计算机专业时文选读之二十二

Digital Light Processing

When one first hears about digital light processing (DLP), it seems almost impossibly complex, even magical —— millions of tiny mirrors on a chip the size of your thumbnail, each of them capable of moving thousands of times per second to create a digital image. In fact, DLP gives new meaning to the phrase “smoke and mirrors” as it applies to computer-related technology.

How DLP Works

In essence, DLP is a nanotechnology implementation of the old survival technique of using a mirror to signal for help ——its purpose is to shine a controlled series of light flashes on a target to send a message. The mirror in this case is part of an optical semiconductor called a digital micromirror device, or DMD. The DMD chip contains not one but an array of up to 2.1 million microscopic mirrors, each just 16 micrometers square (less than one-fifth the size of a human hair) and 1 micrometer apart.

The DMD chip is driven by a digital video or graphic signal in which each digital pixel corresponds to a single mirror on the DMD. Add a light source and a projection lens, and the mirrors can reflect a digital image onto a viewing screen or other surface. Each mirror is mounted on tiny hinges, so it can be tilted 12 degrees toward or away from the light source.

The control electronics direct each mirror to tilt——in other words, to switch on and off——up to 5,000 times per second. When a mirror is switched on more frequently than off, it reflects a light gray pixel; a mirror that's switched off more often reflects a darker-gray pixel. This lets DLP project up to 1,024 shades of gray.

To get color, such as for a TV set, a rotating color wheel (with red, green and blue filters) is put between the white light source and the DMD. The control input delivers separate signals for each of the three colors, and each mirror (i.e., each pixel) is switched on and off as the filter rotates each color between the lamp and DMD.

For example, to project a yellow pixel, a mirror will reflect only red and green light to the projection surface. To project a purple pixel, that mirror will be switched off while the blue filter is in position, and the blue and yellow flashes will alternate so rapidly, our brains will blend them together and we'll see purple. This process allows a DLP system to produce up to 16.1 million colors. Older DLP systems also included a clear segment to bump up overall brightness at the expense of color saturation.

Consumer-grade television monitors use the system described above. For very large projection, such as in movie theaters and auditoriums, a more sophisticated system uses three DMD chips, one for each color, plus an optical prism. The prism splits white light into colors and then recombines the three images before sending them through the projection lens. This system, called DLP Cinema, can produce 35 trillion colors.

In most applications, DLP competes directly with LCD projection. DLP typically offers greater contrast (up to 5,000-to-1 vs. LCD's 800-to-1), with better blacks, while LCD produces greater color saturation. Side by side, an LCD display looks slightly sharper than a DLP in text display applications, but DLP has the edge with moving video, reducing pixelation, or the “screen-door effect.”

The brightest projectors still use LCD technology, which is slightly more efficient, but the smallest, lightest projectors use DLP. In 2003, DLP systems accounted for 13% of the market for large-screen televisions (over 40 inches). In the past year, the number of models of DLP TVs has tripled.

时文选读

数字光处理

当第一次听到数字光处理(DLP)时，它似乎是无法想象的复杂，甚至不可思议——在大拇指指甲大小的芯片上有数以百万计的镜子，为了生成数字图像，它们能每秒转动数千次。事实上，当DLP应用于计算机有关的技术时，它给“欲盖弥彰”这个成语赋予了新的意义。

DLP是如何工作的?

从本质上讲，DLP是用纳米技术实现了一种利用镜子发求救信号的古老求生术——其目的是将受控的闪光照亮目标以发送消息。在这里，镜子是被称做数字微镜器件(DMD)的光半导体的一部分。DMD芯片包含了不是单个而是多达210万个微镜子组成的阵列，每个镜子只有16平方微米(小于人的头发的五分之一)，间隔1微米。

DMD芯片由数字视频或图形信号驱动，其中每个数字像素对应于DMD芯片上的一面镜子。加上一个光源和投影镜头，这些镜子就能将数字图像反射到观看屏幕或其他表面。每面镜子安装在微型铰链上，所以它能对着光源倾斜正负12度。

控制电路指挥每面镜子倾斜——换言之进行开关，速度可高达每秒5000次。当镜子开的频率大于关的频率时，它反射浅灰色的像素，镜子关的状态更多时，就反射深灰色像素。这就让DLP投影高达1024级的灰度。

为获得色彩，如为电视机等，在白光源和DMD之间放置一个旋转的色彩轮(上有红、绿和蓝三种滤色镜)。控制输入为三种颜色分别提供信号，当滤色镜旋转灯泡和DMD之间的每种颜色时，每个镜子(即每个像素)就开关。

例如，要投射黄色像素，镜子只向投影面反射红光和绿光。为投射紫色像素，蓝滤色镜在位的同时镜子处于关，蓝色和黄色交替地快速闪光，我们的大脑会将它们混合起来，我们就看到紫色。此处理方式允许DLP系统产生1610万种颜色。陈旧一些的DLP系统还含有透明片，以牺牲色彩饱和度为代价来提升亮度。

消费级的电视监视器采用上述的系统。对于非常大的投影，如电影院和大礼堂中的投影，系统更复杂一些，采用三个DMD芯片，一个芯片对应一种颜色，外加一个光学棱镜。棱镜先将白光分成单色光，然后在将图像发送出去之前，通过投影透镜将三种图像合并起来。此系统叫做DLP影院，能产生35万亿种颜色。

在大多数应用中，DLP直接与LCD(液晶)投影竞争。通常DLP提供更高的对比度(高达5000∶1，而LCD只有800∶1)和更好的黑色，而LCD的色彩饱和度更好一些。将两者并排放在一起，LCD显示器在文本显示应用中看上去稍微比DLP清晰一些，但对于活动的视频图像，DLP拥有边缘，减轻了像素化，即“屏幕门效应”。

目前最亮的投影机使用的是LCD技术，它的效率也稍为高一些，但最小、最轻的投影机使用DLP。在2003年，DLP系统在大屏幕电视(超过40英寸)市场上的份额为13%。在过去的一年里，DLP电视的型号多了3倍。