Displays. Cathode Ray Tube. Semiconductor Elements. Basic applications. Oscilloscope TV Old monitors. 2009, Associate Professor PhD. T.

Transcription

1 Displays Semiconductor Elements 1 Cathode Ray Tube Basic applications Oscilloscope TV Old monitors 2 1

2 Idea of Electrostatic Deflection 3 Inside an Electrostatic Deflection Cathode Ray Tube Gun creates an electron beam and changes its brightness Electron lens focus (A1) and accelerate (A2) electrons One set of plates (Y) creates an electric field that moves the electron beam vertically, while another set (X) moves the beam horizontally. When the electron beam strikes the phosphor, it illuminates visible light on the fluorescent screen 4 2

3 Magnetic Deflection Basic Idea 5 Inside a Magnetic Deflection CRT One set of coils creates a magnetic field that moves the electron beam vertically, while another set moves the beam horizontally. 6 3

4 Color CRT There are three electron beams (Red, Blue and Green - RGB) that move simultaneously across the screen. All colors on a TV screen are produced as a mixture of red, green and blue. A thin metal screen (shadow mask) has very small holes that are aligned with the phosphor dots (or stripes) on the screen. 7 Flat Displays LCD Displays Application LCD (liquid crystal display) are used in laptop computers, digital clocks, watches, microwave ovens, CD players etc. Advantages They are thinner and lighter and draw much less power than cathode ray tubes 8 4

5 Liquid Crystals Liquid crystals are neither a solid nor a liquid They require an external light source. Liquid crystal materials emit no light of their own U Светлина 9 Liquid Crystal Polarization Twisted nematic (TN), is naturally twisted liquid crystal. It reacts to electric field in such a way as to control light passage. If electric field is applied to liquid crystal molecules, they untwist to varying degrees, depending on the applied voltage. When they straighten out, consequently, no light can pass through that area. 10 5

6 Passive Matrix LCD Uses a simple grid to supply the charge to a particular pixel on the display Has slow response time and imprecise voltage control (normally the pixels around the controlled one also partially untwist, which makes images appear fuzzy and lacking in contrast) Column electrode Pixel Row electrode 11 Active Matrix TFT LCD Thin Film Transistors (TFT) Sub pixels for colors Basic colors and brightness Ensure ability to influence only one pixel at a time. The MOS capacitor is able to hold the charge until the next refresh cycle. 12 6

7 LCD Color Display To can show colors a LCD must have three subpixels with red, green and blue color filters to create each color. Through the careful control and variation of the voltage applied, the intensity of each subpixel can range over 256 shades. An enormous number of transistors are needed. Example For typical laptop with resolution 1,024x768 1,024 columns multiplied by 768 rows by 3 subpixels get 2,359,296 transistors etched onto the glass. 13 LCD Disadvantages Very sensitive to temperature changes Limited display size by the quality-control problems. To increase display size more pixels and transistors are needed and the chance of including a bad transistor also increase. Because of this about 40% of produced displays are rejected which directly affect the price of large display. 14 7

8 Plasma Display Panel (PDP) The basic idea of a plasma display is to illuminate tiny, colored fluorescent lights to form an image. Each pixel is made up of three fluorescent lights - a red light, a green light and a blue light. Just like a CRT television, the plasma display varies the intensities of the different lights to produce a full range of colors. 15 Plasma Display Basic Cell 16 8

9 Cell Construction The xenon and neon gas in a plasma television is contained in hundreds of thousands of tiny cells positioned between two plates of glass. Long electrodes are also sandwiched between the glass plates, on both sides of the cells. The address electrodes sit behind the cells, along the rear glass plate. The transparent display electrodes are mounted above the cell, along the front glass plate. They are surrounded by an insulating dielectric material and covered by a magnesium oxide protective layer, 17 Basic Grid of Horizontal and Vertical Electrodes 18 9

10 Flat-Screen Plasma Display Panel 19 Advantages and Disadvantages Advantages Very wide screen can be produced using extremely thin materials. The image is very bright and looks good from almost every angle Drawback High price. However, falling prices and advances in technology mean that the plasma display may soon edge out the old CRT sets

11 E-paper, e-ink Display Electronic paper, e-paper or electronic ink display is a display technology designed to mimic the appearance of ordinary ink on paper. Electronic paper Reflects light like ordinary paper (unlike LCD that uses backlight to illuminate its pixels) It is capable of holding text and images indefinitely without drawing electricity. Applications e-readers, capable of displaying digital versions of books and e-paper magazines. Mobile phone/watch displays electronic billboards time tables at bus stations electronic pricing labels, general signage. 21 E-Ink Technology E-paper display consists of titanium dioxide particles (e-ink micro capsules) which are dispersed in a hydrocarbon oil so that they can rotate freely. Each particle is a dipole composed of negatively charged black plastic on one side and positively charged white plastic on the other. This mixture is placed between two parallel, conductive plates. Visible images are formed by rearranging charged pigment particles using an applied electric field

12 Mode of Operation When a voltage is applied across the two plates, the particles will migrate to the plate bearing the opposite charge from that on the particles. Reflected white color Light Absorbed Black color Light If a negative electric field is applied at the front (viewing) side of the display, the white particles move to the top of the microcapsule. The surface appears white at that location, because the light is reflected back to the viewer. By reversing this process, the black particles appear at the top of the capsule, which now makes the surface appear dark at that location, because the incident light is absorbed by the colored dye. 23 Mode of Operation Reflected white color Light Absorbed black color Light The polarity of the voltage applied to each pair of electrodes then determines whether the white or black side is face-up, thus giving the pixel a white or black appearance. If the rear electrode is divided into a number of small picture elements (pixels), then an image can be formed by applying the appropriate voltage to each region of the display to create a pattern of reflecting and absorbing regions

13 Advantages and Disadvantages Advantages High contrast in direct sunlight Low-power consumption need energy only when changing image Stable image, which does not need to be refreshed constantly The wider viewing angle Drawback A very low refresh rate (not applicable for interactive applications like using fast moving menus, mouse pointers or scrolling). Gray scale only. There is ongoing competition among manufacturers to provide full-color capability