Chapter 5 Experimental Results and Discussions
5.3 Future Application - Transflective Ch-LCD
Purely reflective displays have poor performance in low ambient lighting conditions, consequently, auxiliary lighting is required. A transflective display is usually the most elegant solution. A transflective display can be used in a transmissive mode with a backlight and also in a purely reflective mode without the backlight. It is well known that bistable Ch-LCD is usually used in a fully reflective mode. However,
with proper design, Ch-LCD can be used in a fully reflective mode as well as a fully transmissive mode. This greatly enhances the usefulness of the cholesteric display technology by enabling its use in low ambient lighting conditions.
The schematic illustration of the transflective Ch-LCD is shown in Fig. 5-29.
Fig. 5-29. Schematic illustration of the transflective Ch-LCD.
R+G+B
The operation principle of the transflective Ch-LCD has already stated in chapter 2, thus we only show the measurement results here. We use standard CCFL as the backlight. The brightness distribution of the backlight and transmitted light brightness of the transflector are shown in Figs. 5-30. (a) and (b), respectively. We can find the peak brightness of the backlight is about 2400 nits and transmitted light brightness of the transflector is about 500 nits. Therefore, the transmittance of the transflector is about 20%.
(a) (b)
Figs. 5-30. (a) Brightness of the backlight and (b) Transmitted light brightness of the transflector.
In reflective mode, the backlight is off and the display is just like reflective Ch-LCD as described before. We use UV band Ch-LCD sample here. The measured brightness and viewing angle distribution is shown in Fig. 5-31.
Viewing angle(°)
Brightness (nits)
Viewing angle(°)
Brightness (nits)
Fig. 5-31. Brightness distribution of reflective mode.
From the result, we find the brightness of the reflective mode is lower than pure reflective Ch-LCD as measured before, due to that the transflector film is not a perfect reflector. Thus, some light transmits through the film instead of reflection by the film.
However, in bright ambience, the brightness of the display is sufficient.
The C.R. of the display in reflective mode is about 2, as shown in Fig. 5-32.
Viewing Angle( °) Viewing Angle( °)
C.R.
Viewing Angle( °) Viewing Angle( °)
C.R.
Fig. 5-32. C.R. distribution of reflective mode.
In dark ambience, the display works in transmissive mode, and the backlight turns on. When the display is in the planar state, the light from the backlight is reflected by the Ch-LC of planar state. The light is then absorbed by the polarizer.
When the display is in the focal conic state, the light from the backlight is weakly
scattered by the Ch-LC of focal conic state and emerges from the display. The result is a bright state from the focal conic state and a dark state from the planar state. The measured brightness of bright and dark states are shown in Figs. 5-33. (a) and (b), respectively.
nitess (ghtnBr
(a) (b) Figs. 5-33. The brightness of (a) bright and (b) dark states.
From the measured results, the display can provide higher brightness than reflective mode. The measured C.R. of the display is shown in Fig. 5-34. The C.R. of the transmissive mode is about 2.
Fig. 5-34. The C.R. of the transmissive mode.
Based on the measurement results, we find the brightness and C.R. of the transmissive mode are close to the reflective mode. However, purely reflective
Viewing angle(°)
is)
Viewing angle(°)
is)
Viewing angle(°)
Brightness (nits)
Viewing angle(°)
Brightness (nits)
ess (nitghtnBr
Viewing Angle(°)
C.R.
Viewing Angle(°)
C.R.
Ch-LCD has poor readability in dark ambience. The transflective Ch-LCD can provide accept performance in dark ambience. The photographs of reflective mode and transmissive mode are shown in Figs. 5-35.(a) and (b), respectively.
(a) (b) black plateblack plate
Fig. 5-35. The photographs of (a) reflective mode (b) transmissive mode.
The photographs are taken in zero field conditions. Fig. 5-35.(a) is a photograph taken of the reflective mode in conventional office lighting. Fig. 5-35.(b) is a photograph taken of the transmissive mode in a nearly dark room. The photographs demonstrate that the transflective Ch-LCD yields sufficient brightness in both situations.
