A hard copy system or image output system is used to produce an analog image on paper or a film from digital image data. Depending on the system, the recording media, resolution, gray level, output size, output rate, cost and stability, are different as compared and shown in Table 8.5.1.
a. Silver halide photography
This is a so-called film recorder which enables the production of film products from digital image data with a light source such as CRT and laser beam. There are two types; the drum type and the flat bed type. The resolution and gray level are excellent. Recently thermal developing systems have become operational replacing the chemical developing system.
b. Electro photography
The negative image is firstly generated on a photo-sensitive drum. Secondly toner is electronically placed according to the negative image. Thirdly the toner is transcribed onto plane paper. The advantage is that the running cost is low. It is widely used as an ordinary hard copy machine.
c. Electro static recorder
Electronically coated paper is firstly given an electric charge in a form of a negative, in accordance with the dot pattern of the image. Toner is secondly placed electro-statically dot by dot. The advantage is that a large size of output can be obtained in a short time with a moderate cost. It is sometimes called an electro static dot printer or simply a dot printer.
d. Thermal transcripter
There are two types; the melting type and the sublimation type. The melting type transcribes a melted ink layer using a thermal head onto plane paper with the use of a coated wax type ink ribbon. As the gray levels, are few only a limited number of color outputs, for example in the case of a classified color map, are available rather than continuous color tones. The sublimation type heats a coated ink sheet ,using a thermal head into vaporized ink, which is transcribed onto a coated paper. The gray level is so many that the image quality is similar to that of a film. The disadvantage of both types is that the paper size is limited because of the site of the ink sheet.
e. Ink jet recorder
A water drop of ink is ejected from a nozzle and is transcribed onto plane paper pixel by pixel. The advantage is that the ink volume can be controlled to produce a continuous gray level onto a large size of paper. The disadvantage is that nozzle maintenance is a problem, because the nozzle hole is sometimes blocked due to irregular ink particles.
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