|MLA Citation:||Bloomfield, Louis A. "Television" How Everything Works 21 Jan 2018. Page 3 of 3. 21 Jan 2018 <http://www.howeverythingworks.org/prints.php?topic=television&page=3>.|
Understanding the photocathode system requires an examination of the interactions of light and metal. Whenever a particle of light—a photon—strikes a metal surface, there is the possibility that the photon will eject an electron from that metal surface. However, each type of metal requires a certain minimum photon energy before it will release an electron. Because infrared light photons carry very little energy, they can only eject electrons from very special metals. The sniperscope contained a very thin layer of one such infrared-sensitive metal.
Actually, this metal layer was deposited on a transparent glass window that formed the front end of a vacuum tube. Light from the scene in front of the sniper passed through a converging lens that formed a real image of the scene on the metal layer. The metal layer was so thin that light striking its front surface through the glass window caused electrons to emerge from its back surface. Electrons ejected from the back of the metal layer were accelerated by a high voltage that was applied between this metal photocathode layer and a phosphor-coated anode layer located very nearby. Each electron acquired so much energy during its brief flight that it caused the phosphors on the anode to glow brightly when it hit them. The electron flight path was short so that electrons emitted by a certain spot on the photocathode would hit a corresponding spot on the phosphor anode and the sniper would see a clear image of the scene in front of the sniperscope.
Because one infrared photon striking the photocathode could lead to the release of dozens of photons from the phosphors on the anode, this sniperscope provided a modest amount of "image intensification." But modern starlight scopes go far beyond this level of amplification. Like the old sniperscope, these modern devices also use a photocathode to turn a pattern of light from the real image of a lens into a pattern of free electrons. But the starlight scope then amplifies these electrons by sending them through narrow channels that have highly charged walls. As the electrons bounce their ways through the channels, they knock out hundreds, then thousands, then even millions of other electrons so that each original photon can release more than a million electrons from the amplifying system. When these electrons strike the phosphor-coated anode, the image they produce is bright and visible, so that the person looking at the anode can effectively see when each photon of light strikes the photocathode and initiates one of these electron cascades. With such incredible light sensitivity, there is no longer any need to actively illuminate the target with infrared light—even starlight is enough illumination to make the target visible through the starlight scope's image intensification system.
In a digital video signal, a physical quantity first represents numbers and then these numbers represent the brightness and color of the spots. The physical quantity representing the numbers doesn't have to be continuous. For example, a current that's on could represent the number 1 while a current that's off could represent the number 0. A certain pattern of on and off currents could represent larger numbers and these numbers could then represent brightness and color. This use of a continuous or non-continuous physical quantity (such as magnetization, charge, or current) to represent numbers and then these numbers to represent a continuous physical quantity (such as brightness) is called digital representation.
One advantage of digital representation is that it's relatively immune to noise. In analog representation, any disturbance in the continuous physical quantity representing the information leads directly to a disturbance in the recovered information. For example, if the strength of a radio wave is representing brightness and color on your television (the current technique), then any disturbance of the radio wave leads directly to a damaged image on your television. But in digital representation, small changes in the physical quantity that's carrying the information won't change the numbers that are obtained from that physical quantity and will thus have absolutely no effect on the recovered information. For example, if the strength of a radio wave is representing numbers in digital format, using binary (base two) encoding, then a small disturbance of the radio wave will not affect the binary numbers that are recovered from the radio wave. To see why that's true, imagine representing the number 1 as a powerful radio wave and a 0 as no radio wave at all. It's pretty easy to tell a powerful radio wave from an absent one so that, even if there is some radio interference around, it's unlikely to confuse the receiver. Moreover, even if noise does occasionally confuse the receiver about a number or two, the digital scheme can include redundant information that allows the receiver to identify errors and to fix them! That's why a compact disk is so immune to noise—even if there is a flaw or dirty spot on the disk, there is enough redundant digital information to reproduce the music flawlessly.
The other advantage to digital representation is that digital compression techniques become possible. A typical video signal contains lots of unnecessary and duplicated information. For example, when two people are standing in a room and the only things that are changing with time are the images of those two people, there is really no reason to keep sending an image of the room itself from the broadcast station to your home. Digital compression can identify redundant information and remove it from the transmission. In doing so, it can use the communication channel more efficiently.
By adopting a digital transmission scheme, the FCC has recognized that broadcasters will be able to send much clearer, more detailed images using digital representations than with the current analog representations, while still occupying the same portions of the electromagnetic spectrum. However, there is a cost—current televisions will not work directly with these new digital signals. To fix that shortcoming, there will be inexpensive converters that receive the new digital signals and recreate the analog signals needed for current televisions. This conversion will allow older televisions to keep working, but the new digital televisions will be designed to make better use of the enhanced details in the transmissions. The new transmissions will contain about 4 times the detail of current transmissions so that the images will be sharper as well as more immune to noise than the current transmissions.
