Chapter 2 Principles of backlight system
2.5 Summary
Ray-tracing method was used to design backlight system in this thesis. Law of reflection and reflection, TIR, and Fresnel’s law, can describe the propagation trajectory and the carried power of light. Thus, the optical software, LightTools, based on the ray-tracing method was used to simulate a backlight system. We then created optical film models which can be characterized by BSDFs of optical films. In order to evaluate the degree of lamp-Mura, the lamp-Mura contrast was defined. The lower the lamp-Mura contrast, less lamp-Mura can be observed. Besides, we found that BEF enhanced brightness but did not improve lamp-Mura well, because the triangular prism structure of BEF had no scattering ability. Therefore, an optical film which integrated brightness-enhancement and light-scattering functions will be designed to suppress lamp-Mura.
Chapter 3
Design and simulation of MPF
A multi-performance film (MPF) with concave-prism structures was designed to replace a conventional brightness enhancement film (BEF) and to improve the optical performance of backlight systems. For saving optical films and suppressing lamp-Mura, two backlight structures with MPFs were designed. By using the simulation software, LightTools, simulation models of real objects were built. Additionally, the curvatures in MPF and MPF-Plate case were optimized to suppress lamp-Mura.
3.1 Design of MPF
The multi-performance film (MPF) was modified from a conventional BEF. As mentioned in Chapter 2, if a non-uniform light distribution was input to a conventional BEF, the oblique incident light could be redirected to enhance brightness at normal direction. But normal incident light was blocked by the prism structure according to the total internal reflection (TIR), and the luminance above lamps was decreased. Additionally, the lamp-Mura still existed after adding the BEF. If the relative maximum and minimum luminance after a BEF can be balanced, lamp-Mura will be improved. Therefore, a multi-performance film (MPF) with concave-prism structures was proposed to replace the original BEF for improving lamp-Mura, as shown in Fig. 3-1. A parabolic curve was proposed as the profile for concave-prism structures in this research. The formula of the parabola is:
2
0 C x
z
z = + , (14)
where z is the vertical distance from the substrate, x is the horizontal position, C is curvature of the parabola and z0 is the vertical shift distance to meet designed prism width.
Fig. 3-1 Schematic of MPFs.
Ray-tracing results were used to describe light propagated though BEF and MPF, as shown in Fig. 3-2. When incident light was input into MPF, concave-prism structures of MPF caused a light divergence which is similar to the scattering property of diffusers. The oblique incident light also was redirected to normal direction to enhance brightness. Therefore, MPF simultaneously included the scattering function as a diffuser and the brightness enhancement function as a conventional BEF. Moreover, the normal incident light partially passed through MPF to enhance normal brightness, and the luminance at the center between two lamps was still remained as a BEF. Thus, we conclude the luminance difference between the maximum and the minimum can be reduced by MPF; on the other hand, the uniformity can be increased, as shown in Fig. 3-3.
(a) (b)
Fig. 3-2 Ray-tracing diagrams of (a) conventional BEF and (b) MPF.
Fig. 3-3 Schematic of backlight luminance distribution by using a MPF.
3.2 Experiment backlight structures
For the propose of saving optical films and of suppressing lamp-Mura, two experiment backlight structures, MPF and MPF-Plate case, were designed to compare with the conventional backlight structure, as shown in Fig. 3-4. For designed MPF case, a MPF and an acrylic transparent plate replaced the BEF III and the highly efficient diffusive plate (HEDP) in the conventional backlight system, respectively. The advantage of MPF case is that the transparent plate has lower cost than the diffusive plate. However, several interfaces of optical films resulted in Fresnel loss to reduce the transmission light efficiency. Therefore, the MPF-Plate, which integrated the MPF and the diffusive plate, was designed to enhance the
output light efficiency. In the following simulation, curvatures of MPFs will be optimized for suppressing lamp-Mura.
(a) Conventional backlight
(b) MPF case
(c) MPF-Plate case
Fig. 3-4 Schematics of experiment backlight structures.
3.3 Specification of demonstrated backlight system
For optimizing MPF in the backlight structure, a common LCD-TV backlight system was adopted to be our design platform, whose specification is shown in Table 3-1. The backlight system had 32-inch display area and 12 lamps on the bottom as shown in Fig. 3-5, and the horizontal and vertical directions were also defined. The specification then was imported to the simulation software to model real objects.
Table 3-1 Specification of demonstrated backlight system.
Item Value
Backlight size (Inch) 32
Backlight thickness* (mm) 20
Number of lamps 12
Lamp pitch** (mm) 33
Lamp height***(mm) 3
Lamp diameter (mm) 3
Input power (per lamp) (W) 5 Brightness (per lamp) (Nit) 20000
* The distance from the reflector to optical films
** The distance between two lamps
*** The distance from the reflector to the lamp
Fig. 3-5 Demonstration of a 32-inch backlight system.
