Scanning Backlight

When moving picture is displayed on conventional LCD, the human eyes perceive blurring resulted from the slow response time of the liquid crystal and hold-type backlight [2-5]. Even if the speed of response is near to zero, the fuzzy edge of the object still exists due to the hold-type property of LCD. This is the main reason why CRT has no create blurring image by the impulse-driving mode as shown of Fig.

1-3 (a). Since CRT is impulse-type, the moving image will be exhibited on the center of the retina, and the edge sharpening can be obtained [6]. In other words, making integral of the moving picture in hold-type LCD by human eyes will perceive the blurring image. It is the reason that the human eyes can track the moving object simultaneously. In reality, the movement of the object and the eyes tracking are both continuous. If the object is presented on the hold-type mode display, the movement of the object will lead to the discontinuousness. It will stay on a specific fixed spot during each frame time. Therefore, under the interaction between these two movements, a continuous, and a discrete, the motion blur happens.

(a) (b)

Fig. 1-3 (a) impulse-type display (b) hold-type display.

CRT (impulse-type display)

(a) (b)

Fig. 1-4 (a) The left side is the situation on the impulse- type display. The moving of black box and real image on the screen can exhibit clear edge. (b) The right side shows the hold-type display. The fuzzy edge occurs during object moving.

Fig. 1-4 shows that the black block horizontally moves along the x-direction. In impulse-type display, the moving of black box exhibits the sharpness edge. If the rapid motion of cinematic image is presented on impulse-type display, the clear image can be observed. However, in the similar case, the fuzzy edge occurs in the hold-type display and the effect of motion blur appearing in LCD will influence the entire quality of display.

In order to overcome the motion blurs, two impulse-driving methods are proposed:

the black frame insertion and the blinking backlight. In the black frames insertion method as shown in Fig. 1-5, the black image is inserted continuously following the real image in each frame, and the backlight is kept constantly on state. Inserting the black frames between each image frame will reduce motion blur. Nevertheless, since the light leakage of LC leads to insufficient dark state [7-10], that will result in lower contrast ratio (CR).

(a) (b)

Fig. 1-5 (a) The black-image-data insertion method and (b) the timing chart in black-image-data insertion method.

The other approach, the blinking backlight system, is realized by iterative on / off state of the backlight. The schematic of the blink backlight system model is shown in Fig. 1-6(b) [11]. The region (1) and (2) indicates the TFT addressing time and the LC response time, respectively. The lamp is turned on following TFT addressing and LC operation, and then turned off before the next frame. The blinking backlight emulates the impulse-type emission like CRT, whereas CCFLs are kept on / off simultaneously.

(a) (b)

Fig. 1-6 (a) The blinking backlight method and (b) the timing chart in blinking backlight method.

Image data

Backlight Time

Image data

Backlight Time

Since the operating time of LC in whole screen is insufficiently fast, the luminous span of the lamps is limited. In other words, the image can not obtain sufficient illumination to perform high brightness level. In order to overcome this inadequate situation, scanning backlight is introduced to improve LC response in a simple way, and do not require overdriving backlight scheme to maintain luminance and contrast ratio.

(a)

(b)

Fig. 1-7 (a) Scanning backlight working sequence during one frame time, and (b) the addressing, response and illumination in the scanning backlight.

As shown in Fig. 1-7(a), the direct scanning backlight system is accomplished by changed in the same direction with the vertical position of the screen. Each lamp was turned on after LC cells are saturated in corresponded region. Each lighting region does not wait the setup of the entire LC of the panel, it can only delay for its corresponding regions of LC that are saturated. Therefore, the scanning backlight can redeem the problem that is brought by the slow response time of the LC.

1.3 Field Sequential Color LCD

In the modern digital multimedia lifestyle, all kinds of information are requested to render high-quality and high-density. Therefore, the high resolution is required to exhibit the vivid image. Displaying color in conventional LCD has been realized by using three kinds of the color filters to lead the Red, Green, and Blue (R, G, and B) colors pass through. However, it is difficult to accomplish high-resolution LC panel by means of the conventional color-filters (CFs) type LCD due to the CFs hinder the resolution of the LCD from high-resolution. The other concerned issue is the optical efficiency that CFs absorb about two third of energy. Using CF-free display is one of the solutions to improve the optical efficiency and resolution. Moreover, the FSC can be expected to have three times higher resolution than that of the same technology applied to conventional CFs type displays [13-16]. However, the switching speed of CCFLs is not fast enough for the pulsed operation, which is required for the field color sequential scheme.

