but having a quicker response to image changes, is the image orthicon. It is constructed much like the vidicon, except that there is a target electrode behind the photocathode (Fig. 29-9). When an electron from the photocathode strikes this target electrode, several secondary electrons are emitted as a result. This is a form of current amplification. The image orthicon acts as a photomultiplier, improving its sensitivity.
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Traveling-wave tubes 549
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29-9 Simplified cutaway view of an image orthicon.
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A fine beam of electrons, emitted from the electron gun, scans the target electrode. The secondary electrons cause some of this beam to be reflected back toward the electron gun. Areas of the target electrode with the most secondary-electron emission produce the greatest return beam intensity; regions with the least emission produce the lowest return beam intensity. The greatest return beam intensity corresponds to the brightest parts of the video image. The return beam is modulated as it scans the target electrode in a pattern just like the one in the TV receiver CRT. The return beam is picked up by a receptor electrode. The main disadvantage of the image orthicon is that it produces significant noise in addition to the signal output. But when a fast response is needed (when there is a lot of action in a scene) and the light ranges from very dim to very bright, the image orthicon is the camera tube of choice. It is common in commercial broadcasting.
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A traveling-wave tube is a form of electron-beam tube that is useful at ultra-high frequencies (UHF) and microwave frequencies. There are several variations on this theme; the two most common are the magnetron and the klystron.
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The magnetron
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Most magnetrons contain a cathode at the center, and a surrounding plate as shown in Fig. 29-10. The plate is divided into sections, or cavities, by radial barriers. The output is taken from an opening in the plate and passes into a hollow waveguide that serves as a transmission line for the UHF or microwave energy. The cathode is connected to the negative terminal of a high-voltage source, and the anode is connected to the positive terminal. Therefore, electrons flow radially outward. A magnetic field is applied lengthwise through the cavities. As a result, the electrons move outward in spirals from the cathode to the anode, rather than in straight lines. The electric field produced by the high voltage, interacting with the longitudinal magnetic field and the effects of the cavities, causes the electrons to bunch up into clouds. The swirling movement of the electron clouds causes a fluctuating current in the anode.
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29-10 A magnetron. Here, only two cavities are shown for clarity; often there are several cavities.
This is the UHF or microwave signal. The frequency depends mainly on the shape and size of the cavities within the magnetron. Small cavities result in the highest oscillation frequencies; larger cavities produce oscillation at relatively lower frequencies. A magnetron can generate more than 1 kW of RF power at a frequency of 1 GHz. As the frequency increases, the realizable power output decreases. At 10 GHz, a typical magnetron generates about 20 W of RF power output. Magnetrons can produce microwaves for use in cooking. The energy at these frequencies excites the molecules in organic substances like meat, vegetables, and grains. Your microwave oven uses a magnetron rated at about 500 W to 1 kW continuous output power.
The klystron
A klystron is a linear-beam electron tube. It has an electron gun, one or more cavities, and a device that modulates the electron beam. There are several different types of klystron tube. The most common are the multicavity and reflex klystrons. A multicavity klystron is shown in Fig. 29-11. In the first cavity, the electron beam is velocity modulated. This means that the speeds of the electrons are made to increase and decrease alternately at a rapid rate. This causes the density of electrons in the beam to change as the beam moves through subsequent cavities. The intermediate cavities increase the magnitude of the electron-beam modulation, resulting in amplification. Output is taken from the last cavity. Peak power levels in some multicavity klystrons can exceed 1 MW (1,000,000 W). A reflex klystron has only one cavity. A retarding field causes the electron beam to periodically reverse direction. This produces a phase reversal that allows large amounts of energy to be drawn from the electrons. The reflex klystron produces low-power UHF and microwave signals, on the order of a few watts.
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