Hard copy refers to the permanent reproduction of displayed or transmitted data. Thus, a hard copy devices produces text and graphical data in form of printout or photograph on plain paper, photographic paper or transparencies etc.
The most commonly used hard copy devices are printers and plotters. Printers are often used for low volume and small representatives whereas the plotters are used for high precision, large sized printouts such as banners.
Printers
Printers are the hardware devices that help to take print out of documents that may contain text or graphics.
On the basis of approach used by printers for printing documents they are classified into two types: impact and non-impact printers.
Impact printers use the approach of hammering a typeface against paper and inked ribbon to produce printouts. The image is formed by pressing an inked ribbon against it or by the use of pressure sensitive paper.
Non-impact printers do not hit or impact a ribbon to print. There is no direct contact between the printhead and the paper. They use thermal, electrostatic, chemical or inkjet technology to produce printouts.
Impact printers are capable of producing multiple copies by the use of carbon papers, which is not possible with a non impact printer.
Impact printers are usually noisy whereas non-impact are quite in operation.
Quality of printer output depends upon the resolution of printer. The resolution is normally expressed in dot per inch (dpi). Thus resolution of a printer refers to the capability of a printer to print number of dots per inch, horizontally or vertically. Higher dpi values increase the sharpness and details of a figure.
Speed of a printer is another important consideration factor. For a high quality printer, speed is usually measured in terms of number of pages printed by a printer in one minute i.e. pages per minute (ppm). Speed is sometimes also expressed in terms of characters-per-second (cps) for character printers and lines-per-minute (lps) for line printers.
1. DOT-MATRIX PRINTER
- Dot matrix printers are character printer that prints one character at a time.
- It is called so because each printed character is formed by a matrix of tiny dots.
- These tiny dots are produced by a printhead that moves horizontally across the paper
- This print head contains a matrix of pins with m columns and n rows such as 5 by 7. The printer can activate each pin independent of others.
- These pins strike the paper against an inked ribbon to form a dot on the paper. Figure shows the character formed by a print head of 5 by 7 i.e. 5 dots vertically and 7 dots horizontally.
- To print a character, the printer activates appropriate set of pins as the print head moves horizontally.
- For faster printing, many dot matrix printers print both ways i.e. while the print head moves from left to right and white it moves from right to left, on return. This method is known as bi-directional printing.
- The quality of printed output depends on the density of dots in the matrix. Most early dot matrix printers had 9 pins and used 5*7 dot matrix to print character. In these printers the bottom 2 pins were used for printing lowercase letters that extends below the line.
- Now-a-days, dot matrix printers have 18 or 24 pins. The best dot matrix printers (24 pins) can produce near letter quality type characters.
- As dot matrix printers are controlled by software and prints in form of dots, they can produce characters of different fonts and sizes. They can also print characters of different shapes from different languages.
- Dot matrix printers are usually slower and their printing speed varies in the range of 50 to 500 characters per second (cps).
- Dot matrix printers use inked ribbon for printing documents and are usually monochrome. However, it is also possible to produce few colors by using multi-colored ribbons. Still these printers lack the ability to print computer generated images at acceptable quality.
- Some of the relative advantages of dot matrix printers are:
- They are capable of producing multi-copies or printouts by the use of carbons.
- They are relatively cheaper and have long life.
- There are several disadvantages that makes dot-matrix printer unpopular:
- They are usually slower is speed.
- They produce a lot of noise due to impact technology used in them.
- They are incapable of producing wide range of colors.
- The printed documents have low resolution.
- They are not capable of printing images and other high quality graphics.
2. DAISY WHEEL PRINTER
- Daisy wheel printer are impact printer that uses a special type of print head (wheel) that resembles a daisy flower. This wheel is usually made of metal or plastic.
- This wheel or print head has number of spokes with a character embossed on each spoke.
- At the time of printing, the wheel is rotated under the hammer until the appropriate character appears just under the hammer.
- After proper positioning of the character the hammer strikes the character which then prints the character on paper through the inked ribbon.
- The petals or spokes of this wheel contains all the possible characters including full stop, question mark, exclamation mark and other punctuations.
