Understanding Display Technology: Monochrome


With 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, so this paper aims to help engineers select the optimum display for their application.

TN-LCD Displays: The start of the user-interface revolution
Better viewing angles with HTN and STN
Addressing contrast issues with FSTN, FFSTN, DSTN & ASTN
Selecting a TN display for your application
Designing with STN/FSTN/FFSTN/ASTN displays
Vertical Alignment Displays: True black background
PMOLED: Low power and slim design
Summary: A quick glance guide to strengths/weaknesses

User experience defines success

The overall user experience is the critical factor that defines the success of any given product’s HMI. It can be tempting to believe that today’s world demands bright colours and complex graphics or animations. However, both monochrome and colour display categories each have their respective strengths. There are also various ways to customise or tailor a solution to meet specific requirements in relation to the user experience or factors such as environmental performance, cost, power consumption, form factor and time to market.

Situations when a monochrome display can be the most suitable solution include:

  • When there are tight cost or power constraints
  • When legacy factors such as an existing MCU affect display choice
  • If design overheads such as software development are a critical factor 

On the other hand, a colour display may be preferred when:

  • Market expectations have already been set
  • A very dramatic impact is required
  • Required to leapfrog established products to deliver demonstrable extra value for money

Part I of this article aims to describe the technologies currently available for monochrome displays, highlighting their strengths, limitations, and closest competitors to help you choose the best design approach and display technologies for your applications.

Monochrome Technologies

A carefully designed user interface based on a well-chosen monochrome display can be clear, simple, easy to understand and respond quickly to the users’s demands. Technical strengths include low power consumption, small form factors, easy software design, and low cost, relative to full-colour alternatives.

For these reasons, monochrome displays continue to offer a viable choice for many high-tech projects, particularly in the industrial and medical sectors. The ability to selectively enhance some monochrome display technologies using coloured filters or backlights, provides extra opportunities to create a truly outstanding user experience.

TN LCD Displays; The Start of the User-Interface Revolution

Twisted Nematic (TN) LCD, followed by Super Twisted Nematic (STN) LCD displays, have been highly influential in the success of poplar electronic products such as calculators, DECT phones and pre-smartphones, digital clocks, and industrial control panels. They consist of liquid crystal material sandwiched between two polarizing plates which are attached to the outside of two glass plates, one upper and one lower. The polarisers are fixed to the upper and lower plates of glass with their polarizing planes oriented at 90° to each other.  When polarisers are arranged in this way they can be described as crossed polarisers. The lower or rear polariser can be reflective, transflective or transmissive. 

 
Figures 1 and 2: The basics of an LCD Cell; Comparison of TN and STN Mode LCD Display 
For an in-depth analysis of this technology, please download our white-paper.

Better viewing with HTN and STN

TN LCD displays can have certain viewing angle limitations, depending on multiplex rates. One way to widen the view angle of TN display is to increase the twist angle from 90° to angles of around 110° for Highly Twisted Nematic (HTN) LCD displays or up to 270° for Super-Twisted Nematic (STN) LCD. These types of displays are available at only a small price premium over basic TN units. HTN and STN LCD allows higher multiplex rates, allowing them to support higher resolution. However, response times of the TN based liquid crystal is far too slow to support fast animations or full-frame-rate video.

Addressing contrast issues with FSTN, FFSTN, DSTN & ASTN

Subsequent development of TN technologies has produced further variants, including FSTN, FFSTN, and DSTN LCD displays.

These are film-compensated displays that contain either one extra layer of compensating film (FSTN) or two layers (FFSTN), in addition to the polarizers, to boost contrast and sharpness. There are potential stability issues when operating in the high end of the display’s operating temperature range causing contrast to deteriorate so the Double Super Twisted Nematic (DSTN) and Advanced Super Twisted Nematic (ASTN) LCD displays were developed to address contrast issues by delivering stable display contrast performance over the entire temperature range of the application; specifically, automotive. Dual-Scan STN displays, introduced a new driving scheme which enabled VGA resolution displays. 

mono-display-for-medical-equipmentSelecting a TN display for your application

The main criteria for selecting TN displays would be a low display resolution requirement, meaning simple segment displays through to small, limited icon designs. Other considerations would be design overheads in terms of MCU or CPU memory availability and ultimately cost considerations.  Many TN display applications do not require LED backlights, and instead utilise a reflective rear polarizer.

