Chapter 2 Principle
2.3 Fresnel Lenses
A Fresnel lens is an optical component which can be used as a cost-effective, lightweight alternative to conventional continuous surface optics as shown in Fig. 2.5.
Fig. 2.6 The schematic structure form conventional continuous surface to Fresnel lens
The design of some simple Fresnel lens will be presented in this section. A grooves-out design directs the facets towards the side of the collimated beam and a grooves-in design orients the facets towards the focal point. Then the Fresnel lens converts a point source to a beam of parallel light as a collimator.
Grooves Out [13]
By Fig 2.6, a set of four equations can be derived.
s αin n sin β (2.14) α γ β (2.15) n sin γ sinω (2.16) tan ω R f⁄ (2.17)
Fig. 2.7 Grooves out type of Fresnel lens
An expression for tan α can be obtained after some substitutions.
tan α γ
γ (2.18) By multiplying and square root, the eq ation 2u .16 can formed a new expression.
n cos γ √n sin ω (2.19) The final form from these equations is
tan α √ ω
ω (2.20)
Grooves In [14]
In a very similar procedure, the Fresnel lens with grooves facing inwards will be presented. According to Fig. 2.7, three equations can be set up to describe this system.
nsinα sinβ (2.21) tan R
f (2.22) ω
β α ω (2.23) Substituting β in equation 2.21 with 2.23 yields an expression for tan α.
tan α (2.24)
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Fig. 2.8 Grooves in type of Fresnel lens
Furthermore, equation 2.24 can express in terms of focal length f and aperture R.
tan α √RR (2.25)
Chapter 3
Design and Simulations
3.1 Introduction
In this chapter, all the design process and the simulation model of viewing-angle-switchable (VAS) backlight were established. Before entering the theme, the basic concept will be introduced briefly. Then the design targets will be decided. Subsequently, the light control film will be described in detail; meanwhile, the brief discussions and optimization processes for VAS backlight unit will be given.
Therefore, the simulation model established to characterize the feature of the VAS backlight as well as the simulation results will also be presented. After all, the VAS backlight model was designed and optimized.
Firstly, the optical performances of the traditional backlight module (direct-type backlight) are shown in below. Figure 3.1(a) is the brightness map with 25 points which could evaluate the uniformity of the backlight system. According to the panel sizes, it could choose different point numbers. The uniformity of VAS backlight was determined with the luminance level of the 9 points as shown the definition in Equation (3.1) and Figure 3.2.
Fig. 3.1 (a) Brightness map and (b) angular distribution of common BL
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Uniformity MM P ,PP ,P PP 100% (3.1)
Fig. 3.2 The 9 points method for uniformity measurement
Fig 3.1 (b) shows the angular distribution of traditional backlight module without brightness enhancement film (BEF). The black line in Fig. 3.1 (b) represents that the luminance is almost the same in any observing direction. Therefore, the traditional backlight could be seen as a lambertian light source with very high brightness and uniformity. After introducing the optical characteristics of traditional backlight, the concepts of VAS backlight will be discussed in the following.
In this thesis, we want to design a BLM which can offer the normal viewing mode as zone I and two oblique viewing modes as zone II and zone III as shown in Fig 3.3. The optical field distribution and the actual image are shown as these schematic presentations in Fig. 3.3. As these pictures, the left parts are the designed angular distribution with three different directions on backlight module and the right parts show the desired viewing situations in actual displays.
According to the schemes in the left parts of Fig. 3.3, the corresponding distribution on relative rectangular candela distribution is shown in Fig. 3.4. The desire angular distribution in Fig 3.4 used to compare with the simulation result of VAS backlight.
(a) Normal viewing mode
(b) Oblique viewing mode
(c) Oblique viewing mode
Fig. 3.3 Schematic presentation for desired viewing angle switchable display
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Fig. 3.4 Schematic presentation for desired angular distribution
After rgets were conce
3.1] Design target of VAS backlight system
3.2 Proposed Module for Viewing-Angle-Switchable Backlight
be refracted into collim
the objective which mentioned in the introduction, the design ta
rned here. Three main design parameters used to set the final goal for VAS backlight included luminance, FWHM of angular distribution, and the uniformity of entire backlight module, and these parameters were compared with common Notebook backlight requirement. Table 3.1 shows the design target of our VAS backlight. All of the requirements shown in this table are indicated that the luminance must be higher than 3500 nits, and the FWHM of viewing angle distribution confined in +/- 20 degrees, and the uniformity was higher than 70% for both normal and oblique viewing modes.
