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What is Temporary Image Retention (TIR)?
Unlike phosphor-based displays such as CRTs and plasma flat panels, LCD displays do not suffer from permanent image burn-in resulting from the prolonged display of static images. This is widely understood as one of the key technological advantages of LCD. However, a temporary retained image may result in such displays, if a static image is displayed for a long time -- an effect known as "image sticking," or temporary image retention (TIR). This effect is not permanent, is not a degradation in overall life, and does not appear at all in typical usage. However, TIR can occur and can be considered undesirable in certain applications where 24/7 display of static images is required.
Every panel specification from every LCD manufacturer that Clarity has ever reviewed includes language about "image sticking." As such, we believe the phenomenon is common to all manufacturers, though there does appear to be some variability both within a single product line and among the various panel manufacturers.
What causes TIR and what can be done about it?
The mechanism of TIR involves the chemistry and drive signals of the LCD, where slight DC components of the drive signals for static displays induce a migration of ions in the Liquid Crystal material to one side of the LCD glass. This migration of ions, given sufficient time, will cause a slight drop in the drive voltage in the pattern of the static image, causing a temporary "ghost" of the image to stick. Thankfully, when the image is changed, these ions diffuse back into the Liquid Crystal and the "stuck" image disappears.
The process of ion migration is something like sedimentation in a wine bottle. Over time, if the wine bottle is left in the same position (a static image is displayed), sediment will appear at the bottom of the bottle (the ions). If, however, the bottle is periodically turned or shaken (a non-static image), then the sediment will remain in solution.
Unlike the burn-in exhibited in phosphor-based displays, TIR is not a permanent condition. Still, it can be problematic in certain applications. The best way to avoid the condition is to limit the amount of static content on the display. When this fix is not really an option, TIR may be avoided in a number of other ways. First, turning the display off (either by power cycle or by displaying a full black field), allows the ions to diffuse back into solution. Second, using a display "orbiter" (included as a menu selectable option in Clarity LCDs) will unobtrusively shift the image by one pixel on a periodic basis to reduce the static nature of the image. Third -- as TIR is somewhat temperature dependent -- ensure the display is not exposed to excessive heat in the installation. Ultimately, if TIR does occur, the quickest way to drive the ions back into solution is to display a full field of black image.
The rate of TIR depends on the image pattern, the duration, the temperature, and slight variations in the LCD. Likewise, the rate of diffusion of the TIR (recovery time) also depends on these factors. Because of panel-based variability, Clarity has always designed its LCD products to accommodate modules from multiple vendors. By working with our suppliers in this way we have been able to consistently ensure the quality of our display devices. Clarity has been designing products using AM-LCDs for eight years and we have yet to see any significant downside to this robust imaging technology.
Why talk about TIR now?
From what we saw at the recent NSCA and GlobalShop tradeshows, the digital signage market is finally poised to explode. Direct view LCD display products are an excellent choice for digital signage applications. As a leader in this space and a longtime advocate of LCD technology, we're keenly focused on product quality and know from experience that the better the technology performs, the faster the market will grow.
We believe that TIR can be greatly improved if not entirely eliminated with tighter process controls implemented during the LCD panel manufacturing process. Ultimately, Clarity would like to see the LCD panel manufacturers respond to the challenge of eradicating TIR all together. In the mean time, AM-LCD remains the most reliable, longest life digital imaging technology available for large screen displays. With knowledge of the cause and effects of TIR, any objectionable effects can be avoided with prudent control of the content and the display environment, even in 24/7 applications.
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A 15-year veteran of the display industry, Mr. Clifton has a solid background of LCD research and design experience that gives him a unique understanding of LCD technology. Prior to joining Clarity, Ben worked at In Focus and Tektronix, where he played key roles in technology groups working on a variety of display-related projects. He also served as VP of Engineering at Motif, a joint venture between In Focus and Motorola. He currently holds 16 patents in the area of flat panel displays and related technologies, and co-invented Active Addressing LCD technology.
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