Understanding Display Technology: Colour


colour--lcd-display-supermarket-scaleWith the increasing pervasiveness of smart objects, and product vendors’ desires to deliver superior user experiences, more and more embedded devices now integrate some type of display. These can range from small displays in IoT nodes, home appliances or retail technology, to larger sizes in equipment like industrial instrumentation or digital signage.

Choices for designers cover a spectrum from simple passively driven segment displays with icons, monochrome dot matrix character or full graphical LCDs, Passive-Matrix Organic LED displays (PMOLED) to high-resolution active matrix displays such as colour TFT-LCDs or AMOLED. The sheer number of different types can be confusing. In Part I we discussed the strengths and choices of monochrome displays available. In Part II we move our attention to colour displays.

Choosing the best display technology for your application

As mentioned in our Monochrome overview, user experience is the critical factor that defines the success of any given product’s HMI and that both monochrome and colour displays have their respective strengths and can both be customised to meet specific user experience requirements. Here we aim to highlight the strengths, limitations, and closest competitors to help you choose the best design approach and display technologies for your applications.

TFT-LCD Displays: A trusted technology 
IPS: Overcoming viewing limitations
AMOLED: A superior alternative
Summary: A quick glance guide to strengths and weaknesses

Colour Displays

The smartphone era has brought colourful, bright, high-resolution displays - capable of crisp graphics and image rendition - to mainstream audiences. These have raised the bar, in terms of user expectations: market demands are perceived to be increasing throughout industrial, scientific, medical, and general consumer sectors. Designers can feel that a colour graphical display is the only way forward, when updating existing products or conceiving new models, to meet those expectations and keep abreast of competitors.

Note that it is worth considering the possibilities offered by the latest monochrome technologies, particularly if cost or power consumption need to be controlled, or if suitable colour effects can be achieved with imaginative use of filters or backlights with displays such as VA types.

It is also important to consider the capabilities of the host system, to determine whether a colour display can be supported. A low-power or performance-constrained embedded host system cold restrict the possible displays size or resolution, colour depth, or frame rate as discussed in a previous article. Software such as drivers is also more complex for colour displays; software-engineering overheads can be offset by using open-source drivers.

If the decision is taken to go ahead and develop a full-colour user interface, the strongest display technologies today are TFT-LCD (thin-film transistor LCD) including In-Plane Switching (IPS) types, or AMOLED (Active-Matrix Organic LED) displays.

TFTs: Mature, trusted and efficient

TFT-LCDs (Thin Film Transistor) are available in a wide range of sizes, from 1-inch to 21-inch. The liquid-crystal behaviour is similar to that in a TN display, relying on 90-degree twisted stacks acting as a waveguide to control the passage of light through the display. ITO electrodes are deposited on the upper and lower glass substrates, and red, green and blue colour filters are deposited on the inner surface of the upper substrate. A thin-film transistor is embedded in every sub-pixel, and connected to the data bus line. This enables the display drive system to address every sub-pixel individually. By controlling the intensity of light transmitted through each colour filter, the TFT-LCD can display millions of colours. 

Figure 1: Structure of a TN-TFT LCD Cell

When a signal is applied to the pixel, the liquid crystal changes from its twisted state to align with the electric field and prevent light passing through – much the same as in a TN display. The TFT-LCD is able to switch rapidly between ON and OFF states, allowing response times fast enough to support rapid display-update rates suitable for presenting video content or smooth animation.

TFT-LCD technology is now quite mature, about 20 years old. As a result, manufacturing processes are efficient and production yields are high, leading to very competitive unit prices. With the emergence of the maker movement, graphical displays have become cost-effective in sizes as small as 1-inch, in addition to sizes that are popular in the PC and mobile markets.

There are some challenges, however. The twisted nematic cells are unable to support high contrast or very wide viewing angles. TFT-LCDs are known to be unable to render true black, and colours also have limited vibrancy. Contrast is dependent on the direction of viewing, and colour shifts, changes in brightness, and contrast inversion (or birefringence) may be experienced. A typical contrast ratio for a standard TN-TFT display is around 400:1 with viewing angles of L70/R70/T70/B60.

IPS: Overcoming viewing limitations

In-Plane Switching (IPS) is a recent technology that overcomes the viewing limitations of conventional TFT-LCDs. It is also known as Super TFT.