3.4 Simulation models
The simulation software, LightTools, was adopted to design the MPF in this thesis.
LightTools can model real objects and it follows the ray-tracing method in simulation. The bidirectional transmissive/reflective distribution function (BTDF/BRDF) of diffusive films and reflectors are shown in Fig. 3-6. A BTDF measurement device and a conoscopic system, which will be introduced in Chapter 4, were used to measure these BTDFs and BRDFs. The BTDFs of top and bottom diffuser were similar and had transmission light peaks at incident angles 0° to 20°. The BTDF of diffusive plate was smoother and wider compared with that of top and bottom diffusers. On the other hand, the diffusive plate had higher scattering ability than the top and bottom diffusers. Besides, the BRDF of the reflector was measured with reflective collimate light source in conoscopic system. Because the reflector had white particles on the surface, the specular reflection and the diffusion were occurred simultaneously. Besides, the models of lamps, BEFs, and MPFs can also be created by the build-in tool in LightTools.
(a) (b)
(c) (d)
Fig. 3-6 Measured BTDFs of the (a) top diffuser, (b) bottom diffuser, and (c) diffusive plate, and BRDF of the (d) reflector.
3.5 Simulation results
To suppress lamp-Mura, the curvatures of MPF and MPF-Plate case will be optimized according to the lamp-Mura contrast result. The lamp-Mura contrast defined in Section 2.2 was calculated according to the spatially luminance distribution form backlight systems. The spatially normal luminance distributions were determined by luminance data of 33 points between two lamps at the backlight system center, as shown in Fig. 3-7. From the figure, the 0 and the 1 pitch are positions above lamps, and the 0.5 pitch is the center position. Compared with MPF case included a MPF, MPF case with a conventional BEF had a higher luminance
at the center position than above lamps, as shown in Fig. 3-7 (a). Additionally, MPF case enhanced the luminance above lamps for a more uniform light distribution. In order to evaluate whether the light distribution is uniform enough for suppressing lamp-Mura, the lamp-Mura contrasts of MPF and MPF-Plate case will be calculated.
(a) (b)
Fig. 3-7 Luminance distributions of (a) MPF case and (b) MPF-Plate case, where C is curvature.
3.5.1 Lamp-Mura contrast and optimized curvature
Lamp-Mura contrasts of different MPF curvatures in MPF and MPF-Plate case are shown in Fig. 3-8. For MPF case, the optimized MPF curvature range from 57 to 81 mm-1 had smaller lamp-Mura contrast than the conventional backlight. For MPF-Plate case, the optimized MPF curvature was about 60 mm-1 the closest to the conventional backlight.
Because the conventional backlight had invisible lamp-Mura, thus, we conclude MPF and MPF-Plate case with optimized curvature also can suppress lamp-Mura.
Fig. 3-8 Lamp-Mura contrast of MPF and MPF-Plate case.
3.5.2 Average luminance
The average luminance between two lamps was also obtained, as shown in Fig. 3-9. The luminance of MPF case decreased when the MPF curvature increased over 70 mm-1. This situation was similar to MPF-Plate case with the threshold MPF curvature of 67 mm-1. Additionally, the luminance of MPF-Plate case in curvature range, 40~75 mm-1, is lower than that of MPF case. At optimized curvature 60 mm-1, MPF-Plate case yielded 2% decrease in luminance (7243 nits Æ 7070 nits) compared with MPF case. However, most important is that both cases had higher luminance than the conventional backlight system.
3.6 Summary
The concave prism profile of MPF was designed as parabola, and the curvature is the designed parameter. For optimizing the MPF curvature, backlight component models were built to create a simulation environment by LightTools. Additionally, MPF and MPF-Plate case with MPFs were designed to compare with the conventional backlight system. In the simulation, lamp-Mura contrast results were obtained. Then, the optimized curvature of MPF and MPF-Plate case, which had less lamp-Mura contrast as the conventional backlight, was found about 60 mm-1. Moreover, a higher luminance was also obtained at curvature 60 mm-1. Thus, we conclude the MPF curvature of 60 mm-1 has best performance to suppress lamp-Mura. We then fabricated MPF by screen printing process.
