Chapter 4 Practical Design Case
4.7 Discussion
Fig. 4-12 Comparison of cross-sectional illuminance distribution between faceted surface reflectors (∆α=1°) and the continuous surface reflector
4.7 Discussion
In terms of simulation results in section 4.5, in order to improve illuminance on the target plane, some free-form reflector solutions for the same illuminated target plane were analyzed.
According to the mathematical model in Fig. 4-2, the reflector curve starts from the origin and extends in the positive x direction. The reflector only utilizes the source-emitted light which ranges from 0 degree to around 80 degrees. To increase the
0
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use range of emitted light, another reflector extends in the negative x direction, which uses the light ranging from 0 degree to around -80 degree. Thus, the reflectors extend in the positive and negative x directions. Both negative and positive x direction reflectors redistribute light to the entire illuminated region, and the reflector curves are plotted in Fig. 4-13. The superposition of the illuminated light from the two reflectors is shown in Fig. 4-14. However, this illumination system suffers the serious issue of shadow of CCFL.
Fig. 4-13 Reflector curves extend in both the positive and negative x directions
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Fig. 4-14 Illuminance distribution charts of superposition of the illuminated light from the two reflectors which extend in both the positive and negative x directions
Furthermore, to solve the lamp shadow issue, by interchanging X(0°) and X(-80°), the negative x direction reflector reflects light and crosses light rays to redistribute on the illuminated region as shown in Fig. 4-15. The negative x direction reflector curve changes slightly (Fig. 4-16) and the simulation results are shown in Fig.
4-17. Compared to the previous design, the illuminance distribution becomes more uniform.
(Lux) x
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Fig. 4-15 Ray-tracing results of light rays cross
Fig. 4-16 Reflector curves for the superposition of the crossed light rays
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Fig. 4-17 Illuminance distribution charts of superposition of the crossed light rays
x (Lux)
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Chapter 5
Fabrication and Experiment
To implement the free-form reflector of Luminaire, a simple Luminaire prototype was fabricated for demonstration. Discussions on experimental results are stated as follows.
5.1 Fabrication technologies and results
5.1.1 Computer Numerical Controlled (CNC) machine [16][17]
Numerical Control is a method of automatically operating a manufacturing machine based on a code of letters, numbers, and special characters. A program is a complete set of coded instructions for executing an operation. The program is translated into corresponding electrical signals for input to motors which run the machine. A computer numerical control (CNC) machine is an NC machine with an on-board computer.
Today, most CNC machines are equipped with continuous-path controllers.
Controllers cause the tool to maintain continuous contact with the part when the tool cuts a contour shape. Continuous-path operations include milling along lines at any angle, milling arcs, and lathe turning as shown in Fig. 5-1. Continuous-path controllers output motion by interpolating each position of the tool. The interpolated positions are determined such that they differ from the exact positions within an acceptable tolerance.
Many continuous-path controllers interpolate curves as a series of straight-line segments. Smaller line segments can achieve high accuracy. (Fig. 5-2)
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Fig. 5-1 Continuous-path tool movement
Fig. 5-2 Interpolation for continuous-path movement
The CNC lathe is a machine tool that is designed to remove material from stock which is clamped and rotated around an axis. Most metal cutting is done with a sharp single-point cutting tool. Modern CNC lathes use turrets to rigidly hold and move cutting tools. A typical CNC turret lathe or turning center is shown in Fig. 5-3.
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Fig. 5-3 CNC lathe
In modern CNC systems, end-to-end component design is highly automated using CAD/CAM programs. The programs produce a computer file that is interpreted to extract the commands needed to operate a particular machine, and then loaded into the CNC machines for production.
5.1.2 Luminaire prototype results
The free-form reflector shape for the Luminaire prototype is depicted in Fig. 4-4.
The free-form surface was modeled by CNC manufacturing methods. Then a specular sheet with high reflectivity was adhered to the model surface through an optical adhesive. Two CCFLs were utilized to enhance illuminance. Fig. 5-4 shows pictures taken of the prototype.
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Fig. 5-4 Luminaire prototype
5.2 Instrument and Measurement setup
5.2.1 Instrument
PM Series™ Imaging Colorimeter and Photometer [18] (Fig. 5-5) was utilized as the instrument to measure light distribution on the target plane. ProMetric systems are capable of capturing images and quantitatively analyzing each individual pixel in these images for its photometric, radiometric and colorimetric characteristics. ProMetric instruments consist of a CCD (Charge-Coupled Device) based camera system, together with instrument control, data acquisition and image processing software.
CCFLs
Free-form surface 11.55
167.58 mm
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Fig. 5-5 PM Series™ Imaging Colorimeter and Photometer
Fig. 5-6 ProMetric software interface
5.2.2 Measurement setup
The measurement setup is illustrated in Fig. 5-7. For convenience, the Luminaire prototype was put in front and below a target plane when light illuminated the target plane. The target plane was a diffuse surface (white paper). CCD camera was positioned in front of the target plane, and the luminance distribution on the target plane was captured by the CCD camera.