Using LEDs as light source is suitable for FSC backlight application due to the response time is sufficiently rapid to switch the different color state. Moreover, the emission spectra of R, G, and B LEDs are narrower than the color filters, as shown in Fig. 1-8(a). Based on the CIE 1931 chromaticity diagram, shown in Fig. 1-8(b), the area of the triangle matched by the FSC LCD can exceed 120% NTSC (National Television Standards Committee), which is anticipated to be better than that of current CCFL LCD.

However, the trigger time of each pixel in FSC LCD should be driven three times higher than conventional display, because each frame is composed of three sub-frames.

Assuming that the frame frequency is 60 Hz, each sub-frame should be less than 1/180 sec (5.5msec) for TFT addressing, waiting for LC rotating, and LED flashing as shown the timing chart of sequential color LCD in Fig. 1-9

(a) (b)

Fig. 1-8 (a) LED emission spectra and corresponding color filter spectra (b) CIE color.

Fig.1-9 Timing chart in FSC LCD with TFT address, LC response time, and backlight lighting time.

(a)

(b)

Fig. 1-10 (a) The mechanism of CBU. (b) Stationary image in FSC display perceived with CBU by eye motion.

Viewing direction of eyes

In contrast to the advantages of FSC LCD, however, waiting TFT address and LC response time cause the shorter illumination time and color break-up (CBU) which degrades display quality. As resemble in motion blur, if a displayed object is moving on an FSC LCD, the edge of the object will appear rainbow colors. Fig. 1-10 exhibits the mechanism of CBU during eye tracking movement.

1.4 Field Sequential Color with Scanning Backlight

To improve the CBU and decrease the required LC response time, a special configuration of backlight is proposed, i.e., a FSC scanning backlight. The conventional FSC backlight sequentially transforms the color states in order of R, G, and B color in each frame, and the entire screen exhibits the same color state at the same time. Furthermore, the traditional scanning backlight is aimed at motion blur, and the light leakage would not introduce an issue because the color state of edge of moving object does not change. Nevertheless, the FSC scanning backlight, the color state is scanned from up to down for whole BLM. If the whole emitting surface of the light guide is divided into 10 portions to illuminate, and the timing relation between the TFT addressing, LC response, and LED flash is shown in Fig. 1-11(b) [17]. If we take the instant moment as a dotted line, it will be important to notice that the two different color states appear on the screen simultaneously. It is obvious that light penetration will occur inside light guide, and that will cause the image color distortion as shown in Fig. 1-12(a).

(a) (b)

Fig. 1-11 (a) FSC scanning backlight system. (b) Time relation on scanning of LC TFT array and LED backlight.

(a) (b)

Fig. 1-12 FSC scanning backlight system (a) without partition; (b) with partitions.

In view of this, each scanning partition of FSC scanning backlight application should be divided into isolating segments to avoid the color mixing error as shown in Fig. 1-12(b). Because the sharp discontinuity between divisions may lead to the unacceptable image, the control of light leakage between each partition is necessary.

Accordingly, light exiting from each partition should be highly directed in order to ensure possibly smallest interference to neighboring divisions.

dotted line

1.5 Motivation and Objective of the thesis

The development of fast-response LCDs, which consists of optically-compensated-bend (OCB) mode LC and high efficiency LEDs for providing three primary colors, have accelerated the accomplishment of the scanning FSC LCD [18-20]. However, this side lit approach is unsuitable for developing the large scale FSC scanning backlight system. In order to maintain the sufficient illumination, the previous researches used the directly view BLM to execute a large size FSC LCD as shown in Fig. 1-13 [21]. Nevertheless, the essential color mixing space should be provided in this mechanism. Moreover, the required partitions are formed by means of shields. The indispensable mixing distance leads the size of display not compact enough. Furthermore, the shields which produce isolating blocks may bring about the discontinuous illumination. The issues of mixing color interval and shields are needed to be further discussed.

Fig. 1-13 A top view of directly type of FSC scanning backlight.

