Good Things Come in Small Packages
Designing small displays that are light, compact, bright, user-friendly, and able to present high-quality images under demanding conditions is not easy, but the demand is there – and so is the ingenuity.
by Steven Vrablik
THE DISPLAY INDUSTRY is striving to bring the most life-like images to the smallest, highest-resolution, lowest-power, and most user-friendly devices possible. It is doing so because the small-to-medium-sized display market is dynamic and offers a multitude of applications. These applications have demanding requirements that are driving various technological advances at breakneck speed. The resulting progress in these displays is validating the old adage that "good things come in small packages."
Display designs are functions of size, volume, weight, front-of-screen performance, and usability, as well as other factors such as power and cost/price. Small- and medium-sized displays cover a very wide range of applications that drive a multitude of display requirements and associated technologies. However, one size does not fit all.
Small displays, generally up to 4 in. on the diagonal, are used in many applications. The mobile-phone application leads the way, using 2-in.-class displays in cellular telephones and pagers and slightly larger displays in smart phones. Audio–video (AV) applications abound in digital still cameras and digital camcorders. Many digital music players utilize small liquid-crystal displays (LCDs) to navigate through the song lists, while a new market in portable media players is emerging.
Color LCDs are used in personal printers for previewing images prior to printing color photographs. Small LCDs are also used in popular handheld gaming devices and handheld global-positioning systems (GPSs). Personal digital assistants (PDAs) tend to use the largest displays in this category – up to 4 in. on the diagonal. Finally, automotive applications include diagnostic and status information, radio, navigation, the instrument cluster, and driver's head-up projection displays.
Mid-sized displays, ranging in size from 5 to 9 in. on the diagonal, are used in applications such as AV, including the popular portable DVD players that typically use displays from 7 to 9 in. on the diagonal. Similar displays can be found in applications such as security monitors, broadcast monitors, airplane video monitors for watching movies, and automotive applications such as front-console navigation and rear-seat entertainment applications, which are manufactured to "automotive grade" specifications to address the rigors of the automotive operating environment. There is also large usage in industrial markets in 5–9-in. sizes for applications such as test and measurement, automated teller machines, and point-of-sale displays. Displays designed for low power consumption are used in handheld mini-PCs and sub-notebooks, as well as for portable test and measurement, diagnostics, and mapping devices. All these displays come in a wide variety of sizes and shapes, with 4:3 and 16:9 aspect ratios being most common.
Fig. 1: By using LTPS or similar technologies for the production of TFTs, it is now possible to build a QVGA display in a size suitable for cellular telephones.
Technologies and Performance
Let us examine the available small- and medium-sized display technologies and their associated front-of-screen characteristics. We will see that although certain technologies are strong in many performance areas, it is generally true that designers have to confront many engineering trade-offs when they develop displays for a particular application or a particular set of customers.
The fundamental question regarding front-of-screen performance is, What information content will be displayed in this application? Will it be characters, simple text, graphics, photographs, or video? Will it be black and white, gray scale, segmented color, or full color?
These questions my seem obvious, but they are essential; they determine whether a simple monochrome passive-matrix display will suffice or whether a full-color active-matrix display is required. Low-end cellular-telephone applications, for example, still use monochrome passive-matrix technology for the main display, but main displays are increasingly becoming active-matrix color displays in telephones with a full feature set where the user interface is more complex or photographs are likely to be displayed. The back-side display of clamshell-type telephones that only display simple alphanumeric data to indicate status, time of day, etc., is relegated to passive-matrix technology. Passive-matrix displays can also be found in applications such as portable music players, automotive information displays, and printers, but they are increasingly being replaced by active-matrix color displays as the need for color increases and the power consumption and price of active-matrix technology decline.
The Next Level
Once an active-matrix display is chosen, the next set of technological choices for fulfilling the desired front-of-screen requirements begins to unfold.
Colors. An early decision must be how many colors are desired in full-color displays? Is six bits per color (for 256K colors) enough? Or do we need eight bits per color (for 16.7 million colors) in very-high-end applications? This decision affects the type of LCD drivers that must be used.
But the number of display colors is just the beginning. What is the saturation range needed for those colors? How life-like should the display be? As measured against the NTSC standard, should the display achieve 45% of NTSC, 72% as achieved by HDTV, or more? This choice affects the selection of color filters and the density of color pigments used. And the color-filter design choices must be combined with the selection of the lighting system: a cold-cathode fluorescent lamp (CCFL) system or a light-emitting-diode (LED) system; either backlit or frontlit, as it may be for reflective displays.