Because the picture tube can't direct its electron beams accurately enough to hit specific red, green, or blue phosphor regions, it needs help from a shadow mask that's located a short distance before the phosphor layer. This thin metal grillwork shades the light-producing phosphors from the wrong electrons. The picture tube has three separate beams of electrons, one for each primary color, and the grillwork ensures that electrons in the red beam are only able to strike phosphors that produce red light. The same goes for the blue beam and the green beam.
The grillwork must stay in perfect registry with the pattern of phosphors on the inside of the picture tube, even as their temperatures change. That's why this grillwork is made of Invar, a special steel alloy that doesn't change size when its temperature changes. Unfortunately, Invar can be magnetized and its magnetic fields can then steer the electrons so that they strike the wrong phosphors. If you were to hold a strong magnet near the face of a computer monitor, you would probably magnetize the Invar shadow mask and spoil the color balance of the images on the monitor.
To demagnetize the Invar, you must expose it to a magnetic field that fluctuates back and forth and gradually diminishes to zero. The Invar's magnetization would also fluctuate back and forth and would dwindle to nothing by the time the demagnetizing field had vanished. Traditionally, this demagnetizing was done with a large wire coil that was powered by alternating current so that its magnetic field fluctuated back and forth. This coil was gradually moved away from the picture tube so that the influence of its magnetic field slowly diminished to zero, leaving the Invar completely demagnetized. In good computer monitors, this coil and an automatic power source for it are built in. When you push the degauss button, you see a burst of colors as the demagnetizing coil's fluctuating magnetic field erases the magnetization of the shadow mask and also steers the electrons wildly.
Apparently, degaussing circuitry has been built into all color televisions sets for the past 20 or 30 years. When you turn on your television, a demagnetizing coil activates briefly and removes minor magnetization from the television's invar mask.
Color sets create the impression of full color by mixing the three primary colors of light—blue, green, and red—right there on the inside surface of the picture tube. A set does the mixing by turning on and off three separate electron beams to control the relative brightnesses of the three primary colors at each location on the screen. The shadow mask is a metal grillwork that allows the three electrons beams to hit only specific phosphor dots on the inside of the tube's front surface. That way, electrons in the "blue" electron beam can only hit blue-glowing phosphors, while those in the "green" beam hit green-glowing phosphors and those in the "red" beam hit red-glowing phosphors. The three beams originate at slightly different locations in the back of the picture tube and reach the screen at slightly different angles. After passing through the holes in the shadow mask, these three beams can only hit the phosphors of their color.
Since the shadow mask's grillwork and the phosphor dots must stay perfectly aligned relative to one another, the shadow mask must be made of a metal that has the same thermal expansion characteristics as glass. The only reasonable choice for the shadow mask is Invar metal, an alloy that unfortunately is easily magnetized. Your son has magnetized the mask inside your set and because moving charged particles are deflected by magnetic fields, the electron beams in your television are being steered by the magnetized shadow mask so that they hit the wrong phosphors. That's why the colors are all washed out and rearranged.
To demagnetize the shadow mask, you should expose it to a rapidly fluctuating magnetic field that gradually decreases in strength until it vanishes altogether. The degaussing coils I mentioned above plug directly into the AC power line and act as large, alternating-field electromagnets. As you wave one of these coils around in front of the screen, you flip the magnetization of the Invar shadow mask back and forth rapidly. By slowly moving this coil farther and farther away from the screen, you gradually scramble the magnetizations of the mask's microscopic magnetic domains. The mask still has magnetic structures at the microscopic level (this is unavoidable and a basic characteristic of all ferromagnetic metals such as steel and Invar). But those domains will all point randomly and ultimately cancel each other out once you have demagnetized the mask. By the time you have the coil a couple of feet away from the television, the mask will have no significant magnetization left at the macroscopic scale and the colors of the set will be back to normal.
Incidentally, I did exactly this trick to my family's brand new color television set in 1965. I had enjoyed watching baseball games and deflecting the pitches wildly on our old black-and-white set. With only one electron beam, a black-and-white set needs no shadow mask and has nothing inside the screen to magnetize. My giant super alnico magnet left no lingering effect on it. But when the new set arrived, I promptly magnetized its shadow mask and when my parent watched the "African Queen" that night, the colors were not what you'd call "natural." The service person came out to degauss the picture tube the next day and I remember denying any knowledge of what might have caused such an intense magnetization. He and I agreed that someone must have started a vacuum cleaner very close to the set and thus magnetized its surface. I was only 8, so what did I know anyway.
Finally, as many readers have pointed out, many modern televisions and computer monitors have built-in degaussing coils. Each time you turn on one of these units, the degaussing circuitry exposes the shadow mask to a fluctuating magnetic field in order to demagnetize it. If your television set or monitor has such a system, then turning it on and off a couple of times should clear up most or all of the magnetization problems. However, you may have to wait about 15 minutes between power on/off cycles because the built-in degaussing units have thermal protection that makes sure they cool down properly between uses.
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