Direct-view devices include the venerable cathode-ray tube
and the newer flat-panel technologies of LCD
, and OLED
. They differ from projector
s in that the image is created at the viewing surface. Projectors are split into two categories, front- or rear-projection. Rear-projection
monitors look like direct-view screens, but differ in that the image is not generated at the screen surface.
A screen’s aspect ratio relates to how it is shaped. The old TV standard uses an almost square screen, with an aspect ratio of 4:3. A movie screen is wider, and the new HDTV standard uses a widescreen ratio of 16:9.
The various Digital Television standards involve an important concept – interlaced vs. progressive images. An interlaced image is what standard TV uses. It takes a 480-line image and breaks it into two fields made up of 240 alternating lines each, displaying them one after the other to create a smoother-looking picture. A progressive-scan image is what a computer generates, where the entire image’s lines are displayed all at once.
The digital TV standard allows producers to use one or all of several resolutions, interlaced or progressive, depending upon the quality of the image desired. Resolutions include 480, 720, and 1080 with a small “i” or “p” to designate the scan method. Progressive scan is the technique of choice for large-screen monitors, and many of them contain line doublers to convert standard interlaced signals into progressive images.
Most displays in the home still use a cathode ray tube, or CRT at its core. Originally invented in 1931, the CRT-based television quickly found its way into most people’s homes. It creates an image by shooting a controlled stream of electrons from a device in the neck of the tube onto a layer of colored phosphors on the inside of the face of the tube, causing them to glow in a way that creates the image.
CRT’s advantages are high brightness and contrast, along with a wide viewing angle. A little-recognized advantage is that a CRT has a very long operational life, working for upwards of 15 or 20 years in most cases. The disadvantages are that a tube-based monitor is large and heavy. A recent development in CRT technology is the flat-faced tube. Advantages to a flat face with square corners as opposed to a curved face with rounded corners is that there is less glare from reflected light, and very little image distortion. They cost more than their rounded cousins, but are still less than the other flat-screen alternatives. Of course, a flat face CRT is still a large and heavy device.
A plasma display uses phosphors similar to that in a CRT, but it causes them to glow using an energized gas, called plasma, contained in thin tubes running along the inside of the screen. The biggest advantage to a plasma monitor is that it is flat, enabling the creation of screen sizes impossible to realize using CRT technology. It is common to find a plasma screen with a diagonal measurement greater than 50 inches. Other advantages include high brightness and contrast, along with a decent viewing angle. The biggest drawback to a plasma screen is cost. Even the lowest-priced models cost four to eight times as much as a large-screen CRT. Other drawbacks include phosphor burn-in faster than that of a CRT, and a significantly shorter operational life.
An LED display is currently used only in outdoor applications like billboards and outdoor monitors like the ones in Times Square. This is because the distance between the pixels (the dots that make up the image) is too great for a screen less than 10 feet wide or larger, and the high cost. This will change as LED technology advances.
An LCD monitor is basically a large high-performance version of the one used with a computer. Although they can be used as such, they differ in that they have much better color response, higher refresh rates to support video, and wider viewing angles. LCD TVs also have a much higher quality control at the factory, as computer CRTs are acceptable with a bad pixel or two. An LCD creates an image using a bright backlight shining through a sheet of liquid crystals that change the light much as a stained-glass window does. The best LCD monitors rival plasma screens in image quality. Advantages include the capability to make smaller screens than is cost-effective with plasma, for desktop and bedroom nightstand applications, and a relative immunity to image burn-in. Disadvantages include color range, contrast, and cost.
Organic light-emitting diode (OLED) technology is something you will probably hearing a lot of in the future, but it is not yet ready for prime time. It has a great deal of promise, but will only appear in portable devices until the technology matures a little more.