How to choose a scanner

- Aug 17, 2018 -

First we need to know what is called resolution. All types of scanners are marked with its optical resolution and maximum resolution. The unit of resolution is dpi, dpi is the abbreviation of English Dot Per Inch, which means the number of pixels per inch. After understanding the meaning of resolution, let's take a look at how important the parameters of these scanners are to us.

Optical resolution refers to the actual amount of information that can be acquired by the optical system of the scanner, that is, the resolution of the photosensitive element of the scanner. For example, a scanner with a maximum scanning range of 216 mm × 297 mm (suitable for A4 paper) can scan a maximum width of 8.5 inches (216 mm), and its photosensitive element contains 5100 units with an optical resolution of 5100 ÷ 8.5 inches = 600 dpi. . Common optical resolutions are 300 x 600 dpi, 600 x 1200 dpi, 1200 x 2400 dpi or higher.

The color resolution is also called color depth, color mode, color bit or color gradation. In short, it is an index indicating the degree of color or grayness of the scanner. Its unit is bit. The exact meaning of the color bit is how many "bits" are used to represent a pixel that is scanned. For example: 1bit can only represent black and white pixels, because the numbers in the computer use binary, 1bit can only represent two values (2 1=2) ie 0 and 1, which represent black and white respectively; 8bit can represent 256 gray levels (2 8=256), they represent different gray levels from black to white; 24bit can represent 16777216 colors (2 24=16777216), generally more than 24bit colors are true colors, of course, 30bit, 36bit, 42bit model. In theory, the more color bits, the more realistic the color.

Let's talk about the relationship between resolution and file size. The general scanning application can automatically calculate the file size when you preview the original draft, but knowing how to calculate the file size will help you make the appropriate choices when managing scanned files and determining the scanning resolution. The calculation formula of the black and white image file is: horizontal size × vertical size × (scanning resolution) 2 ÷ 8. The calculation formula of the color image file is: horizontal size × vertical size × (scanning resolution) 2 × color depth × 1/8. For example, by scanning a normal color photo (3.5×5 inches) with a 24-bit color RGB method and a scanning resolution of 300 dpi, the obtained image file length is 5×3.5×300 2×24×1/8=4725000 Byte or 4.7 MB. .

The last concept to be elaborated is OCR. It is an abbreviation for English Optical character recognition. Its function is to read the text image information on the printed matter through an optical input device such as a scanner, and use the pattern recognition algorithm to analyze the morphological characteristics of the characters to distinguish different Chinese characters. Chinese OCR is generally only suitable for identifying printed Chinese characters. The use of a scanner and OCR can partially replace the function of inputting Chinese characters into the keyboard, which is a labor-saving and quick text input method. Common OCRs include Tsinghua Wentong, Shangshu, and Mengyu.