Screens

Displays of the future

From LCD televisions to the latest force-sensitive touchscreen technology, electronics and photonics are pushing the envelope ever outward.

The take-away

  • Though widely used, LCD and OLED displays still have disadvantages like little resistance to water.
  • Touchscreens used in smartphones could be more sensitive to the force of the applied touch and more robust to ambient conditions. Denmark’s WaveTouch is working on the touchscreens of the future.

When you look at a screen, what you’re really seeing is an electro-optical layer that’s manipulating light in order to produce a visual image. The better this layer performs, the more successful the product will be. Current market-leading display technologies are based on liquid crystals and organic light-emitting diodes. The demand for high-definition displays with touch functionality is also becoming increasingly important, and will likely be the new battlefield for competing technologies.

Liquid Crystal Displays

First Appearance

Liquid crystal displays (LCDs) appeared on the market in the 1970s. Within 20 years, they became the leading technology for watches and calculators as well as for laptop screens, desktop monitors and televisions. LCDs compromised the long-standing success of cathode-ray tubes and large-screen flat plasma display panels.

How it works

In LCDs, each pixel – the switchable unit in a display – consists of a layer of liquid crystal (LC) material trapped between two polarising glass plates. Transparent electrodes on the glass plates apply an electric voltage that switches the LC on and off. In many LCDs, an additional backlight illuminates the LC material, for example blue light-emitting diodes (LEDs) combined with broad spectrum phosphors provide a white light source. Traditional colour LCDs use red, green and blue filters.

Strengths and weaknesses

The image quality of LCDs has improved over the years, and photonics occasionally lends an additional hand: for example, quantum dots – nanometre-sized semiconductor particles – can be incorporated into backlight units to obtain more vibrant colours without the need for bespoke filters. Nevertheless, LCDs have issues with contrast, depending on the light conditions, and a limited viewing angle.

Organic Light-Emitting Diodes

First Appearance
Around 2012 a different kind of display started to overtake LCDs. The new technology, building on basic research that dated to the 1960s, was based on organic light-emitting diodes (OLEDs), in which organic semiconductors replace the materials found in conventional LEDs.

How it works

In OLED displays, thin films of organic material are placed between two electrodes. A voltage is applied, causing the organic semiconductor layers to emit light at specific wavelengths. Red, green and blue OLEDs can be arranged as pixel arrays to produce full-colour images. The emission wavelengths of the OLEDs can be tuned by varying the applied voltage.

Strengths and weaknesses

OLEDs offer several promising features for flat display panels. They don’t require backlighting, which reduces the thickness and overall weight of the device. OLED displays produce vivid colours, achieve better contrast and guarantee wide viewing angles. The versatile fabrication process of OLED displays has also allowed companies to explore previously uncharted territory, such as flexible and transparent displays. The latest smartphones by Samsung and Apple are based on OLEDs. On the downside, this technology is more expensive than LCD, and the materials involved can degrade, for example due to exposure to oxygen and water.

TECHNOLOGIST photonics Screens article - Displays of the future

Touchscreens

First Appearance

In the 1960s, a group of researchers at the University of Illinois developed an optical touch screen based on scanning infrared technology for the computer learning system they were developing, PLATO (Programmed Logic for Automatic Teaching Operations). Decades later, devices such as Apple’s first iPhone began to incorporate touch functionality. Now it is increasingly sought after in all types of displays.

How it works

The idea behind scanning infrared technology is relatively simple: a grid of infrared light beams (often generated by LEDs) covers the screen. Phototransistors translate an interruption in the light as a touch input. This technology is durable and can be scaled to large areas, but it requires a large number of light sources and detectors. Most present-day existing touchscreens in smartphones and tablets rely on electrical rather than optical technologies. For example, in projected capacitive touchscreens, the input given by the user’s finger tapping on the surface of the display changes ever so slightly the capacitance between conductive electrodes.

Strengths and weaknesses

Everyone agrees that touchscreens are a success, but the ideal solution still hasn’t been found. For example, sensitivity to the force of the applied touch would be desirable. Touchscreens can also become challenging under certain conditions, such as if the surface is wet.

Future Touchscreens

First Appearance

In 2011, researchers at the Technical University of Denmark (DTU) presented an optical touchscreen based on waveguide sensing. The DTU team’s idea has now been taken up by WaveTouch, a joint venture between OPDI Technologies, a Danish optics innovation company, and O-Net Communications Ltd in Hong Kong. The optical nature of this touchscreen technology makes it – in principle – more robust to ambient conditions traditionally disruptive to electronics-based touch-sensitive platforms. WaveTouch will be targeting niche markets where their features may have an advantage against available technologies – for example, for outdoor portable navigation.

How it works

A single laser light source is placed in one corner of the screen, producing the light beams that constitute the touchsensitive grid. The beams propagate in a planar waveguide (in this case a transparent plastic plate) through total internal reflection (TIR). Here, a touch input given by a finger or stylus on the sensitive surface breaks the TIR, allowing some light to escape the waveguide. TIR in the context of touchscreens is not an entirely new concept, but the specific realisation devised at DTU is novel.

Strengths and weaknesses

Because WaveTouch technology requires only one light source and a small detector array, the number of components is drastically reduced. The touchscreen is also sensitive to the force of the applied touch, which opens the door to an even broader range of functionalities.

WaveTouch, photonics.com, Handbook of Visual Display Technology