All iPads Understanding mobile display technology

Discussion in 'iPad' started by lianlua, Nov 21, 2012.

  1. lianlua macrumors 6502

    Jun 13, 2008
    Since there seems to be a lot of ignorance, confusion, and general trolling on what's involved in a very complicated and expensive piece of technology, maybe it would be helpful to explain some of the basics about what goes into making a display panel. Warning: this is a long post.

    First, the basics.
    All liquid crystal displays (LCDs) work by using an electric current to change the physical position of liquid crystals, nematic (rod-shaped) organic polymers (complex molecules made up of long chains of repeating structures). By rotating the chain of liquid crystals, you alter how much light is allowed to pass through, forming the basis of an image.

    TFT, or thin film transistor, refers to the overall architecture. TFTs get their name from the very thin layers (<1 micron) of semiconductor materials deposited onto the substrate (base). Not all TFT displays are LCDs (Samsung's OLED panels are TFT displays, too), and not all LCDs are TFT (though all the ones that matter for consumer products are).

    Next you have active matrix and passive matrix. Again, this is a fundamental architectural parameter and refers to underlying mechanics, not to a unique property of LCDs. Passive matrix displays, like cheap monochrome LCDs in watches and calculators as well as e-ink devices like a Kindle, are "passive" because they are bistable, meaning that once their on/off state has been set electrically, they don't need additional power applied until they have to change state again. These displays use very little power, but they don't change state quickly, don't work very well with color filters, and are highly susceptible to crosstalk (accidental changes in state when adjacent pixels are meant to change). Active matrix displays require constant power to maintain their state, but they react much more quickly and have much better color reproduction. Some display types are exclusive one type (e-ink is always passive; plasma TVs are always active), and others can be built either way (e.g., LCD, OLED).

    Panel Types
    Here's where things get interesting. LCDs are broadly classified by two parameters: semiconductor process and liquid crystal process.

    Liquid Crystal Processes
    These are the terms you've probably heard in passing: TN, STN, MVA, IPS, S-IPS, etc. All of these refer to the physical structure and alignment design for the liquid crystals themselves.

    TN (Twisted Nematic) panels are the most basic and the cheapest. The liquid crystals are arranged in a spiral pattern between electrodes on two plates of glass. Light transmission is controlled with an electrical current sent between the two electrodes, causing the crystals to "un-twist" and scatter some of the light. Nearly all consumer devices use TN panels, including most tablets, smartphones, TVs, and virtually all notebook computers and consumer desktop monitors.

    IPS (In-Plane Switching) panels are much more expensive and difficult to manufacture. They get their name from the fact that instead of the crystals untwisting to float perpendicular to the glass, they rotate ("switch") parallel (in the same "plane") as the glass panels, which scatters less light in unwanted directions and improves viewing angles and color reproduction. However, because the electrodes are arranged differently, it was for a very long time difficult to get them to respond as quickly as TN panels.

    Because of the cost and complexity of IPS, MVA (multi-domain vertical alignment) was offered as a way to improve color and viewing angles over TN panels, but at a lower price and performance compared to IPS. These work by arranging crystals at a variety of orientations to reduce the undesired part of the scattering inherent in TN, but while still using the cheaper, simpler construction methods. MVA/PVA is a mixed bag--better color, contrast, and viewing angle over TN but at the cost of being slower, dimmer, and more expensive.

    There are lots of other variations on these technologies, and today the speed issues are largely resolved with IPS and MVA, while TN color and viewing angle has improved somewhat. MVA/PVA has largely fallen out of use as newer low-cost variants (PLS, FFS) on IPS become more common.

    Semiconductor processes
    Totally separate from the LCD configuration above is the question of what TFT manufacturing process is used. The main question in this part of the display has to do with electron mobility, which affects how fast a transistor can respond (and consequently its power efficiency), how much light transmission you can achieve, and how small you can make the pixels.

    What you're most likely looking at right now is some flavor of amorphous silicon (a-Si). This is an established, efficient, reliable, and cheap method. Unfortunately, electron mobility caps out at around 1-10 (the unit is square centimeters per Volt-second, but let's just keep it simple) and pixel densities max out around 280-300 ppi. LCDs made with a-Si can be TN or IPS, retina or non-retina, small or large. All iPads, retina and non-retina, mini and full-size, are made with a-Si, along with all other tablets, laptops, desktop monitors, and TVs.

    The current superstar is low temperature polysilicon (LTPS). The low temperature bit is important because it makes it easier to make very small crystals using the industrial vapor deposition processes we have. LTPS has electron mobility of 60-100, allowing for much smaller, faster, more power efficient backplanes (the circuitry that makes an LCD work). It is, however, astronomically expensive to manufacture, which is why it is used almost exclusively on high-end smartphones like the Nexus and iPhone lines.

    A compromise in price and performance is the use of a metal oxide backplane, and right now all the buzz is around IGZO (Indium Gallium Zinc Oxide). Because it uses a much cheaper process compared to LTPS and it has electron mobility of 10-30, it makes a great option for high-density displays that are larger than a smartphone or camera viewfinder. Metal oxide backplanes are also moving into high-end televisions and large desktop monitors, not to achieve higher densities, but to achieve faster switching speeds (240Hz is near the physical limit for a-Si, so HDTVs need a new transistor technology for different reasons). It is less than half as good, but also substantially cheaper than LTPS so it's a strong contender for use in small tablets, but right now is still difficult to manufacture in large sizes.

