CRTs, or video monitors, are the most common output device on computers today. The figure below illustrates the basic structure of a CRT. A CRT is an evacuated glass bottle, with a heating element on one end and a phosphor coated screen on the other. When a current flows through this heating element, called a filament, the conductivity of the metal filamant is reduced due to the high temperature. This cause electrons to pile up on the filament, because they can not move as fast as they would like to (Notice I'm wearing my "electrons-are-particles" hat). Some of these electrons actually boil off of the filament.
These free electrons are attracted to a strong positive charge from the outer surface of the focusing anode cylinder (sometines called an electrostic lens). However, the inside of the cylinder has a weaker negative charge. Thus when the electrons head toward the anode they are forced into a beam and accelerated by the repulsion of the inner cylinder walls in just the way that water is speeds up when its flow though a smaller diameter pipe. By the time the electrons get out they're going so fast that they fly past the cathode they were heading for.
The next thing that the electrons run into are two sets of weakly charged deflection plates. These plates have opposite charges, one positive the other negative. While their charge is not strong enough to capture the fast moving electrons they do influence the path of the beam. The first set displaces the beam up and down, and the second displaces the beam left and right. The electrons are sent flying out of the neck of the bottle, called a yolk, until they smash into the phosphor coating on the other end of the bottle. The impact of this colission on the out valence bands of the phophor compounds knocks some of the electrons to jump into the another band. This causes a few photons to be generated, and results in our seeing a spot on the CRT's face.
CRTs were embraced as output devices very early in the development of digital computers. There close cousins, vacuum tubes, were some of the first switching elements used to build computers. Today, the CRT is a the last remaining vacuum tube in most systems (Even the flashing lights are solid-state LEDs).
Most likely, oscilloscopes were some of the first computer graphics displays. The results of computations could be used to directly drive the vertical and horizontal displacement plates in order to draw lines on the CRT's face. By varying the current to the heating filament the output of the electron beam could also be controlled. This allowed the intensity of the lines to vary from bright to completely dark.
These early CRT displays were called vector, calligraphic or affectionately stroker displays. The following demostration gives some feel for how they worked.
By the way, this demo is an active Java applet. You can click and drag your mouse inside of the image to reorient the CRT for a better view. Notice the wireframe nature of the displayed image. This demo is complicated by the fact that it's a wireframe simulation of a wireframe display system. Notice how the color of the gray lines of the CRT vary from dark to light indicating which parts of the model that are closer to the viewer. This technique is called depth-cueing, and it was used frequently on vector displays. The intensity vriations seen on the teapot, however, are for a different reason. Eventaully, the phosphors recover from their excited state and the displaced electrons return back to their original bands. The glow of the phosphor fades. Thus, the image on the CRT's face must be constantly redrawn, refreshed, or updated.
The two primary problems with vector displays are that they required constant updates to avoid fading, thus limiting the drawn scene's complexity, and they only drew wireframes.
During the late 50s and early 60s, broadcast televison, really began to take off. It had been around for a while, but it didn't become a commodity item until about this time. Televisions are basically just oscilloscopes. The main difference is that instead of having complete control over the vertical and horizontal deflection, a television sweeps its trace across the entire face in a regular fixed pattern (the actual details are slightly more complicated, but that's the jist of it). This scanning pattern proceeds from the top-left of the screen to the bottom-right as shown in the diagram. The final result is that the entire screen is painted once every 1/30th of a second (33 mS).
Televisions were mass produced and inexpensive. For a computer to paint the entire screen it needs only to synchronize its painting with the constant scanning pattern of the raster. The solution to this problem was to add a special memory which opreated synchronous to the raster scanning of the TV. But, while televisions were cheap, memory wasn't. So there was a long period where the patterns were scanned out of a cheap high-density read-only memories, called character generators. The trick was to use a single 8 bit code to specify an 8 by 12 character pattern from the ROM, and with a few addressing tricks one could build a nice display (80 by 25 character) with only 2 kilobytes of memory. Thus the era of the CRT-terminal was born.
