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1. ELECTRONICS ASSIGNMENTSUBMITTED BY:DATE: 4/28/2011<br /> LCD: LIQUID CRYSTAL DISPLAY<br />Short for liquid crystal display, a type of display used in…
  • 1. ELECTRONICS ASSIGNMENTSUBMITTED BY:DATE: 4/28/2011<br /> LCD: LIQUID CRYSTAL DISPLAY<br />Short for liquid crystal display, a type of display used in digital watches and many portable computers. LCD displays utilize two sheets of polarizing material with a liquid crystal solution between them. An electric current passed through the liquid causes the crystals to align so that light cannot pass through them. Each crystal, therefore, is like a shutter, either allowing light to pass through or blocking the light.<br />BASIC DIAGRAM OF LCD<br />WORKING:<br />Each pixel of an LCD typically consists of a layer of molecules aligned between two transparent electrodes, and two polarizing filters, the axes of transmission of which are (in most of the cases) perpendicular to each other. With no actual liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second (crossed) polarizer. In most of the cases the liquid crystal has double refraction.<br />The surface of the electrodes that are in contact with the liquid crystal material are treated so as to align the liquid crystal molecules in a particular direction. This treatment typically consists of a thin polymer layer that is unidirectionally rubbed using, for example, a cloth. The direction of the liquid crystal alignment is then defined by the direction of rubbing. Electrodes are made of a transparent conductor called Indium Tin Oxide (ITO).<br />Before applying an electric field, the orientation of the liquid crystal molecules is determined by the alignment at the surfaces of electrodes. In a twisted nematic device (still the most common liquid crystal device), the surface alignment directions at the two electrodes are perpendicular to each other, and so the molecules arrange themselves in a helical structure, or twist. This reduces the rotation of the polarization of the incident light, and the device appears grey. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are almost completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer. This light will then be mainly polarized perpendicular to the second filter, and thus be blocked and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray. This electric field also controls (reduces) the double refraction properties of the liquid crystal.<br />LCD PRINCIPLE DIAGRAM<br />MONOCHROME LCD :<br />LCD capable of detecting only two colours.Monochrome LCD images usually appear as blue or dark gray images on top of a grayish-white background. Color LCD displays use two basic techniques for producing color: Passive matrix is the less expensive of the two technologies. The other technology, called thin film transistor (TFT) or active-matrix, produces color images that are as sharp as traditional CRT displays, but the technology is righttop expensive. Recent passive-matrix displays using new CSTN and DSTN technologies produce sharp colors rivaling active-matrix displays. Most LCD screens used in notebook computers are backlit, or transmissive, to make them easier to read. <br />The picture displays that Monochrome LCDS usually appears as dark grey images.<br />MILITARY USE OF LCD MONITORS:<br />41541704785995LCD monitors have been adopted by the United States of America military instead of CRT displays because they are smaller, lighter and more efficient, although monochrome plasma displays are also used, notably for their M1 Abrams tanks. For use with night vision imaging systems a US military LCD monitor must be compliant with MIL-L-3009 (formerly MIL-L-85762A). These LCD monitors go through extensive certification so that they pass the standards for the military. These include MIL-STD-901D - High Shock (Sea Vessels), MIL-STD-167B - Vibration (Sea Vessels), MIL-STD-810F – Field Environmental Conditions (Ground Vehicles and Systems), MIL-STD-461E/F – EMI/RFI (Electromagnetic Interference/Radio Frequency Interference), MIL-STD-740B – Airborne/Structureborne Noise, and TEMPEST - Telecommunications Electronics Material Protected from Emanating Spurious Transmissions.<br />The figure shows a MIL-STD-901D 20.1" LCD Monitor.<br />DRAWBACKS OF LCD:<br />Before we end, here are some disadvantages of LCD displays, as opposed to other display technologies such as CRT and plasma. <br /><ul><li>As opposed to CRTs, LCDs cannot form multiple resolution images. LCD's can only produce clear images in their native resolution or a small fraction of it.
  • 2. The contrast ratio for LCD images is less than its CRT and plasma counterparts.