    Building an LCD, layer by layer
    It may surprise you to learn that only about 5-6% of the light generated in a typical LCD display actually ends up coming out of the cover glass and traveling to your eye. But that fact should help to understand why the bright backlight is such a power hog in a mobile device. When you see all the pieces of an LCD, it might make more sense. Working from back to front on an edge-lit design typical of mobile devices and notebooks:

    A. Backlight Unit (BLU)
    1. Backing sheet. An opaque reflector seals the back of the unit and provides an even color base.
    2. Backlight. This is a row (or rows) of LEDs or CCFL (fluorescent bulbs) and curved, mirrored reflectors used to project light out underneath the display.
    3. Light guide. This is an optical film meant to distribute the harsh light from the LEDs at the edge of the display to make an even light source so you can see what's going on. The namesake plate is a sheet of clear material, frequently PMMA (which you might know as perspex, acrylic glass, or Lucite) that is etched with a pattern of dots and ridges. Light travels along the sheet parallel to the screen, pumped in at the edges, until it hits a dot or ridge, which scatters the light in a bunch of directions (loosely toward your face). The pattern of dots and ridges/channels that are etched into the PMMA is carefully engineered to scatter less light near the illuminated edge and more light farther away to help compensate for the brightness gradient over distance.
    4. A diffuser is a translucent sheet of plastic that smooths out the dot and ridge pattern in the light guide.
    5. A prism film is a clear lens sheet with a triangular sawtooth pattern cut into it designed to take the light that has bounced off the dots on the light guide and focus it all so that it is pointing toward the front of the monitor.
    6. A brightness enhancing film (BEF) captures and recycles some of the light that is going in the wrong direction to get through the polarizer. The prism and BEF are sometimes integrated into a single layer called a DBEF (dual BEF).

    B. LCD Panel
    7. Rear polarizer. This filter ensures that only light that is exactly aligned gets into the LCD (otherwise, it would be chaos later!).
    8. Backplane. This layer contains all the control circuits and transistors that do the work of making images on the screen.
    9. LCD. Here it is! This is a glass sandwich, filled with the crystals. The glass layer on the front has an aperture grille on it (a pattern of holes), each one individually manipulated to allow a certain percentage of light through, controlling the brightness of the red, green, and blue channels of each pixel.
    10. Front polarizer. Same as the rear polarizer, but rotated 90 degrees. Without the front polarizer, all you'd ever see with the monitor turned on is a solid white screen. Other films can also be applied to enhance color saturation, viewing angles, and other display attributes.

    C. Cover Layer
    11. Touchscreen matrix
    12. Cover glass (the part your finger actually can touch)

    In antiglare (matte finish) displays, there is another optical film placed on top of the glass, textured to soften specular (mirror-like) reflections by scattering that light in multiple directions at the expense of creating a hazier image with less color saturation across the whole display. You can also use privacy films to reduce viewing angles and scratch-resistant and/or oleophobic films as well.

    Some of the above layers can be combined into multi-function optical films; others can be separated out into multiple layers, but these are the basics. Some of the layers can be fused together as well (in-cell touch matrices, bonding cover glass to the touchscreen and/or LCD panel, etc.), but all of the above parts are present in some capacity. There are also many other parts that aren't directly in the stack (LCD drivers and controllers, backlight power inverters and ballasts, etc.).

    Hopefully this paints a picture for those of you who are confused by "technobabble".
  2. zhenya macrumors 603


    Jan 6, 2005
    Thanks very much for that lianlua. Amazing the amount of technology that goes into these screens that we interact with every day.
  3. lianlua thread starter macrumors 6502

    Jun 13, 2008
    Absolutely. And when you start to dig even deeper, looking at what goes into manufacturing some of these films and layers, and moving on to other performance factors (contrast, color saturation, color accuracy, controlling crosstalk and image retention), it gets even crazier!

    There are whole armies of engineers working on these issues, and Apple actually employs a group of optics and display engineers who invented much of the technology needed to create the first retina displays. They didn't just write a check and sit around waiting for Samsung to do the work. Display manufacturers of course also have huge in-house talent working on big things to push the envelope, and then there are other partners--places like 3M, BASF, and Corning and industrial process shops like Coherent and CVD that develop the machines to sell to Sharp and Samsung and JDI that make it all possible.
  4. nuckinfutz macrumors 603


    Jul 3, 2002
    Middle Earth
    Thank you

    I wince every time I read

    "Retina displays = more battery requirements"

    without any regard to the type of panel used.
  5. lianlua thread starter macrumors 6502

    Jun 13, 2008
    Any time.

    That statement is technically true, though not for the reasons most people here think. A retina display will always increase battery requirements, regardless of panel type used. We do not currently have any LCD technology that fully offsets* the increased power demands resulting from doubling pixel density.

    * The possible exception is taking an a-Si standard display and moving to LTPS with retina, and even there it's iffy, as the comparison is typically between a middling efficiency a-Si display and a top-efficiency LTPS display, rather than a top efficiency a-Si display for an apples-to-apples match. LTPS displays are currently only available up to around 5 inches anyway, so it doesn't help for tablet/laptop/TV discussions.
  6. nuckinfutz macrumors 603


    Jul 3, 2002
    Middle Earth
    What'll be interesting to see is the power draw of a typical Retina class display using IGZO. I've seen a bunch of data about power savings using tablet sized or very large smartphone sized screens but what about the typical laptop size display? We'll see.
  7. lianlua thread starter macrumors 6502

    Jun 13, 2008
    We already know. It's up to about 25% less than existing products.

    It's unlikely that IGZO will make its way into mid-sized panels (laptop/desktop monitors) for a while--large format, low density panels like HDTVs are a much more likely candidate once smartphones and tablets are up and running. IGZO has major problems with stability right now that are holding it back. That's why it's being launched in limited markets and relatively low-volume products.

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