Simulations and Discussions
3.4 The Determination for the Type of the Micro Structure
In order to present the color correctly, the angular distribution of the out-coupled rays must be well controlled for the FSC LCD backlight. In other words, the light reflected by the out-coupler should have the character of high directivity and should be normal to the output surface. As discussion in chapter 2.4.2, the reflective out coupler seems the better candidate than the refractive type. According to the reference, [12] it also shows that the micro groove is much suitable then the bump or hole-type micro-reflector owing to the micro-reflector will lead to the wider angular distribution as the result depicted in [Fig 3.3]. Recently, some literatures brought up advanced micro structures named double prisms, [15] to narrow the light distribution further with no brightness enhance film or diffuser. [Fig 3.4] However, considering the cost of production and the manufacture technology, the discrete arrangement of the micro groove structures fabricated by diamond turning was the elected method to be used.
(a) (b)
[Fig 3.3] (a) Angular light distribution measured on a bare linear light guide with uniformly positioned micro-prisms on the front and gradationally positioned micro-prisms on the rear. (b) Angular light distribution on a bare linear light guide with micro-reflector array on the rear.
[Fig 3.4] The all radiated light from double prism (Mode1 is the radiated light from the sub prism and Mode2 is the radiated light from the main prism).
Besides, although the wedge-type light pipe has higher light efficiency than the linear light pipe, the direction of most illuminating rays deviates from the normal of the output surface. Those out-coupled rays leaved the pipe at grazing angle are not practical for most applications. Therefore, the external correction plate such as a TIR prism sheet should be combined with the pipe. [Fig. 3.5] [16]
[Fig. 3.5] The wedge-type light pipe needs the TIR prism sheet to modify the illumination light.
3.5 Optimization of FSC LCD Backlight System 3.5.1 Optimization of the light-collimating bars
In this section, all of the work is done by my cooperator Cho-Chih Chen. As mentioned before, the light-collimating bar was used to reduce the X-Z plane divergence of the LED. To do such function, there are several different kinds of micro-structures were built on the out-coupling side of the bar and they can be classified into three types of zones, as shown in [Fig 3.6].
x
[Fig 3.6] The zones of light-collimating bar: ○1 The transmitting zone corresponded to the light source which had an incident angle approximately within ±15o. ○2 The blaze zone where saw-toothed shapes were formed was defined as the area within the incident light with angles larger than 15o but smaller than about 30o. ○3 The prism zone with isosceles triangular prism pattern was utilized to refract the light with angles larger than 30o. This zone was illuminated not only by the corresponding LED but also by adjacent LED.
The angular distribution of the extraction light from the light-collimating bar was shown in [Fig 3.7]. According to this result, the light-collimating bar remarkably reduced the X-Z plane divergence of the incident light.
-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 9090
[Fig 3.7] The angular distribution of the extraction light from light-collimating bar.
3.5.2 Optimization of the linear light pipe
After establishing the modal of light-collimating bar, the distribution of the light from the bar’s out-coupling surface had been obtained already and it can be treated as the new light source at this time for the following simulation. The related parameters of the micro grooves over the bottom of the linear light pipe should be clearly defined first. As shown in [Fig 3.8], the parameters of the groove structure include the orientation of the groove orientation, groove pitch p , groove depthd , and groove width along the Z-axiswz , along the X-axis wx . In fact, the groove orientation affects the angular distribution of the illuminated rays while the groove width and pitch determine the illumination uniformity.
[Fig 3.8] The parameters of the micro-groove structure
[Fig 3.9] The optimization flowchart
The optimization flowchart is depicted in [Fig 3.9]. First, the desired direction of the out-coupled rays could be modified by choosing a proper groove orientation. The simulation in this part was simply carried out under a uniform distribution of the micro grooves due to the grooves arrangement does not influence the directionality.
From the discussion in chapter 2.4.2, the relation about the out-coupled angleand the prism orientation was depicted in [Fig 2.20] where the gray region corresponds to the rays guided in the pipe by TIR. The line represented the out-coupled angle 00intersects to the curves of the prism orientation at300and 450. Accordingly, the angular distribution simulated under the various groove orientations between 300and 450. The result was shown in [Fig 3.10]. The angular distribution at groove angle430had the narrowest divergent angle of about
300
and symmetry to the normal of output surface compared with other angles. It implied that a tolerance of 20around the optimal groove orientation was acceptable and the requirement could be achieved by most of the fabrication process. However, the orientation has a little different from the analytical solution. Considering the
average direction of propagation has value of680, the calculated value of the groove angle was 34.50.
