Optics & Laser Technology 44 (2012) 1301–1306

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Optics & Laser Technology

0030-39

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/optlastec

Improvement of luminance and uniformity of light guide panel usingscatterer pattern by laser processing

Sohee Park, Yongjin Shin n, Eunseo Choi, Hyejoon Ma, Seungsuk Lee

Department of Physics, Chosun University, Seosuk-dong Dong-gu, Gwangju 501-759, Korea

a r t i c l e i n f o

Article history:

Received 7 November 2011

Received in revised form

21 December 2011

Accepted 21 December 2011Available online 23 January 2012

Keywords:

Back-light unit (BLU)

Light guide panel (LGP)

CO2 laser processing

92/$ - see front matter & 2011 Elsevier Ltd. A

016/j.optlastec.2011.12.040

esponding author. Tel.: þ82 62 230 6638, fax

ail address: yjshin@chosun.ac.kr (Y. Shin).

a b s t r a c t

Light guiding panel (LGP) is major device in back light unit (BLU) of liquid crystal display (LCD) and

determines the optical property of the LCD in terms of average luminance and uniformity. Previously

proposed scatterer patterns had problem of non-uniform luminance such as high concentration of

luminance near an entrance of a light source. In this paper, V-groove based scatterer patterns were

proposed to improve the performance of the LGP. With changing interval between V-grooves and depth

of V-groove itself, various patterns were prepared. The feasibility of the designed model was checked

with simulation, which calculates luminance value over the LGP surface based on ray tracing technique,

and the obtained results were analyzed to determine optimized fabrication conditions in experimental

implementation. In modeling of the scatterer patterns, two different kinds of the patterns were

considered: one is the pattern composed with regular V-groove interdistance at the same depth along

the LGP length, and the other has regular arrangement along the length but different depth. From

the results of simulation, experimental inscription with CO2 laser was done. The implemented pattern

consisting of V-grooves with linearly increasing depth of the slope of 3.21 has proved good possibility.

The proposed LGP pattern is expected to be a good alternative to expensive and time consuming

fabrication process, which could be a good solution for highly efficient LGP for LCD in the near future.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Back light unit (BLU) used as a dimmer in liquid crystal display(LCD) makes a line source such as a cold cathode fluorescent lamp(CCFL) or an array of light emitting diode (LED) convert a surfacesource through a light guiding panel (LGP). The transformed lightsource after the LGP can cover the entire LCD panel with uniformillumination. From the reason, the LGP is core device of the BLUand determines the overall performance of the BLU in terms of lowenergy consumption, high luminance and uniformity. Researcheson the LGP are mainly focused on the development of scatterers’pattern in polymethyl methacrylate (PMMA) that is widelyexploited as a material of the LGP [1,2].

Detailed transformation process of the light distribution is asfollows: the light incident from a side of the LGP is the line source.The incident light is dispersed through a diffuse sheet, whichcan firstly achieve rather uniform luminance. Uniformity of theincident light is changed further after passing through a prismsheet, which contributes enhancement of luminance performanceon the LCD panel. Efforts to reduce fabrication cost and improvethe luminance performance have been made continuously with

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applying various methods for technical standardization of aproduct and reduction of the number of the sheets. Among thesemethods, the study to increase the luminance as well as theuniformity has been in process by optimizing size, shape andgeometrical distribution of scatterer. In previous results, lessscatterers can improve the luminance due to low scattering loss,but it may yield poor uniformity. But higher density is not alwaysa good choice because higher scattering loss should be involved.Also because of absorption of the LGP itself, high density scat-terers’ pattern can reduce the luminance. From this point of view,the efficient design of scatterers’ pattern is one of the importantissues. As an example, reduced interdistance between scatterers inpattern produced scatterer’s density and the uniformity wasimproved [3,4].

Fig. 1 shows the scatterer patterns reported in the previousresearches. Molding method as seen in Fig. 1(a) had an advantageof high luminance but it was costly and time consuming infabrication and modification of the mold. The significant variationof LGP performance with the pattern fabrication conditions wasalso another hurdle to be overcomed in this method. V-cuttingmethod could be adopted for automatic processing. It was able toproduce uniform brightness as well as superior luminance. But itwas difficult to apply various patterns in fabrication, and residuallittle particles generated in the course of the processing requiredadditional post-treatments. Other candidate to overcome these

Fig. 1. Side view of a LGP having an engraved pattern; (a) a LGP with surface

pattern on the bottom, (b) a LGP with an inner pattern composed of three-

dimensional scatters, (c) a LGP having both surface and inner patterns, and (d) the

proposed LGP model based on, laser processing pattern.

