Liquid Crystal Display PDF

Liquid Crystal Display We all know that matter takes the form of gas, liquid, or solid. A liquid crystal is a material state between that of a liquid and a solid. A liquid crystal has the property of a highly ordered molecular structure (a crystal) and the property of viscosity (a fluid). Liquid c...

Liquid Crystal Display We all know that matter takes the form of gas, liquid, or solid. A liquid crystal is a material state between that of a liquid and a solid. A liquid crystal has the property of a highly ordered molecular structure (a crystal) and the property of viscosity (a fluid). Liquid crystal materials are linear organic molecules (Fig. 17.4) that are electrically charged, forming a natural molecular dipole. Consequently, the liquid crystals can be aligned through the action of an external electric field. Display Characteristics LCDs are fashioned pixel by pixel. The LCD has a very intense white backlight that illuminates each pixel. Each pixel contains light-polarizing filters and films to control the intensity and color of light transmitted through the pixel. The differences between color and monochrome LCDs involve the design of the filters and films. Color LCDs have red-green-blue filters within each pixel fashioned into subpixels, each with one of these three filters. Most medical flat panel digital display devices are color LCDs. Fig. 17.5 illustrates the design and operation of a single pixel. A backlight illuminates the pixel and is blocked or transmitted by the orientation of the liquid crystals. The pixel consists of two glass plate substrates that are separated by embedded spherical glass beads of a few microns in diameter that act as spacers. In addition, bus lines---electric conductors---control each pixel with a thin-film transistor (TFT). Spatial resolution improves with the use of higher megapixel digital display devices. Medical flat panel digital display devices are identified by the number of pixels in the LCD. A 1-megapixel display will have a 1000 × 1000 pixel arrangement. A high-resolution monitor will have an 8-megapixel display or a 2160 × 3840 pixel arrangement. Table 17.3 reports the matrix array for popular medical flat panel digital display devices. Image Luminance The LCD is a very inefficient device. Only approximately 10% of the backlight is transmitted through a monochrome monitor and half of that through a color monitor. This inefficiency is partly attributable to light absorption in the filters and polarizers. Because a substantial portion of each pixel is blocked by the TFT and the bus lines, efficiency is reduced still further. The portion of the pixel face that is available to transmit light is the "aperture ratio." Aperture ratio is to a digital display device as "fill factor" is to a digital image receptor. Aperture ratios of 50% to 80% are characteristic of medical LCDs. Aperture ratio is a measure of image luminance of LCDs. LCDs have good grayscale definition. LCDs are not limited by veiling glare or reflections in the glass faceplate; thus, good contrast resolution is attained. The intrinsic noise of an LCD is low, and this also results in better contrast resolution. Light-Emitting Diode Display Any material that emits light in response to an outside stimulus is called a phosphor and the resulting visible light is called luminescence. A number of stimuli, including electric current (the fluorescent lamp), biochemical reactions (a lightning bug), visible light (a watch dial), and x-rays (Roentgen's discovery), cause luminescence in materials. Luminescence is similar to characteristic x-ray emission. However, luminescence involves outer shell electrons. When a luminescent material is stimulated, the outer shell electrons are raised to excited energy levels. This situation effectively creates a hole in the outer-electron shell, which is an unstable condition for the atom. The hole is filled when the excited electron returns to its normal state. This transition is accompanied by the emission of a visible light photon. The range of excited energy states for an outer shell electron is narrow, and these states depend on the structure of the phosphor. The wavelength of emitted light is determined by the level of excitation to which the electron was raised and is characteristic of a given phosphor. Two types of luminescence have been identified. If visible light is emitted only while the phosphor is stimulated, the process is called fluorescence. If, on the other hand, the phosphor continues to emit light after stimulation, the process is called phosphorescence. There is yet a third, more recently engaged luminescence: electroluminescence. Electroluminescence was first described early in the 20th century and developed into a commercial product as a light-emitting diode (LED) by Texas Instruments in the 1960s. A diode is an electronic device that allows electric current flow in one direction only. Solid-state semiconductor diodes appeared before transistors and integrated circuits and helped in the discovery of LEDs, which are in widespread use today. The symbol for an LED is shown in Fig. 17.6. An LED emits light when electrically stimulated. The first LED semiconductor material was gallium arsenide (GaAs), and it emitted infrared photons. Newer semiconductors are employed in the commercial production of LEDs that emit photons spanning across the ultraviolet, visible, and infrared spectrum of radiation. The use of blue, green, and red LEDs is widespread today in many lighting products, including black/white and color digital display devices. Backlight In addition to digital display devices and video monitors, LEDs are now commonly found in general lighting applications, automobile headlights, traffic signals, and instrument panels. High-power white-light LEDs are rapidly replacing incandescent and fluorescent lighting and LCD medical image monitoring. Actually, the LED does not replace the LCD digital display monitor; it simply provides the backlight for such a monitor. The LED replaces the fluorescent lamp in earlier LCDs. All video monitors today are really LCDs with a more intense backlight, the LED. There are a number of advantages to the use of LEDs as the backlight. Such digital display devices are thinner and have a larger active area for the visual screen. They have inspired the newest generation of curved video screens and ever-larger video screens. LED monitors have a longer life by at least a factor of two over fluorescent backlit digital display devices. They have lower power consumption, which is registered as a reduced electric power bill for the hospital. They produce measurably less heat, which is reflected in the hospital power bill as well as increased radiology reading room comfort.