The purpose of the thesis is to establish a large size configuration in FSC scanning backlight system, which can define each partition without shields as well as reduce the mixing color space.

1.6 Organization of this Thesis

The thesis is organized as following: the principle of the components, light guides combine with prismatic micro-bump structures, and the optical theories will be presented in Charter 2. In Chapter 3, the specification of the proposed module will be given, and the simulated optical efficiency and light leakage will be obtained.

Furthermore, the fabrication process for micro structure, the method of measurement, and the experimental results will be obtained in Chapter 4. Finally, the conclusions and the future works will be described in Chapter 5.

Chapter 2

Principle

2.1 Radiometry and Photometry 2.1.1 Radiometry

Radiometry is a science which is used to measure the radiation and principally deals with the radiant energy of any wavelength. No matter what kinds of optical system design, the purposes are receiving and transmitting the radiation or communication energy. Therefore, the radiation should be quantified and defined clearly. The fundamental radiometric quantities are shown in Table 2-1.

Table 2-1 Radiometry Quantities

The Q denotes the propagating energy of electromagnetic radiation, and its basic unit is the Joule. Sometimes, the amount of photons is also defined as the radiation energy, and the energy of a single photon ish .

The radiant flux is the rate of flow of the energy with respect to time, dQ/dt, and the unit is Watt (W). The recommended symbol for power is φ.

The radiant intensity is power per unit solid angle, and the unit is W/sr. One

an area on the surface of the sphere equal to that of a square of side of length equal to the radius of sphere. The intensity is the derivative of the power with respect to the solid angle, dφ/dw. The symbol is I.

The radiance is power per unit projected area per unit solid angle, and the unit is sr incident from all direction in a hemisphere whose base is that surface. The symbol is E, and it is the derivate of the power with respect to area, dφ/dA. A similar quantity is radiant exitance, which is the power per unit area leaving a surface to a hemisphere whose base is that surface. The symbol is M.

2.1.2 Photometry

Photometry is the measurement of light which is defined as electromagnetic radiation that is detectable by the human eye. This range corresponding to wavelength is 380 to 830 nanometer (nm). Unit symbols are subscripted with v to denote visible, and unit names are prefixed with the term luminous. The unit of luminous flux (φ ) is _v called a lumen (lm). The fundamental photometric quantities are similar to the radiometric as shown in Table 2-2. The only difference between radiometry and photometry is that the radiometry includes the entire optical radiation spectrum, while photometry is limited to visible spectrum as defined by response of human eye.

Fig.2-1 shows the both measurement system.

Table 2-2 Photometry Quantities

Fig. 2-1 The unit of radiometric and Photometric.

From the lumen definition, there are 683 lumens per watt at 555 nm. This is the wavelength that corresponds to the maximum spectral responsivity of human eye. The conversion from watts to lumens in any other wavelength involves the product of power (watts) and the V(λ) in the wavelength of interest. The luminous flux in any wavelength can be calculated by the following equation:

Where φ is the luminous flux, _v φ_λλ is the corresponding radiant spectrum of radiation source, and V_() is the photopic spectral luminous efficiency function as shown in Fig. 2-2.

Fig. 2-2 Human visual response function.

One thing to watch out for is that the radiometry measurement system uses the same units of Watt as used for electric measurements. In both measurement systems, the term Watt refers to the energy per unit time generated by radiation source. Thus, it is possible to define two different measurements with the same units. The term radiant efficiency, with units W/W, refers to the energy conversion efficiency of radiation source in converting electrical power into radiant flux. The term luminous efficiency, with unit lm/W, refers to the energy conversion efficiency of the light source in converting electrical power into luminous flux. The term luminous efficiency, with unit lm/W, refers to the ratio of luminous flux to radiant flux generated by the light source.

λ – wavelength (nm)

) ( V

2.2 The light pipe concept

The function of the light pipe transforms a light source into a desired light distribution. When light is propagating inside the light pipe by total internal reflection (TIR), the pipe surface can selectively covered the outcoupling zone whose function consists in extracting part of light out from the pipe. The shape of the pipe may change depending on the desired optical operation and the object to be illuminated.

Fig.2-3 shows different kinds of the light pipe applications: the light guide plate and the circular light pipe.

(a) (b) Fig. 2-3 (a) Light guide plate. (b) Circular light pipe.

