How scanning really works?

To understand the scanning process you need to know this followings:

Resolution

Resolution is the number of pixel, an image content. Pixel appear as tiny dots, which, when placed together creates an illustration of continuous colors and shades. These colors and shades define the shape of an image.

Scaling

Scaling refers to the physical size of an image in inches or pixels, not to its resolution size. A small image requires less disk space and processing time then a larger image but a high resolution scan increase the file size tremendously.

Moire

When an image is scanned at a high resolution, the scanner may create moire patterns. These patterns are caused by interference between two sets of fine pattern grids, the scanner samples, and the halftone screen in the original image. The pattern can be formed by using any scanner.

Images printed on a printing press are printed with halftone screen patterns. These images are composed of a pattern of dots, which can be seen with the help of magnifying glass. If it’s a black & white image then the halftone dots are printed entirely in black. In case of color images there are four screens in each of the three primary colors, such as cyan, magenta, yellow and black (CMYK color system). These fine dots causes’ optical problems in a scanned image because the scene image also composed of the fine dots.

DPI, PPI and LPI

Different output device use different methods to display images and have different limitations on their output resolution. Following factors need to be taken into account when an image is scanned

DPI is the number of dots of ink per inch for an image printed on paper. Most laser printer can print between 300 to 1200 DPI, depending on the quality of the printer. It is good to set the resolution of the image to a lower value than the resolution of the printer to ensure good quality prints.

PPI is the number of pixels per inch in an image. The PPI reflects the actual sampling of the scanned photograph where each sample becomes a pixel. The PPI should normally be greater than the DPI in a printed image. It allows you to resize the image.

LPI is the number of lines per inch in halftone printing. These lines content halftone dots which, when viewed as a whole, forms an image. The LPI is related to the DPI as both deals with printing resolution. However, LPI is used for printing of an entire publication.