Four types of scanning techniques are used by manufactures to generate soft copy of color images or documents:
1) Multi Pass Technology
2) Single Pass Technology
3) Photo Multiplier Tubes(PMT) Technology
4) Contact I Image sensing Technology.
Multi Pass Technology:This scanning system was used in traditional scanners. Three different light sources (i.e. Red, Green and Blue) are used to scan the image.
First red light is switched on and green and blue are left off. Scanning is done with red light source. In second pass red and blue are left off and image is scanned with green light source. In third pass image is scanned with blue light by placing red and green sources in off condition. Result obtained from these three scans is combined to get the final image scan.This problem has the problem that all the scans must be precisely aligned so that when images are combined image with sharp edges can be generated.
Single Pass Scanners: Most of the scanners used today are single pass scanners. Two types of technologies are used to implement single pass scanning.
a) Using Color Filters: In this approach single white light is used.
When this light falls on the document to be scanned, it is reflected and is filtered to red, green and blue color by rotating color filter. This filtered light is then sensed by single CCD device. 
b) By using Tri –Chromatic Prismin Tri-Chromatic prism technology a rotating prism is used that segregates the deflected white light into different colors.
Photo Multiplier Tubes (PMT) Technology: This technology is used in drum scanners. In this type of scanners single transparent cylindrical drum is used. Image is placed over it and light is passed through drum. The reflected light from image is captured by photo multiplier tubes. These tubes are light sensing tubes and are very much sensitive.
Contact Image Sense Technology: This technology is used in portable hand held scanners. In this technology colored LED’s are used instead of cold cathode lamps. LEDs can be switched on or off very quickly. These devices has the advantage that LEDs are required to placed few millimeters below the glass surface and making them too close can damage them. This types of scanners are also not successful to scan binded books because LED can’t generate this much light that can be reflected back by the folded portions of the book.
1) Multi Pass Technology
2) Single Pass Technology
3) Photo Multiplier Tubes(PMT) Technology
4) Contact I Image sensing Technology.
Multi Pass Technology:This scanning system was used in traditional scanners. Three different light sources (i.e. Red, Green and Blue) are used to scan the image.
First red light is switched on and green and blue are left off. Scanning is done with red light source. In second pass red and blue are left off and image is scanned with green light source. In third pass image is scanned with blue light by placing red and green sources in off condition. Result obtained from these three scans is combined to get the final image scan.This problem has the problem that all the scans must be precisely aligned so that when images are combined image with sharp edges can be generated. Single Pass Scanners: Most of the scanners used today are single pass scanners. Two types of technologies are used to implement single pass scanning.
a) Using Color Filters: In this approach single white light is used.
When this light falls on the document to be scanned, it is reflected and is filtered to red, green and blue color by rotating color filter. This filtered light is then sensed by single CCD device. 
b) By using Tri –Chromatic Prismin Tri-Chromatic prism technology a rotating prism is used that segregates the deflected white light into different colors.
Photo Multiplier Tubes (PMT) Technology: This technology is used in drum scanners. In this type of scanners single transparent cylindrical drum is used. Image is placed over it and light is passed through drum. The reflected light from image is captured by photo multiplier tubes. These tubes are light sensing tubes and are very much sensitive.
Contact Image Sense Technology: This technology is used in portable hand held scanners. In this technology colored LED’s are used instead of cold cathode lamps. LEDs can be switched on or off very quickly. These devices has the advantage that LEDs are required to placed few millimeters below the glass surface and making them too close can damage them. This types of scanners are also not successful to scan binded books because LED can’t generate this much light that can be reflected back by the folded portions of the book.
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