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1、1 功率最小化背光的TFT - LCD的亮度和显示并行 对比缩放 摘要 本文提出了一种并行亮度和对比度 缩放CBC内的冷阴极荧光灯技术 CCFL作为背光的TFT - LCD显示器。此技术的目的 在节约电力减少了背光照明而保留通过图像保存图像保真度对比。首先我们如何解释和CCFL作品展示如何模型之间的背光照明和非线性功率消耗。接下来我们提出的对比度失真指标以量化背光缩放后的图像质量损失。最后我们制定和优化解决CBCS优化问题以尽量减少目标的忠诚和力量度量。实验结果表明平均的3.7X可实现节能只有10的对比失真。 1简介 使用电池供电的电子产品上的研究指出冷阴极荧光灯CCFL背光的液晶显示器显示主
2、宰了整个系统的能量消耗1。在SmartBadge制度例如显示器消耗28.628.6和50的总功率活跃闲置待机模式分别1。为了减少电力消耗的背光研究人员2 3有提出了背光尺度的概念。该背光缩放动态调暗背光技术以保护它的权力消费的同时增加LCD的透射率小组以补偿的图像保真度下降造成的损失背光。图像保真度之间的相似性定义为原来与背光-缩放图像。如果背光缩放图像是相同的原始图像的亮度范围每个像素这些方法被称为亮度不变背光缩放那么就没有后背光保真度损失缩放。然而当一个更加积极的背光缩放政策是用来获得更大的权力储蓄亮度不变性不再是实现和引起的失真降低了图像的保真度。图像保真度可衡量扩大后的背光亮度差异。然
3、而亮度指标太严格有效的背光缩放政策。由于调光的背光直接限制的动态 范围内的图像亮度亮度不变的背光缩放政策往往是过于保守提供巨大的能量 储蓄。在本文中我们提出利用作为一个图像对比度指标以衡量背光缩放后的图像保真度?2 1.1术语 下面的说明光度数量约1背光的TFT - LCD显示器如图1。光通量流明的 光的能量发射率修正为标准人类视觉光谱响应。发光强度坎德拉被定义为每一个steradiam光通量流明锶 - 单位固体角。夜光intensitycan用来表征 发出的光功率从1点光源如光线灯泡。照度LUX的定义为1流明的光通量单位面积cd/m2的。照度可用于表征光从表面发射功率。大多数光米例如摄影的目
4、的测量照度数量。该光通量不得乘坐合格后表面平行 这样的旅行距离为光强度降低增加。亮度流明定义为每单位面积流明 steradiamlm/m2/简。亮度是用来率最高亮度的CRT或LCD显示器4。 图1CCFL背光和光度方面插图 在本文中我们使用背光因子表达背光照明的百分比并以透射表达了在TFT - LCD的半透明。该背光因素和TFT - LCD的透射率决定认知亮度 从TFT - LCD显示器。 1.2背光的TFT - LCD显示器 一个背光或透射的主要成分的TFT -液晶显示子系统包括视频控制器框架 缓冲区视频接口TFT - LCD面板以及背光。该帧缓冲区是一个由软件使的内存部分申请提供视频数据的
5、视频控制器。该从应用的视频数据存储在帧缓冲区 CPU的。视频控制器和视频数据的获取产生适当的模拟VGA或数字DVI视频在视频接口信号。视频接口携带之间的视频控制器和TFT - LCD的视频信号 3 显示。在TFT - LCD显示器和视频数据接收产生适当遮荫 - 透射 - 每个像素 根据其像素值。所有关于像素的透射液晶面板是从后面照亮背光。对于观察员显示的像素是光明的如果它的透过率高即在对状态这意味着它传递背光。关于另一方面显示的像素看起来是黑暗的如果它的透过率低即在关闭状态这意味着它阻止背光。如果透射率可以调整到两个以上的不同层次之间的对和关闭状态则可以显示的像素在灰度。如果灯罩颜色可以作为红
6、绿或蓝使用不同的颜色过滤器然后可以显示像素的颜色混合3分在不同的颜色在不同的像素灰度。换言之一个像素的亮度知觉取决于它的透射率和背光照明。 现有的TFT - LCD显示器大多使用的CCFL背光其无与伦比的亮度密度感谢 - 发光的最光在最短的形式因素。冷阴极管可旨在生成任意颜色这是至关重要的音响纯白色的背光应用。该冷阴极管制造技术是成熟的以便它的成本减至最低。该CCFL背光的功耗然而是相当高的比较在TFT -液晶面板。 一个透射观察对象的亮度为L产品的光源亮度的b和对象吨透射率5。对于一个像素背光的TFT -液晶显示器其透过率是其像素值x的函数因此它的亮度L的观察 L tx?b 1 周围环境的光
7、线是不考虑在这里因为它没有为透射TFT - LCD的效果较一反射或半透之一。图2描述的关系方程1假设透过率是一个线性函数 像素的值。 图2标准化的像素亮度值函数右是背光因子b和在TFT - LCD产品透射函数中心。 4 在非背光规模的TFT - LCD显示器背光b总是固定在满功率。该背光技术在缩放2 3中B日益减少的像素值从x到x的 xxb 2 xx/b 3 保持同样属这些做法有以下缺点 ?方程2不能保持亮度不变根据公式1。 ?不饱和像素间的反差是不失真考虑。 ?该软件为基础的方法具有较高的能量/性能开销。 ?不正确的CCFL照明建模为一个线性功能的权力。 在本文中我们提出解决方案以克服上述上
8、述缺点。在第二节我们的CCFL特性照明和电力消耗。我们建议在第3节调整透过率函数T的像素值而不是十最佳CBCS问题介绍了第0。部分5提出的结论之后实验结果第6条。 2灯管照明和电源建模 CCFL背光单元组成的荧光灯等驾驶的DC - AC逆变器光反射。阿CCFL是一与电极两端密封的玻璃管。管填充用惰性气体氩气和汞。玻璃的内表面 管涂有荧光粉当它发出可见光激发光子。波长或颜色的可见光依赖于气体和荧光粉的类型。在液晶显示器背光应用红色绿色适当组合和蓝色荧光粉生产所需的三波段白光。否则显示的图像将颜色改变。 冷阴极管转换成可见光电力能源所谓的气体放电现象。当一个高电压 应用到电极上的转向灯电动弧生成的
9、电离气体并让电流来流。中移动的离子注入能量的碰撞汞原子。汞原子的电子接收能源和跳跃到更高能级的发光其次 紫外线光子时回落到原来的能源水平。在电离气体进行的电流。该导体阻抗的气体不同的5 是金属导体具有非线性行为减少为目前的增加。因此CCFL的驱动已被替代电流交流以避免潜在的爆炸。 一个DC - AC逆变器通常是用来驱动一个灯管电池供电的应用。一个DC AC逆变器基本上是一个开关振荡器电路用品从一个高电压交流电低电压的电池。名义交流变频现代CCFL是避免闪烁在50-100千赫范围。该额定工作电压要高于500 VRMS的保留惰性气体电离。 为了节省电池供电应用的能源调光控制是为DC - AC逆变器
10、所需的功能。不同冷阴极灯管的调光方法已被使用包括线性电流脉冲宽度调制电流斩波6。在一的DC - AC逆变器具有调光控制模拟或数字输入信号暴露调整灯管照明。最受设计的DC - AC逆变器具有较高的电效率gt 80和线性响应输出电力输入功率。大多数荧光灯然而光效率低lt20和非线性的输出光功率与反应输入功率7。 2.1 CCFL特性 冷阴极管照明是一个复杂的驾驶功能当前环境温度预热时间灯管年龄驾驶波形灯尺寸和反射设计7。对于CBCS只有驱动电流是可控的。因此我们模型湟跫苷彰髯魑坏那缌骱凸芎雎云渌问涞牡湫凸叵档腃CFL照明和在动力图3a所示。冷阴极管照明增加单调上升的动力前达到80的全部动力。超过8
