Journal of the Korean Physical Society, Vol. 58, No. 4, April 2011, pp. 756∼760 Optimization of the Buffer Gas in a Flat Fluorescent Lamp for Brightness Improvement Taewon Kim School of Electrical and Electronic Engineering, Chung-Ang University, Seoul 156-756, Korea Kangwhan Lee∗ School of Information Technology, Korea University of Technology and Education, Cheonan 330-708, Korea (Received 17 June 2010, in final form 17 February 2011) The influence of the buffer gas pressure and composition on the performance of a flat fluorescent lamp (FFL) i.e., brightness, is investigated for a 4000 FFL. To find the optimal buffer gas condition for the 4000 FFL’s maximum performance, specifically brightness, the total gas pressure was varied from 30 Torr to 5 Torr with Ne:Ar composition ratios of 80:20, 90:10, and 0:100. As a result, when the total gas pressure and the composition were 25 Torr and Ne:Ar with a ratio of 80:20, respectively, the maximum brightness could be obtained. This agreed well with the result of a simulation that was performed with the volume-averaged (global) model of the plasma chemistry of Ne and Ar gases and their mixtures (including constant Hg gas). PACS numbers: 52.80.Yr Keywords: Flat fluorescent lamp, Buffer gas, Discharge DOI: 10.3938/jkps.58.756 I. INTRODUCTION that has a cylindrical bar shape with a very small ra- dius. The CCFL has a high brightness and a long life of use. However, when a LCD uses a conventional CCFL, The flat fluorescent lamp (FFL) is a flat light source, the brightness uniformity becomes a problem. There- emitting light from a large active area. It does not need fore, to increase the brightness uniformity, the backlight a light distribution element, thus reducing the cost for unit, which uses the CCFL as a light source, needs op- the whole lighting panel. Therefore, FFLs are suitable tical members, such as a light guide panel (LGP), a dif- for large-area flat light sources, such as photolithogra- fusion member and a prism sheet. Consequently, the phy light sources and broadcast light sources. The liquid LCD device using the CCFL becomes large in size and crystal display (LCD) is also a well-known application of heavy in weight due to the optical members. To solve the FFL. The LCD, which features a thin lightweight this problem, a flat fluorescent lamp as backlight unit configuration and low electricity operability, has shown light source has been suggested. Figure 1(a) shows a rapid expansion in its range of applications with improve- 3200 flat fluorescent lamp. This FFL backlight unit offers ments in liquid crystal materials and development of fine longer lifetimes and better light uniformity compared to pixel processing techniques, and is widely used in house- that of the typically cylindrical CCFL. Another bene- hold television sets, desktop/notebook computer moni- fit is that the number of FFLs required to illuminate tors, large-sized flat panel television sets and so forth. an LCD is much less than the number of CCFLs. For However, unlike the cathode ray tube or the plasma dis- instance, a 4000 LCD TV will typically require at least play panel, the liquid crystal display panel does not emit 10 CCFLs while a FFL-based backlight system require any light by itself, but merely changes the orientation or just one FFL. Even at larger sizes, only one FFL will be the arrangement of the liquid crystal. This characteris- enough. tic of the LCD makes it need, at the rear of the liquid As the range of the application of liquid crystal dis- crystal display panel, a backlight unit to supply the light plays becomes broad, great effort has been made to im- to the information display surface uniformly. In a con- prove their electro-optical characteristics, such as view- ventional LCD device, the light supplying backlight unit ing angle, contrast ratio, gray-scale capability, and so generally uses a cold cathode fluorescent lamp (CCFL) forth. Thus, as a basic requirement, brightness improve- ment is still the critical issue. In this paper, to find ∗E-mail: [email protected]; Author to whom correspondence should the maximum brightness of a flat fluorescent lamp, for be addressed which Argon and Neon have been widely used as the -756- Optimization of the Buffer Gas in a Flat Fluorescent Lamp ··· – Taewon Kim and Kangwhan Lee -757- flat fluorescent lamp. The channels are spaced apart at a regular interval. A cross section of each of the channels has a semi-elliptical shape and an aspect ratio of about 0.24. The aspect ratio is defined by the channel height divided by the channel width of the channel cross sec- tion. The shape of channel cross section and the aspect ratio are also good parameters that could be optimized in the future. The channels are connected to