19. Write several sentences about advantages and disadvantages of crTs and lcDs.

20. Read, translate the text. Learn the words in bold type: Printers

Printers fall into one of two categories. They are either impact printers or non-impact printers. The difference is that impact printers use a mechanism that strikes an inked ribbon which creates an imprint on the paper whereas non-impact printers spray ink, use heat or pressure to create images on the paper.

Impact printers are usually very noisy. Dot-matrix and line printers are impact printers. They are usually used by companies for routine jobs like printing labels, invoices, or multi-part forms. Most impact printers use continuous-form paper, in which the sheets are connected together, but perforated so they can be easily separated from each other. It also has holes on the sides to help feed it through the printer. You might think these are old-school, but companies so still use these, especially places like auto repair shops, retail stores and factories because they are able to withstand extreme environments.

Ink-jet printers are the most popular for home use because of their low cost and reasonable print quality. They spray tiny dots of ink onto paper and will even print on photo or glossy paper. They can also print on other materials like address labels, envelopes, card stock, and transparencies. Some ink-jet printers come with software that helps users create greeting cards, banners, resumes and the like. There are two factors that determine the quality of an ink-jet printer, one is it's resolution which is measured by the number of dpi (dots per inch) it can output, and the other is it's speed which is measured by the number of ppm (pages per minute) it can produce. Most ink-jet printers offer resolutions of up to 4800 by 1200 dpi and can print from 5 to 25 ppm. With the price of ink-jet printers dropping drastically over the years, some people joke that it would be cheaper and wiser to upgrade to a new printer when their ink runs out than to replace the cartridges! Of course, an alternative would be to buy an ink refill kit, but they can be very messy and on occasion cause problems with the printhead or nozzle, especially if the ink has a chance to dry in the cartridge before it is refilled. Graphic artists use very large and complex printers called large-format printers that are capable of producing high-quality photo-realistic prints, signs, posters, maps, diagrams, and the like.

Now, let's take a moment to talk about laser printers. Also a non-impact printer, they can produce high-quality printouts very quickly. A laser printer uses a laser beam and powdered ink called toner when it creates printouts. Here's how it works: the document information is sent from your computer to the printer where it is stored temporarily in its own memory. The printer then uses software that interprets the information into a page description language (PDL), in essence, telling the printer where the ink should go on the paper. The laser beam produces an image on a cylindrical drum as the light from the laser alters the electrical charge wherever it strikes the surface of the drum. This charge is what causes the toner to stick to the drum. The drum then rotates and transfers the toner to the paper as it feeds through. Lastly, a set of rollers uses heat and pressure to permanently fuse the ink to the paper. Generally, laser printers can print text at speeds of 9 to 30 ppm though there are some large businesses that use printers that can output several hundred ppm! Another reason businesses like laser printers is because they can print at very high resolutions, usually ranging from 600 to 2400 dpi.

There's yet another printer that will produce color photographic-quality prints... the thermal printer! They generally cost more than ink-jet printers, but are less expensive than many color laser printers. Thermal printers push electrically heated pins against heat-sensitive paper. There are very basic low-quality thermal printers meant for printing receipts and such, but there are also two sorts that offer high print quality. One is the thermal wax-transfer printer which uses heat to melt colored wax onto heat-sensitive paper that will not smear. The other is the dye-sublimation printer which uses heat to transfer colored dye to specially coated paper. Medical labs, photography studios, and many print shops use these high end printers although they can be very expensive, especially if they have the ability to print a wide array of sizes. Lower-end dye-sublimation printers that can only print a couple sizes are used at home or by small businesses because they are less expensive, but the trade-off is that they print much slower than ink-jet or laser printers.

(based on http://www.ehow.com)