Touch display technologies

Most of us buy mobile phones and tablets based on display specifications as one of the evaluation parameter. I will try and explain what are the various types of display technologies and some terms which you should know about.

The touch based smartphone screens are a complex assembly and consist of the following layers

• Base layer - display (LCD, LED)
• Middle layer - Touch panel (Resistive, capacitive)
• Top layer - protective glass (gorilla, etc)

Structure of a touch panel display 

The above assembly is one of the most expensive component of a smartphone. It is also one of the most important part of the hardware specification which most customers use for evaluation. So before making a decision you should know what it is.

The various display technologies are

• LCD - Liquid Crystal Display
• TFT - thin-film transistor
• IPS - in-plane-switching
• LED - Light Emitting Diodes
• OLED - Organic Light-Emitting Diode
• AMOLED - Active-Matrix OLED
• Super AMOLED

• Retina Display

The above terms are explained below (not too much details, just sufficient)

LCD Display

An LCD, or Liquid Crystal Display, is a type of screen that is used in many computers, TVs, and cell phones. LCDs are very thin, but are actually composed of several layers. Those layers include two polarized panels, with a liquid crystal solution between them. Light is projected through the layer of liquid crystals and is colorized, which produces the visible image.

Picture : Structure of a LCD pixel

The liquid crystals do no emit light themselves, so LCDs require a backlight. That means that an LCD requires more power, and could potentially be more taxing on your phone’s battery. LCDs are thin and light, though, and generally inexpensive to produce.

Two types of LCDs are primarily found in cell phones : TFT (thin-film transistor) and IPS (in-plane-switching). TFT LCDs use the thin-film transistor technology to improve image quality, while IPS-LCDs improve on the viewing angles and power consumption of TFT LCDs.

TFT LCD

TFT stands for Thin Film Transistor, and is a type of technology used to improve the image quality of an LCD. Each pixel on a TFT-LCD has its own transistor on the glass itself, which offers more control over the images and colors that it renders.

While TFT-LCDs can deliver sharp images, they also tend to offer relatively poor viewing angles, meaning they look best when viewed head-on. If you view a TFT-LCD from the side, it can be difficult to see. TFT-LCDs also consume more power than other types of cell phone displays. In general, TFT-LCDs are found on more low-end smartphones or feature phones, and on basic cell phones.

IPS LCD

IPS stands for in-plane switching. An IPS-LCD is a type of thin display that offers better viewing angles than TFT-LCDs. IPS-LCDs feature two transistors for each pixel, where TFT-LCDs use just one. This requires a more powerful backlight, which delivers more accurate colors, and allows the screen to be viewed from a wider angle. The downside is that an IPS-LCD may consume more power than a TFT-LCD.

OLED Displays

OLED, or Organic Light-Emitting Diode, displays are able to deliver sharper and brighter images than LCDs, while also using less power. Just like LCDs, OLED displays come in a variety of types. Here are the types of OLED displays you're likely to find on today's cell phones and smartphones.

Structure of the OLED pixel

OLED, or Organic Light-Emitting Diode, displays are able to deliver sharper and brighter images than LCDs, while also using less power. Unlike an LCD, which requires a backlight, OLED screens can emit their own light. This allows OLED screens to display brighter, sharper images and text, and to offer better viewing angles. It also means that OLED screens are thinner and consume less power than their LCD counterparts.

OLED screens also offer faster refresh rates than LCDs, so they can deliver smoother video playback. But there are a few drawbacks to OLED displays. For one, the organic materials used to make them have a more limited lifetime than the components of other display types. OLED screens also have the potential to be more easily damaged by water or other liquids. Also, when viewed in direct sunlight, the images on an OLED display are not as bright as what you'd see on an LCD.

AMOLED

AMOLED is the abbreviation for Active-Matrix OLED, a type of display that is found in TVs and mobile devices, like cell phones. AMOLED displays actually pair part of a traditional TFT display with an OLED display. This allows them to offer faster response times than regular OLED displays, which can be prone to ghosting when displaying fast-moving images. AMOLED displays also offer greater power savings than traditional OLED displays.