- In order to change the types of font, user need to change daisy wheel which is detachable and replaceable.
- Many daisy wheel printers also support bold, double strike or triple strike facility.
- Daisy wheel printers were not so popular and are not used because of the following limitations:
- They are noisy as they use hammering technology to print as character.
- In order to produce varying fonts, users need to replace the daisy wheel.
- They print only characters and symbols and cannot print graphics.
- They are generally slow with a printing speed of about 10 to 75 characters per second.
3. CHAIN/BAND PRINTER
- Chain/band printers are line printers that print one line at a time.
- It contains a metallic chain or band where all the characters and symbols of a character set supported by printer are embossed. A standard character set may have 48, 64 or 96 characters.
- Each character set contains all possible upper-case, lower case alphabets and special characters.
- In order to enhance printing speed, the chain or band contains several sets of embossed characters. For example, a chain of 48 character set may have 5 sets of 48 characters on it, there by having 240 (48*5) characters.
- There is a set of print hammers in front of chain. The total number of hammers is equal to the total number of print position i.e. if a printer supports 132 print position there will be 132 hammers.
- The paper and inked ribbon is placed between the chain and hammers.
- The chain rotates at a high speed. In order to print at a desired print position, the printer activates the appropriate hammer when the character embossed on the chain passes below it.
- In order to use different fonts and languages, users need to change the band.
- Chain printer has an edge over dot matrix printer as it is faster in speed and prints one line at a time. Its speed is in range of 400 to 3000 lines per minute.
- Like dot matrix printer it suffers from several limitations:
- It is noisy as it also uses hammer to print a character on paper.
- It is not capable of printing graphics such as charts and graphs.
- It cannot produce text of varying fonts and sizes.
4. DRUM PRINTER
- A drum printer is a line printer that also prints one line at a time.
- It consists of a solid cylindrical drum where characters are embossed on its surface.
- These characters are arranged in form of circular bands with each band containing all the characters of a character set.
- The total number of band on a cylinder is equal to the maximum number of characters or print positions that the printer can print on a line. For example, if there are 132 bands (i.e. characters in one line) and 46 characters in one set, then there will be 6336 (132*46) characters on entire drum surface.
- Just like chain printer, drum printer also has a set of hammers for each band that is mounted in front of drum. The total number of hammers is equal to the total number of bands.
- The paper along with inked ribbon is placed between the hammer and drum.
- The drum usually rotates at a high speed.
- To print a character at a specific position, the printer activates the appropriate hammer that strikes the paper with inked ribbon.
- The hammer strikes when character (embossed on drum) appears at desired location.
- In order to print one line, the drum has to make one full revolution. Therefore, all the characters on a line are not printed at same time.
- Drum printer is unpopular because of its various limitations:
- Being impact printer it produces a lot of noise.
- It is not easy to change font of printed characters. Changing the font would require the change of drum. This operation is usually expensive.
- It is not suitable for printing graphics such as charts and graph.
- It is not possible to print characters with varying font sizes.
5. INKJET PRINTER
- Inkjet printers are the non-impact printers that create an image by spraying small droplets of ink onto paper. An inkjet printer is shown in figure.
- The droplets sprayed on papers are extremely small (usually between 50 and 60 microns in diameter).
- The print head of an inkjet printer contains a series of tiny nozzles that are used to spray drops of ink.
- These printers are both monochrome and color. A monochrome inkjet printer uses a single cartridge with black ink, whereas color printer uses two ink cartridges-black and tricolor. The tricolor cartridge contains red, blue and green colors. These colors can be combined with each other or with the black color to produce other desired colors.
- Inkjet printer are based on two different working technologies:
- Thermal Bubble or Bubble Jet – In this technology, tiny resistors are used to create heat. Because of this heat, ink vaporizes and creates a bubble. As the bubble expands, some of the ink is pushed out of the nozzle onto the paper. When this bubble pops out or collapses, a vacuum is created. This pulls more ink into the print head from the cartridge. A typical bubble jet print head has 300 or 600 tiny nozzles and all of them can fire a droplet simultaneously. This technique is used by Canon and Hewlett Packard (HP).