[Table
As shown in Fig. 3.5, the divergent rays of point source can
ating light by single lens (i.e., collimator) in geometrical optics. If the point light source located on the optical axis of lens as in Fig 3.5 (a), the output light would propagate along the direction of optical axis of this lens. When the point source located off the optical axis with the included angle α, the output rays would travel parallelly with the same angle as Fig 3.5 (b). From this simple idea, the viewing direction adjustable backlight system was possible by placing a lens in front of the light source and provides the directional optical field via different locations of the light sources. However, the lens shape would increase much thickness of entire
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(a) (b)
Fig. 3.5 (a) O direction.
After the basic concept of viewing angle control, the original opto-mechanism setup
backlight module in practice. In order to overcome this problem, a designed light control film could used to replace the role of common lens to compose the VAS backlight.
n-axis source provide collimating rays in normal (b) Oblique direction rays via off-axis light source.
included the designed prism layer and three LED light bars as shown the cross-section in Fig 3.6, and Fig 3.7 is the scheme of three-dimensional picture. The normal mode is achieved by a designed prism layer and a LED light bar on the optical axis as the green rays in Fig. 3.6. In addition, the oblique mode is achieved by the same prism layer and an off axis LED light bar as the cyan rays. Because of the limit of size and the difficulty of manufacturing, a commercially available cylindrical Fresnel lens was used to replace the prism layer. In the following sections, the cylindrical Fresnel lens was called “Fresnel lens” in this thesis for convenience. The Fresnel lens played well a role in controlling the rays to provide directional optical fields and the peak value of the oblique direction is about 40 degrees.
Fig. 3.6 Schematic configuration of proposed backlight module
Fig. 3.7 The actual setup of proposed backlight module in 3-D
In this opto-mechanism, two parameters could be adjusted: the distance from the surface of LED light bar to the Fresnel lens and the power radio of these LED light bars. Besides, the detail specification of Fresnel lens and LED light were added in chapter four. In addition, it deserved to be mentioned that the focal length of the Fresnel lens was 5 cm.
The distance (i.e. d) of parameter which mentioned above was the first concern in this design procedure. In the beginning, the main assignment was to find out the suitable distance of normal viewing mode. By this reason, we could simulate this configuration with difference operating distance by optical software (i.e. LightTools).
The simulation results are shown as radar sketch in Fig. 3.8. According to Fig. 3.8, if
the LED light bar is held 5 cm (i.e. d = focal length) away from the Fresnel lens as the blue line, the angular distribution would be too sharp to apply on normal viewing mode. However, if the LED light bar is placed on the one-third of the focal length (i.e.
d/3 = 1.67 cm) as the green line, the FWHM of the angular distribution would be too wide to use as the normal mode.
Fig. 3.8 The angular distribution of normal viewing mode with various distances in radar sketch
Fig. 3.9 The FWHM of angular distribution V.S. various distances in normal viewing mode
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Fig. 3.9 shows the relationship between FWHM and the distance d. According to the design parameters in Table 3.1, d/2 was the most suitable distance for our design target.
However, how about the optical performances of oblique viewing mode?
normal mode, and I would discuss in the following.
various distances in oblique de
According to Fig. 3.10 and Fig. 3.11, the higher distance have inadequate angle for ob
We could repeat the similar process by optical software to find out the angular distributions in oblique viewing mode. However, it did not easily make a decision like
Fig. 3.10 The simulation results of radar sketch with
mode. The higher distance had inadequate angle for oblique viewing mo and the FWHM of the angular distribution was too wide to use in lower distance.
lique viewing mode and the FWHM of the angular distribution are too narrow to use as the black, brown, and the green lines. However, if the distance decreases to one-third focal length (i.e. d/3 = 1.67 cm), it will increase the FWHM for normal mode which shown in Fig. 3.8 and had much stray light in other viewing directions.
Therefore, we have a trade-off between the normal and oblique modes. So, we chose
d/2 (i.e. half focal length 2.5 cm) as the distance between the Fresnel lens and LED light bars. After the distance chosen by the simulation results above, the simulation of our opto-mechanism could be started. And the simulation result was shown in Fig.
3.12.
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Fig. 3.11 The FWHM V.S. various distances at oblique viewing mode.
Fig. 3.12 The illuminance map of normal viewing mode.
There was a hot zone at the center in this map and the uniformity was lower than 70%.
The lower distances caused the inadequate FWHM.
s a hot spot area appear by using this simple setup as the VAS
Via this way, we could reduce the dimension of screen in Fig 2.4 from 3-D into 1-D.