Like traditional OLED displays, though, AMOLED displays may have a more limited lifetime, because of the organic materials used to make them. Also, when viewed in direct sunlight, the images on an AMOLED display are not as bright as what you'd see on an LCD.

Super AMOLED

Super AMOLED displays, which are found in many Samsung smartphones, are designed to improve on the performance of traditional AMOLED displays. Samsung says its Super AMOLED displays offer significantly better performance than AMOLED displays when viewed in direct sunlight. They are also supposed to offer brighter images while consuming less power than their AMOLED counterparts.

Retina Display

Apple calls the display on the iPhone a "Retina Display," saying it offers more pixels than the human eye can see -- a claim that has been disputed by some experts.

The iPhone 4's Retina Display features a resolution of 326dpi (dots per inch). When announcing the phone, Apple's Steve Jobs said that 300dpi is a "magic number," because it's the limit of the human retina to distinguish pixels. Because the iPhone 4's display features a resolution higher than the magic number, Apple says it will always look smooth and clear.

Screen Size and Resolution

Apart from the display technology there are a few more parameters of the display which are important

a) Display Size - this is the diagonal length of the viewable part of the display or length from one corner of the viewable display to the diagonally opposite corner. 

b) Display Resolution - it determines the amount of content that can be displayed at any given time. It defines the number of pixels horizontally and vertically. So when we say the resolution is 800 x 480, it means the display has 800 vertical pixel lines and 480 horizontal pixel lines. The various resolutions available are listed below.

QQVGA	160 x 120		XGA	1024 x 768
HQVGA	240 x 160		WXGA	1280 x 720
QVGA	320 x 240		WXGA	1280 x 768
WQVGA	400 x 240		WXGA	1280 x 800
HVGA	480 x 320		WXGA	1360 x 768
VGA	640 x 480		WXGA	1366 x 768
WVGA	800 x 480		XGA+	1152 x 864
FWVGA	854 x 480		WXGA+	1440 x 900
SVGA	800 x 600		SXGA	1280 x 1024
DVGA	960 x 640		SXGA+	1400 x 1050
WSVGA	1024 x 576		WSXGA+	1680 x 1050
WSVGA	1024 x 600		UXGA	1600 x 1200
			WUXGA	1920 x 1200

c) Pixel density - the total number of pixels within a physical constraint. It's calculated in pixels per inch (ppi), which is fundamentally a measure of how tightly pixels are squeezed together.The pixel density depends on the above 2 factors (size and resolution)

What looks better? 

Let's take an example. If you have a smart phone with a 3.5 inch display and a resolution of 480 x 800 and a tablet with a 7 inch display and the same resolution 800 x 480, 

a) the size of each pixel on the 7 inch display is larger than on the 3.5 inch display 

b) the pixel density on the 3.5" phone is higher (266 ppi) than that on the 7 inch screen (133 ppi)

In other words, the pixel density (pixels per inch) of the 3.5 inch phone is double that of the 7 inch tablet with same resolution, so pictures look more crisp on the smaller screen with the same resolution. 

Note that the resolution terminology is reversed (480 x 800 vs 800 x 480) because the display on the smart phone is in the portrait form and the tablet is landscape, but both resolutions are the same. 

Touch panel 

Touch panel is that part of the display that responds to the user's touch. It performs actions as per the action defined at the location of the touch. There are 2 major types of touch on smart phones and tablets available at this time - Resistive and Capacitive.  

Resistive Touch panels 

Resistive touch screens possesses two layers of conductive material with a very small gap between them, which acts as a resistance. When the resistive touchscreen is touched with a finger (or a stylus) the two layers meet at the point of touch and thus makes a circuit there. Resistive touch screens are not as responsive as capacitive touch screens and hence, often require a stylus. This technology is cheaper to make and is used only in low end phones. 

