问题 问答题 简答题

像质计金属丝底片对比度公式讨论提高对比度的主要途径,并说明通过这些途径提高底片对比度可能会带来什么缺点?

答案

参考答案:

象质计金属丝底片对比度公式△D=-0.434μpγσ•d/(1+n),提高对比度主要途径及由此带来的缺点:

(1)增大μp值。在保证穿透力的前提下,尽量采用能量较低的射线,得这样会使曝光时间增加。

(2)增大γ值。可选用γ值更高的微粒胶片;由于非增感型片胶片γ值和黑度成正比,也可通过提高底片黑度增大γ值。但高γ值的微粒胶片感光速度往往较慢,需要增大曝光时间,提高黑度也需要增加曝光时间,此外,黑度的提高会增大最小可见对比度△Dmin,对灵敏度产生不利影响。

(3)提高σ值,可选择焦点尺寸小的射源,或增大焦距,这样做也会使曝光时间延长。

(4)减少n值。要减小散射线,就要使用铅窗口和铅屏蔽,这些也将降低工作效率,使曝光时间延长。

单项选择题
单项选择题

Advertisers tend to think big and perhaps this is why they’re always coming in for criticism. Their critics seem to resent them because they have a flair for self-promotion and because they have so much money to threw around. "It’s iniquitous," they say, "that this entirely unproductive industry (if we can call it that) should absorb millions of pounds each year. It only goes to show how much profit the big companies arc making. Why don’t they stop advertising and reduce the price of their goods After all, it’s the consumer who pays."
The poor old consumer. He would have to pay a great deal more if advertising didn’t create mass markets for products. It is just because of the heavy advertising that consumer goods are so cheap. But we get the wrong idea if we think the only purpose of advertising is to sell goods. Another equally important function is to inform. A great deal of the knowledge we have about household goods derives large from the advertisements we read. Advertisements introduce us to new products or remind us of the existence of ones we already know about. Supposing you wanted to buy a washing-machine, it is more than likely you would obtain details regarding performance, price, etc. from an advertisement.
Lots of people pretend that they never read advertisements, but this claim may be seriously doubted. It is hardly possible not to read advertisements these days. And what fun they often are, too] Just think what a railway station or a newspaper would be like without advertisements. Would you enjoy gazing at a blank wall or reading railway by-laws while waiting for a train Would you like to read only closely-printed columns of news in your daily paper A cheerful, witty advertisement makes such a difference to a drab wall or a newspaper full of the daily ration of calamities.
We must not forget, either, that advertising makes a positive contribution to our pockets. Newspapers, commercial radio and television companies could not subsist without this source of revenue. The fact that we pay so little for our daily paper, or can enjoy so many broadcast programs is due entirely to the money spent by advertisers. Just think what a newspaper would cost if we had to pay its full price !
Another thing we shouldn’t forget is the "little ads", which are in virtually every newspaper and magazine. What a tremendously useful service they perform for the community ! Just about anything can be accomplished through these columns. For example, you can find a job, buy or sell a house, announce a birth, marriage or death in what used to be called the "hatch, match and dispatch" columns; but by far the most fascinating section is the personal or "agony" column. No other item in a newspaper provides such entertaining reading or offers such a deep insight into human nature. It’s the best advertisement for advertising there is!

An argument made by critics of advertisers is that______

A. advertising makes contribution to the pockets. B. readers claim they never read advertisements.
C. advertising may entail a price rise for goods. D. little ads invariably appeal to baser instincts.