Optical Performance Considerations
When designing an LCD panel, some of the most important choices are made with respect to the optical performance. In mobile applications, these choices are also impacted by the amount of power available for backlighting.
The key choices are:
With LCDs, the light is typically generated by a cold cathode fluorescent tube (CCFT) — especially for larger size panels. White LEDs are increasingly preferred for smaller sizes. The light is channeled through an optical light guide and then through a series of diffusers. Various layers are placed inside the LCD structure to control the cone at which the light emerges.
For displays that do not require wide viewing angles, you have the option of directing all of the light on axis. In doing so, you can drive it with far less power. Most of the power in a display system is consumed by the backlight, so the efficiency and methods of utilizing light are crucial in determining the power requirements.
Power consumption is also affected by thickness. The thicker backlights are more efficient as light collectors. This determines the numerical aperture of the optical system. The acceptance angle of an optical system determines how much light you are able to collect. Using a CCFT, most of the light that is generated by the tube — even if you put a reflector behind it — is lost. Some of it simply misses it by going right past it, and some is reflected from that first surface between plastic and air, and some part is collected. So the biggest reduction in light (loss of power) is getting the light into the light guide. There are additional losses in every other part of the system — at every air-to-plastic interface, ~4% of the light is lost. One has to start with an enormous amount of light just to get enough going into the display.
There are further losses in the display itself. Each display has two polarizers with light losses. The theoretically ideal efficiency of polarizers is only 50% and the reality is much less. Inside a TFT display, the transistor and buss bars are opaque. The amount of area known as the aperture ratio - the ratio of the open aperture to the opaque areas — is a critical factor in terms of the amount of light throughput. Since the light can reflect off the front off the transistors and buss bars to your eye, the contrast ratio is also determined to a large extent by the aperture ratio. One can put in a black absorbing layer on top of those reflective structures to further enhance these devices, but every step, every layer, every change in optical index is another place for light to adversely reflect from the front or cause a loss in efficiency of light getting to the eye.
For higher resolution displays, more of the space is used for transistors and buss bars. It is always more difficult to get higher contrast ratios and better transmissivity with higher resolution formats. When wide viewing angles are also required, it is especially difficult. One must put light in a wide cone at the same time you have substantial losses coming through the panel.
The technologies that achieve the widest viewing angle in terms of uniformity of color — today’s Advanced Super View technology – does not work absolutely uniform from edge-to-edge of the pixel. The result is some loss of transmissivity in these panels, so more light is required. Film compensation techniques require less light, but achieve lesser viewing angle improvements.
To achieve very high contrast ratios, some of the new Advanced Super View displays are normally black — meaning when no field is applied, these displays will look black. Conventional amorphous Silicon TFT displays are normally white. (Almost all Sharp’s competitors produce only normally white displays). By going to a normally black display, it is also much easier to achieve zero bright pixel defects.
Advanced Super View is much more tolerant of cell-gap non-uniformities than a traditional TN display, so we can get much wider viewing angles and much higher contrast ratios, and uniformity of blacks. What we give up is transmissivity — you need a bigger backlight behind it, requiring more power and overall thickness of the envelope.
|