The CRT (Cathode Ray Tube)
- better known as "the screen" -
The CRT is a large, cone-shaped glass tube that site on it's side, so that a flat portion faces you. It is a vacuum inside, and it charged with about 25,000 volts of electricity !!! There are three electrodes inserted in the back called "guns" which are only charge with a few hundred volts. This huge different in voltage causes a lightning bolt to flow through the tube, and it smacks into the faceplate - lighting up the phosphor coating.
For dark or black areas, the TV must turn off the guns off. To do this, the voltage is increased to about 700 volts. Now, it would seem that - if you have 700 volts on one end, and 25,000 volts on another, that an electron beam would surely occur. However, since there is quite a distance between the guns and the faceplate, it takes an extreme voltage difference, and actually, there is a cutoff point where no beam flows, even though there is still a large difference in voltage.
NOTE: the three cathodes, or "guns" are situated as a triangle for Shadow Mask CRT's and they are In-Line for Aperture Grill CRT's - you will see why later.
Note that in the top picture, the beam is bending - this is called "deflection". This is necessary, because the beam actually "paints" lines across the screen, and must move back and forth at a high speed. The resulting lines that appear on the screen are called "scan lines". The lines are scanned out in the same order as you would read a book. It is not shown in the diagram, but a dense, series of coiled wires, called a "deflection yoke", are place along the sloped sides of the CRT, and scanning signals are sent through these wires, creating a magnetic field to bend the beam left, right, up, and down.
Deflection Yoke - pictured below, wraps around the neck on the CRT and extends partway up along the sloped sides. The yoke has signals running through it's wires that create a strong electrical/magnetic field - rapidly deflecting the electron beam, scanning across the screen to paint full frames at a rate of 60 frames per second. The entire concept is truly amazing.
There are actually 4 sets of coils - two sets at the top and bottom to control vertical movement - and two sets on either side to control horizontal movement. Since they are used in combination, the beam can be made to move so that it hits the faceplate anywhere on the screen - all it takes is a a combination of vertical and horizontal magnetic field.
The coils are comprised of hundreds of copper coils. A large number of coils are required, since it takes a strong magnetic field to penetrate the thick glass walls of the picture tube and move the beam left and right. The beam scans a picture across the screen, using the same pattern as reading a book does.
Masks - Refining the Electron Beams
Televisions have the same type of faceplate as monitors do - with a an inner coating of color phosphor. However, monitors require much tighter control, because they must be able to display sharp, clear, small text - and therefore they have tighter specs, such as dot pitch, number of dots or lines, etc.
The electron beams, upon leaving their respective Red Green and Blue guns, disperse slightly, and lose their sharp edges. It was found that if a sheet of metal was placed in front of the phosphor with tiny holes drilled into it - it "chopped off" the blurry edges of the circular beam and directed an exact, tight circle of energy onto the phosphor. This was known as a "shadow mask"
Today, manufacturers now place an ultra-thin metal covering behind the phosphor in all monitors, that directs and refines the electron beams. There are two types - shadow mask, and aperture grill. The shadow mask has been around a long time, and is simply a sheet of metal perforated with holes. The other type, introduced by Sony with their Trinitron Monitors, and now very common - is called the "Aperture Grill". As the name implies, a "grill" with vertical slats is laid down behind the phosphor. The metal strips allow the beams to penetrate as vertical slats that run up and down the screen.
Shadow Mask CRT's offer the sharpest details and are therefore the best choice for text. Aperture Grill CRT's offer the most vivid color saturation and are the best choice for images.
Inversion of the Electron Beam (this is shown in the images below) - earlier we told you that the three cathodes, or guns, in the neck of the picture tube - are situated as a triangle for Shadow Mask CRT's and they are In-Line for Aperture Grill CRT's . When they emit their beams, they are directed toward the faceplate, and when they pass through the mask - they are inverted. The shadow mask has holes, and all three beams pass through the same hole simultaneously. Since they enter the hole at an angle, the three beams diverge. Since the beams need a little bit of space after they pass through the mask (in order to disperse outward) a slight amount of diffusion occurs. This is actually good, since it lessens the amount of black space between dots (shadow mask) or slots (aperture grill).
Shadow Mask
The shadow mask is a thin metal sheet with tiny holes. For a 21-inch screen, there are about 400,000 trios of dots (dot triads). These dots concentrate and contour the color beams to add sharpness to the picture. Without it, the colors would bleed into one another.
It does have a disadvantage of allowing only a small portion of the beams to pass through, reducing the overall brightness - but this is compensated for by simply increasing the beam strength. The mask is not placed directly onto the phosphor coating - these is just a small amount of space to allow the 3 beams to diverge. Note how the three beams pass through one hole in the mask, and diverge in the opposite direction before hitting the phosphor on the faceplate.
Television CRT's and Computer CRT's are basically the same structurally, and therefore we use the computer display for images here. The shadow mask - as opposed to the aperture grill - has the advantage of fine dots, which result in sharp text.
NOTE: if you stand back 10-15 feet from the screen, these images take shape.
Note that unlike the aperture grill, you can see the black areas of the mask surrounding each dot. This is the disadvantage of the shadow mask - it impedes too much light. However, I have heard that companies have perfected the shadow mask by enlarging the dots (Enhanced Shadow Mask).
