Cathode Ray Tube. Cathode Ray Tube, CRT. Additive Colour Triangle. Cathode Ray Tube, CRT. Colour CRT. Additive Colour. phosphor screen RED

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1 Cathode Ray Tube Cathode Ray Tube, CRT electron gun focusing deflection horizontal vertical phosphor electron beam phosphor screen energised electrons raise phosphor atoms from ground state to higher energy level atoms fall back to ground state emitting light colour of light (frequency) depends on difference between levels which depends on material of phosphor duration of light emission depends on type of phosphor 03 December 2002 Devices 1 03 December 2002 Devices 2 Cathode Ray Tube, CRT Additive Colour Triangle electron gun cathode produces electron beam control grid voltage controls energy (brightness) focusing system electrostatic or magnetic acts as lens to give small spot on phosphor deflection system electrostatic or magnetic horizontal and vertical causes movement of electron beam across screen GREEN RED BLUE 03 December 2002 Devices 3 03 December 2002 Devices 4 Additive Colour Colour CRT applies to mixing lights, used in displays primary colours red, green, blue secondary colours cyan = green + blue yellow = red + green magenta = red + blue white = red + green + blue black = no light combination of phosphors emitting different coloured light human eye merges colours from adjacent areas of screen beam penetration CRT two layers of phosphor : red, green colour depends on electron beam energy slow red, fast green, medium orange & yellow used in random scan screens 03 December 2002 Devices 5 03 December 2002 Devices 6

2 Colour CRT Phosphor Triads shadow mask CRT three phosphor dots per pixel emit either red, green or blue three electron guns - one per colour dot shadow-mask screen all three beams deflected and focused as group onto mask where holes aligned with dots beams pass through hole and activates dot triangle colour variations due to beam intensity phosphor dots arranged in rows three per pixel 03 December 2002 Devices 7 03 December 2002 Devices 8 Shadow Mask Aperture Grille electron beams mask triad screen Alternative to shadow mask Mask with vertical stripes Rely on electron beam to define pixel Brighter, sharper images Stripes less stable technology than mask 03 December 2002 Devices 9 03 December 2002 Devices 10 Terminology dot pitch distance between centres of two same colour phosphor dots resolution sharpness or detail in image number of pixels in horizontal and vertical directions relates to dot pitch and beam size aspect ratio ratio of width to height of display screen commonly 4:3 Terminology gamma non-linear function which represents relationship between the input (beam intensity) and output (brightness) gamut range of colours that a device can display persistence time for light to decay to 1/10 of original intensity microseconds redraw: refresh rate high persistence: good for complex, static pictures low persistence : good for animation, need high refresh rate 03 December 2002 Devices December 2002 Devices 12

3 Random Scan random scan displays beam directed where line is to be drawn display list for picture definition refresh cycle through commands in display list 30 to 60 times per second delay if too few lines designed for line drawing applications high resolution, smooth lines 03 December 2002 Devices December 2002 Devices 14 Raster Scan raster scan displays beam directed pixel from left to right, horizontal retrace scan line from top to bottom, vertical retrace frame buffer contains picture definition to refresh screen programming buffers bitmap (black & white) pixmap (colour) pixel scan line 03 December 2002 Devices December 2002 Devices 16 raster op to construct picture applied to rectangles in bitmap or pixmap destination = source op destination op may be one of 16 boolean functions commonly used replace, copy dest = source or dest = source or dest xor dest = source xor dest and dest = source and dest refresh from frame buffer rate : 60 to 80 frames per second interlacing alternate scan lines displayed screen filled in two passes standard display device good for area fill 03 December 2002 Devices December 2002 Devices 18

4 RGB monitors take beam intensity from frame buffer 24 bits per pixel (true colour) 8 bits red, 8 bits green, 8 bits blue 256 settings per gun ~16 million colours how many can the eye distinguish? 8 bits per pixel (common in older systems) 4 bits per pixel (Windows95 GUI) Colour Lookup Tables (CLUT) frame buffer value are used as index into table e.g. table has bit values pixel value 8-bits reduces size of frame buffer select 256 colours from full palette for table entries can change content of table under API control 03 December 2002 Devices December 2002 Devices 20 CLUT Example frame buffer 8 bits per pixel points to table entry CLUT 8 bits per R,G,B controls the 3 beams = 24bits Mitsubishi Electronics Diamond Scan monitors, 19-inch (18.0-inch diagonal viewable image) Flat-square CRT with Invar shadow mask 1600 x 1200/75 Hz or 1280 x 1024/85 Hz non-interlaced addressable resolution Ultra-fine 0.25 mm dot pitch 03 December 2002 Devices December 2002 Devices 22 Example 17-inch Sun Color Monitor 15.7-inch viewable image size 0.28mm dot pitch 1024 x 60Hz, 75Hz 1-inch FD Trinitron Color Monitor 21-inch Flat Screen AG CRT 19.8-inch viewable area 0.24mm aperture grille pitch 67Hz, 76Hz CRT Display Issues needs high quality shadow mask takes too much power prone to misfocusing and misconvergence of electron beam cause harmful electromagnetic radiation from high voltage electronics and magnetic fields too big, too heavy 03 December 2002 Devices December 2002 Devices 24

5 Flat Panel Displays emissive: convert electrical energy to light light emitting diode (LED) semiconductor diode emits light usually red plasma panel gas filled region between two plates horizontal/vertical electrodes in the plates firing causes plasma glow at intersection needs refresh 60 times per second Flat Panel Displays non-emissive : needs light source liquid crystal display (LCD) nematic liquid crystal cell opposed polarisers on each side pair of conductors intersect at cell (pixel) when cell is on, crystals align to twist light & it is transmitted when cell is selected (off), crystals align so that light is blocked needs refresh from frame buffer light from surroundings or internal (back lighting) no power state gives light screen 03 December 2002 Devices December 2002 Devices 26 LCD Panel backlighting cell LCD Panel vertical conductors electrodes liquid crystals polarising filter alignment layer horizontal conductors electrodes colour filter 03 December 2002 Devices December 2002 Devices 28 LCD Cell nematic liquid crystal twisted LCD Cell nematic liquid crystal aligned light beam light beam ON normal state alignment layer & polariser 03 December 2002 Devices 29 OFF polariser 03 December 2002 Devices 30

6 LCD Displays colours from 3 elements per pixel shades of colour varying voltage varies twist, varies amount of light passed may be limited to 64 shades (6bits) 256 (8bits) for true colour passive matrix relies on persistence to maintain state slow to change ghosting, smearing LCD Displays active matrix transistor at each pixel for each colour to control voltage TFT - thin film transistor extra layer maintains state between scans eliminates slow response problems expensive to produce accurately no power state gives dark screen 03 December 2002 Devices December 2002 Devices 32 Example Sun TFT LCD Monitor 18.1-inch actual viewable area (equivalent to 19"/20" CRT) 0.28mm pixel pitch true 24-bit, 256 gray scale levels, 16.7M colours 60Hz or 76Hz (5:4 aspect ratio) +/- 80 degree viewing angle, both horizontal and vertical LCD Issues no problems with convergence => crisp text refresh rate full screen visible area problems cause by flawed cells => bright or dark spots backlighting => uneven brightness visual interference => ghosting, streaking moiré patterns => jitter limited lifetime compared with CRT 03 December 2002 Devices December 2002 Devices 34