You’ll spend most of your time looking at the display more than anything else but what are smartphone display panels made of?
As with any technology, smartphone display panels are a mega business on their own. Japanese component and notebook giant Toshiba has had to brush aside rumours that Apple was planning to invest in a new production line built by Toshiba Mobile Display, such is the demand of high-quality, high-resolution display panels.
But when you start looking at the specs of smartphone displays, it can get very confusing very quickly. You might think you’re just looking at a bunch of pixels but how those pixels are created can affect everything from the price of your phone to how long the battery lasts.
The type of display your smartphone has is typically described by an alphabet soup – LTPS, AMOLED, SLCD, Super AMOLED and TFT LCD all represent different technologies used in the production of display panels. Knowing what each type does, its benefits and drawbacks will help you understand just how good (or not) your phone is.
LTPS – Low-temperature polycrystalline silicon
If you see these written as a display type, forget it – LTPS is a description of a manufacturing process, not a display technology. Low-temperature polycrystalline silicon can be used to make different types of screens – AMOLED as well as standard LCDs. It’s a way of creating tiny silicon crystals that go into making the pixels of a display. The “low temperature” part is important because it means this process can create screens using low temperatures, allowing low-cost substances such as plastics to be used as the backing material on which the display panel is infused or created. As a result, it also means you can create more flexible display panels.
AMOLED – Active-matrix organic light-emitting diode
Okay, this is a type of screen technology. OLEDs or organic light-emitting diodes have been around for a while now and they have one significant benefit: to produce black, you simply turn an OLED off. To produce a light colour, they have to produce light. So they have huge potential for power savings in mobile devices.
The “Active matrix” describes how each OLED is addressed or controlled. The alternative is a passive matrix display where rows or columns of OLEDs are addressed rather than individual pixels. As a result, AMOLED displays are not only brighter, use less power, they’re also faster.
The problem is that AMOLED panels are in high demand, with that demand exceeding supply.
The other issue with AMOLEDs is that because of the fabrication process, they can be difficult to see if viewed in direct sunlight. AMOLED panels are typically three layers, the AMOLEDs, the touch-panel sensor layer made of glass and then the top glass protective surface with air in between each layer. The diffusion of light through all three layers causes the AMOLED light to be diffused and difficult to see.
AMOLED panels are used in a number of phones including Google’s Nexus One and early versions of the HTC Desire.
Super AMOLED
So Korean giant Samsung decided to come up with a different method that combined the top glass layer and the touch-panel glass layer into one.
This promotional video gives you a brief overview of Samsung’s Super AMOLED technology.
By reducing the number of layers and removing one air gap, light dispersal is reduced, making these AMOLED displays easier to see in bright light.
Samsung uses the Super AMOLED panel in its Galaxy S phone and is expected to use it inside the upcoming Nexus S.
SLCD – Super liquid-crystal display
LCD has been the mainstay for display panels from PDAs to notebooks to TVs over the last 15 years or so. What makes Super LCD so super is said to be improved light bleeding so that blacks actually look a bit more like black than they typically used to, giving better overall contrast. In comparisions with AMOLED, some reviews suggest that SLCD gives warmer colours than AMOLED. However, battery life appears to be worse with SLCD displays.
SLCD shouldn’t be confused with S-LCD, which is the name for the Samsung/Sony joint venture for manufacturing LCD panels.
Smartphone maker HTC began using SLCD panels in its Desire smartphones in August 2010 due to shortages in AMOLED panels from Samsung. If you have an early Desire, it’ll more likely have an AMOLED panel whereas those manufactured after August 2010 will have an SLCD panel instead.
IPS – In-plane switching
Apart from poor contrast ratios, the other issue with LCD panels is poor viewing angles. The further you move of the centre axis of an LCD panel, the worse the image becomes until you begin to see the reflected negative of that display. In-plane switching is a more expensive solution to the viewing angle problem by changing the direction in which the liquid crystal molecules move. So instead of the normal right-angle or perpendicular switching, IPS panels switch molecules in the same plane as the panel. It means light transmitted through the molecules can be seen at (almost) any angle.
IPS technology is most often used in LCD monitors – and usually at prices three times the going rate. It’s the technology behind Apple’s Retina display in the iPhone 4.
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#1 by Barry Young on December 17, 2010 - 8:24 am
You are to be commended for trying to deal with such a difficult subject. Most of what you write is reasonable, but I have a few comments:
1. The low temperature of LTPS does not refer to an actual temperature, it is a relative term addressing the difference between using a laser and using annealing — rapid thermal annealing (RTA), which operates at very high temperatures (>600 degrees C. LTPS operates at temperatures comparable to a-Si processing 300 to 400 degrees C. Therefore LTPS is “too hot” for use directly with flexible backplanes. Manufacturers, such as Samsung make the active matrix on glass and then use a lift-off process to take it off the glass and put it on the flexible material.
2. OLEDs have a number of benefits vs. LCDs, other than the darkest blacks; they switch ~1000 times faster; contrast ratio is not reduced when looking at the display off axis as with LCDs, yes even with IPS the contrast ratio declines as the off axis increases, they are thinner, and actually trace the gamma curve more accurately. Power consumption depends on the image and in most cases OLEDs use less power than LCDs, especially for video applications.
3. Passive matrix and active matrix designs are addressed by sub-pixel not by pixel for color displays.
4. OLEDs are challenged in bright sunlight as are LCDs. because they have a cathode layer which reflects light. The more reflective the greater the interference. When a touch capability is added external to the display, there is a gap between the display and the touch material, which causes greater reflectivity. By eliminating the external touch and integrating it into the display, there is no gap. Ambient light reflection is also reduced by minimizing the reflectivity of the cathode material and by running the display at a high brightness. These are all issues of design, not fabrication.
5. OLEDs have excellent color reproduction and the display designer can select what level they want to achieve. Most choose to use highly saturated colors greater than the NTSC standards, which the reviewers choose to assess negatively. The color saturation has nothing to do with the Pentile approach, which produces colors and lines consistent with the standards
6. Display makers have several ways to improve the viewing angles and IPS is just one method. In IPS, the liquid crystal is positioned horizontally, instead of vertically and one of the conductors is patterned on the color filter instead of the active matrix. However, IPS monitors are not 3X the price of other designs, they are competitively priced.
7. The term retina display is a marketing definition and is beginning to mean greater than 300 ppi.