The shadow mask is a fine mesh made of (64% iron & 36% nickel) just in front of the faceplate which guides the three electron beams onto three colored phosphor dots on the inside of the faceplate. Surprisingly, only about 20-30% of the electron beam actually passes through the holes in the mask. You can imagine that as the beams travel through the tube, their circular shape becomes distorted. The mesh, called a "shadow mask", is a film of metal, with holes cut out, that direct and refine the beams. :
The inside of the faceplate is coated with phosphor dots - Red, Green, and Blue (called RGB), and when the electron beams hits the phosphor dots, they glow. If the beams did not move, we would see a white dot in the center of the screen (equal amounts of R, G, and B create white). That used to happen with older TV's when you turned them off, since the dot would stop moving, and the phosphor would glow for a couple of seconds. New TV's do not allow the beam to settle in the middle that way, because it can burn out that spot of phosphor, leaving a black dot when you view television programs. Here is a close-up snapshot of a Shadow Mask monitor while tunred off - the brightness of the image has been increased so that you can see the dots :
You cannot make out the color of the phosphor dots. They actually have a very low-level of color, that when excited by the beam, becomes bright and saturated.
Dot Triad (RGB Triangles of Dots) - not to be confused with "pixels". During normal operation, the beams scan very quickly across the faceplate, creating glowing "lines" on the phosphor. The lines are close together, so that we cannot discern that they are lines (unless you get up close and look). Here is a magnified section of a typical CRT line, which is a row of the 3 primary colors mentioned. Note that they are arranged in triangles. That way, when the circular beams hits the metal mesh in front of the faceplate of phosphor, the 3 beams are refined into 3 exact tiny circular beams, which hit the 3 dots. As the three beams it scan left to right, they illuminate the first set of 3 dots, then the second set of 3 dots, etc - until they reach the rightmost set of 3 dots :
NOTE: reading across the scan line - the dot triad inverts itself every other triad. You can see the first triad (skipping the half-blue dot) has a Red dot on the bottom, and a Green then Blue dot on the top. The next triad has a Green then Blue dot on the bottom, and a Red dot on the top.
Varying Colors Intensity - Although the faceplate voltage is kept constant, at a very high voltage - the voltage applied to the 3 color guns varies with the color signal. If you are viewing a golf tournament and there is a patch of grass, the signal would send higher voltages to the Red and Blue guns, and a low voltage to the Green gun. The bigger the difference in voltage between the guns and the faceplate - the stronger the electron beam. By lowering the Green gun voltage, the attraction of the electrons to the faceplate is increased, and the green beam zaps through the tube, while the Red and Blue are limited. In this scenario, one line would look something like this if magnified :
Example of Shadow Mask Display
Now look at a close-up of the Netscape icon - you will see the solid blue area is composed of no large blue dots, small green dots, and no red dots. I was expecting to see blue dots, and black where there should have been green and red dots. All this means is the the shade of blue is actually not pure blue for the icon. If it was pure blue then the colors would be R=0, G=0, B=255. In that case you would still the black areas between the blue dots, since 2 out of every 3 dots are shut down. To check out how much of each color existed in that area, I used the eyedropper (color picker) in Photoshop, to select the color, and took a digital macro photo of the screen :
Photo of Screen
Zoom of the Icon
When zooming in 1600% and viewing the same portion of the icon, you can see that the blue is represented by two colors, which are averaged by the eyes (they cannot see that fine of a resolution). The average color of the blue area is RGB=1,66,194. Note that the photo of the screen shows the tiny green dots between the larger blue dots. The monitor can display finer colors that the icon file is capable of, so it averages the grey and pure-blue pixels and displays them as a lower intensity (194) of blue dots and green (66) dots.
Aperture Grill
The aperture grill has the advatage of brilliant color - since the slats allow more beam saturation to pass through. But it has the disadvantage of a bit of blurring between dots, since there are no holes cutout in the mask, and bleeding can occur. In addition, since the structure has vertical metal pieces, they tend to move and stray left and right, and it was found that a couple of horizontal retaining wires had to be run across the screen about the 1/3 of the way from the top - and 1/3 of the way from the bottom. These wires create two slender horizontal gray lines which cannot be removed. Aperture grills have the advantage of bright, vivid, saturated colors. The problem with blurry text has been largely eliminated in recent years. Note how the three beams pass through the grill, and diverge the opposite direction before hitting the phosphor on the faceplate.
Example of Aperture Grill Display
The picture below shows the aperture, with it's slotted colors, and the retaining wires (which are actually extremely slender, and barely visible). An even smaller area is blown up for inspection. In the right top corner it is "white" - and you can see the repeating pattern of the vertical slots . . Red-Green-Blue-Red-Green-Blue etc.
NOTE: if you stand back 10-15 feet from the screen, these images take shape.
Note that in the detailed pic on the right - you cannot see the vertical grill (which would be black strips if it was wide enough). The aperture grill is so fine, that it allows the color to come streaming in with very little blockage. The small amount of of distance between the grill and the phosphor, disperses the beams slightly and causes the black slats to vanish
This image shows the dispersion effect (CRT top view), greatly exaggerated for clarity -