Designing with STN/FSTN/FFSTN/ASTN Displays

When considering STN/FSTN/FFSTN displays, the following considerations should be considered: Black and White positive-mode displays are optimised with FSTN; Black and White negative-mode displays are optimised with FFSTN. For high-temperature applications such as automotive-grade LCDs requiring stable contrast performance over the entire specified temperature range, DSTN and ASTN displays offer the best contrast performance.

Vertical Alignment Displays

Vertical Alignment (VA) LCDs can deliver excellent performance where a crisp, clear display is required, with high contrast and a deep black background.

As with TN-type LCDs, Vertical Alignment displays rely on changing the orientation of liquid crystal molecules to block or permit the passage of light through the display. As the name suggests, when the display is in off-state, the molecules are aligned vertically on specially textured inner surfaces of the two plates of glass, unlike TN mode displays where the LC molecules are aligned parallel to the glass.

             

 

This vertically aligned arrangement prevents light from passing.  As a result, the display produces a background that is deep black, enabling VA displays to achieve very high contrast ratios of more than 1000:1, and with sharply delineated characters.  Wide viewing angles are another important strength of this type of display. 

As with many LCD technologies, a backlight is required as the display works only in the transmissive normally black mode.  Almost any backlight colour can be used to deliver the required visual effect, like cool white for a high visual impact and easy readability. Because the background is dark, and the characters are lit, colour effects work particularly well with VA displays. This can be achieved using a special backlight design or with optical filters, and selective colour is also possible to ensure specific characters such as warning symbols, can stand out against other characters to capture the user’s attentions. In this way, VA displays can offer a lower-cost alternative to a full-colour TFT-LCD panel, benefiting from the presence of colour to improve interaction or enhance the user experience.

They are ideal where only one or two areas of colour are needed, and full-colour TFT-LCD would be excessively expensive or power hungry. Like TN types, VA displays can be built to custom specifications cost-effectively and quickly.

PMOLED

Organic LED (OLED) displays in general are emissive displays that require no backlight, unlike LCDs, and so can be made thinner than LCDs. In fact, they can be made so thin that flexible displays are possible. They can also be more efficient, since the power required to drive the organic LEDs is lower than that required for a conventional backlight – whether LED or CCFL.

Among the types available, the Passive-Matrix OLED (PMOLED) operates by driving each line of the display sequentially, one at a time. The display comprises an array of horizontal and vertical conductors. Pixels are formed at the intersections between these conductors, and can be monochrome or – by including red, green and blue sub-pixels - colour. There is no storage capacitor, but ensuring a bright display demands a high applied voltage. This can shorten the lifetime of the PMOLED.

Moreover, because the lines are driven one at a time, duty cycle limitations restrict the maximum possible number of lines and hence the overall display size and resolution. For this reason, PMOLED displays are often applied in smaller sizes, up to about 3-inches. This makes them well suited to use in wearable devices, smart retail shelves, domestic appliances, and industrial control panels.

Summary

Parameter/relative strength

Best

OK

Worst

Response time

OLED

VA

TN

Contrast

OLED

VA

TN

Blackness when unlit

OLED

VA

TN

Viewing angle and effects at extremes

OLED

VA

TN

Outdoor readability

TN

-

OLED, VA

Readability in bright ambient light

OLED, VA

TN

-

Readability in low ambient light

OLED, VA

-

TN

Temperature range: low-temperature performance

OLED

TN

VA

Temperature range: High temperature performance

VA

OLED

TN

Power consumption

OLED

TN

VA

BOM cost

TN

VA

OLED

Size ranges

OLED

TN, VA

-

Hardware design challenges

No distinction

No distinction

No distinction

Software design challenges

TN, VA

-

OLED

 

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

Part II of this white paper compares the merits of Colour LCD technology.

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.



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