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Optics & Laser Technology 39 (2007) 1437–1442

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Analysis of the laser patterning inside light guide panel

Taehun Kima, Sohee Parkb, Hungkuk Oha, Yongjin Shinb,�

aDepartment of Mechanical Engineering, Ajou University, Wonchun-dong Yeongtong-gu, Suwon 443-749, KoreabDepartment of Physics, Chosun University, Seosuk-dong Dong-gu, Gwangju 501-759, Korea

Received 9 August 2006; received in revised form 2 October 2006; accepted 2 October 2006

Available online 13 November 2006

Abstract

The objective of this research is to evaluate the feasibility of the internal patterning in the light guide panel (LGP) by applying laser

engraving. LGP fabricated by the internal patterning is proposed as it offers better efficiency than is provided by bottom surface

patterning. The patterns fabricated by laser engraving system could improve efficiency by approximately 40%, requiring less energy

consumption in average brightness and uniformity than required by bottom surface patterned devices. Internal scatters were fabricated

by Q-switched 2nd harmonic Nd:YAG laser engraving system. The performance of the fabricated LGPs was measured and its results

analyzed. Modification of the shape of the LGP patterns from the simple geometry has been investigated to control the uniformity. The

proposed internal scatter embedded LGP with laser engraving could provide an alternative method to conventional bottom surface

scatters type with optimized patterns and geometry.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Laser engraving; Back-light unit (BLU); Light guide panel (LGP)

1. Introduction

A conventional liquid crystal display (LCD) is com-posed of a liquid crystal panel (LCP) and the back-lightunit (BLU). Because LCP itself cannot emit light fordisplaying its digitized information, the light produced bythe BLU is transmitted through the LCP. The BLUconsists of a light source, reflective sheet, light guide panel(LGP), diffuser sheet, and two prism sheets as shown inFig. 1 [1,2]. The LGP has scattering patterns inscribed onthe bottom surface. They convert point- or line-shapedillumination from the side of LGP into surface-shapedillumination at the top surface by changing the propagat-ing direction of the incident light, as shown in Fig. 1. Theyare generally inscribed on the surface of LGP by a varietyof methods, such as printing, injection, and directmarking.

The laser engraving method exploits material evapora-tion at a highly focused spot region when the material isilluminated using a high-power pulsed laser, as shown inFig. 2. The laser engraving method could create internal

e front matter r 2006 Elsevier Ltd. All rights reserved.

tlastec.2006.10.002

ing author. Tel.: +8262 230 6638, fax: +82 62 225 6659.

ess: yjshin@chosun.ac.kr (Y. Shin).

cracks, which are visualized as white point due to scatteringof light. In order to engrave the material without damagingthe surface, the material is mounted on an x–y translationstage that moves in front of the laser beam, which isfocused onto the material by lens after being expanded byoutput telescope, as in Fig. 2(b) [3–5].In this paper, we propose a novel LGP that has internal

scattering patterns inscribed within it by processing usingthe laser engraving method. This proposed LGP couldoffer improved efficiency in emitting the available light

Fig. 1. Diagram of LCD BLU and ray scattering.

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Fig. 2. The laser engraving and measurement system: (a) laser focusing

diagram, (b) schematic of laser system, and (c) schematic of measurement

setup.

Table 1

Advantages and disadvantages of patterning method (* very bady**** very

Suitability for mass

production

Luminance efficiency Flex

mod

Printinga **** ** **

Injection moldinga **** *** *

Laser engraving *** **** ****

aReferred from Ref. [6].

T. Kim et al. / Optics & Laser Technology 39 (2007) 1437–14421438

energy than an LGP, which has patterns on the surface.The laser engraving method could provide a cost-effectiveLGP without additional pre- and post-processing stepsrequired in conventional fabrication methods. It may alsoprovide a method to control the uniformity of theillumination, by virtue of the ability to create a 3Dgeometrical shape. The advantages and disadvantages ofthis laser engraving method are shown in Table 1 [6]. Themost prominent advantages of the laser engraving methodare the increase of luminance efficiency, flexibility inpattern modification and elimination of pre- and post-processing for patterning.

2. Experiment

The material for the experiment is a transparentpolymethyl methacrylate (PMMA) commonly used forthe fabrication of LGPs. The physical properties of

good)

ibility in pattern

ification

Elimination of pre- and

post-processing

Controllability of

luminance uniformity

** **

** **

**** ****

Fig. 3. The proposed LGP model (unit is mm).

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the PMMA used are a transmittance of 0.93 and arefractive index of 1.49. The dimensions of the LGPs usedwere 50� 40� 5mm3 (in length�width� height) and thepatterning area was 38� 38� 4mm3, as seen in Fig. 3. Noother sheets were applied on the LGP, so that thepropagation of light incident from a source would beonly affected by boundaries of the LGPs and by scatterswithin the LGPs. The scattering patterns on the surfaceof and inside the LGP were designed by conven-tional modeling and design software (Autodesk, Inc.,AutoCAD).

A Q-switched 2nd harmonic Nd:YAG engraving lasersystem (LOTIS TII Co., Model: LA-2136-E4) was utilized,as shown in Fig. 2(b). The system operates at a wavelengthof 532 nm, with 30mJ maximum pulse energy, and 50 mmspot size. For the experiment, a pulse energy of 15–16mJ at50Hz repetition rate was used.