Measurement Setup
Measurement List
Menus & Toolbar
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Fig. 5-7 CCD camera measurement setup
5.3 Experimental results and discussions
5.3.1 Experimental results
According to the measurement setup illustrated in Fig. 5-7, luminance distribution on the target plane from the Luminaire prototype is shown in Fig. 5-8. The color bar represents the luminance distribution which was normalized by the
maximum value. ( Luminance 100%
maximum Luminance ) By comparison, the normalized luminance distribution result of the conventional Luminaire, whose luminaire reflector is not specified for uniform lighting purpose, is shown in Fig. 5-9. Cross-sectional normalized luminance distribution of the Luminaire prototype and the conventional Luminaire are shown in Fig. 5-10 and Fig. 5-11, respectively.
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Fig. 5-8 Luminance distribution of proposed Luminaire prototype
Fig. 5-9 Luminance distribution of the conventional Luminaire
x
x
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Fig. 5-10 Cross-sectional normalized luminance distribution of the Luminaire prototype
Fig. 5-11 Cross-sectional normalized luminance distribution of the conventional Luminaire
%
x
%
x
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Furthermore, Light Meter was used to measure illuminance (Lux) on the target plane. Several points on the plane were chosen as measuring positions. Comparison of cross-sectional illuminance results between the conventional Luminaire and the Luminaire prototype are shown in Fig. 5-12.
Fig. 5-12 Comparison of cross-sectional illuminance results between the conventional Luminaire and the Luminaire prototype
5.3.2 Discussion
According to the above results, uniformity deviation of the Luminaire prototype is 14.5% while uniformity deviation of the conventional Luminaire is about 40%. The proposed Luminaire obviously improved illumination uniformity, except that there were certain issues due to fabrication errors.
The illuminance on the target plane was lower along the positive x direction.
Since CNC Lathe dimension tolerance was ±0.03 mm, interpolating and cutting errors for the reflector surface were more sensitive to a large angle θ incidence, resulting in darker region. Besides, there were errors during the process of adhering the specular sheet to the model surface, for example, mismatch and bubble, which changed optical
0
position on the target plane (cm)
Illuminance
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property for the reflecting surface. Moreover, two CCFLs were utilized in the Luminaire prototype to improve average illuminance, so the output light distribution from sources slightly deviated from the calculated model.
Some dark and bright stripes appeared across the luminance distribution. This phenomenon resulted from scratches and notches on the model surface as shown in Fig.
5-13. These scratches and notches were created due to cutting errors. Rays struck these scratches and notches and then reflected to different directions. The result was similar to the faceted surface reflector analysis in section 4.6.
Fig. 5-13 Scratches and notches on the model surface
5.4 Performance of lighting upon a reflective display
The merits of the Luminaire prototype lighting upon a reflective display was evaluated. Light emitted from the Luminaire prototype illuminated a Cholesteric LCD and a poster, respectively. Results of the Luminaire prototype were compared with that
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of the conventional Luminaire which is not specified for uniform lighting purpose.
The lighting performance results on the Cholesteric LCD are shown in Fig. 5-14, and results on the poster are shown in Fig. 5-15.
Fig. 5-14 Comparison of lighting performance on the Cholesteric LCD (a) the conventional Luminaire (b) the proposed Luminaire prototype
(a) Conventional Luminaire (b) Proposed Luminaire prototype x
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Fig. 5-15 Comparison of lighting performance on the poster (a) the conventional Luminaire (b) proposed Luminaire prototype
From the above pictures, comparing the Luminaire prototype with the conventional Luminaire, uniformity improvement perceived by the human eye was not as significant as the results captured by the CCD camera. This is because of adaption to brightness by the human eye, so the difference between maximum and minimum luminance was not seen. The issue can be further improved by utilizing CCFL that emits higher flux. The difference will become distinguishable when the illuminance level is raised.
In addition, there were still perceivable dark and bright stripes on the display from the Luminaire prototype, which resulted from fabrication errors discussed in section 5.3.
(a) Conventional Luminaire (b) Proposed Luminaire prototype x
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Chapter 6
Conclusions and Prospects
Reflective LCDs, without back-light, utilize ambient light to display images.
With progress in display technologies, reflective LCDs have been applied to electric billboards and electric posters. The image qualities are ideal in well-lit locations or under diffuse sunlight. At night or in dim indoor environments, front lighting is required due to insufficient surrounding illumination. Conventional front lighting Luminaires for the large-size reflective displays generally suffer issues of non-uniform illumination, resulting in poor image quality.