Fig. 2. Schematic of CO2 laser processing system including laser focusing

Fig. 3. Scatterer pattern processed by CO2 laser processing system; (a) the proposed L

length, and (c) microscope images of laser processing V-grooves having different proce

S. Park et al. / Optics & Laser Technology 44 (2012) 1301–13061302

shortcomings was the laser injection method. This method couldprovide merits of high speed fabrication processing owing toutilization of fast scanning technique and easy modification.Moreover, it was easy to control the uniformity. This non-contactprocessing technique produced no redundant particles, which isdirectly related to removal of the post-treatment job. This methodproved to be low cost and time saving while keeping similarquality in terms of the uniformity compared with other existingsurface treatment methods.

Fig. 1(b) shows schematic with the scatterers formed inside theLGP, which uses three-dimensional laser processing system tocontrol each position of the scatterer. When the method wasemployed, the performance of the LGP proved the possibility ofthe internal scatterer in the application of the LGP. A disadvantageof this method was low luminance due to low density of scatterers[5,6]. In Fig. 1(c), a hybrid method is seen. With applying three-dimensional scatterers inside of the LGP, uniform brightness couldbe achieved. Additional surface pattern was added in this LGP. Thissurface pattern inscribed by the CO2 laser was able to enhance theperformance of luminance [7]. Fig. 1(d) is the schematic drawingof our proposed LGP. This method can hold the advantages to beobtained from the internal scatterers and the surface pattern. Thistechnique could be a good solution for the problems of high costand time-consuming complex process to be required for internal3-D processing and external surface treatment. To appreciate thefeasibility of the proposed method, simulation was performedwith models depth variant V-groove. The linear increase in thedepth of each V-groove produces slope in scatterer pattern [8].

Fig. 2 is the schematic of the laser system used for patterninscription at the bottom of the LGP. Beam scan type CO2 laser(SYNRAD’s HC-20 model, Max. output 17 W, Average output14 W, TEM00 mode) was exploited in the experiment [9,10].

diagram. Inset presents pattern processed with focused light beam.

GP model, (b) side view of implemented LGP with different V-groove depth along

ssing depth.

S. Park et al. / Optics & Laser Technology 44 (2012) 1301–1306 1303

2. Experiment

As a material of the LGP, PMMA (refractive index of 1.49) is atransparent acrylic resin having high mechanical strength. Thismaterial has superior characteristics such as fragile-resistant, strain-resistant, lightweight and fire-resistant property. Figs. 3 and 4present PMMA sample (50 mm in length�40 mm in width�3 mm in thickness) used for design and implementation, which issimilar to size of a LCD panel of a mobile phone. The height of themodel was set to 2.7 mm or less by considering the maximumprocessing depth of the V-groove. This value is 90% of the maximumallowable thickness. This condition was applied to both the simula-tion and the implementation.

Fig. 4. The proposed LGP model (unit is mm) and their pattern area.

Fig. 5. Luminance measurement system; (a) measurement setu

To evaluate the feasibility of the proposed model, ray tracingsoftware (LightTools, Optical Research Associates) was used. Thatcan perform design and analysis of optical devices. Based on thecalculated results, unique pattern based on the existing V-cuttingmethod was implemented in the area of 38 mm�38 mm. Lightsources used in the experiment were white LEDs having photo-metric flux 4.4 lm and view angle 701. These sources arecommonly used for mobile devices. Three arrayed-LEDs in theinterval of 10 mm were placed at the left side of the fabricatedLGP as seen in Fig. 4.

A non-contact plane luminance meter (Minilta’s Chroma MeterCS-100A), which is able to measure luminance, chromaticity, andcolor of reflective subject, was used to measure the luminance anduniformity of the fabricated LGP. The measurement was carriedout at different sections defined in Fig. 5(b). At each section, themeasurement of the luminance was performed at equal conditions.The section was largely classified into three parts such as incidencepart (A, B, and F), intermediate part (C, G, and H), and distant part(D, E, and I). The measured value was used to quantify the averageluminance and uniformity of the LGP.