Chapter 4
Fabrication technologies and measurement instruments
Optical devices with micro-structures, such as prisms, micro-lens and diffractive optical elements, are generally used in micro-electro-mechanical systems (MEMS), display devices and projectors, et al. For devices with feature size of 1μm ~ 1mm, photolithography and etching techniques (standard semiconductor processes), diamond machining and excimer laser micro-machining (laser ablation) are applicable[20]. These methods are complex and are not efficient in fabricating the proposed MPF concave-prism structures. Therefore, an easy and economical fabrication method, screen printing process, supported by E vendor was adopted to fabricate MPF. Moreover, several instruments for measuring MPF profiles and optical performances of designed backlight systems are also introduced in this chapter.
4.1 Screen printing process of MPFs
The concept to fabricate the MPF concave-prism profile is to fill a UV-curable material on a conventional BEF, where this material has the same refractive index to the prism structure. The MPF fabrication schematic by using screen printing process is shown in Fig.
4-1. The dimension of the used BEF is 25μm height and 50μm width as shown in Fig. 4-2.
For controlling the amount of material filled, first, a mask (so-called screen) with a designed aperture ratio distribution was placed on a conventional BEF. Second, the UV-curable
material was dropped on the mask. Third, a squeegee, which is a flexible rubber, pressed and horizontally moved over the mask. Then, the UV-curable material flowed through mask apertures to fill the prism grooves on the BEF. Finally, a filled concave profile was formed after curing the filled material in UV-exposure.
According to the designed aperture ratio of the mask, the amount of filled material could be controlled to make different concave-prism profiles of MPFs. In this thesis, the term
“filling ratio (%)” depended on the mask aperture ratio was applied to classify different filling profiles of MPFs.
Fig. 4-1 Schematic of screen printing process.
Fig. 4-2 Schematic of used BEF structure.
4.2 Measurement instruments
After the fabrication, fabricated MPF profiles were examined and measured by an optical microscope (OM) and the surface profiler, Alpha-Step IQ[21], respectively. Then, optical performances of backlight systems were measured using a conoscopic system and a
charge-coupled device (CCD). In order to set up the diffusive plate and diffuser models in our simulation, our designed bidirectional transmittance distribution function (BTDF) measurement device was used to extract scatter properties of diffusive films to import into the simulation software, LightToolsTM.
4.2.1 Conoscopic system
The conoscopic system applies Fourier transform lens to transfer light beams emitted from the test area to a CCD array. Therefore, the angular properties could be easily measured on the CCD sensor plane. The CCD array consists of various directional CCD sensors to detect brightness, color, and angular distribution of transmissive light. Besides, it has two modes for measuring transmissive or reflective light. In our experiment, the transmissive mode is employed because the backlight system can emit light itself.
The options for testing under illumination are based on the combination of Fourier Optics and a cooled CCD sensor head. As shown in Fig. 4-3, the measurement is used in transmissive mode, where the first lens provides a Fourier transform image of the display surface. Each light beam emitted from the test area at incident angle, θ, could be focused on the focal plane at the same azimuth and at a position x=F(θ). The angular characteristics of the sample are thus measured simply and quickly, without any mechanical movement.
4.2.2 BTDF measurement device
In order to measure BTDFs of diffusive films, a light source device which can provide collimating and different-incident-angle light is required. Accordingly, we designed a device for this purpose, as shown in Fig. 4-4. Nine light-emitting diodes (LEDs) sited on the arc are chosen as the light sources. The LED is green light with a peak wavelength about 520 nm.
Each LED represents each incident direction, from 0 to 80, step by every 10 degrees. Besides, the small aperture on the exit plane of LEDs and the small hole on the top plate ensure the exit light is collimated.
Fig. 4-4 Schematic of our designed device for BTDF measurement.
When measuring BTDFs of diffusers, the BTDF measurement device is placed under the conoscopic system to serve as the light source. The luminance information in each direction can be measured by the conoscopic system. A photograph of the experimental setting is shown in Fig. 4-5.
Fig. 4-5 Diagram of BTDF measurement.
4.3 Summary
Before the MPF fabrication, simulation software LightTools built models of backlight components to do simulations to find optimized design results. The BTDF measurement device measured the BTDF value of optical films to import into LightTools to build corresponding models. The optical property of simulation models is similar to real objects.
The conoscopic system not only can measure the BRDF of the reflector but also the optical performances of backlight systems, such as angular luminance distribution, color information, et al.
To fabricate the concave-parabolic prism structure of MPFs, the idea in this thesis was to fill some material in the prism groove of conventional BEFs to be the concave-parabolic profile. Screen printing process was adopted to accomplish this idea. By adjusting the aperture ratio of the mask (screen), the amount of filled UV-curable material in the prism groove was controlled; on the other hand, different prism profiles of MPFs could be realized. Compared with the imprint method, the cost of a mask in the screen printing process is lower than that of a mold. Thus, we conclude that screen printing process has an advantage of high design flexibility to find the optimized MPF in simple process and low cost. Moreover, fabricated optimum MPF is possible to transfer to a mold for roll-to-roll mass production.
