Screen flicker

Published by: mike 2008-11-22

Cathode-ray tube computer monitors 'flicker' as the electron beam scans relatively slowly down the face of the tube, then quickly flies back to the top, with the whole process repeating quickly. I understand that this 'flickering' is postulated to be the cause of many ills attributed to extended computer use. Briefly, (an in-depth technical description is unnecessary, just a very brief overview), how do the newer flat screen LCD type full size colour monitors operate (or for that matter, the laptop display panels which have been around for a while)? i.e. on a microscopic scale, in what manner are the pixels constructed, and by what means are they operated? Does their method of operation mean that they produce no flickering (assuming a stationary display of desktop icons, GUI etc) - and hence none of the potential for the ills which might be attributable to the flicker innate in the design of a cathode ray display?

Dr. Dobbs | Implementing Flicker-Free Motion | September 1, 1995::
To convincingly move an object across a screen, you need smooth motion without flicker. Paul uses Genus Microprogrammings graphics libraries and the Phar Lap 286
http://www.ddj.com/184409630HOME

Hello gan! As you surmised at the end of your question, it would appear that LCD displays don't precisely have a screen flicker because of the way they work - either the cells are on or they're not: "Q: What does "refresh rate" mean to me in an LCD monitor? A: For LCD monitors, the refresh rate is much less relevant. LCD monitors display a stable, flicker-free image at 60Hz. There are no visible differences between 85Hz and 60Hz. Q: What does Refresh Rate mean for LCDs? A: Unlike CRT display technology in which the speed the electron beam is swept from the top to the bottom of the screen determines flicker, an active matrix display uses an active element (TFT) to control each individual pixel and thus refresh rate is not really applicable to LCD technology." Phillips LCD Monitor FAQ http://www.lcdmonitor.philips.com/faq_general.php "A CRT has three electron guns whose streams must converge faultlessly in order to create a sharp image. There are no convergence problems with an LCD panel, because each cell is switched on and off individually. This is one reason why text looks so crisp on an LCD monitor. There's no need to worry about refresh rates and flicker with an LCD panel - the LCD cells are either on or off, so an image displayed at a refresh rate as low as between 40-60Hz should not produce any more flicker than one at a 75Hz refresh rate." Refresh Rate: Flat Panel (LCD) vs. CRT monitors http://home.houston.rr.com/djsmith7/refresh.htm The same page discusses the configuration and operation of an LCD panel: "An LCD consists of two polarising filters with their lines arranged perpendicular (at 90 degrees) to each other, which, as described above, would block all light trying to pass through. But in-between these polarisers are the twisted liquid crystals. Therefore light is polarised by the first filter, twisted through 90 degrees by the liquid crystals, finally allowing it to completely pass through the second polarising filter. However, when an electrical voltage is applied across the liquid crystal, the molecules realign vertically, allowing the light to pass through untwisted but to be blocked by the second polariser. Consequently, no voltage equals light passing through, while applied voltage equals no light emerging at the other end." Refresh Rate: Flat Panel (LCD) vs. CRT monitors http://home.houston.rr.com/djsmith7/refresh.htm For a little more information about how LCDs work, I headed for one of my favorite places on the Web - HowStuffWorks.com. They have a habit of explaining everything in an easy to understand format. For the whole story on LCDs, start here: How LCDs Work http://computer.howstuffworks.com/lcd.htm ...but pay particular attention to this part, which discusses active matrix displays and continues on to the operation of color LCD: "Active-matrix LCDs depend on thin film transistors (TFT). Basically, TFTs are tiny switching transistors and capacitors. They are arranged in a matrix on a glass substrate. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. Since all of the other rows that the column intersects are turned off, only the capacitor at the designated pixel receives a charge. The capacitor is able to hold the charge until the next refresh cycle. And if we carefully control the amount of voltage supplied to a crystal, we can make it untwist only enough to allow some light through. By doing this in very exact, very small increments, LCDs can create a gray scale. Most displays today offer 256 levels of brightness per pixel." LCD Systems http://computer.howstuffworks.com/lcd4.htm (The page also includes a nifty visual explanation of how pixels are controlled in an LCD color display.) Some other pages of interest: The LCD FAQ (very comprehensive, and quite easy to understand) http://www.microtipsusa.com/products/faq.html How is the LCD in a laptop computer so bright? http://computer.howstuffworks.com/question580.htm LCD Displays and Fluorescent Light http://www.cloanto.com/users/mcb/19960719lcd.html This was great fun to look into for you! Of course, if you need me to look for additional information or just need a bit more assistance, please don't hesitate to ask for a clarification. I'll be happy to help! --Missy Search terms: [ "refresh rate" LCD ], and look-up of LCD on HSW

IBM Human Ability and Accessibility Center | Developer guidelines ::
Avoid causing the screen to flicker with a frequency greater than 2 Hz and less than 55 Hz. Screen flicker. Hardware Self Contained, Closed Products
http://www-03.ibm.com/able/guidelines/peripherals/phwflicker.htmlHOME

CRT Vs LCD - Which monitors will be best suited for your environment?::
the conventiaonal CRT monitor against the fashionable LCD screen. Screen Burn. Screen Flicker. Interference. Power Consumption. Dead Pixels. Response Time
http://www.pantherproducts.co.uk/Articles/Monitors/CRT_LCD.shtmlHOME

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