11. Bits for pictures. Performance characteristics of monitors.

The characters and pictures that we see on the screen are made up of dots, also called picture elements (pixels).

In a bit-mapped display, the dots displayed on the screen correspond, pixel by pixel, with bits in the main memory of the computer. The bits are held in an area of the memory called the 'refresh buffer' and are stored in groups that represent the horizontal and vertical position of the pixels on the screen and whether the pixels are on or off. On monochrome systems, one bit in this 'map' represents one pixel on the screen and can be either 'on' or 'off' (black or white). On colour systems, each pixel is a certain combination of the three primary colours: red, green and blue. The total number of colours which can be shown on the screen is called the colour palette

The performance of a monitor is measured in the following parameters:

 Brightness - the luminance of images is measured in cd/m2 (candela per square metre).

 Screen size (viewable image size) is measured diagonally, in other words, a 17" screen measures 17 inches from the top left corner to the bottom right. For CRTs, the viewable size is typically one inch (25 mm) smaller than the tube itself.

 Resolution - the clarity of the image depends on the number of distinct pixels contained on a display, horizontally and vertically. A typical resolution is 1024 x 768.

 Colour depth - the number of colours a monitor can display.

 Refresh rate is the number of times in a second that a display is illuminated.

 Response time is the amount of time a pixel in a monitor takes to go from active (black) to inactive (white) and back to active (black) again

 Contrast ratio is the ratio of the luminosity of the brightest color (white) to that of the darkest color (black) that the monitor is capable of producing.

 Power consumption is measured in watts.

 Aspect ratio is the ratio of the horizontal length to the vertical length. 4:3 is the standard aspect ratio, for example, so that a screen with a width of 1024 pixels will have a height of 768 pixels.

 Viewing angle is the maximum angle at which images on the monitor can be viewed, without excessive degradation to the images. It is measured in degrees horizontally and vertically.

12. Main functions of printers. Give description to mfp, mfd, aio, virtual, dot-matrix, ink-jet, laser, thermal transfer, thermal printers and plotters.

In computing, a printer is a peripheral which produces a hard copy or printout of documents stored in electronic form, usually on physical print media such as paper or transparencies.

In addition, a few modern printers can directly interface to electronic media such as memory sticks or memory cards, or to image capture devices such as digital cameras, scanners; some printers are

combined with scanners and/or fax machines in a single unit, and can function as photocopiers.

Printers that include non-printing features are sometimes called Multifunction Printers (MFP), Multi-Function Devices (MFD), or All-In-One (AIO) printers. Most MFPs include printing, scanning, and copying among their features. A Virtual printer is a piece of computer software whose user interface and API resemble that of a printer driver, but which is not connected with a physical computer printer. A dot-matrix printer uses a group, or matrix, of pins to create precise dots required to shape a character. A print head containing tiny pins strikes an inked ribbon to make letters and graphics. They print text and graphics and nowadays some of them can print up to 500 characters per second (cps). They are slower than laser printers but much cheaper.

An ink-jet (also called bubble-jet) printer generates an image by spraying tiny, precise drops of ink onto the paper.. This type of printer is quite fast, silent and not so expensive as a laser printer. A standard ink-jet has a three-colour cartridge, plus a black cartridge. A laser printer uses a laser beam to fix the ink to the paper. A laser works like a photocopier – it scans the image with a laser beam and transfers it to paper with a special ink powder called toner which is attracted to paper by an electrostatic charge and then fused on by a hot roller. Laser printers are fast and produce a high resolution of 1,200 to 2,400 dpi, so they are ideal for businesses and for professional graphics work. They are constantly being improved.

Thermal transfer printers use solid sticks of colored ink, which are melted and fed into a piezo crystal operated print-head which sprays the ink on a rotating, oil coated drum. The paper then passes over the print drum and the image is transferred, or transfixed, to the page. Drawbacks of the technology include high power consumption and long warm-up times from a cold state.

Thermal printers work by selectively heating regions of special heat-sensitive paper. They are silent and considered to be inexpensive.

A plotter is a vector graphics (using geometrical primitives such as points, lines, curves, and shapes or polygon(s), which are all based on mathematical equations, to represent images in computer graphics) printing device which uses ink and fine pens held in a carriage to draw detailed designs on paper. It's used in computer- aided design, maps, 3-D technical illustrations, construction plans, engineering drawings and other technical illustrations.