The resistive touchscreen consists of a flexible top layer made of Polyethylene (PET) and a rigid bottom layer made of glass. Both the layers are coated with a conducting compound called Indium Tin Oxide (ITO) and then spaced with spacers. While the display is operational, an electric current flows between the two layers. When a touch is made, the flexible screen presses down and touches the bottom layer. A change in electrical current is hence detected and the coordinates of the point of touch is calculated by the controller and parsed into readable signals for the operating system to react accordingly.   

                                     

These systems transmit only 75% of light from the display. The resistive touchscreen is further divided into 4-, 5-, 6-, 7- and 8-wired resistive touchscreen. While the constructive design of all these modules is similar there is a major distinction in each of its method to determine the coordinates of touch.

The Resistive Touchscreen works well with almost any stylus-like object.

Capacitive Touch panels 

Capacitive touch screens are more responsive to human touch when compared to resistive touch screens. Hence, the user experience for touch is much better. Capacitive touchscreens are used in high-end smartphones. A capacitive screen is smooth to operate as it just requires the presence of your finger, without any pressure.

The Capacitive Touchscreen Technology is the most popular and durable touchscreen technology used all over the world at most. It consists of a glass panel coated with a capacitive (conductive) material Indium Tin Oxide (ITO). The capacitive systems transmit almost 90% of light from the display. There are various capacitive technologies available as explained below.

Surface-Capacitive screens, in this technique only one side of the insulator is coated with a conducting layer. While the display is operational, a uniform electrostatic field is formed over the conductive layer. Whenever, a human finger touches the screen, conduction of electric charges occurs over the uncoated layer which results in the formation of a dynamic capacitor. The computer or the controller then detects the position of touch by measuring the change in capacitance at the four corners of the screen.

In the Projected-Capacitive Touchscreen Technology, the conductive ITO layer is etched to form a grid of multiple horizontal and vertical electrodes. It involves sensing along both the X and Y axis using clearly etched ITO pattern.

In the Projected-Capacitive Touchscreen Technology, the conductive ITO layer is etched to form a grid of multiple horizontal and vertical electrodes. It involves sensing along both the X and Y axis using clearly etched ITO pattern. The projective screen contains a sensor at every intersection of the row and column, thereby increasing the accuracy of the system. There are two types of projected capacitive touchscreen: Mutual Capacitance and Self Capacitance

Capacitive touch screens also support multi-touch or plural technology, which can detect two or more touches over its display area at the same time. Some of the common functionalities that require multitouch interface are zooming in, zooming out, rotating objects, panning through a document, virtual keyboard, etc.

The capacitive touchscreen can only be operated by bare finger or multiple fingers for multi touch.   

Protective glass

The glass on top of the display assembly is a glass which provides strength to the assembly and protects the touch panel and display from damage from scratches, impact, heat, etc. The most known glass is the Gorilla glass made by Corning. The other type being sapphire glass also used in wrist watches as well.

Corning Gorilla Glass is an environmentally friendly alkali-aluminosilicate thin-sheet glass designed specifically to function as a cover glass for portable display devices. Its superior composition allows a deeper layer of chemical strengthening than is possible with other chemically strengthened glasses, giving it an increased ability to resist and withstand damage.   

A variety of features come together in Corning’s Gorilla Glass to create a strong, damage-resistant glass that is ideal for smartphones and tablet PCs, including:

• Chemically strengthened to withstand high-volume user interaction and repetitive motion
• Highly durable cover glass for increased mobile Tablet PCs lifespan
• Adds protection without impacting pen response
• Increased ability to resist and withstand damages compared to device without Gorilla Glass
• Retains its performance advantage even when used in thin form factors
• Retains optical characteristics after extended use and abuse
• Remains strong over time
• Water, chemical and disinfectant resistant coating: easy to clean and remove dust, dirt, or other elements.