Displays for portable devices

The strong customer demand for portable gadgets in Japan has triggered research on alternatives to the mainstream liquid crystal modes TN and STN , which, because of low transmittance and thus high power consumption, are more suitable for backlit displays for desktop monitors.

With the goal of bringing reflective display quality close to paper print, brightness, contrast, and color saturation must be improved. Since polarizers effectively cut off 50% and color filters 66% of the incoming ambient light energy, several new modes without color filter or polarizers have been developed. One such mode is guest-host in which an anisotropic dye (guest) is incorporated in a LC (host). The applied electric field reorients both the LC and the dye molecules, which due to its anisotropic absorption will switch between transparent and opaque states. Displays employing the guest-host mode are bright, have wide viewing angle, but slow response. It is therefore mainly used in watches and portable digital assistants ( PDA s)

Another way to switch light is by controlling the amount of scattering in a mixture of polymers and LC droplets. Whereas the polymer matrix is fixed, the LC droplets can be reoriented as usual by an electric field. For certain directions, the refractive indices of the droplets and polymer are matched and light therefore goes through without scattering. On the other hand, orienting the droplets in such a way that the indices are mismatched causes increased scattering and therefore a lower brightness. Displays based on this principle still needs color filters but the brightness is greatly improved over the two-polarizer conventional TN display. Applicatons include projectors and large-size shutters for window glass.

Another type of reflective displays utilize light control via diffraction by changing the LC phase electrically. Cholesteric LC s, for example, have a periodic structure that either selectively backscatters light of a certain wavelength or transmit the remaining ones. Applying an electric field will change the cholesteric phase to focal conic which efficiently scatters incoming light and the display will appear dark. In the reflective mode, the helical pitch determines the wavelength of the diffracted light so cholesteric LCD s appear colored.

The electrically induced phase change is reversible but has a hysteresis with built-in bistability. Therefore, display contents will be maintained even after the power has been switched off. Moreover, Ch LCD s do not require active matrix driving because of the bistability and displays with extremely high resolutions (300+ ppi) are therefore within the reach. There are still, however, several problems that have to be solved prior to commercialization. Switching is slow (typically hundreds of ms) and requires high voltages (typically 40 V) incompatible with battery power. Because of the bistability, means to achieve full color are also an issue.

Ferroelectric LC s ( FLC s) play a special role in LCD s becuase of an intrinsic fast response in the microsecond range. The surface stabilized FLC ( SSFLC ) mode is bistable and does not require any active matrix driving. With the bistability, however, grey scale (or color) must be simulated by spatial dithering. Manufacturing SSFLC devices is very challenging since it requires a laterally homogeneous inter-substrate spacing of less than 2 micrometers. Canon has, nevertheless, commercialized a 15-inch SXGA (1280x1024) display using the SSFLC technology. Meanwhile, Toshiba is working on an active-matrix anti - FLC which does not suffer from bistability and, consequently, not from any lack of grey scale capability. Similarly, Denso has developed prototypes of a passive-matrix AFLC using a partial phase-change mode. Although FLCD s look promising from a matieral point of view, unreliable production, limited operating temperature range, and lower contrast are still issues.

Field Emission Displays

Field emission displays ( FED s) have many similarities with conventional cathode ray tubes ( CRT ). In fact, one company calls its FED product "flat CRT ". As for the CRT , electrons are accelerated in vaccuum towards phosphors which then glow. The main difference is that the electrons are generated by field emission rather than thermal emission so the device consumes much less power and can be turned on instantly. Instead of one single electron gun, each pixel comprises several thousands sub-micrometer tips from which electrons are emitted.

To achieve a low operating voltage, the tips are made of a low-work function material such as molybdenum and is shaped into very sharp tips so that the local field strengths become high enough for even a moderately low gate voltage. The state-of-the-art FED s can operate at gate voltages as low as 12 V. Sucked out of the tips, the electrons are accelerated towards the phosphor screen by either a low or high voltage. A low voltage simplifies the device design but disables the use of highly efficient and mature CRT phosphors. The design problems are rapidly being overcome and the mainstream FED is therefore of the high-voltage type.

Since it is difficult to control the current of each individual tip, the display operates in a saturated mode with each pixel turned either on or off. Thanks to the fast response of the device (ns range), grey scale can be obtained by pulse-code modulation ( PCM ).

Much of the FED research is still focused on suitable emitter matierals which can lower the driving voltage. One of the most exciting ones is diamond which enables field emission at voltages as low as 1-2 V. Manufacturing such tips, however, is a hurdle and the commercial FED s are therefore still using metal tips.

Since the FED is a vaccuum device, atmospheric pressure becomes a severe problem for large-area panels. In particular, internal support posts which prevent the device from imploding, must be thin enough to fit into space between pixels. This together with lifetime issues and bringing down the driving voltage are the main challenges ahead for FED developers.

Typical FED applications include portable ruggedized instruments, pocket video players, mobile videophones, aircraft video displays, and, eventually, laptop computers. Compared to TFTLCD s, FED s are far superior with a wider viewing angle, faster response, higher color saturation, and lower power consumption. Despite this, however, manufacturing and lifetime problems have prevented a full-scale commercialization of FED s as have furious investment in the TFTLCD production capacity, resulting in unhealthy price drops.

Surface conduction emitter display ( SED ) is a new FED -type display type pursued by Canon. Thhe emitter consists of a thin film of ultra-small palladium oxide (PdO) particles which is patterned into narrow gaps (10 nm) where the film has been removed. As electrons are driven in the surface film, they tunnel through the gap, are multiply scattered against the other edge and finally accelerated by the anode voltage. SED devices are not new but emission of previous materials (mainly metals) have proven unstable and thus unsuitable for display applications. SED prototypes have promisingly demonstrated luminances more than twice of that of PDP s at a lower power consumption so Canon is aiming at introducing the technology in consumer products.

