Please be patient
(LARGE SECTION - to explain Monitors thoroughly - I had to include many images in this
section - make sure to read Monitors - Part2 as well)
Part 1
Author's Note : As far as I know, the close-up photos of aperture grills and shadow masks included on this page are the first of their kind. The photos are quite interesting, since you can actually see each microscopic dot of the screen (or in the case of the aperture grill, each slot).
The monitor is the most important PC component, at least as far as the user is concerned. It is your interface, it is what you stare at . . . hour after hour. It decides what kind of experience you will have with your PC. Make no mistake - the choice of monitor is extremely important. If you use your PC frequently, then this is one area you do not want to go cheap !!
Monitors range from old, small, amber or green screens made for the text screens of the MS-DOS world, to large, lush models priced in the thousands. This section will cover how a monitor works, and most importantly, how to select the right monitor for you.
There are thousands of monitors on the market, and not surprisingly, a large number of pricey models that have a poor picture !! If I can do anything for you here on this page, it will be to steer you towards a decent selection, and I will do just that.
Make sure to read through the last third of this page, on resolution. Everyone misunderstands it - and it will really help you when you work with images, displaying, resizing, and printing them.
CRT (Cathode Ray Tube)
The CRT, or "Picture Tube", is the bulk of the Monitor. The rest is circuitry and adjustment controls. The CRT is a hard molded glass v-shaped "tube", with all air removed (a vacuum). The faceplate is the screen that you view, and the inside of it is covered with a material called "phosphor", which glows when exited electrically.
An extremely high, constant, positive voltage is applied to the faceplate (the "anode"). Smaller voltages are applied to the electron guns (the "cathodes"). Positive voltage attracts electrons, but only if there is a different is voltage. For example, if the anode was +10,000 volts, and the cathode was +10,000 volts - then no current would flow, and the phosphor would not glow - no picture.
But - the anode is usually +20,000 to +30,000 volts, and the electron guns are kept at a low voltage. The great difference causes a strong beam of electricity, like a lightning bolt, to surge forth from the electron guns, splatting into the faceplate, and lighting up a point on the phosphor.
For dark or black areas, they need to 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 30,000 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.
Note that 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. There is one main difference - the monitor scans the lines in an "interlaced" pattern (field 1), where it scans the odd lines first, and then returns to the top to scan the even lines (field 2). The example below shows a screen with only 8 scan lines, which is two fields - 4 lines each. Actual monitors have hundreds of lines per field. For more on interlacing, see the Monitor2 section :
NOTE: . Actually, there are no "black bars" - these are only there to show you the pattern of scanning the lines across the faceplate of the CRT. In actuality, the phosphor glows for a very short time after the beam passes across it - so the previous scanned field is merely "painted over" again . . . updated continuously - just like a picture show. Also, the beam must "retrace" (travel back up to the top) after each field is scanned - this is invisible to you, since during the retrace, the beam is turned off.
CRT Sizes - computer monitors come in standard sizes of 13", 15" 17" 19" 21" and 22". You can get huge monitors (see The Big Picture), but they are extremely pricey, and cause you to continually move your eyes and head left and right as you work, The only reason you would even need a 29" monitor would be for presentations - and even better, go out and a buy a PC Video Projector for that. The way the monitor is measured used to be very sneaky and underhanded - and they did the same thing with TV's as well. It is measured diagonally from corner-to-corner. However, the edges are covered over by a plastic housing, which chops off some real estate. I just measured my 22" monitor diagonally, and the actual viewable diagonal line is 19.75". I also measured my daughter's 17" monitor and the viewable diagonal line is 15.75". The manufacturers measured the CRT from corner-to-corner "BEFORE" the housing is placed around it. Recently, they have become honest, and list the "viewable" measurement - which is the actual diagonal measurement of what we can see.
Flat vs Curved Faceplate - monitors always has a curved faceplate, although the flat monitors get around that fact. The curve gave structural strength to the monitor glass, which is under tremendous pressure (remember the air has been vacuumed out). In the last few years, Flat or "near-Flat" monitors have been the rage, and they are beautiful. It gives the effect of staring at the side of a fish tank. The monitors actually still have the curved faceplate, but they add additional glass to the front wich corrects the curvature.
Distortion - since the electron beam is curved before it hits the faceplate phosphor - it is a very precise yet trickly configuration to have it scan lines to form a perfect rectangle. There are many variables, and no two new monitors are manufactured to scan identically. Therefore, every monitor has user adjustable parameters to correct for these conditions of distortion. The diagram below shows the main types of distortion that commonly occurs. The manual adjustments counter these effects, and square up the display.