There were a few attempts at building systems with downloadable or programmable character generators. And a few systems added an extra byte to specify the foreground and background colors of the character cell. Lots of tank/maze arcade games in the 70's worked this way. But by the late 70's and early 80's the price of memory started a free-fall and the graphics terminal was born. Next lecture we'll go into a lot more detail about how the notion of a framebuffer is fundamental to modern computer graphics.
Color CRT's are more complicated than the simple monochrome models summarized above. The phosphors on the face of a color CRT are laid out in a precise geometric pattern. There are two primary variations, the stripe pattern of in-line tubes shown on the left, and the delta pattern of delta tubes as shown on the right.
Within the neck of the CRT there are three electron guns, one each for red, green, and blue (the actual beams are all the same color-- invisible). There is also a special metal plate just behind the phosphor cover front face, called a shadow mask. This mask is aligned so that it simultaneously allows each electron beam to see only the phosphors of its assigned color and blocks the phosphor of the remaining two colors.
The figure shown above shows the configuration of an example in-line tube. On page 44 of Hearn & Baker you'll see a similar diagram for a delta electron gun configuration
A significant portion of the electron beam's energy strikes the mask rather than the phosphors. This has two side effects. The shadow mask has to be extremely rigid to stay aligned with the phosphor patterns on the CRT face. The collision of electrons with metal mask causes the mask to emit some of it absorbed energy as eletromagnetic radiation. Most of this energy is in the form of heat, but some fraction is emitted as x-rays. X-rays can present a health hazard. This wasn't a large problem for television because the intensity of the x-ray radiation falls off quickly as you move away from the screen. However, computer monitors are supposed to be viewed from a short distance. This health concern along with the high voltages and power dispations of CRTs has motivated the development of new display technologies.
For more information on CRTs check out the following links:
When LCDs are used as optical (light) modulators they are actually changing polarization rather than transparency (at least this is true for the most popular type of LCD called Super-twisted Nematic Liquid crystals). In their unexcited or crystalline state the LCDs rotate the polarization of light by 90 degrees. In the presence of an electric field, LCDs behave like a liquid and align the small electrostatic charges of the molecules with the impinging E field.
The book Hearns & Baker is a little confusing in describing how LCDs work (pp. 47-48). They call the relaxed state the "On State" and the excited state the "Off State". Their statement is only true from the point of view of the pixels when the LCDs are used in a transmissive mode (like on most laptops). The opposite is true when the LCDs are used in a reflective mode (like on watches).
The LCD's transition between crystalline and liquid states is a slow process. This has both good and bad side effects. LCDs, like phosphors, remain "on" for some time after the E field is applied. Thus the image is persistent like a CRT's, but this lasts just until the crystals can realign themselves, thus they must be constantly refreshed, again, like a CRT.
Rather than generating light like a CRTs, LCDs act as light values. Therefore, they are dependent on some external light source. In the case of a transmissive display, usually some sort of back light is used. Reflective displays take advantage of the ambient light. Thus, transmissive displays are difficult to see when they are overwhelmed by external light sources, whereas reflective displays can't be seen in the dark. You should also note that at least half of the light is lost in most LCD configurations. Can you see why?
The LCD's themselves have extremely low power requirements. A very small eletric field is required to excite the crystals into their liquid state. Most of the energy used by an LCD display system is due to the back lighting.
I mentioned earlier that LCD's slowly transition back to their crtstalline state when the E field is removed. In scanned displays, with a large number of pixels, the percentage of the time that LCDs are excited is very small. Thus the crystals spend most of their time in intermediate states, being neither "On" or "Off". This behavior is indicative of passive displays. You might notice that these displays are not very sharp and are prone to ghosting. Another way to building LCD displays uses an active matrix. The individual cells are very similar to those described above. The main difference is that the electic field is retained by a capacitor so that the crystal remains in a constant state. Transistor switches are used to transfer charge into the capacitors during the scanning process. The capacitors can hold the charge for significantly longer than the refresh period yeilding a crisp display with no shadows. Active displays, require a working capacitor and transistor for each LCD or pixel element, and thus, they are more expensive to produce.
More resources:
Next time well discuss the computer side of raster displays.
Back to OutlineThis page was last modified 8/21/96