The optical source applied in the experiment comprisedthree LEDs (3.5 cd, 1201 viewing angle, 10mm separated

Fig. 4. Engraved LGP samples with modified pattern: (a) slope pattern wit

Fig. 5. Schematic of proposed patterns: (a) bottom surface pattern, (b) inner lin

pattern which has gradually varying density.

with each other), as shown in Fig. 3. Fig. 4 shows the sideand top views of the pattern-engraved samples.A quantitative analysis of average brightness anduniformity of brightness of engraved LGPs was undertakenusing an imaging colorimeter (Radiant Imaging Co.,PM 1400 Series), as shown in Fig. 2(c). It has0.005–1010 cd/m2 measuring range, 73% accuracy, and16 bit CCD range.Experiments were carried out by following two steps.

Firstly, the effect of the internal patterns within the LGPon the average brightness and uniformity of the bright-ness was analyzed. We have chosen two types of patterns(inner linear slope pattern and inner curved slopepattern). All patterns were composed of spherical-shapedelements, which have 50 mm diameter. They werearranged with centers separated by 210 mm. To investi-gate the controllability of uniformity by adjustmentof the element’s density, the inner curved slope patternwas applied by gradually varying the density. In this

h uniform density and (b) slope pattern with gradually varying density.

ear slope pattern, (c) inner curved slope pattern, and (d) inner curved slope

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case, the pattern was divided into five sections, with theseparation between elements gradually decreasing awayfrom the entrance of light. Distances between centers ineach section were 250, 230, 210, 190, and 170 mm,respectively. Based on the results obtained from theprevious steps, to investigate the variation trends onaverage brightness and uniformity of brightness by thegeometrical shapes, modifications of pattern’s geome-trical shape were made.

Fig. 6. Measured brightness of the LGP having laser engraved internal scatters

figures) and left bottom (right 3D figures) side: (a) bottom surface pattern, (b

curved slope pattern which has gradually varying density.

3. Experimental results and analysis

The various patterns applied were shown in Fig. 5.The curve of the inner curved slope pattern shown inFigs. 5(c) and (d) has 200mm radius. Fig. 5(d) shows thepattern which has gradually varying density. Fig. 6 showsthe measured brightness of the LGP according to thepatterns in Fig. 5. Figs. 6(a–c) show the dependency oflocation and shape of patterns. Comparing Fig. 6(d) with

according to the patterns. Optical source was placed at the bottom (left 2D

) inner linear slope pattern, (c) inner curved slope pattern, and (d) inner

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Fig. 8. Schematic of modified patterns: (a) 1st modification and (b) 2nd

modification.

Table 2

Performance of LGP in average brightness and uniformity of brightness

Average brightness

(cd/mm2)

Uniformity of brightness

(Min./Max.)

Improvement of average

brightness vs. bottom

surface pattern (%)

Improvement of

uniformity vs. bottom

surface pattern (%)

Bottom surface pattern 1.083 0.457 — —

Inner linear slope pattern 1.895 0.521 74.976 14.004

Inner curved slope pattern 1.556 0.629 43.674 37.636

Inner curved slope pattern

having various densities

1.071 0.540 �1.108 18.161

Fig. 7. Graphical comparison of LGP in average brightness and

uniformity of brightness.

T. Kim et al. / Optics & Laser Technology 39 (2007) 1437–1442 1441

Fig. 6(a), it can be seen that the careful adjustment of thescattered density allows control over the uniformity ofbrightness.

The results are summarized in Table 2 and Fig. 7. Theaverage brightness for the patterning area of an LGP,which has bottom surface slope was 1.08 cd/mm2. TheLGPs, which had an inner linear slope pattern or an innercurved slope pattern exhibited average brightness of 1.89

and 1.55 cd/mm2, respectively. The uniformity of bright-ness on the LGP’s surface was calculated as the minimumvalue divided by maximum value of the brightnessmeasured at the LGP’s surface. When the value ofuniformity is 1.000, there is no difference betweenminimum and maximum value and its LGP has uniformlight intensity at any area on it. The uniformity of theLGPs, which had bottom surface pattern, inner linear slopepattern, and inner curved slope pattern were 0.457, 0.521,and 0.540, respectively. The average brightness anduniformity of LGPs having inner linear slope pattern andinner curved slope patterns were improved by about 75%and 14%, and about 43% and 37%, respectively,compared with that offered by the LGP having a bottomsurface pattern. From these, we conclude that innerpatterns enhance both the average brightness and basicuniformity of brightness of LGP.A number of alternative patterns, shown in Fig. 8, were

investigated, with the measurement results shown in Fig. 9.The difference between the brightness at the entrance(length is 0) and at the exit (length is 40) shown in Fig. 6(b)is improved, as shown in Figs. 9(a) and (b), by modifyingthe reference pattern. From this comparison, partialmodification of pattern’s geometry in inner patterningcould offer more control over the uniformity of brightnessthan is possible using conventional bottom surfacepatterning. And the graphical comparison of these isrepresented in Fig. 10.

4. Conclusion

The use of an inner pattern in LGPs is proposed, and itsimpacts on brightness and uniformity have been analyzed.From the experiments, the average brightness and uni-formity of the LGP is improved by up to 75%. We feel thatthese improvements are due to more efficient use of thelight from the light source than is possible using bottomsurface pattern because, the inner pattern can be located inthe LGP with a sloped angle to the light propagatingdirection that is not possible in the case of surface pattern.In addition to these improvements, the inner patternapproaches could offer more options for controlling theuniformity of brightness such as modification of the

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Fig. 9. Measured brightness of the LGP which is having modified pattern shape to increase uniformity. Optical source was placed at the bottom (left 2D

figures) and left bottom (right 3D figures) side: (a) 1st modification and (b) 2nd modification.

Fig. 10. Graphical comparison of LGP having modified pattern shape to

increase uniformity.

T. Kim et al. / Optics & Laser Technology 39 (2007) 1437–14421442

pattern’s geometrical shape, not possible in the surfacepattern.

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