5.4 Summary
After the whole fabrication of the Ch-LCD test cells, a measurement system
“ConoScope” was utilized to measure the reflective spectra, brightness and contrast ratio. Based on the measurement results, Ch-LCDs with UV band or infrared band LC materials have wide band reflection in bright state. Therefore, the black and white reflective Ch-LCD can be demonstrated. The C. R. of Ch-LCD with infrared band LC material is about 10~15. The contrast is sufficient for reflective display applications.
In term of driving voltage, Ch-LCD with infrared band LC material has low driving voltage of 22V, which is much lower than conventional Ch-LCD’s. The infrared band Ch-LCD has advantages of high contrast and low driving voltage. Besides, the
transflective Ch-LCD can yield reasonable contrast in both the transmissive mode as well as the reflective mode. This extends the cholesteric display technology to enable its use to low ambient lighting conditions.
Chapter 6 Applications
6.1 Introduction
As the Internet and computer related industries growth continues, the electronic publishing is a huge market. People can download articles or books from the Internet.
However, everyone wants to read them like a real book instead of the images on a bulky monitor.
The electronic book (e-book) is a reading device, as shown in Fig. 6-1. This portable reading machine is mostly employed to read all types of electronic publications like news, books, textbooks and all Internet related information.
Fig. 6-1. Photograph of latest black and white reflective Ch-LCD[35]. Nowadays, it is known that handheld devices, such as e-books, palm PCs, mobile phones, etc., are high volume markets. All kinds of handheld devices are battery operated. The power consumption is one of the key issues.
6.2 E-Book Application
As a portable electronic product, low cost and good performance are of first important. Low power consumption is the second important issue. Comfortable holding and easy carrying is another important requirement. In order to meet all the basic requirements, bistable reflective Ch-LCD is the best solution to e-book application. Due to the image-memory of Ch-LCD, the e-book based on it will not only be the lowest cost for CPU and its peripheral components, it will also consume extremely low power.
The e-book is usually made up of the CPU, the memory, the display, the battery, and some peripherals like AC link (include touch screen, audio), the PC link, USB, etc.
The display is the key component for e-book. A low power consumption display can dramatically reduce the battery size. The image-memorized Ch-LCD makes it to be one of the best candidates for e-book due to its real low power consumption and its paper-like static image without any flicker.
The e-book should have such features as portability, superior readability, and lightweight. The built-in Internet connection allows you easily to download documents. The e-book can hold up to 100K pages on removable memory card.
Reflective Ch-LCD is the most power saving display and one of best performance displays due to its memory effect. As discussed above, Ch-LCD maintains the images under zero external applied voltage. The power is consumed only when the images are changed. The energy dissipation per update is about 50mJ for VGA resolution and less than 100mJ for 720×720 resolution. When you view the images 10 seconds longer or more, the other displays like STN-LCD or AM-LCD will consume 20 more times energy than Ch-LCD. If you read the VGA image on Ch-LCD spending one minute, you can save 250 times the energy of other refreshing reflective LCD consumes.
0 6.3 The Driving Scheme of E-Book
The driving voltage is an important issue for applications. The electro-optical response of Ch-LCD is shown in Fig. 6-2. The curves are obtained under zero field condition after driving.
Fig. 6-2. Electro-optical response of Ch-LCD.
We use three pulses to drive Ch-LCD, which is passive matrix driving method.
A 32V high voltage pulse with 2ms wide is applied to drive the whole display to the field induced nematic state. After a short relaxation, a low voltage pulse about 12V is applied to drive the display to an incomplete focal conic state. We call these two stages as image erasing. Right after image erasing to dark state, we address the display row by row by using a 32V high pulse with about 0.3 ms wide to planar
“bright” state and a 24V pulse with 0.3 ms wide to focal conic “dark” state.
Threshold voltage Vt is defined that the LC can change from planar state to focal conic state. At the high voltage pulse Vr, the LC is driven to the nematic structure by the end of the pulse but then relax quickly to the planar reflecting state when the voltage is suddenly dropped to zero. The low voltage pulse Vs is to drive the LC to incomplete focal conic state after image erasing to a focal conic dark state. The three pulse driving method can not only be easily implemented by standard STN driver, but also achieves high addressing speed.
6.4 Full color cholesteric displays
Full color displays are more appealing to user. Conventional full color Ch-LCD was fabricated by stacking three layers of RGB (Red, Green and Blue) colors of cholesteric cells, as described in chapter 2. However, there are some drawbacks of the method. Stacking three cholesteric cells results in the device thick and heavy and parallax will decrease the resolution.
