Specifications of Smartphones - The Display Unit

The Display

There are lot of display types used in mobile phones. They can be either color or monochrome. Monochrome displays on the other hand can be alphanumeric or graphic.
Alphanumeric displays can show only symbols with a constant size, while graphic displays can show fonts of different sizes and animations.

The color displays usually are

• CSTN - Colour Super Twisted Nematic, 
• TFT - Thin Film Transistor
• TFD -  Thin Film Diode
• LCD - Liquid Crystal Display
• SLCD - Super Liquid Crystal Display
• OLED - Organic Light Emitting Diode
• AMOLED - Active-Matrix Organic Light Emitting Diode
• IPS - In Plane Switching
• Retina

There are also two types of touchscreen displays - capacitive and resistive, which are both based on TFT technology.

The Touchscreen

CAPACITIVE OR RESISTIVE

CAPACITIVE touchscreens work by sensing the electrical properties of the human body, while RESISTIVE ones operate by sensing direct pressure applied by the user.

Capacitive touch sensors are used either as buttons or on touchscreens. They work by sensing the electrical properties of the human body instead of pressure and generally they don't work with a stylus so they don't allow handwriting recognition.
However, capacitive touchscreens feel more sensitive than their resistive counterparts.
Capacitive touch screens are also considered more durable than resistive touch screens.


The RESISTIVE type can be activated by pressing not only with human skin but also with a stylus and thus allow handwriting recognition input.
Resistive touchscreens operate by sensing direct pressure applied by the user. It can be activated by pressing it not only with a finger but also with a stylus.

A resistive touch screen consists of a touch layer placed on top of a standard display. The touch layer normally includes two transparent electrical layers separated by a small gap.
Pressing the display's surface causes the two separate layers to come into contact, which creates an electrical connection that can be sensed and located.

Display Types

Liquid Crystal Display (LCD)

A liquid crystal display is the most common display type among mobile phones because of its low power consumption and good image quality. LCD is made up of an array of liquid crystals that get illuminated by a back-light. They are generally easy to read, even under direct sunlight.

However, this back-lighting means that blacks tend to appear gray and LCDs therefore have less contrast than other display technologies.

Some of the types of LCD displays are STN, TFT and TFD.
STN offers low cost and low power consumption, but low image quality.
TFT features greater image quality and response time, yet its displays are more expensive and need more power.

Three other types of LCDs are Transmissive, Reflective, or Transflective.
Transmissive displays offer nicer image quality in low or medium ambient light, while
Reflective ones work best in bright ambient light.
Transflective displays combine the best of both.


Super LCD (SLCD)
Super LCD (SLCD) is a display technology used by numerous manufacturers for mobile device displays. It is mostly used by HTC, though Super LCD panels are actually produced by S-LCD Corporation.
Super LCD differs from a regular LCD in that it does not have an air gap between the outer glass and the display element. This produces less glare and makes the user feel "closer" to the display itself. Super LCD's benefits also include lower power consumption and improved outdoor visibility. Super LCD has been succeeded by the newer Super LCD2 displays.


CSTN - Color Super Twisted Nematic
Super Twisted Nematic (STN) is a type of LCD display technology. STN is black and white while CSTN is the color version. (C)STN displays are used on lower end devices.
Typically an STN display has worse image quality and response times than a TFT LCD, but is cheaper and more energy efficient.


Thin Film Transistor
TFT is one of the best Liquid Cristal Display technologies in terms of image quality and response time. However, it also consumes more power and is more expensive.
TFT, like TFD, is an active-matrix technology. This means a transistor is located next to each pixel, allowing it to be turned on and off individually. This ensures faster response time and greater contrast.


Thin Film Diode
TFD is a kind of Liquid crystal display (LCD) technology. It is an active-matrix technology which means that a diode is situated next to each pixel making it possible for the pixels to be turned on and off individually. This allows a quicker response time and more contrast than passive-matrix technologies.
TFD takes the excellent picture quality and the fast response of TFT displays and combines them with the low power consumption and cost of the STN ones.


Organic Light Emitting Diode (OLED)
A display technology that consists of small dots of organic polymer which emit light when charged with electricity.
OLED displays have several advantages over the LCDs. They are thinner, lighter, brighter, need less power, have better viewing angles, contrast and response time for video and animation. OLEDs are also cheaper and easier to manufacture.
On the other hand, LCDs offer better legibility in bright ambient light.

As opposed to LCD panels, which are back-lit, OLED displays are 'always off' unless the individual pixels are electrified. This means that OLED displays have much purer blacks and consume less energy when black or darker colors are displayed on-screen. 

Active-Matrix Organic Light Emitting Diode
Active-matrix OLED displays provide the same performance as their passive-matrix OLED counterparts, but they consume significantly less power.
This advantage makes active-matrix OLEDs well suited for portable electronics where battery power consumption is critical.
AMOLED displays have a very fast refresh rate but on the down side are not as visible in direct sunlight.

Screen burn-in and diode degradation due to their organic nature are other factors to consider.

On the positive side, AMOLED screens can be made thinner than LCDs as they don't require a back-lit layer and they can also be made flexible.

Difference between AMOLED and Super AMOLED

Super AMOLED displays reduce the thickness of the screen by integrating the touch response layer with the display itself.

Super AMOLED displays handle sunlight better than AMOLED displays and are also better on power consumption. 

In-Plane Switching
IPS (In-plane switching) is a screen technology for liquid crystal displays (LCDs).
IPS is a further improvement on TFT LCDs whereby the way the crystals are electrically excited is different and the orientation of the crystal array is rotated. This orientation change improves viewing angles, contrast ratio and color reproduction which were the limitations of TFT LCDs. 
Energy consumption is also reduced compared to TFT LCDs.
Although IPS is better than TFT LCDs they are also more expensive when put on a smartphone.

Retina

Retina is another marketing term, this time from Apple. A Retina display is not defined by any particular characteristic, other than that it is supposedly of sufficient resolution that the human eye can't discern pixels at a normal viewing distance.

This measurement obviously changes depending on the size and resolution of the display.

credit : Gsmarena and Wikipedia