Similar to the process mentioned in chapter 2, the deriving details were shown below.
Fig. 3.13 1-D linear nd illumination distribution By Fig. 3.12, there wa
backlight, and the poor uniformity of normal and oblique mode was the chief defect in this configuration.
In order to solve the uniformity problem, we could review the photometry example shown in Fig 2.4. The result was equation (2.13) which also means the cos θ law. It meant that the irradiance on a flat surface which illuminated by a small point source was direct proportional to cubic of cosθ. Therefore, the main purpose of this study was to modify the viewing angle of observer in a horizontal direction in VAS backlight.
light source a
According to equation (3.2), the illumination distribution of a point source was proportional to cos2θ, and the hot spot issue was formed by this term. Therefore, we could use this property to increase the uniformity of the VAS backlight. As shown in Fig 3.8, the blue line represents the illumination curve caused by the point source. If the source intensity, Is, proportioned to cos-2θ, the illumination distribution on the screen would become a straight line which means high uniformity. Based on this property, the uniformity could be improved.
Fig. 3.14 The blue line represented illuminance distribution via point source and the designed luminance intensity was red curve.
The discussion about the concept in Fig. 3.14 was an approximate process.
Therefore, we could set 1-D linear light source in optical software to confirm this approximation was a correct method. According to the result in Fig. 3.15, the ideal and simulation curves were similar. It could believe that this reduction progress was usefully.
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Fig. 3.15 The gray line represented the simulation curve which could fit the cos θ approximation
By this idea as motioned above, the light sources could be separated into three directions which as Fig. 3.16 to make the inverse cosine-square distribution introduced in the previous pages. Therefore, we could modify the opto-mechanism which was motioned in Fig. 3.6. In addition, a new opto-mechanism was proposed as Fig. 3.17 and the power ratio of each light bar could be adjusted to improve the uniformity.
Fig. 3.16 The light sources could be separated into three directions based on theoretic result to make the inverse cosine-square distribution, and then the hot spot issue could be fixed
Fig. 3.17 The new opto-mechanical structure of VAS backlight unit and the middle section (CDE) was used for normal viewing mode, and the other cluster (AB, and GF) were used for oblique viewing mode.
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In brief, Fig. 3.17 shown the middle section was used for normal viewing mode as shown in the green dotted line. On the other hand, the other two clusters (AB and GF) were used for oblique viewing mode. Then, the inclined light bars (BCEF) were used to increase the luminance around the edge of the partition. In order to fit the cos θ optical field distribution, the power ratio of group (ADG) to group (BCEF) was 3, and the inclined angle of tilt plane with respect to the horizontal was 55°.
Fig. 3.18 A 7 inch prototype was constructed by three sections of LED backlight units with the Fresnel lens
Since we are constrained by the fabrication issue, a 7 inch prototype was constructed by three LED backlight units with the Fresnel lens, as shown in Fig. 3.18.
After confirming the final opto-mechanism, we could check the simulation result of uniformity improvement. The simulation outcomes were shown in Fig. 3.19 and Fig.
3.20. The uniformity of one unit was over 70%, which is better than our design target in Table 3.1. In addition, the FWHM of angular distribution is +/- 20 degree, which meet the requirements of design target [Table 2.1].
Fig. 3.19 The simulation outcomes of normal mode uniformity
Fig. 3.20 The simulation outcomes of angular distribution in normal mode
3.3 Simulation results for VAS backlight arrays
Fig. 3.21 represented the layouts of the VAS backlight units, turned on the middle and side parts of LED light bars were normal and oblique mode respectively.
The uniformity of normal mode was over 80% that could fit in with the targets as shown in Fig.3.22. In addition, the FWHM of angular distribution of normal mode was confirmed between +/- 20 degree as Fig. 3.23 which could fit in with the design targets in Table 3.1.
Fig. 3.21 The simulation setups for (a) normal viewing mode and (b) oblique viewing mode in the optical simulation tool.
Fig. 3.22 The simulation result of uniformity in normal viewing mode
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Fig. 3.23 The angular distribution of normal viewing mode and the FWHM was +/-20°
However, the first compromise improved the uniformity of normal mode, but uniformity of oblique mode will not be good enough as shown in Fig. 3.24. If the power ratio was tuned for the uniformity of oblique mode again, it would decrease the location of peak value to 20 degree. Therefore, we had a trade-off between the uniformity and angular distribution of normal and oblique modes. If we wanted to increase the uniformity of oblique viewing mode, the viewing angle of oblique mode would be influenced. Therefore, a diffuser film will added into the entire opto-mechanism to solve the problem and then to improve the uniformity in oblique viewing mode.