11、0冷阴极管照明开始饱和。这种饱和现象因为封闭电离气体已完全履行和不能释放更多的光子。此外增加温度和管内压力进一步抑制放电7 8。这一观察表明减少光冷阴极管效率的地区是不饱和青睐功率的应用程序。 2.2冷阴极管照明/动力表征 我们使用的照明功能逐步表征CCFL的功耗作为一种照明功能 6 该背光factorb01代表正常化背光照明这是动态的CBCS可控政策。 该直流空调模拟或数字调光控制输入逆变器并不总是成正比的线性输出 背光照明。需要仔细校准推导出正确的映射之间的背光因子B和调光控制输入的Qb项。阿诸如精密亮度米9提供准确的读数绝对照度。这些昂贵的米然而通常无法使用电子实验室。我们发现绝对照度读
12、数无须校准的CCFL背光 缩放应用。准确的摄影测光表可以达到目的到目前为止因为它是能够感应轻微亮度差异。我们以此作为重规模轻米调整背光和TFT - LCD的同时而保持同样的照度。我们开始与测量照度的最高CCFL背光当b 1时申请调光控制的Qb 1时和最低液晶透射率x 0时0255。在透射X是获得显示纯灰色图像其中红色绿色蓝色 x对于每一个像素。 x是的透过率增加直到光米可以给人一种变化和不同的阅读报告。然后减少通过减少直至背光调光控制Q因子B报告表上读数。由于改变了TFT LCD液晶显示器灰度透射众所周知在改变背光是断言是相同的。记录Q表调光控制值的背光因子B 255 - X的/ 256。在同
13、时间背光Pbacklight功耗也是测量和记录。重复上述过程在x 0 . 255。插后我们可以得到的Qb和Pbacklight二。该为彩色背光的TFT - LCD 10结果显示在图3a。入式4堵得到以下参数PLin0.4991 PSat0.1489 CLin0.1113 CSat0.6119 Bs0.8666 5这种权力模式将被纳入第1解决问题的最优CBCS。 7 3结论和未来工作 我们已经提出了冷阴极背光技术的CBCSTFT - LCD显示器。该建议旨在保护技术 通过降低功率的背光照明同时保留形象逼真的图像通过对比保存。我们如何解释和CCFL的作品展示了如何模型非线性度之间的背光照明和电力消
14、费。我们提出了相反的失真指标以量化后背光缩放的图像质量损失。我们已制定和优化解决CBCS优化问题随着减少保真度和功率指标的目标。实验结果表明一个平均的3.7X省电可以实现10的对比度失真。该CBCS我们在此提出的技术文件仅适用于静态图像。未来研究但是应该考虑将其应用于视频申请。自背光因素决定是基于每个帧单独的背光因素可能会改变显着因为在连续帧的直方图差异显着。在巨大的变化将背光因素引入帧间亮度失真的观察员。何时该CBCS技术是适用于视频应用如1 MPEG2解码器该背光因素的变化应有限的这样的变化太微妙被觉察人类眼睛。 8 Power Minimization in a Backlit TFT-
15、LCD Display by Concurrent Brightness and Contrast Scaling Abstract This paper presents a Concurrent Brightness and Contrast Scaling CBCS technique for a cold cathode fluorescent lampCCFL backlit TFT-LCD display. The proposed technique aims at conserving power by reducing the backlight illumination w
16、hile retaining the image fidelity through preservation of the image contrast. First we explain how CCFL works and show how to model the non-linearity between its backlight illumination and power consumption. Next we propose the contrast distortion metric to quantify the image quality loss after back
17、light scaling. Finally we formulate and optimally solve the CBCS optimization problem with the objective of minimizing the fidelity and power metrics. Experimental results show that an average of 3.7X power saving can be achieved with only 10 of contrast distortion. 1 Introduction Previous studies o
18、n battery-powered electronics point out that the cold cathode fluorescent lamp CCFL backlight of an LCD display dominates the energy consumption of the whole system 1. In the SmartBadge system for instance the display consumes 28.6 28.6 and 50 of the total power in the active idle and standby modes
19、respectively 1. To reduce the power consumed by the backlight researchers 23 have proposed the concept of Backlight Scaling. The backlight scaling technique dynamically dims the backlight to conserve its power consumption while increasing the transmissivity of the LCD panel to compensate for the ima
20、ge fidelity loss due to reduced backlight. Image fidelity is defined as the resemblance between the original and backlight-scaled image. If the backlight-scaled image is identical to the original image in terms of the brightness of each pixel these approaches are called brightness-invariant backligh
