The most common display device in an image processing environment is a CRT. A CR1T consists of an electron gun and a phosphor screen, as shown in Figure 2.28. The electron gun produces a beam of electrons that are focused in a narrow region on the phosphor screen through the electrostatic lens, which involves tli use of an electrostatic and a magnetic field. The electron beam excites the phosphor to generate light. For monochrome tubes, one beam is used. For monochrome tubes, three separate beams are used to excite the phosphor for each of the three colors. The screen is typically scanned from left to right and from top to bottom to cover the whole screen.
The phosphor chemicals are generally light metals, such as zinc, in the form of sulfide and sulfate. The phosphor material is processed to obtain very fin particles, which are applied to the inside of the glass plate. For monochrome tubes, the phosphor coating is a uniform layer. For color tubes, the phosphor deposited in dots or vertical layers for each color.
When the high-velocity beams excite the phosphor, electrons in the atoms the phosphor move to a high energy level. As the electron beams move to different spot on the screen in raster scanning, the electrons move back to a low energy level and emit light. The radiation of light from the screen is called luminescence. When the light is extinguished, the screen fluoresces. The time takes for light emitted from the screen to decay to 1% of the maximum value called the screen persistence. For medium to short persistence, generally used for television monitors, the decay time is about 5 msec. Long persistence phosphor exists, but can cause merging of two frames, resulting in substantial blurring for sequence of images with motion.
Since their screen persistence is short, most CRTs used for display are bright coed tor short periods of time as the electron beams scan the screen. To display a still image or a sequence of images, the screen must he refreshed continuously. Display monitors often have semiconductor random access memory (RAM) for refresh. The refresh memory, which can typically hold more tlia;i one image frame of a reasonable size. is used to refresh the screen.
The display monitor used in an image processing environment is typically a high quality industrial type. A typical monitor has an aspect ratio (width to height) of I arid is typically calibrated to display a square frame. Standard display sizes are 512 x 512 and 1024 x 1024, but other sizes, such as 640 x 480 and 1024 x 1280, are available. The refresh rate has traditionally been 30 frames/sec with a 2:1 interlace the same as for television broadcasting. Individual scan lines are sharply defined on high quality monitors. A sharp horizontal line, therefore. may appear only in the odd field or only in the even field, so the flicker rate for this scan line would become 30 cycles/sec. At this rate, the flicker may be visible. To avoid this problem, displays which refresh the screen at 50 frames/sec or 60 frames/sec without interlace are available. This provides a display that is essentially flicker free and is more pleasant to the human viewer.
In many displays, the refresh memory is used not only or refreshing the screen hut also for processing. The displayed tmaee caii be zoomed, scrolled, or processed with a simple filter in the refresh memory in real time. As hardware advances continue, the display unit will gain even more sophisticated capabilities.
An image displayed on a CRT screen is considered soft since it is in a sense a temporary image. One approach to making a hard copy is to photograph the CRT screen on film. Special camera systems that have their own small CRT screens are available for such purposes. There are many additional hard copy devices. To obtain multiple copies, for example, methods such as letterpress, lithography (offset), and gravure exist.