- Piezoelectric – This technique uses piezo crystal and has been patented by Epson. A crystal is located at the back of the ink reservoir of each nozzle. The crystal receives a tiny electric charge that causes it to vibrate. When the crystal vibrate inwards, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls some more ink into the reservoir to replace the ink sprayed out.
- An inkjet printer offer several advantages:
- Inkjet printers produce high quality output. A typical inkjet printer providers a resolution of 600 dots per inch.
- As these printers produce output as a pattern of tiny dots, they are capable of printing any shape and size. Hence, it is possible to print text with varying font style, size.
- These are also suitable for printing high quality graphics such as charts, graphs and images.
- As they are non-impact they are much quiet than dot matrix and other impact printers.
- They are also compact and are easily portable.
- Inkjet printers also suffer from few disadvantages:
- Inkjet printers incur additional cost of refilling the cartridge on regular basis. This makes it relatively expensive.
- They are slower in speed than dot-matrix printers and can print 40 to 300 characters per second.
- Lifetime of inkjet printouts produced by inkjet printers is limited. They eventually fade and the color balance may change.
- If not used for longer period, the print head dries up.
- Inkjet nozzles get clogged very easily and need to be cleaned.
6. Laser Printer
- Laser printers are the most advanced printers. They are similar to photocopy machine and are based on Xerographics principle.
- The major components of laser printer are laser beam, multi-sided mirror, photosensitive or photo receptor drum, and a toner i.e. a powered charged ink.
- The working of laser printer is explained as follows:
- It uses photosensitive rotating drum. Initially, this drum is given a total positive charge by the charge corona wire (a wire with an electrical current running through it). Some printers use a charged roller instead of a corona wire for the same purpose.
- To print a page, the printer focuses the laser beam on the electrostatically charge drum by the spinning multi-sided mirror.
- The laser beam removes the positive charge from the drum except for the area to be printed. In this way, the laser draws the letters and images to be printed as a pattern of electric charges. This is called as electrostatic image.
- After this, the negatively charged toner (ink) powder sticks to this positively charged area (forming characters or image) of the drum i.e. all those parts of drum surface which are exposed to the laser beam.
- A little later, the toner coated drum is pressed against the paper in order to transfer this black powder onto the paper.
- The printer then applies heat and pressure to fuse the toner permanently on the paper. This is done by passing the paper through a fuser (a pair of heated rollers). This fuser also heats up the paper itself, which is why pages are always hot when they come out of laser printer.
- After the toner is deposited on the paper, the drum is rotated and cleaned with a rubber blade in order to remove the toner sticking to its surface so as to prepare it for next page printing. This is done by passing the drum surface through discharge lamp.
Thus, electrical image is erased by exposing this drum to a bright light.
- Initially most commercial laser printers were limited to monochrome printing (i.e. black color toner). The color printers work in the same way as monochrome printers. However, the above described entire printing process is repeated four times one pass for cyan (blue), one for magenta (red), one for yellow and last one for black. By combining these four colors of toner in varying proportions, it is possible to produce a large variety of colors.
- Laser printers are controlled through page description languages (PDLs). In order to support hundred of different fonts and graphics, print controller and the host computer communicate via PDL. The print controller is the laser printer’s main onboard computer that talks to host computer through either USB port or a parallel port.
There are two de-facto standards for PDL :- PCL (Printer Control Language): It was developed by Hewlett Packard (HP).
- Postscript: This is the de-facto standard for Apple Macintosh printers
- Laser printers are high quality printers and have resolution of 600 dpi and 1200 dpi. There are certain electrographic printing plate laser printers with resolution of 2400 dpi used for printing on plastic sheet.
- Thus laser printers offers several advantages :
- Laser printers are noiseless as they are non-impact printers.
- They have high printing speeds and can print 4 to 12 pages per minute (ppm).
- They offer high quality printing and can produce characters of different font style and size. They can also print graphics such as charts, graphs and images.
- As compared to inkjet printer, laser printers incurs low cost per page.
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