IPS derives its name from the fact that the liquid-crystal molecules are aligned in parallel with the glass plates, whereas the TN principle adopted in conventional TFT displays is based on perpendicular alignment of the molecules. In an IPS display, the crystals remain oriented in parallel whether the pixel is turned on or off. Electrodes are placed on the same sheet of glass, instead of on opposing sides, so that when the signal is applied, the crystals rotate horizontally in the same plane.

IPS TFT displays deliver several improvements to the end-user experience. Because the pixels block light when in the off state (the opposite situation to conventional TFT), IPS TFT exhibits high contrast and the background is true black when the display is powered down. In addition, in the event that a transistor fails, the associated pixel becomes dark. This is less disturbing than bright-pixel defects in a conventional TFT. Colour reproduction is also more accurate, contrast and colour are more consistent over a wider viewing angle, and optimum visibility is achieved throughout wide-ranging conditions and environments, including daylight.

AMOLED: A superior alternative

We previously discussed the merits of PMOLEDs in our Monochrome section which can display colours by incorporating red, green and blue sub-pixels. Because only one line can remain on at any one time, the display brightness depends on the duty cycle, which can place a practical limit on size and resolution.For applicatios requiring a colour display that should also be large and have high resolution, for example to present precise graphics or animations, an Active Matrix OLED (AMOLED) can offer a superior alternative.

Figure 2: LCD vs OLED structure

An AMOLED display contains a transistor at each pixel, which enables the pixel to remain turned on for as long as needed in any given frame. This overcomes the trade-offs between duty cycle, brightness and resolution that limit the performance of colour PMOLEDS.

AMOLED technology has become extremely successful in smartphone markets, and is adopted by leading handset brands. Displays can offer several advantages over TFT, and IPS (Super TFT), including light weight, a thin profile, wide temperature range, and excellent optical performance. In addition, their low power consumption answers a key demand within today’s mobile markets, which require long run times from smaller and smaller batteries.

These characteristics also make AMOLED a strong candidate for a wide range of industrial or medical applications. Low power allows modes in which the display remains permanently on, instead of having to power down to conserve energy. This can be a major advantage in situations where the user needs to see the displayed information immediately, without first having to wake the screen.

The wider temperature range is an obvious advantage in industrial environments or equipment that is used or permanently installed outdoors.

In addition, AMOLED displays can be made from flexible substrates, giving great flexibility to create curved displays or to install a display on a non-flat surface, avoiding the complexity of a cutting an aperture.

Summary  

Parameter/relative strength

Best

OK

Worst

Response time

AMOLED

IPS-TFT

TN-TFT

Colour rendition

AMOLED

IPS-TFT

TN-TFT

Blackness when unlit

AMOLED

IPS-TFT

TN-TFT

Viewing angle and effects at extremes

AMOLED

IPS-TFT

TN-TFT

Outdoor readability

TN-TFT

IPS-TFT

AMOLED

Readability in bright ambient light

AMOLED

IPS-TFT

TN-TFT

Readability in low ambient light

AMOLED

IPS-TFT

TN-TFT

Temperature range: low-temperature performance

TN-TFT

AMOLED

IPS-TFT

Temperature range: High temperature performance

TN-TFT

IPS

AMOLED

Power consumption

AMOLED

TN

IPS

BOM cost

TN

IPS

AMOLED

Size ranges

AMOLED

TN

IPS

Hardware design challenges

No distinction

No distinction

No distinction

Software design challenges

No distinction

No distinction

No distinction

Why not download this white paper. Our downloadable version contains even more technical information.

Conclusion

Designers must contend with many choices to identify the display most suited to their application.

The choice of whether to use a monochrome or colour display, to deliver the required user experience, is one of the first decisions to be made when creating the human-machine interface for any given application. Whichever the chosen route, several options are available, and making the right choice can be based on a variety of criteria.

We hope this paper's aims to help navigate these options and compare their relative strengths has helped you choose the optimum display type to meet the applicable requirements. Getting expert assistance, preferably from an early stage in the project, can help achieve a superior result that is also more cost efficient and faster to market.

If you would like to dicuss your project with one of our skilled engineers, please contact us today.



Latest From The Knowledge Centre

Tackle EMC compliance early to avoid problems later

Our differences on display so you can design for customisation

Putting your product design to the test

Custom Displays - Flexible, Robust and Accessible

Engineers - The unsung heroes of tech

Understanding EMC Conducted Noise Immunity for Touchscreens

Displaying Engineering at the Electronics Design Show, Stand G30

Business evolution keeps Anders on display

How has your technological world changed?

Please call us +44 (0)207 388 7171