21、t scaling then there is no fidelity loss after backlight scaling. However when a more aggressive backlight scaling policy is used to gain greater power savings the brightness invariance is no longer attainable and the induced distortion degrades the image fidelity. Image fidelity can be measured by
22、the brightness variance after backlight scaling. However the brightness metric is too strict for efficacious backlight scaling policies. Since dimming the backlight directly limits the dynamic range of the image brightness a brightness-invariant backlight scaling policy is usually too conservative t
23、o deliver great energy savings. In this paper we propose using the image contrast as a metric to measure the image fidelity after backlight scaling. 1.1 Terminology The following photometric quantities are illustrated around a backlit TFT-LCD display in Figure 1. Luminous flux lumen is the emission
24、rate of light energy corrected for the standardized spectral response of human vision. Luminous intensity candelais defined as one lumen of luminous flux per steradiam sr - unit of solid angle. Luminous intensitycan be used to characterize the optical power emitted from a spot light source such as a
25、 light bulb. Illuminance lux is defined as one lumen of luminous flux per area cd/m2. Illuminance can be used to characterize the luminous 9 power emitted from a surface. Most light meters e.g. for photographic purpose measure the illuminance quantity. The luminous flux may not travel in parallel af
26、ter passing the surface so that the light intensity decreases as the travel distance increases. Luminance nit is defined as lumen per area per steradiam lm/m2/sr. Luminance is used to rate the maximum brightness of CRT or LCD monitors 4. Figure 1: Illustration of CCFL backlight and photometric terms
27、. In this paper we use backlight factor to express the percentage of the backlight illumination and transmissivity to express the translucence of the TFT-LCD. The backlight factor and TFT-LCD transmissivity determine the perceived luminancefrom the TFT-LCD display. 1.2 Backlit TFT-LCD display The ma
28、jor components of a backlit or transmissive TFT-LCD display subsystem include the video controller frame buffer video interface TFT-LCD panel and backlight. The frame buffer is a portion of memory used by software applications to deliver video data to the video controller. The video data from the ap
29、plication is stored in the frame buffer by the CPU. The video controller fetches the video data and generates appropriate analog VGA or digital DVI video signals to the video interface. The video interface carries the video signals between the video controller and the TFT-LCD display. The TFT-LCD di
30、splay receives the video data and generates proper shade transmissivity for each pixel according to its pixel value. All of the pixels on the transmissive LCD panel are illuminated by the backlight from behind. To the observer a displayed pixel looks bright if its transmissivity is high i.e. in the