Cathode Ray Tubes

Invented in 1897, the cathode ray tube ( CRT ) is still the most common display type. The picture is rasterized by rapidly scanning an electron beam in a vacuum tube whose inner front surface is covered by red, green, and blue phosphors. Electrons generated by a heated electron gun are accelerated towards the phosphors by a static high-voltage field and deflected by magnetic fields which, together with the electron beam current, is controlled by a video signal. As the electrons impact on the screen, phosphors are excited and emit colored light ( See Principle of a cathode ray tube.. )

The CRT is a very simple and matured device and the production costs have been trimmed. It features advantages such as high response speed suitable for high-frame rate, high-resolution video, wide viewing angle, saturated colors, high peak luminance, and high contrast.

However, due to the high-voltage field, oscillating magnetic field, and Bremsstrahlung (X-rays) generated by electrons hitting the screen, the CRT has been regarded as hazardous for long-term use. During the last decade, though, several of these problem have successfully been solved and modern computer monitors are today being designed according to strict environmental standards such as TCO -95.

Another common cause of eye fatigue is flickering which occurs from the short emission life time of the phosphors. An NTSC TV signal with a 30 Hz frame frequency is therefore interlaced at 60 Hz to reduce flicker. Because of the higher resolution of computer monitors, the limited response speed of the video electronics makes it difficult to increase the frame frequency of non-interlaced signals (standard computer output). Recent state-of-the-art video electronics can handle SXGA (1280x1024) at 75 Hz or more, though.

In addition to its size, weight, and high power consumption, traditional CRT s with cylindrically or spherically curved surfaces have suffered from geometrical distortion, particularly at the edges. Recently, however, flat-surface CRT s have been introduced by companies such as Sony, Sharp, and Matsushita (Panasonic) which eliminate such distortion.

Although several of the drawbacks above have been removed by new technologies, CRT s will eventually face problems with high-resolution displays requiring finer pixel pitches. Rather than thinness, lower power consumption and weight, it is for this reason TFTLCD s will be a serious competitor to CRT s.

Vacuum fluorescent displays

Vacuum fluorescent displays ( VFD s) is another display that utilizes thermal emission (640ºC filament) of electrons and phosphor excitation to generate color. In contrast to a CRT , however, the electrons are accelerated by a much lower voltage and the pixels are switched on/off by changing the sign of the electric potential at the target anode. A positively charged target will attract electrons wheras a negatively charge target will repell them. Attracted electrons excite the phosphors which thereby emit light. Contrary to a CRT , the phosphors can be patterned in any shape and VFD s are therefore suitable for displaying icons in consumer electronics. Due to their ruggedness and high luminance, they are also employed in automobile dashboard- and headup displays. The two major companies in Japan pursuing VFD s are Ise Electronics and Futaba.

In prinicple, VFD s could be used in larger displays for monitor applications but since it is a vacuum device, the mechanical construction could be a problem. An advantage over FED s, though, is that no spaces are needed between the pixels.

Plasma Display Panels

A plasma display panel ( PDP ) is essentially a matrix of tiny fluorescent tubes which are controlled in a sophisticated fashion. There are two main types, DC - and AC of which the latter has become mainstream because of simpler structure and longer lifetime. This section treats the AC -type.

A plasma discharge is first induced by the positive period of an AC field and a layer of carriers is shortly thereafter formed on top of the dielectic medium. This causes the discharge to stop but is induced again when the voltage changes polarity. In this way, a sustained discharge is achieved. The AC voltage is tuned just below the discharge threshold so the process can be switched on/off by adding a relatively low voltage at the address electrode.

The discharge creates a plasma of ions and electrons which gain kinetic energy by the electric field. These particles collide at high speed with neon and xenon atoms, which thereby are brought to higher-energy states. After a while, the excited atoms return to their original state and energy is dissipated in the form of ultraviolet radiation. This radiation, in turn, excite the phosphors which glow in red, green, and blue ( RGB ) colors, respectively. Since each discharge cell can be individually addressed, it is possible to switch on and off picture elements (pixels).

To generate color shades, the perceived intensity of each RGB color must be controlled independently. While this is done in CRT s by modulating the electron beam current, and therefore also the emitted light intensities, PDP s accomplish shading by pulse code modulation ( PCM ). Dividing one field into eight sub-fields, each with pulseweighted according to the bits in an 8-bit word, makes it possible to adjust the widths of the addressing pulses in 256 steps. Since the eye is much slower than the PCM , it will intregrate the intensity over time. Modulating the pulse widths will therefore translate into 256 different intensities of each color. The number of color combinations is therefore 256 x 256 x 256 = 16,777,216.

At first, PDP s had problems with disturbances caused by intereference between the PCM and fast moving pictures. By fine-tuning the PCM scheme, however, this problem has been elliminated.

While PDP s are relatively light weight and can be manufactured at a thickness of 3-4 inches, they still consume prohibitively much power. The luminous efficiency, i.e. the amount of light for a given amount of supplied electric power, is still at approximately 1 lm/W, about 10% of some other FPD technologies. Also, the discharge process causes sputtering of the cells which inevitably reduces life time. With a new protective dielectric layer of MgO, however, this problem has largely been solved.

Despite these problems, PDP s are promising because of their modest requirements on manufacturing technology. Compared to TFTLCD s, which use photolithographic and high-temperature processes in clean rooms, PDP s can be manufactured in less clean factories using low-temperature and inexpensive direct printing processes. Also, PDP s feature wide viewing angles, no susceptibility to magnetic fields, and are easy to scale up for wall-hanging TV applications.