how crt and lcd monitors work

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  • 7/30/2019 How CRT and LCD Monitors Work


    How CRT and LCD monitors work

    We all spend an awful lot of time sat in front of computers. Whether we're gaming or working, we are at themercy of what many would argue is the most important element of any system - the monitor.

    A well-defined monitor can make using a system a pleasure. Likewise, being forced to squint at a 15" CRT at60Hz can make us weep in pain and long for a nice LCD to while away our hours at. A good display makes all thedifference.

    Monitors are widely used and rarely understood. Sure, you know that the difference between LCD and CRT isthat one is flat and one is massive and heavy. But do you really understand the technology that goes into thesethings?

    In this article, we're going to investigate how CRTs and LCDs work, and also examine some of the issuespertaining to monitors, such as Refresh Rate and Vsync as well as looking into our crystal ball to see the future ofdisplays.

    For a primer on resolutions, you might like to check out our previous articlehere.

    The Basics

    So let's start with the easy stuff. The picture that appears on your monitor comes from the graphics card in yourcomputer, and the job of the graphics card is to render the picture suitable for the monitor. A wired output runsfrom the graphics card to the monitor.

    But you knew that already.

    Both the graphics card and monitor adhere to the same set of specifications, so that they can happily talk to eachother. The standards are set out by VESA, which defines things like how monitors identify themselves to the

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    CRT stands for Cathode Ray Tube, and is descriptive of the technology inside that chunky monitor you mighthave on your desk.

    CRTs receive their picture through an analogue cable, and that signal is decoded by the display controller, whichhandles the internal components of the monitor - think of it as the mini-CPU for the monitor.

    CRTs have a distinctive funnel shape. At the very back of a monitor is an electron gun. The electron gun fireselectrons towards the front through a vacuum which exists in the tube of the monitor. The gun can also bereferred to as a cathode - hence the electrons fired foward are called Cathode Rays.

    These rays correspond to to the red, green and blue channels of the display and video card.

    At the neck of the funnel-shaped monitor is an anode, which is magnetised according to instructions from thedisplay controller. As electrons pass the anode, they are shunted or pulled in one direction or the otherdepending on how magnetic the anode is at that time. This moves the electrons towards the correct part of thescreen.

    The electrons pass through a mesh, and this mesh defines the individual pixels and resolution on the screen.

    Electrons that pass through the mesh then hit the phosphor coating which is on the inside of the glass screen.

    When the particles hit the phosphor, they immediately light up - causing the light to shine through the front of the

    monitor, thus making up the picture on the screen. There are three differently coloured phosphours for each pixel

    (known as phosphor triads), and depending on which phosphor the electron hits, that's which colour the pixel will

    light up.Differences in components

    Different monitors differ in quality, and this is often dependent on the technology and components used internally.

    Some CRT monitors use a single electron gun at the rear of the monitor to produce the electrons that will becomethe red, green and blue electron rays. However, higher quality monitors have an individual gun for each, whichcan increase picture quality.

    The metal used for the mesh at the front of the monitor will also affect quality. Electrons also produce ionsbecause of imperfections in the vacuum, and these electrons are destructive to image quality if they hit thephosphor. Consequently, meshes are made of relatively thick metal to prevent phosphor damage. However, inbetter quality monitors, a thinner, yet tougher metal alloy is used for the mesh. Because it's thinner, it meansmore light can get through, making for increased brightness and higher contrast.

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    Aperture Grille v Shadow Mask

    Each CRT has a sheet of metal at the front of the monitor which (partly) defines the pixels on the screen. Shadowmask is an older technology, and is l iterally a piece of metal with mil lions of holes in it which allow the differentcathode rays through to hit the phosphour. Because a shadow mask covers the whole back of the screen,protecting the phosphor from stray ions, it also limits the strength of the rays, reducing the brightness of themonitor.