Our approach to full color application is to achieve a broad band reflection covering the entire visible spectrum, i.e., from 450 to 650 nm. We used full spectrum reflective method to realize a black and white reflective cholesteric display. Since the reflected light is white, so we can pattern conventional color filters for obtaining full color displays, as shown in Fig. 6-3. The fabrication processes are compatible with conventional process.
Figs. 6-3.(a) Structure and (b) Configuration of full color Ch-LCD.
6.5 Summary
Finally, we summarize the advantages of Ch-LCD e-book products.
1. Long-term image memory.
2. Low power consumption.
3. Real static images: No flicker.
4. Lightweight.
5. Wide viewing angle: ±80° can be achieved.
6. Low cost.
7. No image parallax.
8. Excellent sunlight readability.
However, full color ability and low operation voltage are important issues for future application. For full color application, we proposed full spectrum reflective method to achieve a black and white cholesteric display. With wide band reflection in bright state, the reflected light is white. Therefore, by conventional color filters process, full color reflective cholesteric displays can be realized. Besides, high operation voltage about 50V is a drawback of cholesteric displays. From the experimental results, we find the operation voltage of infrared band cholesteric LC can be below 25V, thus the display can be compatible with standard driver IC. As a result, the low operation voltage full color cholesteric displays are more suitable for E-book application.
Chapter 7
Conclusions
As the Internet and the computer related industries growth continues, the electronic information display is an important technology. High brightness, high readability, wide viewing angle, low power consumption and high color saturation are the main concerns. Low power consumption and light weight are main advantages of reflective LCDs. Among all reflective LCDs, bistable reflective Ch-LCDs are best solution to e-book application due to the merits of lower power consumption, low cost, and good readability. However, due to the limitation of cholesteric LC materials, the reflective spectrum is narrow band. Therefore, the display is usually monochromic appearance, which often can not satisfy the user’s requirement. Black and white displays are the least desired for viewers. In order to solve the problem of narrow band reflection, a new method “Full Spectrum Reflective Method” is proposed. The characteristic of this method is to use two reflective spectra: one is the spectrum of cholesteric LC, the other is the spectrum of reflector compensating each other to broaden the spectrum of the display. Wide band reflection can display white images instead of monochromic images. Besides, the dark state is created by cholesteric’s scattering effect in focal conic state and polarizer’s filtration effect. Therefore, black and white reflective cholesteric LCD can be demonstrated. A typical LCD cell process was utilized to carry out the Ch-LCD test cells due to its convenient manufacturing process. Finally, “ConoScope” was utilized to characterize the properties of the fabricated test cells and compared with simulation results.
In the simulation, we established a simulation model by LCD simulation
software “DIMOS” used to characterize the features of the reflective Ch-LCDs. We utilized green band, UV band, and infrared band Ch-LC materials to optimize the optical properties of Ch-LCDs. From the simulation results, Ch-LCD with UV band Ch-LC can be wide band reflection, thus, enabling a black and white display.
In the experiments, “ConoScope” was utilized to measure reflective spectra, reflectance, contrast ratio, viewing angle and voltage response. In term of reflective spectra, UV band Ch-LCD has the widest reflective spectrum and high reflectance of 50%, which agreed with the simulated results. In term of contrast ratio, infrared band Ch-LCD has the highest C.R. of 10, which is much higher than conventional reflective Ch-LCD’s. Therefore, the proposed method can improve the image quality of Ch-LCD. Besides, high operation voltage about 50V is the drawback of conventional Ch-LCD’s. However, by using infrared band Ch-LC material, the operation voltage can be decreased to 25V, which can be implemented by standard STN driver. As a result, the cost and manufacturing yield can be improved.
We used full spectrum reflective method to achieve wide band reflection covering the entire visible spectrum to realize black and white reflective cholesteric displays. For full color applications, since the reflected light is white, so we can pattern conventional color filters for obtaining full color displays.
Finally, it can be concluded that UV band or infrared band Ch-LCDs with full spectrum reflective method can have advantages of wide band reflective spectra, high contrast and low driving voltage. The Ch-LCDs are suitable for E-books applications.
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