3. Experimental results and analysis

In design of the LGP pattern, reduction of scattering loss andimprovement of the luminance uniformity due to the pattern ofthe LGP were major interest. Two different kinds of patterns weredesigned: one was the pattern with same processing depth alongthe length, and the other was the pattern with different proces-sing depth that increased along the length. The schematic of theprocessed pattern was shown in Fig. 6. From Fig. 6(a) to (c), depth

p, (b) measurement area classified by sections on the LGP.

Fig. 6. Side view of a LGP model based on the proposed pattern; (a) same depth of

0.1 mm, (b) same depth of 0.3 mm, (c) same depth of 0.5 mm, (d) slope of 4.11,

(e) slope of 3.21, and (f) slope of 2.31.

Fig. 7. Simulation results of luminance of the proposed LGP. Optical sources were pla

0.1 mm, (b) same depth of 0.3 mm, (c) same depth of 0.5 mm, (d) slope of 4.11, (e) slo

S. Park et al. / Optics & Laser Technology 44 (2012) 1301–13061304

at pattern was changed into 0.1 mm, 0.3 mm, and 0.5 mm,respectively. From Fig. 6(d) to (f), the depth at pattern was notthe same but increased with resulting different slope of 4.11, 3.21,and 2.31, respectively. Linearly varying slope gives that themaximum processing depth was 2.7 mm (90% of the samplethickness), 2.1 mm (70% of the sample thickness), 1.5 mm (50%of the sample thickness). The width of respective V-groove wasfixed to 0.3 mm, and this value was not changed at the experi-mental implementation. The simulation of the designed patternswas firstly carried out after modeling and the results wereanalyzed in terms of the average luminance and uniformity.

After determining of processing depth as explained above, theinterval between V-grooves was adjusted to present better luminanceperformance. The optimized value was applied to laser processingconditions. Figs. 7 and 8 presented the luminance performance underthe condition of the interval of 3.0 mm and 1.5 mm. The simula-tion deduced important design factors in pattern. As the intervalincreased, incident light did not propagate further through theV-groove in the LGP, which means that the incident light was notdispersed widely and uniformity was directly degraded. When theinterval was narrowed to 1.5 mm, the overall results shown in Fig. 7were changed to those seen in Fig. 8. When the depth was 0.1 mm,the average luminance and uniformity was 571.4 cd/mm2 and 11.7%from the calculated results shown in Fig. 8(a). The obtained averagevalue was the maximum among the results of Fig. 8. As the depthincreased to 0.3 mm, the average luminance and uniformity waschanged to 559.4 cd/mm2 and 23.2%. At the depth of 0.5 mm, thevalue in average luminance and uniformity was varied to 502.2 cd/mm2 and 31.5%. These results tell that the deeper the V-groove depth,the lower the average luminance but the higher the uniformity. Thedepth of 0.5 mm rather than 0.3 mm or 0.1 mm resulted in highlyconcentrated luminance distribution near the entrance with provid-ing the lowest average luminance.

To cope with the problem, the pattern at equal depth werereplaced with that of increasing depth as well as the position of

1000

900

800

700

600

500

400

300

200

100

0Surface Receiver

Luminance(cd/mm2)

ced at the top side. Interval between V-grooves was 3.0 mm; (a) same depth of

pe of 3.21, and (f) slope of 2.31.

1000

900

800

700

600

500

400

300

200

100

0

Surface ReceiverLuminance(cd/mm2)

Fig. 8. Simulation results of luminance of the proposed LGP. Optical sources were placed at the top side. Interval between V-grooves was 1.5 mm; (a) same depth of

0.1 mm, (b) same depth of 0.3 mm, (c) same depth of 0.5 mm, (d) slope of 4.11, (e) slope of 3.21, and (f) slope of 2.31.

Table 1Performance of the designed LGP in average luminance and uniformity according

to the patterns. Simulation was carried out at the interval between V-grooves

of 1.5 mm.

Average luminance

(cd/mm2)

Uniform of brightness

Min/Max (%)

(a) Plane (depth 0.1 mm) 571.4 11.7

(b) Plane (depth 0.3 mm) 559.4 23.2

(c) Plane (depth 0.5 mm) 502.2 31.5

(d) Slope (angle 4.11) 684.9 83.1

(e) Slope (angle 3.21) 677.3 84.3

(f) Slope (angle 2.31) 644.8 72.9

Table 2Performance of the implemented LGP in average luminance and uniformity

according to the patterns.