  • 3. Due to their longer response time, LCDs may show ghosting and mixing when images change rapidly.
  • 4. The viewing angle of an LCD is narrower than a CRT or plasma display, thereby restricting the number of people who can conveniently view the image on the screen at one time.
  • 5. Image persistence is a common phenomenon with LCDs. This is something comparable to screen burn-in on CRTs.</li></ul>42284654364355DIFFERENCE BETWEEN LED AND LCD :<br />LED TV’s are still LCD TV’s. They are just considered to be the newer LCD TV version because of the new backlight system used. The LED TV uses Light Emitting Diodes, while the standard LCD TV uses fluorescent lamps. Although they both still use Liquid Crystal Display technology. The main difference is the part behind their screen, which is the backlight.<br />LED TV’s are LCD TV’s with a new backlight system. They are newly developed for LCD’s, because Light Emitting Diodes are said to give more balance in color saturation, and use less power than the fluorescent lamps. LED TV’s are the newest version, and that’s why they are currently more expensive than the standard LCD TV’s.<br /> PHOTO DIODE <br /> <br />39706551136650A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation. The common, traditional solar cell used to generate electric solar power is a large area photodiode.<br />Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction.<br />32658054084320PRINCIPLE OF OPERATION:<br />A photodiode is a PN junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron (and a positively charged electron hole). This mechanism is also known as the photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimised to enhance the sensitivity of the device.<br /> Photovoltaic mode:<br />When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. This mode exploits the photovoltaic effect, which is the basis for solar cells – in fact, a traditional solar cell is just a large area photodiode.<br />Photoconductive mode:<br />In this mode the diode is often reverse biased, dramatically reducing the response time at the expense of increased noise. This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance (and to the irradiance).<br />Although this mode is faster, the photoconductive mode tends to exhibit more electronic noise. The leakage current of a good PIN diode is so low (< 1nA) that the Johnson–Nyquist noise of the load resistance in a typical circuit often dominates.<br />Photodiodes are P-N junctions specifically designed to optimize their inherent photosensitivity. Photodiodes can be used two ways -- in a photovoltaic (here it becomes a current source when illuminated -- see solar cell), or photoconductive role. To use a photodiode in its photoconductive mode, the photodiode is reverse-biased; the photodiode will then allow a current to flow when it is illuminated.<br /> <br />Photodetector from a CD-ROM Drive. 3 photodiodes are visible.<br />CURRENT VOLTAGE CHARACTERISTICS OF A PHOTO DIODE:<br />391668054254FEATURES:<br /><ul><li>RESPONSIVITY:</li></ul>The ratio of generated photocurrent to incident light power, typically expressed in A/W when used in photoconductive mode. The responsivity may also be expressed as a Quantum efficiency, or the ratio of the number of photogenerated carriers to incident photons and thus a unitless quantity.<br /><ul><li>DARK CURRENT:</li></ul> The current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by calibration if a photodiode is used to make an accurate optical power measurement, and it is also a source of noise when a photodiode is used in an optical communication system.<br />43497504509135APPLICATIONS OF PHOTO DIODE:<br /><ul><li>P-N photodiodes are used in similar applications to other photodetectors, such as photoconductors, charge-coupled devices, and photomultiplier tubes.
  • 6. Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, and the receivers for remote controls in VCRs and televisions.
  • 7. In other consumer items such as camera light meters, clock radios (the ones that dim the display when it's dark) and street lights, photoconductors are often used rather than photodiodes, although in principle either could be used. LIGHT APP ADAPTER
  • 8. Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a better, more linear response than photoconductors.
  • 9. They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators) or instruments to analyze samples. They are also used in pulse oximeters.
  • 10. PIN diodes are much faster and more sensitive than ordinary p-n junction diodes, and hence are often used for optical communications and in lighting regulation.
  • 11. P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser range finding.</li></ul>REFERENCE:<br />Sources from where data was collected are-<br /><ul><li>www.wikipedia.org
  • 12. www.answers.com
  • 13. www.webinstituteforteachers.com
  • 14. www.tpub.com
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