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由X线管焦点辐射出的X线穿过被检体时，受到被检体各组织的吸收和散射而衰减，使透过的X线强度的分布呈现差异，到达屏-片系统，转换成可见光强度的分布差异，并传递给胶片，形成银颗粒的空间分布，再经显影处理成为二维光学分布，形成X线照片影像。 决定X线“质”的因素主要是
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X线胶片特性曲线是描绘曝光量与所产生的密度之间关系的一条曲线，由于这条曲线可以表示出感光材料的感光特性，所以称之为特性曲线。特性曲线的横坐标为曝光量，以对数值lgE表示；纵坐标为密度，以D表示。特性曲线由足部、直线部、肩部和反转部组成。足部密度的上升与曝光量不呈正比，曝光量增加很多，密度只有较小的增加。直线部密度与曝光量的增加呈正比，密度差保持一定，此时曲线沿一定的斜率直线上升。肩部密度随曝光量的增加而增加，但不呈正比。反转部随曝光量的增加密度反而下降，影像密度呈现逆转。特性曲线可提供感光材料的本底灰雾(Dmin)、感光度(S)、对比度(γ)、最大密度(Dmax)、宽容度(L)等参数，以表示感光材料的感光性能。 下列叙述正确的是
Israel is a "powerhouse of agricultural technology", says Abraham Goren of Elbit Imaging (EI), an Israeli multinational. The country’s cows can produce as much as 37 liters of milk a day. In India, by contrast, cows yield just seven liters. Spotting an opportunity, EI is going into the Indian dairy business. It will import 10,000 cows and supply fortified and flavored milk to supermarkets and other buyers. So will EI lap up India’s milk market Not necessarily. As the Times of India points out, its cows will ruminate less than 100 miles from the headquarters of a formidable local producer—the Gujarat Co-operative Milk Marketing Federation, otherwise known as Amul. This Farmers’ Co-operative spans 2.6m members, collects 6.5m liters of milk a day, and boasts one of the longest-running and best-loved advertising campaigns In India. It has already shown "immense resilience" in the face of multinational competition, says Arindam Bhattacharya of the Boston Consulting Group (BCG). Its ice-cream business survived the arrival of Unilever; its chocolate milk has thrived despite Nestlé. Indeed, Amul is one of 50 firms—from China, India, Brazil, Russia and six other emerging economies— that BCG has anointed as "local dynamos". They are prospering in their home market, are fending off multinational rivals, and are not focused on expanding abroad. BCG discovered many of these firms while drawing up its "global challengers" list of multinationals from the developing world. The companies that were venturing abroad most eagerly, it discovered, were not necessarily the most successful at home. Emerging economies are still prey to what Harvard’s Dani Rodrik has called "export fetishism". International success remains a firm’s proudest boast, and with good reason: economists have shown that exporters are typically bigger, more efficient and pay better than their more parochial rivals. "Exporters are better" was the crisp verdict of a recent review of the data. Countries like India and Brazil were, after all, once secluded backwaters fenced off by high tariffs. Prominent firms idled along on government favors and captive markets. In that era, exporting was a truer test of a company’s worth. But as such countries have opened up, their home markets have become more trying places. Withstanding the onslaught of foreign firms on home soil may be as impressive a feat as beating them in global markets. BCG describes some of the ways that feat has been accomplished. Of its 50 dynamos, 41 are in consumer businesses, where they can exploit a more intimate understanding of their compatriots’ tastes. It gives the example of Gol, a Brazilian budget airline, which bet that its cash-strapped customers would sacrifice convenience and speed for price. Many Gol planes therefore depart at odd hours and make several hops to out-of-the-way locations, rather than flying directly. Similarly astute was India’s Titan Industries, which has increased its share of India’s wristwatch market despite the entry of foreign brands such as Timex and Swatch. It understood that Indians, who expect a good price even for old newspapers, do not throw their watches away lightly, and has over 700 after-sales centers that will replace straps and batteries. Exporters tend to be more capital-intensive than their home-bound peers; they also rely more on skilled labor. Many local dynamos, conversely, take full advantage of the cheap workforce at their disposal. Focus Media, China’s biggest "out of home" advertising company, gets messages out on flat-panel displays in 85,000 locations around the country. Those displays could be linked and reprogrammed electronically, but that might fall foul of broadcast regulations. So instead the firm’s fleet of workers on bicycles replaces the displays’ discs and flash-cards by hand. The list of multinationals resisted or repelled by these dynamos includes some of the world’s biggest names: eBay and Google in China; Wal-Mart in Mexico; SAP in Brazil. But Mr. Goren of EI is not too worried about Amul. The market is big enough for everybody, he insists. Nothing, then, is for either company to cry about. Which of the following would the author most probably agree
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It is hard for modern people to imagine the life one hundred years ago. No television, no plastic, no ATMs, no DVDs. Illnesses like tuberculosis, diphtheria, pneumonia meant only death. Of course, cloning appeared only in science fiction. Not to mention, computer and Internet. Today, our workplace are equipped with assembly lines, fax machines, computers. Our daily life is cushioned by air conditioners, cell phones. Antibiotics helped created a long list of miracle drugs. The bypass operation saved millions. The discovery of DNA has revolutionized the way scientists think about new therapies. Man finally stepped on the magical and mysterious Moon. With the rapid changes we have been experiencing, the anticipation for the future is higher than ever. A revolutionary manufacturing process made it possible for anyone to own a car. Henry Ford is the man who put the world on wheels. When it comes to singling out those who have made a difference in all our lives, you cannot overlook Henry Ford. A historian a century from now might well conclude that it was Henry Ford who most influenced all manufacturing everywhere, even to this day, by introducing a new way to make cars—one, strange to say, that originated in slaughter houses. Back in the early 1900s, slaughter houses used what could have been called a "disassembly line." That is, the carcass of a slain steer or a pig was moved past various meat-cutters, each of whom cut off only a certain portion. Ford reversed this process to see if it would speed up production of a part of an automobile engine called a magneto. Rather than have each worker completely assemble a magneto, one of its elements was placed on a conveyer, and each worker, as it passed, added another component to it, the same one each time. Professor David Hounshell, of The University of Delaware, an expert on industrial development tells what happened: "The previous day, workers carrying out the entire process had averaged one magneto every 20 minutes. But on that day, on the line, the assembly team averaged one every 13 minutes and 10 seconds per person." Within a year, the time had been reduced to five minutes. In 1913, Ford went all the way. Hooked together by ropes, partially assembled vehicles were towed past workers who completed them one piece at a time. It wasn’t long before Ford was turning out several hundred thousand cars a year, a remarkable achievement then. And so efficient and economical was this new system that he cut the price of his cars in half, to $260, putting them within reach of all those who, up until that time, could not afford them. Soon, auto makers over the world copied him. In fact, he encouraged them to do so by writing a book about all of his innovations, entitled Today and Tomorrow. The Age of the Automobile had arrived. Today, aided by robots and other forms of automation, everything from toasters to perfumes is made on assembly lines. Edsel Ford, Henry’s great-grandson, and a Ford vice president: “I think that my great-grandfather would just be amazed at how far technology has come." Many of today’s innovations come from Japan. Norman Bodek, who publishes books about manufacturing processes, finds this ironic. On a recent trip to Japan he talked to two of the top officials of Toyota. "When I asked them where these secrets came from, where their ideas came from to manufacture in a totally different way, they laughed, and they said. ’Well. We just read it in Henry Ford’s book from 1926: Today and Tomorrow.’\ The last paragraph implies that
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