Once the number of colors and color saturation is decided, we must decide how bright the display should be? Will it be used primarily indoors, outdoors, or both? If, at this point, we could freely design-in the luminance required, the designer's life would be considerably easier. Unfortunately, brightness does not come free.
A brighter display generally requires more power or, in the case of reflective or transflective displays, more costly processing steps to improve the outdoor performance. The type of light source is key in determining brightness and power consumption, whether that light source is the common CCFL or an LED as used in several emerging applications.
CCFLs are designed for many different applications, from thin and highly efficient for low-power portable applications to larger and more robust for industrial applications. There are also CCFLs that are specially designed for demanding applications such as low-temperature start-up in automotive applications.
Display brightness is also affected by the presence of brightness-enhancement films for light collimation or polarization and combinations thereof. Further affecting the brightness performance is subpixel aperture size – the larger the better. Advanced technologies for making thin-film transistors (TFTs) – such as low-temperature polysilicon (LTPS) and continuous-grain silicon (CGS) – can improve aperture size.
As has been described, providing the desired front-of-screen performance presents a challenging set of requirements for choosing and optimizing display technologies for a particular application, but what exactly does "front" mean? If we define front narrowly, i.e., if we provide a narrow viewing angle for privacy or to increase the brightness in a well-defined viewing direction, we are led to a different set of design choices than if we provide a wide viewing angle so the display can be seen by several users simultaneously. And will the display be used in multiple orientations, as in the case of a camera cellular telephone, which consumers use to take both portrait and landscape photographs? They will then expect to view those photographs equally well on the main display.
Toshiba Matsushita Display Technology Co., Ltd.
Fig. 2: This system-on-glass LTPS LCD, which was shown at the author interviews at SID 2005, has the ability to capture touch input by sensing the shadow image projected by a finger approaching – and then leaving – the display.
A variety of viewing-angle technologies are employed in the small- and medium-sized display markets to accomplish these diverse objectives. Film-based approaches using optical compensation films are practical and low in cost, but more sophisticated techniques are now available.
A variety of cell-based technologies have emerged that provide better viewing angles than film-based approaches, but they may come with a cost premium and they may degrade brightness. Examples include Advanced Super View (ASV) from Sharp Corp. and the similar Super PVA (S-PVA) from Samsung, in-plane switching (IPS) from LG.Philips LCD and Hitachi, and optically compensated bend (OCB) which was recently demonstrated by Toshiba America Electronic Components (TAEC).
A final front-of-screen performance factor relates to response time: How quickly should the images on the screen be refreshed? The answer depends, first of all, on the type of imagery that will normally be displayed? Will there be still images or real-life video images? Will these images only be viewed in moderate ambient-temperature conditions or must they be seen in harsh environments, such as in automotive rear-seat entertainment applications for use in cold or hot ambient conditions?
Display manufacturers have responded to these challenges by adopting liquid-crystal material with different viscosity characteristics and have also narrowed the cell gap through improved process control to improve response times. New approaches include OCB technology, which can have a response-time capability of less than 5 msec. To date, OCB has only received commercial application in large-sized TV sets, with Toshiba Matsushita Display Technology Co., Ltd. (TMD) exploring other implementations.
Making a Display Small
What does it take to make a display "small" and still satisfy the expectations of its users in a particular application? Not surprisingly, many of these applications are portable or mobile. Therefore, the device should be small and light in weight; yet, based upon the application, it is likely that the desired image will require high information content and will need to be life-like.
An example is today's high-end cellular telephones, on which photographs, Web sites, news, and even movies will be viewed! Amorphous-silicon (a-Si) displays have evolved over time to incorporate smaller driver ICs with higher densities, enabling higher-resolution formats with fewer components and greater reliability. Improvements have been made in lighting, power, and cell structures.
The most significant improvements have come from the adoption of LTPS and similar glass-processing approaches, such as CGS from Sharp Corp. which increases the electron mobility within the TFTs, thus enabling integration of the driver ICs and even other circuits directly onto the glass substrate.