31、on state meaning it passes the backlight. On the other hand a displayed pixel looks dark if its transmissivity is low i.e. in the off state meaning it blocks the backlight. If the transmissivity can be adjusted to more than two different levels between the on and off states then the pixels can be di
32、splayed in grayscale. If the shade can be colored as red green or blue by using different color filters then pixels can be displayed in color by mixing three sub-pixels in different colors at different grayscales. In other words the perceived brightness of a pixel is determined by its transmissivity
33、 and the backlight illumination. Most of current TFT-LCD displays use CCFL backlighting thanks to its unrivaled luminance density emitting the most light within the minimum form factor. The CCFL can be designed to generate arbitrary color which is critical to reproducing pure white in the backlighti
34、ng applications. The technology of manufacturing CCFL is mature so that its cost has been minimized. The power consumption of the CCFL backlight however is considerably high compared with that of the TFT-LCD panel. The observed luminance of a transmissive object L is the product of the luminance of
35、the light source b and the transmissivity of the object t 5. For a pixel on a backlit TFT-LCD display its transmissivity is a function of its pixel value x. Thus its observed luminance L is L tx.b 1 10 The ambient light is not considered here because it has little effect for a transmissive TFT-LCD w
36、hen compared with a reflective or transflective one. Figure 2 depicts the relation in Equation 1 assuming that the transmissivity is a linear function of the pixel value. In a non-backlight-scaled TFT-LCD display the backlight bis always fixed at full power. The backlight scaling techniques in 23 re
37、duce b while increasing the pixel value from x to x by xxb 2 xx/b 3 to maintain the same L. These approaches have the following drawbacks: ? Equation 2 cannot preserve brightness invariance according to Equation 1. ? The contrast distortion among the unsaturated pixels is not considered. ? The softw
38、are-based approach has high energy/performance overhead. ? The CCFL illumination is incorrectly modeled as a linear function of power. In this paper we propose solutions to surmount the above-mentioned drawbacks. In Section 2 we characterize the CCFL illumination and power consumption. In Section 3
39、we propose adjusting the transmissivity function t rather than the pixel value x. The optimal CBCS problem is introduced in Section 0. Section 5 presents the experimental results followed by conclusions in Section 6. 2 CCFL Illumination and Power Modeling A CCFL backlight unit consists of the fluore
40、scent lamp the driving DC-AC inverter and the light reflector. A CCFL is a sealed glass tube with electrodes on both ends. The tube is filled with an inert gas argon and mercury. The inner glass surface of the tube is coated with phosphor which emits visible light when excited by photons. The wavelength or color of the visible light depends on the type of the gas and phosphor. In the LCD backlighting application a proper mix of red green and b.