    Aperture grille is a newer technology which defines the gaps through which electrons pass using a mesh of wiresrather than a sheet with holes in. Whereas a shadow mask is made of circular holes, the grill is made of verticalslots. Because it is by its nature thinner, it allows for brighter displays. However, the grill is fragile and prone tobeing knocked around. The grill is therefore strapped to the monitor using stiff horizontal wires - this is whatcauses the distinctive pair of lines across high-end aperture grille monitors.

    Invar mask is a variant of shadow mask, and uses a thinner, stronger metal to form the mask, allowing for betterimage quality whilst remaining cheaper than aperture grill to produce.

    Sony's Trinitron brand and Mitsubishi's Diamondtron brand are both variants of Aperture grill.

    Dot pitch and resolution

    Each pixel on the CRT screen is defined by lighting up combinations of the red, blue and green phosphors thatmake up the pixel. With a varying strength of electron gun operating on each phosphor, different colours are

    produced - with red, blue and green all fired on maximum strength, that means bright white is produced.Dot pitch is measured on most monitors as the distance, diagonally, between two phosphors of the same colour.However, some manufacturers quote dot pitch on monitors as the horizontal distance between phosphors, which

    can make them appear better specified, on paper, than perhaps they are.

    Dot pitch combined with viewable image area defines the maximum resolution of the screen. For example, if you

    have a 21" monitor with a viewable area of 401mm x 298mm, and a dot pitch of 0.26mm, you will have a CRT

    capable of displaying a maximum resolution of 1758 horizontally. How so?

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    Well, if we take 1 as the diagonal dot pitch, Pythagorus dictates that the horizontal dot pitch (ie the gap between

    pixels as rendered horizontally by the graphics card) will be 0.87. 0.26 diagonal dot pitch multiplied by 0.87

    makes for a diagonal equivalent pitch of 0.228 horizontally. 401mm horizontal viewable screen area multiplied by

    0.228 is 1758, hence a maximum of 1758 pixels are usable on the screen.


    Flat panel monitors are a relatively recent product to enter the computer market. The clue to LCD technology is inthe name - crystals that are in liquid form. Because they are in a liquid form they are easily manipulable, and thisallows us to play with the way that light interacts with them. If you have a flat panel in front of you, try justpressing gently on the surface - you can see the crystals move around and alter the picture.

    LCD panels are fairly simple to understand. The signal comes in and, as with a CRT, the signal from the videocontroller is decoded and understood by a display controller on the monitor itself. The controller has two things tocontrol - the electrics of the pixels and the light source.

    The actual image on a TFT is made up of a matrix of pixels. Unlike with CRTs, there's no complex equation of dotpitch and image area to try and calculate - the native resolution of the monitor is simply the number of pixelscontained in the matrix. If i t's a 17" monitor, chances are there are 1280 pixels in the matrix horizontally, and1024 vertically.

    Perspective view

    Each pixel is made up of three sub-pixels, which have red, green and blue filters in front of them, just as eachpixel on a CRT has RGB phosphors. The subpixels are made up of a group of liquid crystal molecules. These

    molecules are suspended between transparent electrodes and are mashed between two polarising filters.

    The two filters are exact opposites of each other. As the light from the light source behind the first filter comes in,the filter effectively whites it out - which means that if it was to pass through the liquid crystals with no interaction,the filter on the other side would polarise it back to black, leaving no colour being emitted. In fact, alternatecurrent - leaving the crystals 'dead in the water' - is how black is created on a panel.

    However, if the electrodes apply current to the liquid crystals they twist and change the way that the light ispassed through, altering its polarisation and this then results in the correct colour coming out of the secondpolarising filter and being displayed to the user.

    The backlight itself is a cold cathode. Depending on how expensive the display is, there will be either a singlecathode at the top, or one at the top and one at the bottom, or two at the top and two at the bottom for optimumbrightness and clarity. These cathodes are diffused through a layer of plastic and then throu