This integration helps to more easily implement high pixel density and eliminates mechanical connections for greater reliability. For example, LTPS has made it possible to create QVGA (240 x 320) displays for cellular-telephone applications in sizes as small as 1.8 in. on the diagonal, having a pixel density of 223 pixels per inch (ppi). This pixel format is normally used in PDA applications with sizes ranging from 3.5 to 4.0 in. and with densities of 114–100 ppi using a-Si technology. Therefore, it is possible to display the same amount of information in a cellular-telephone display that is only one-quarter the size of a typical PDA display (Fig. 1).
LTPS is also enabling high-resolution displays in mini-PC and sub-notebook applications. LTPS and similar approaches also improve subpixel aperture ratios. The transistors used in every subpixel are smaller than the associated transistor in a-Si TFT displays, due to the greater electron mobility in LTPS displays. Thus, with improved aperture ratios, display manufacturers achieve greater brightness and/or lower power consumption for the same application, typically resulting in greater battery life for the portable/mobile device.
The size and weight of the LCD is a critical determinant of the size and weight of the end-user product that embodies it, and the main contributor to the weight of the LCD is the amount of glass used. Thus, display manufacturers have been working to produce thinner glass for their modules, often using chemical or mechanical glass-processing techniques. In the thinnest displays, thickness and weight have been reduced by as much as 50% or more. These thin displays must and do stand up to the usual rigors of mechanical system testing, but the additional processing steps come with an associated cost premium.
Fig. 3: By using LTPS and system-on-panel techniques, Toshiba Matsushita Display Technology Co., Ltd., built this display with an image-scanning function integrated into the display itself. The device was exhibited at SID 2004.
Making Displays "Green"
With increasing urgency, environmental considerations are driving changes in display technologies. The European Union's adoption of the Reduction of Hazardous Substances (RoHS) Directive, effective July 2006, has driven most display manufacturers to introduce RoHS-compatible displays that have no more than 0.1% by volume of homogeneous lead and other listed materials in its circuits and connections.
Currently, the CCFLs used in display modules contain mercury, but they are exempted from the directive's requirements if each lamp contains less than 5 mg of mercury. But the trend toward mercury-free products is driving the LCD industry toward adopting LED-basedlighting in an increasing number of applications. These applications range from cellular tele-phones and PDAs today to media players, auto-motive displays, notebook PCs, monitors, and even TV sets in the future, as LED technology becomes more prevalent and lower in cost.
The Next Frontier
How will small- and medium-sized displays continue to evolve? Two technologies deserve mention: system-on-glass (SOG) and organic light-emitting diodes (OLEDs).
System-on-Glass. SOG is the evolution of LTPS, CGS, and similar technologies toward greater and greater integration of the peripheral circuitry directly onto the glass substrate. As electron mobility increases even more through processing improvements, we can foresee a day when most of the system will be integrated directly onto the glass substrate.
Some novel approaches have already been demonstrated by TMD and others. For example, at SID 2005, TMD presented a paper describing an advanced LTPS LCD with the ability to capture touch input, much like a touch-screen device, by sensing the shadow image projected by an approaching finger towards the display and its release (Fig. 2). LTPS technology enables us to integrate other functions onto the glass display screen. Com-mercial products may someday use the display to capture images of objects placed on top of it,as demonstrated at SID 2003 and 2004 (Fig. 3).
SOG technology is already used to eliminate some of the peripheral circuitry surrounding the LCD in cellular-telephone applications, thus enabling higher-density displays with fewer electrical connections and peripheral silicon circuits.
OLEDs. OLED displays are emissive displays that do not require a light source. They achieve this by using organic materials that emit light when an electric current is applied. These materials have an extremely fast response time and provide a wide viewing angle. As a result, OLED displays are much thinner and lighter than LCDs, and they eliminate the need for the inverter that is required for CCFL-based displays and the driver circuitry required for LED-based displays. The color saturation in OLEDs yields a vibrant, life-like display (Fig. 4).
To date, adoption has been slow because of the inherent difficulties in bringing this new technology to the commercial market. These difficulties include the introduction of the materials into the display-manufacturing process, and the control and management of the materials over the life of the product. However, some bold inroads have been made both in smaller passive-matrix OLEDs and active-matrix OLEDs used in automotive in-dash applications, handheld appliances, and digital still cameras. Someday it will be possible to adapt OLED technology to a much wider range of small- and medium-sized applications. It is likely that their first appearance in these new applications will be in AV-related applications, where a life-like fast-response video image is highly desirable. •
Fig. 4: The high contrast and saturated colors typical of OLED displays are evident in this 3.5-in. display prototype developed by Toshiba Matsushita Display Technology Co., Ltd.