In this thesis, the front illuminating system for uniform illumination on the target display was proposed and studied. A free-form mirror reflector redistributed light from CCFL source. The shape of the free-form reflector was numerically calculated according to the normal distance between the source and the target plane, the illuminated region, and the positions of the source and the target plane. For the practical design case, the parameters were shown in Fig. 4-1 when z= 50 cm.
Based on simulation results, compared to CCFL directly illuminating case, the proposed illumination system consisted of the free-form reflector improved illumination uniformity while uniformity deviation was within 5%. The average illuminance was 167 Lux.
According to faceted analysis for free-form reflector surface, the uniformity deviation was more close to the continuous surface reflector when the partition angle
∆α was smaller. Furthermore, smaller partition angle ∆α might relieve dark and bright
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stripes problems.
A simple Luminaire prototype was fabricated for demonstration. The free-form surface was modeled by CNC manufacturing methods. Then a specular sheet with high reflectivity was adhered to the model surface through an optical adhesive. Two CCFLs were used to enhance illuminance.
Experimental results for the luminance distribution on the target plane captured by CCD camera verified uniformity improvement (uniformity deviation δ=14.5%), except the issues due to fabrication errors including interpolating and cutting errors, which were limited to accuracy of utilized manufacturing methods.
6.1 Limitations of the front illuminating system
There are some limitations to reflector design based on the linear model.
Referred to Fig. 3-3 and equation (3.8), illuminating point on the target plane X( ) is linear to incident ray angle , which was accurate in mathematical description.
However, in view of Radiometry, the illuminating ray projection angles on the plane are slightly different as shown in Fig. 4-5, leading to brightness variation between upper and lower plane. Thus, the brightness on the lower plane will decrease when the illuminating ray projection angles are seriously deviated from those on the upper plane. In addition, the reflector in the proposed illumination system only utilizes the source-emitted light which ranges from 0 degree to around 80 degrees. Simulation results in section 4.7 showed that the reflectors extend in both the positive and the negative x directions increased the use range of emitted light, but they suffered the issue of CCFL shadow. The issue remains as the future work.
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6.2 Future work
For fabricating the Luminaire prototype, interpolating and cutting errors for the reflector shape can be improved by using CNC machines with smaller dimension tolerance. Reflector shape with higher accuracy can control ray-directions precisely.
Scratches and notches on the reflector surface can be alleviated through surface treatment or optical adhesive. High flux output CCFL is preferable to enhance illumination level.
In fact, the process of adhering the specular sheet to the model surface will cause reflecting properties to change slightly. Instead of adhering the specular sheet, coating a layer of high-reflective material on the model surface is an alternative.
Furthermore, considering the specular reflection issue for the Cholesteric LCD surface, microstructure design can be applied to redirect light to the viewing requirement. The additional prism array film can be attached to the front surface of the Ch-LCD to redirect the light distribution and enhance the image quality.
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Reference
[1] http://en.wikipedia.org/wiki/File:Scooter_headlights.jpg
[2] http://www.parc.com/research/publications/files/5706.pdf
[3]Andreas Timinger et al. Designing Tailored Free-Form Surfaces for General Illumination, Proc. of SPIE Vol. 5186 (2003)
[4] New Buildings Institute, Inc. Advanced Lighting Guidelines (2003)
[5] Yi-Pai Huang, Applications of Microoptical Components for Image Quality Enhancing on Portable Liquid Crystal Displays, PhD thesis, NCTU(2004)
[6] Magink display technologies, http://www.magink.com/
[7] Bahaa E. A. Saleh and Malvin Carl Teich, Fundamentals of Photonics, John Wiley
& Sons, Inc, p.4 (1991)
[8] Frank J. Pedrotti and Leno S. Pedrotti, Introduction to Optics, Prentice Hall, p.10-15 (1992)
[9] http://note.j-i-n.name/2005/06/photopic-luminous-efficiency-function
[10] Roland Winston, Juan C. Minano and Pablo Benitez, Nonimaging Optics, Elsevier Academic Press, p.415-418 (2005)
[11] Yankun Zhen, Zhenan Jia, Wenzi Zhang, The Optimal Design of TIR Lens for Improving LED Illumination Uniformity and Efficiency, Proc. of SPIE Vol. 6834, 68342K, (2007)
[12] Kuan-Ting Chen, The Design of Illumination System and its Applications to UV Illumination, Mater thesis, NCTU (2007)
[13]
http://en.wikipedia.org/wiki/Runge-Kutta
[14] Kendall Atkinson, Elementary Numerical Analysis 2nd edition, p. 329-330 (1993)
59 [15]
Optical Research Associates, http://www.opticalres.com/index.html
[16] James V. Valentino and Joseph Goldenberg, Introduction to computer numerical control, Prentice Hall
[17] http://en.wikipedia.org/wiki/CNC
[18] ProMetric 9.1, http://www.radiantimaging.com/