Chapter 5
Experimental results and discussions
After fabricating MPFs by screen printing process, surface profiles of fabricated MPFs were measured by a surface profiler. Then, the relationship between filling ratios and fitting curvatures of MPFs was also obtained. The experimental results, which included brightness, viewing angle, output light efficiency and real luminance images, of three experiment backlight structures (conventional, MPF and MPF-Plate case) will be presented and discussed.
5.1 Fabrication results
The optimized MPF curvature among fabricated MPFs can be obtained by measuring the surface profiles. The surface profiles between two prisms width were measured by Alpha-Step IQ[20], as shown in Fig. 5-1. The result showed that the higher filling ratio, the flatter surface profile. In order to verify whether the fabrication result meets the designed MPF curvature, measured MPF profiles were fitted with a parabolic curve to find an approximate curvature.
For example, fitted results of filling ratio 10% and 25% MPFs are shown in Fig. 5-2. Then, the relationship between filling ratio and fitting curvature of MPFs were obtained, as shown in Fig. 5-3. Among the results, the approximate curvature of filling ratio 10% MPF of 58 mm-1 was the closest to the optimized curvature 60 mm-1 of MPF and MPF-Plate case. Thus, we conclude that filling ratio 10% MPF is the optimized MPF which can suppress lamp-Mura as the conventional backlight system. In following experiments, filling ratio 10% MPF will be
adopted to do experiments for verifying our conclusion.
Fig. 5-1 Measured MPF surface profiles.
(a) (b) Fig. 5-2 Fitting profiles of filling ratio (a) 10% and (b) 25% MPFs.
Fig. 5-3 Relationship between filling ratio and curvature.
5.2 Experimental results
Filling ratio 10% MPF was applied to MPF and MPF-Plate case to do experiments. Then, optical performances, such as lamp-Mura contrast, normal brightness, viewing angle and output light efficiency, of three experiment backlight structures (conventional, MPF and MPF-Plate case) were measured and compared.
5.2.1 Lamp-Mura contrast and average luminance of MPF case
To verify that the optimized MPF curvature had best performance of suppressing lamp-Mura, lamp-Mura contrasts of MPF case with fabricated MPFs were obtained to compare with simulation results. For the conventional backlight and MPF case, 9 points luminance between two lamps at the center of the 32” backlight system was measured by the conoscopic system, as shown in Fig. 5-4. Then, lamp-Mura contrasts were calculated according to the measured luminance distribution. The simulation and experimental results of lamp-Mura contrast and average luminance are shown in Fig. 5-5.
The lamp-Mura contrasts in the experimental and simulation results had similar trend. In
the experimental result, the filling ratio 10% MPF yielded minimum lamp-Mura contrast, 0.010, lower than the conventional backlight (0.012), which agreed with the simulation result.
On the other hand, MPF case with filling ratio 10% MPF resulted in invisible lamp-Mura as the conventional backlight. Moreover, for the MPF-Plate case with the filling ratio 10% MPF, a lamp-Mura contrast, 0.018, was also obtained by experiments. However, this lamp-Mura contrast higher than the conventional does not mean that lamp-Mura is appeared. The following experimental will verify whether MPF-Plate can suppress lamp-Mura.
Moreover, the average luminance of MPF case was lower than the conventional backlight. Compared with simulation results, the average luminance of MPF case was higher than that of the conventional backlight when the curvature was below 70. The mismatch result of the average luminance will be discussed at the end of this chapter.
Fig. 5-4 Measured luminance distributions of conventional backlight and MPF case.
(a) (b)
Fig. 5-5 Comparisons of simulation and experimental results in the MPF case, (a) lamp-Mura contrast and (b) relative average luminance.
5.2.2 Normal brightness and viewing angle
The experimental result of the angular luminance cross-section was measured, as shown in Fig. 5-6. Calculated values of luminance angle and of relative normal brightness are shown in Table 5-1. For MPF case, the FWHM was improved to 90°, and the normal luminance can be maintained as high as the conventional backlight. For MPF-plate case, although the normal luminance was reduced to 90% of that of the conventional backlight, the FWHM was improved further to 116°. Compared the MPF case with the MPF-Plate case, MPF case with high normal brightness is adaptable to a high brightness backlight module; MPF-Plate case with wider FWHM is suitable for a wide viewing angle LCD.
(a) (b)
Fig. 5-6 Diagrams of luminance cross-section in (a) horizontal and (b) vertical.
Table 5-1 Relative normal brightness and luminance viewing angle.
Table 5-1 Relative normal brightness and luminance viewing angle.