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| November 2005 |
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| Contributing to even better image quality for liquid crystal displays |
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| With the continued growth in the popularity of flat-screen televisions in recent years, demand for backlights (CCFLs) used in liquid crystal displays (LCDs) is increasing. Here we will present the features and future trends of the phosphors that affect the quality of CCFLs. |
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| What role do phosphors play? |
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A CCFL (Cold Cathode Fluorescent Lamp) is a long, thin fluorescent lamp tube with an outside diameter of approximately 1.5 to 4 millimeters and a length of about 200 to 1,300 millimeters. CCFLs owe their pervasive use as backlights in LCDs to superior characteristics such as their compact size, low price, and long life. The CCFL tubes are bent into L-shapes or U-shapes according to their applications, with shorter ones being used for backlights in devices like notebook computers and longer ones being used in televisions. The phosphors are three colored powders (red, blue, and green), only a few microns thick, applied to the inside of the CCFL tubes. The coloration of these phosphors makes the CCFL glow white, allowing it to function as a backlight. The white light of the CCFL is again divided into three primary colors of light through the color filter, and creates images on the display screen (see figure).
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Phosphors for use in CCFLs. Under natural lighting all three phosphors look like white powder (left), but they emit their respective colors when exposed to ultraviolet light (right). |
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| How are our phosphors superior? |
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Long life is one of the product's strengths.
[A phosphor lifespan test] |
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Each phosphor particle is analyzed. (size in µms) |
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Having resolved problems with former weaknesses in viewing angle and response time, LCDs are headed toward larger and larger screen sizes.
As LCD screen size expands, longer CCFLs will naturally be required. The three colors of phosphors used in CCFLs are combined to produce the best whiteness for a backlight. However, if the phosphors are not applied evenly, unwanted variations in whiteness will be produced. This can lead to one end of the CCFL tube looking more reddish or bluish than the other, which is called "tube end color difference". Therefore, the longer CCFL tubes become, the more color difference gets produced.
If a certain color looked different depending on where it appeared on the screen, due to tube end color difference, it would be unacceptable. Yet, in conventional phosphor application processes in CCFL manufacturing, there is no way to avoid slight unevenness in the phosphor coatings and the resulting color differences. Is there any way to keep the color differences to an acceptable level without altering the phosphor application process? At Kasei Optonix, we have solved this problem by controlling the physical properties of the phosphors. We have succeeded in developing phosphors that allow for CCFLs with extremely small tube end color difference applicable to large-screen LCDs.
Another required characteristic of LCDs is color reproducibility. This is how closely the color on the screen can match the color of the real thing. In displays using CCFLs, this is largely determined by the coloration of the three-color phosphors themselves. We have improved the colors emitted by the phosphors, developing high color reproduction phosphors with a dramatically increased color range (approximately 140% over our previous phosphors). |
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| How did Kasei Optonix achieve these new developments? |
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The total luminous flux (the total amount of light emitted in all directions from a light source) is measured with an integrating sphere that evenly mixes multiple types of light. |
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Our backbone product up to now has been the phosphors for CRTs (picture tubes) used in conventional televisions. Recently, however, we began developing phosphors for CCFLs, the backlights for LCDs, in response to the popular emergence of flat-screen televisions such as plasma and LCD televisions.
In the development process, ideas that came out of our daily discussions internally and with users became a major breakthrough. Since the product sees slightly different applications depending on the user, we have stayed in close touch with users, searching for and establishing control points that had not previously existed, and strived to provide a reliable product that is easy for all kinds of users to employ. Also, in order to have the new high color reproduction phosphors gain recognition, we needed to get not only the lamp manufacturers who are the direct users of the product, but also computer and television manufacturers to understand the benefits of the product. This process also required a significant amount of time to be structured.
The phosphors that provide for very low tube end color difference in CCFLs have been earning high praise for their use in large-screen LCDs. Our phosphors offering high color reproduction are also starting to get attention as an element in the next generation of LCDs. |
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| What are the future challenges and overall outlook? |
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The flat-screen television market is expected to continue growing, with forecasts for dramatic market expansion, in particular over the next five years.
In sales of various goods on the Internet where the consumer makes buying decisions sight unseen, the accurate reproduction of the colors of items on the display is important. From this standpoint, phosphors providing high color reproducibility are very attractive elements for both televisions and personal computers.
Meanwhile, the light emitting diode (LED) has made its debut as a major player in the backlight market. LEDs have advantages such as high color reproducibility and non-use of mercury. However, they cannot compete with CCFLs in terms of price and product consistency. This means that if CCFLs continue to achieve high color reproducibility on par with LEDs, CCFLs are likely to maintain an active role in the market in the future.
CCFLs using our phosphors are already beginning to be adopted in large-screen liquid crystal televisions, and with information that several companies are set to release new models of high color reproduction televisions in the near future, orders for phosphors are expected to increase even further.
As flat-screen high-definition televisions continue to gain popularity and screen sizes continue to grow, the level of quality demanded from backlights will keep getting higher. Since phosphors hold the key to better CCFLs, we will continue to develop and improve our phosphors, always pushing the technology forward through ceaseless research and development. |
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Kasei Optonix, Ltd.
1060 Naruda, Odawara-shi, Kanagawa 250-0862
TEL: [+81] (0)465-36-1111 |
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