each other by a gas ‘pass-hole’. Through these holes, the buffer gas and the mercury vapor can be diffused uniformly. The upper and the lower substrates are bonded with each other by a sealing paste, such as a frit including metal and glass. The frit has a lower melting point than glass. The frit is disposed along the edge portions of the upper and the lower substrate, and both substrates are com- pressed when the frit is heated so that both substrates are combined with each other. Then, air in the channels is expelled, and the discharge gas is injected into the channels. The getter, including mercury, is attached to the flat fluorescent lamp and then heated. In that case, the mercury in the getter is vaporized and then injected Fig. 1. (Color online) (a) Real flat fluorescent lamp for a and diffused into the lamp, and the getter is removed. 3200 liquid crystal display backlight unit and (b) sketch of a Usually, in the case of a 4000 flat fluorescent lamp, 80 flat fluorescent lamp including the inside layer structure. mg mercury is used in the getter. The inner surface of upper substrate except for the portion corresponding to the channels makes contact with the lower substrate due buffer gas, including mercury as a 253.7 nm wavelength to the pressure difference between the light-emitting dis- ultraviolet light generation source, we focused on the op- charge spaces and the atmosphere. The discharge gas timization of the buffer gas pressure and the composition includes argon (Ar), neon (Ne) and mercury (Hg) vapor. ratio. The paper is organized in four sections as follows: Band-shaped electrodes are made in both edges on the Firstly, in Section II, I shall briefly explain the flat fluo- outer surfaces of the upper and the lower substrates cor- rescent lamp and the processes to produce the lamp used responding to both ends of the channels. The first and in this experiment. Section III is devoted to the descrip- the second electrodes extend in a direction that is sub- tion of the measurement processes, apparatus and the stantially perpendicular to the direction in which the simulation with a volume-averaged (global) model. The channels extend. Both electrodes include a metal like measurement and simulation results are discussed in Sec- copper (Cu) having a high electrical conductivity. The tion IV. The conclusion and future work are presented first and the second electrodes are also formed on the in Section V. outer surface of the lower substrate at the same posi- tions as the upper substrate electrodes. When a dis- charge voltage is applied to the discharge gas through the first and the second electrodes, the discharge gas, II. FFL PRODUCTION PROCESSES mercury, generates ultraviolet light. The glass surfaces of the lamp which contact the discharge gases are coated The flat fluorescent lamp consists of a upper glass sub- with Y2O3 to prevent sodium ions from flowing out into strate and a flat lower glass substrate. Soda lime glass the discharge spaces. is used as both substrates. When the lower and the up- The flat fluorescent lamp further includes a phosphor per glass substrates are combined with each other, to layer and a light-reflecting layer in the case of the lower make a number of channel-shaped light emitting dis- substrate. The light-reflecting layer is formed on the in- charge spaces, the upper glass substrate is manufactured ner surface of the lower substrate. The fluorescent layer by using a glass-forming process. In detail, a flat glass is disposed on the light-reflecting layer. The fluorescent plate is heated; then, the air above the heated glass is layer, which transforms the ultraviolet radiation gener- 3 inhaled through many tiny air holes in a desired-shape ated from the mercury excited species (6 P1) into visible mold right over the heated glass. Spontaneously, the light, is optimized for 253.7 nm wavelength ultraviolet heated glass is brought into the final shape of the inte- light. The fluorescent layer includes red, green and blue rior of the mold to form the upper substrate with chan- phosphors. The red, green and blue phosphors trans- nels. The upper glass substrate includes a plurality of form the ultraviolet light into red, green and blue light, channels (33 channels in the case of a 4000 FFL). The respectively. Substantially same amounts of red light, channels extend substantially parallel to an edge of the green light and the blue light generate white light. A -758- Journal of the Korean Physical Society, Vol. 58, No. 4, April 2011 light-reflecting layer having a high optical reflectivity is generation source. The detailed simulation procedure formed on a lower substrate and is made with aluminum and results will be published in another paper in the fu- oxide (Al2O3).