(a) (b)
Fig. 3.24 The (a) uniformity and (b) angular distribution of oblique mode
Chapter 4
Experimental Results
4.1 Introduction
The concept of proposed viewing angle switchable backlight has been introduced in Chapter 3. Compared with the simulation, the experimental results of entire viewing angle switchable backlight module will be exhibited in this chapter.
The VAS backlight can be functioned as two kinds of modes (i.e. normal and oblique viewing modes). In normal viewing mode, the backlight offers a narrow optical field in the normal direction. On the other hand, the backlight offers an optical field in a fixed direction in oblique mode. The target of viewing angle control can be achieved by switching the LED light bars in our VAS backlight system.
4.2 Light Source Properties
The LED light bars were located at the bottom of the VAS backlight module, and each VAS backlight unit possessed seven light bars. One linear light bar contains 48 LED chips and phosphor layer as shown in Fig. 4.1. These two pictures showed the entire appearance of LED light bar, the placements of two electrodes, and the light
Fig. 4.1 The white light LED light bar (iLED L03)
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emitting region. In the experiment, the chromaticity of the LED light bar was (x, y) = (0.3035, 0.3199). In addition, the relative spectral distribution of the light bar was shown in Fig. 4.2. It is notable that the spectrum had two peaks in the ranges of blue and yellow wavebands which were induced by the LED chips and phosphor. The specification of the LED light bar was expressed in Table 4.1 in detail. The driving voltage can be adjusted according the table to control the actual luminance in our opto-mechanism and reach the inverse cosine square distribution [Chapter 3.2].
4.3 Light Control Prism Layer : Fresnel Lens
In the beginning of this thesis, a designed prism layer was proposed to redirect the light rays from the sources. Owing to the limit of some fabrication issues, the commercially available cylindrical Fresnel lens by Edmund Optics was adopted to replace the original prism layer. Hence, the following would be the detail information of this lens which is marshaled in Table 4.2. In addition, Fig 4.3 shows the cylindrical Fresnel lens by Edmund Optics. The red dash line in this picture represents the symmetric axis of this lenticular lens.
420 480 540 600 660 720 780
0.0
Fig. 4.2 Spectrum distribution of white light LED light bar (iLED L03)
[Table 4.1] Electrical and optical characteristics of LED light bar (iLED L03)
[Table 4.2] The specifications of Edmund Fresnel lens Size
Fig. 4.3 The Edmund cylindrical Fresnel lens
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4.4 Experimental Setup
This paragraph showed the entire mechanism setup of the backlight module. As shown in Fig. 4.4, the VAS backlight unit included cylindrical Fresnel lens and seven LED light bars, and the locations of these light bards were coincident with Fig. 3.7.
Furthermore, Fig. 4.5 exhibited the combined Fresnel lens. The original length of this lens was 12 inch as mentioned in Table 4.2. Therefore, the Fresnel lens was divided into three parts in practical mechanism. Eventually, 7 inch VAS backlight module could be fabricated with twenty one LED light bars and the combined Fresnel lens.
Fig. 4.6 shows the upper Fresnel lens and the bottom LED light bars simultaneously, and Fig. 4.7 performs the actuated backlight module we’ve manufactured.
Fig. 4.4 The Schematic configuration of entire mechanism setup for the actual VAS backlight module
Fig. 4.5 The combined Fresnel lens
Fig. 4.6 The apparatus was demonstrated which included the upper Fresnel lens and the bottom LED light bars
Fig. 4.7 The entire VAS backlight module that we’ve manufactured was turned on
4.5 Measured Results : Normal and Oblique Viewing Modes
The results of cross-sectional luminance and angular distribution in one unit were exhibited for normal and oblique viewing modes in this section. As shown in Fig.
4.8 and Fig. 4.9, the green curves presented that the uniformity of normal and oblique
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modes could reach 80% and 75%, respectively. In addition, the FWHM of angular distribution of these two modes were +/- 20 and +/- 15 degree as shown in Fig.4.8 and Fig. 4.9. All above data could reach the design target as introduced in chapter 3.1.
Furthermore, the peak value of oblique mode in Fig. 4.9 (b) was located at 30 degree.
The FWHM and the peak value of oblique mode were different from the simulation
The FWHM and the peak value of oblique mode were different from the simulation