[Fig 3.10] The angular distribution simulated under the various groove orientations After choosing the proper groove’s angle, the uniformity and efficiency should be enhanced with the optimized micro groove distribution. There were two encoding schemes, as illustrated in [Fig 3.11], to be considered. The first approach keeps all the size of the micro groove and modulates the groove spatial frequency along the Z-axis.
The other method tends to keep the groove spatial frequency constant and to vary the groove widths along the X-axis and Z-axis. Naturally, the spatial frequency and the groove dimension could be changed simultaneously for the generation of more complex lighting distribution. In this place, the first approach was taken into account for the initial optimization, and then the grooves widthwz along the Z-axis modulated according to the Gaussian function will be discussed.
[Fig 3.11] Encoding schemes of (a) modulating frequency and (b) varying groove size (a)
(b)
The illuminating cones produced by each groove Near-field effect
[Fig 3.12] Near-field discontinuities
However, here an indicative criterion should be considered that the maximal groove depth dmax must smaller than the height of the pipe twenty times at least to diminish the visible groove pitches.
max
The height of the pipe
d 20 (3.1)
The smaller groove width also allows producing a smoother illumination distribution.
In other words, it can prevent the perturbation in uniformity because of the near-field discontinuities shown in [Fig 3.12]. The initial value of the groove widthwz was chosen as10um, which is capable for the diamond turning fabrication.
By means of the linear decreasing of the groove pitch, the corresponding decreasing ratio for uniformity of higher than 80% for a single pipe was then found.
In this optimization process, the groove pitch was varied from 155 to 200μ m and the decreasing ratio of 0.9995was determined, where the decreasing ratio was defined as pi/pi1(p was the pitch between i i groove to (th i1)thgroove). As the result depicted in [Fig 3.13], the illumination at the regions marked by the dot line still lower than the other portion and should be improved further.
In order to enhance the illumination at some regions mentioned above and achieve higher uniformity, a Gaussian function was introduced to modulate the grooves widthwz along the Z-axis.
[Fig 3.13] Illumination map of light guide plates with a single division lit up The Gaussian function is defined as
2 0 2
( )
( ) exp[ ]
2 z z Gau z A
(3.2)
where z and 0 are the average position and deviation of the Gaussian wave packet respectively, z could be any point along the Z-axis and A means the weighting. The [Fig 3.14] shows two Gaussian wave packets symmetrically arranged and connected at z0 80mm (the center point of the pipe). The groove width wz varied from 10.3um to 10.7um, which based on the Gaussian distribution with
1.07
A , 20, and z0 45mm and 115mm.
△=20 △=20
Z0=45 Z0=115
[Fig 3.14] The relation between the groove width and the Z-axis position
[Fig 3.15] Illumination map of light guide plates with a single division lit up According to the optimized groove width distribution shown in [Fig 3.14], the simulation result was revealed in [Fig 3.15]. As seen in the figure, the uniformity was indeed improved by the Gaussian function distribution and the angular distribution depicted in [Fig 3.16] is almost the same as that in fixed groove width. The uniformity of higher than 85% and the divergent angle of the extracted light less than 310 on the x-z plane were achieved. In addition, the light leakage from each pipe of light guide to adjacent divisions merely accounted for 3.6%, shown in [Fig 3.17].
However, the total optical efficiency attained only to 60%.
Degrees
[Fig 3.16] Rectangular candela distribution plot for divergence of light extraction on the x-y and y-z plane, respectively.
60%
0.03051
60%
0.03051
3.6 Summary
A spatial-temporal scanning backlight for field-sequential-color display system was designed and simulated. The optimal directivity of extraction light occurred in the prism orientation adapted as 430 , meanwhile, the divergent angle of the extracted light less than 310 on the x-z plane were achieved. The color dependence can be considerably diminished in the sequentially driven backlight scheme. The uniformity of higher than 85% was achieved and the light leakage was well suppressed to 3.6%. However, the optical efficiency still has a space to be further improved afterward.
[Fig 3.17] Illumination map of light guide plates with a single division lit up
Chapter 4
Experimental Results and Discussions
4.1 Introduction
The field-sequential color backlight system shown in [Fig 4.1] was demenstrated using the light sources of 60 packages of R, G, B, 3-in-1 LEDs. The optimized light pipes with the micro-groove structures arranged on the bottom were fabricated by the diamond knife micro-machining technology. In this chapter, the optical characteristics of the backlight module and the light pipe will be introduced and discussed. In the first part, the integrating sphere was utilized to measure the optical property of the 3-in-1 LED. After that, the fabricated micro-groove structures were examined with the optical microscope and the alpha step. In addition, the Conoscope detected the collimating degree of the illuminating light from the light pipe output surface. Finally, the light leakage from the single lit up light pipe to adjacent light pipe and the uniformity of the whole panel were captured by the CCD camera.
[Fig. 4.1] FSC LCD backlight system