Which Monitor Should I buy ??
There are many excellent monitors, and many poor monitors. For the price and quality, the Dell 19" Trinitron (OEM Sony) is my favorite. For the power user, the 22" Flat-Screen NEC FP1350 can't be beat - I have one and it is a joy !!! I actually do not think the Sony monitors are worth the money - but go ahead and check them out, since they seem to change every year, and it is impossible to keep up. I do like the Mitsubishi Diamond series, so you may want to check those out. I don't know about recent Cornerstone monitors - but a few years ago, I paid a hefty fee for a 19" Cornerstone 45/101sf shadow mask, and the colors are all pastel at best ! IIyama supposedly makes an awesome Monitor, but I have never seen one.
As far as size, do not go below 19" - it just isn't worth the small savings, considering you will stare at that screen for thousands of hours.
How a Monitor Works
An Overview - Lines, Fields, and Frames - As you already know . . . the monitor, or "screen", or "display" - is similar to a TV screen, in that it displays images and motion video as a series of rapidly changing still images - called "frames". The trick, is that since the images flash on the screen so quickly, that we are unable to discern that, and our own mind blurs the frames together. It has been shown, that for most people, if still images are flashed at a rate of 60 times per second or higher, we simply cannot detect the images, and it is seen as smooth, flowing, motion.
The monitor actually displays 30 frames per second (actually 29.997 to be exact - discussed later, so assume 30 fps for now). This is due to a process known as "interlacing", where an odd "field" is displayed by scanning the odd lines onto the screen using a powerful electron beam, and then and even field is shown by scanning the even lines onto the screen.
As these fields are painted onto the screen as a series of horizontal lines, what you are actually seeing is a chemical coating inside the glass called "phosphor". The phosphor glows when the electron beam runs across it, and the golw last for a short time, so that is does not turn black again as soon as the beam goes down to the next line. The phosphor is made so that on each line of the screen, it continues to glow, just long enough, until the beam once again makes the full trip and runs across that same line again. The lines are so small and close together, that we cannot detect them, and instead it just looks like a full image. Film projectors actually do that - they flash an entire image, one after the other.
Televisions and monitors, however, do not have that capability, because they use a large, glass tube with a flat plate in the front, and a narrow neck in the back with 3 "guns", one for each primary color. Again, the faceplate is charged with very high positive voltage (20,000 to 30,000 volts), while the electron guns in the neck have a low voltage. The great difference in voltages causes a line of electricity called an "electron beam" to surge from each gun in the neck of the tube, pass through a mask or grill, and finally slamming into the faceplate phosphor.
Refresh Rates - the refresh rate is actually the same as the number of fields per second, and is measured in Hz (Hertz), which is cycles per second. Each time a field is painted onto the screen, the screen is essentially "refreshed". Refresh rates are adjustable within Windows under the Display settings, but it is best to simply select "Optimal" and let Windows manage the rate for you (unless you are seeing flicker - then you should manually change it to a higher rate, 75 to 85 Hertz is fine).
Masks - Refining the Electron Beams
Televisions have the same type of faceplate as monitors do - with a an inner coating of color phosphor. A problem occurren when they tried to use TV screen for computer monitors - the text was blurry. It seemed the CRT could not define crisp, clear edges. This is fine for television, since the text that is used is just credits at the end of a show, and usually it is quite large. Monitors require much tighter control.
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.
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).
Aperture Grill
The aperture grill 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.
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 -
Shadow Mask
The shadow mask has the disadvantage of allowing only a small portion of the beams to pass through, and therefore is less vibrant and bright. Note how the three beams pass through one hole in the mask, and diverge the opposite direction before hitting the phosphor on the faceplate.
It 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 :
Now look at a close-up of the Netscape icon from my Daughter's shadow mask monitor. 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. 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 then looked at a reading of the components :
The left image shows the small area that is blown up on the right image (I shaded the rest in yellow so that you can tell which part I used). Notice the area on the left is white, and correspondingly, the values or RGB are 255, 255, 255. In the middle of the image we have our shade of Blue with a hint of green (Blue = 180 Green = 25 Red = 0). It is surprising to see that in the Blue area, the blue dots are much more saturated, and they also are larger that in the white area.
An oddity - the Blue component is 255 in the white area - but 180 in the blue area, where the dots are obviously more saturated and larger.