Average luminance

(cd/mm2)

Uniform of brightness

Min/Max (%)

(a) Plane (depth 0.1 mm) 69.1 12.3

(b) Plane (depth 0.3 mm) 235.0 14.7

(c) Plane (depth 0.5 mm) 253.6 34.4

(d) Slope (angle 4.11) 566.1 67.0

(e) Slope (angle 3.21) 571.2 86.8

(f) Slope (angle 2.31) 601.0 81.7

(a) (b) (c) (d) (e) (f)

Fig. 9. The average luminance and uniformity of the implemented LGP; (a) same

depth of 0.1 mm, (b) same depth of 0.3 mm, (c) same depth of 0.5 mm, (d) slope of

4.11, (e) slope of 3.21, and (f) slope of 2.31.

S. Park et al. / Optics & Laser Technology 44 (2012) 1301–1306 1305

the V-groove was moved backward from the entrance. Conse-quently, use of different depth, or linearly increasing depth, washelpful for improvement of the average luminance and unifor-mity. Table 1 listed the simulation results obtained from thepattern with non-uniform depths. The average luminance rose torange of 644.8–684.9 cd/mm2 while the uniformity increased to83.1% and 84.3% at 4.11 and 3.21. Exceptionally, at the slope of2.31, the uniformity was 72.9% that is rather lower value. Theresults present that the pattern with the slope of 3.21 shown inFig. 6(e) was best choice for improving the performance ofthe LGP.

Table 2 and Fig. 9 were obtained from the implemented CO2

laser-processed LGP, which was fabricated with conditions givenin the simulation. When the incident light comes to pattern ofFig. 6(a), which has the depth of 0.1 mm, the average luminanceand uniformity were measured as 69.1 cd/mm2 and 12.3%,respectively. At the depth of 0.3 mm, the measurement valueswere 223.0 cd/mm2 and 14.7% in the average luminance anduniformity. When the depth reached 0.5 mm, the average lumi-nance and uniformity increased to 253.6 cd/mm2 and 34.4%. Fromthe measurement results, the deeper depth showed the better

average luminance. These results were not matched with thoseof the simulation. It could be explained that the specification ofthe LEDs used in the experiment was not matched with that of

S. Park et al. / Optics & Laser Technology 44 (2012) 1301–13061306

simulation. The dependency of the luminance on the slope wasseriously affected to the divergence angle of LED and distortedbeam shape (that is not circular one but elliptic shape).

As the results of measurement, the average luminance wasplaced in range of 566.1–601.0 cd/mm2. The variation was notsignificant according to the pattern variation, but the valueincreased by twice compared with those of pattern with the uniformdepth. In case of pattern with the slope of 4.11, the incident lightmay propagate farther but the uniformity was reduced to 67.0%.When the slope was changed to 3.21 and 2.31, the uniformitysignificantly changed up to 86.8% and 81.7%. Those were wellmatched with the results from the simulation.

In conclusion, the pattern having the interval of 1.5 mm andthe slope of 3.21 were chosen to the best conditions for efficientLGP performances providing the improvement by twice in theaverage luminance and 7 times in the uniformity.

4. Conclusion

To improve the performance of the LGP, the scatterer patternsin the LGP was specially designed based on the V-groove pattern,which u linearly increasing depth along the LGP. The possibility inthe application of proposed patterns to improve the performanceof the LGP was firstly checked by the simulation, and then theexperimental implementation was carried out using the para-meters determined from the simulation results. Analysis of thesimulation and experimentally measured values on the averageluminance and uniformity was presented to demonstrate thefeasibility of the proposed technique on the enhancement of theperformance of the LGP. The pattern with the interval of 1.5 mmand the slope of 3.21 were the best conditions for efficient LGPperformances.

Introducing the optimized depth of V-groove and intervalbetween V-grooves is expected to maximize the advantage of

the proposed technique. Determination of experimental proces-sing parameters based on the simulation research can reducetime wasting in specialized pattern design, fabrication and eva-luation of adaptability of the proposed design with simplifyingschedule of entire processing.

Acknowledgments

This study was supported by research funds from ChosunUniversity, 2010.

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