The following subsections will introduce and evaluate the characteristics of different types of the outcouplers. These outcouplers are classified by their working principles.

2.2.1 Surface & Volume scattering light pipes

It is well known that the polished light pipe propagates the light by TIR without any optical losses. However, any regional defects of the surface will cause the optical scattering. Actually, the light is partially coupled out from the pipe by the surface roughness which can be generated on purpose in specific location by manufacturing process as shown in Fig. 2-4(a). The main drawback of the outcoupling by surface

As shown in Fig 2-4(b), when the light guided in the light pipe meets the volume scattering section, part of light will not respect the TIR anymore and then they couple out from the light pipe. This volume scattering results from the refraction of the light generated by particles which are poured into the pipe by fabrication process. The illumination should be theoretically controlled by modulation of doping the density of the particles. However, the technical limitation of the fabrication will not allow an exact control of doping distribution. Therefore, the uniformity of the illumination will be difficult to obtain in the volume scattering light pipe.

(a) (b)

Fig. 2-4 (a) Surface scattering light pipes (b) Volume scattering light pipes 2.2.2 Variable cross-section light pipes

The variation of the pipe cross-section can be implemented and classified by continuous or discontinuous cross-section light pipe. The discontinuous cross-section type is illustrated as Fig. 2-5(a). The flux coupled out from the pipe is directly size of the respective cross-sections. The partition of extracted flux is determined as

i propagation angle α of the guided ray is changed at each intersection with the inclined angle. When the propagation angle α becomes smaller than critical angle, the ray will not respect the TIR condition anymore, then the ray is extracted from light pipe.

If the inclined angle of the pipe surface and the incident angle are defined as and

1

i , respectively. Then the new propagation angle_iis defined as





_i  _i1 2 (2-4)

(a) (b)

Fig. 2-5 (a) Discontinuous cross-section light pipes; (b) continuous cross-section light pipes

2.2.3 Refractive & Reflective Outcouplers

The principle of the refractive outcouples consists in placing discontinuities along the pipe surface in order to locally exceed the TIR condition. In such situation, the light escapes from pipe by refraction as shown in Fig. 2-6(a). Where α is the incident angle and ρ is the inclination of refractive surface. The indices of the refraction of the PMMA and air are defined by n_pmma and n_air. Accordingly, the exit angle β can be

The relationship between exit angle, incident angle, and incline angle is shown in Fig.2-6 (b). The gray region covers the incident angle which relates to the light guided by TIR in the pipe. Furthermore, it is obvious that the light extracted from the pipe appearing large incline angle distribution. It is not practical for general light pipe which requires to illumination normal to the surface. Therefore, this kind of approach usually demands a correction plate to collimate the direction of light.

(a)

(b)

Fig. 2-6 (a) Refractive type outcouplers; (b) exit angles achieved by refraction for different orientations of the refractive surface.

The same to the refractive approach, the reflective outcouplers use discontinuities

along the surface of light pipe which deflects the guided ray out from the pipe. Two kinds of the reflective micro prism can be selected: coated micro-prisms and the TIR micro-prism. As illustrated in Fig. 2-7 (a), the reflective coated micro-prism is placed along the pipe surface to deflect part of guided light. The incident angle and the prism orientation are defined by α and ρ, respectively. Therefore, the exit angle can be

Fig. 2-7 (a) Outcoupling produced by a reflective micro-prism (b) Exit angles achieved by reflection for different orientations of the reflective micro-prism.

orientations of the micro prism. Different with the refractive case which has grazing angle of the extracted light, the reflective coated micro-prisms approach is more flexible. Therefore, for given any incident angle, there exists a better corresponding orientation to get the desired outcoupling distribution. However, the metallic coating on working face of prism is difficult to be done and the metallic absorption affects the optical efficiency of outcoupling rays. The cumulative optical losses will significantly deteriorate the optical efficiency of the light pipe.

The TIR micro prisms have an advantage that will not require any coating.

Moreover, TIR will not cause any optical losses. However, the TIR only occurs when rays reaching the prism which incident angle is greater than critical angle as

c obviously that the shadowing effects increase for grazing incident angle (α > 90). A

c obviously that the shadowing effects increase for grazing incident angle (α > 90). A