Main technical parameters of the scanner
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Optical resolution refers to the degree to which the scanner can actually sample the image when it scans the image, expressed in dpi (dots per inch) or ppi (pixels per inch).
For flatbed scanners, optical resolution is further divided into horizontal and vertical resolution. The horizontal resolution mainly depends on the number of pixels of the line CCD and the width of the scan (resolution = number of CCD pixels / maximum width of scanning). The vertical resolution is based on the number of lines that the stepper motor in the scanner moves by dragging the scan head per unit time, so the vertical resolution is related to the stepper motor and the mechanical drive section. Of the two resolution indicators of the scanner, the more important is the horizontal resolution. It is generally mentioned that optical resolution refers to horizontal resolution.
For example, for a scanner with a 5000 pixel CCD, the scanner's maximum scan width is 8.3 in, then:
Optical resolution = 5000 / 8.3 = 600dpi
For a roller scanner, the optical resolution is mainly determined by the width of the number of scanning lines, that is, the distance that the roller is fed in one turn. The finer the scan line, the higher the resolution; conversely, the lower the resolution.
The higher the optical resolution, the larger the amount of image information that can be acquired, and the more details are included in the scanned output image. The optical resolution of the scanner is related to the maximum magnification that can be magnified by the image formed by scanning with this scanner and the maximum number of screen lines at the time of printing. The relationship between scanning resolution, image magnification and number of printed screen lines is:
Scanning resolution = magnification × number of screen lines × quality factor (1.5 ~ 2)
It can be seen that when the number of screens for printing is constant, the scanning resolution of the scanner limits the magnification of the image. When the maximum magnification of the image is limited by the scanning resolution, only the magnification ratio can be reduced, and only the magnification or the number of screen lines can be reduced.
●Maximum resolution
On the basis of optical resolution, if the image formed by scanning is pixel-inserted by software, the resolution of the image can be improved. The maximum resolution is the resolution obtained after maximizing the insertion of pixels, so it is also called interpolation resolution, which refers to the resolution that can be achieved by calculation. The maximum resolution is different from the optical resolution. Although the maximum resolution can improve the resolution of the scanned image, it cannot actually increase the amount of information in the image, but will make the image look blurry. Software interpolation makes the resolution higher and the image quality worse. However, in this way, scanning of high magnification images can be realized from software. The maximum resolution is usually 2 to 4 times the optical resolution.
By understanding the meaning of the above two resolutions, we can correctly determine the quality of a scanner when purchasing a scanner.
●Maximum density range
The maximum density range, also known as the maximum density dynamic range, is the range of densities that the scanner can recognize for changes in the level of the original. The small maximum density range will cause the detail level of the dark portion of the original to be lost. Although the image of this area still has a slight change, the photosensitive device cannot distinguish and the output signal is the same, so the area on the scanned image becomes ungraded. The same hue of change. Only scanners with a wide range of densities can reflect the details of these dark portions. So dark adjustment is the key to verifying scanner performance.
Generally, the reflected original density range is less than 2.0D, and the transmitted original has a maximum density of 3.5D, so scanning the transmissive original requires a much higher scanner.
●Color bit depth
The color bit depth is the number of layers the scanner can recognize for each color. Early scanners had only one bit and only recorded two gray levels, black and white. At present, the color number of commonly used scanners is 8 digits, 10 digits, 12 digits, and 16 digits, which are commonly known as 24-bit, 30-bit, 36-bit, and 48-bit scanners. In theory, a 24-bit scanner can distinguish between 256-level grayscale and 16.77 million colors; a 30-bit scanner can distinguish between 1024 grayscales and 1 billion colors; a 36-bit scanner can distinguish between 4096 grayscales and 68.7 billion colors. The 48-bit scanner can distinguish between 65,536 gray levels and 281 million colors. Therefore, the higher the color number of the scanner, the richer the captured color, and the more the scanned image hierarchy, the larger the dynamic range.
● Zoom ratio
The zoom ratio is the magnification at which the scanner reduces or enlarges the original. Zooming is a processing program that produces larger or smaller images in the scanning software. When the scanned image scaled by the scanning software is sent to the image editing program, there is no need to change the size of the image. In the scanning software, the zoom ratio is inversely proportional to the optical resolution, and the larger the zoom ratio of the image, the lower the optical resolution. When using the maximum resolution, the zoom ratio can only be less than 1.
●Scan originals
The original scanned by the scanner mainly refers to the manuscript, the manuscript, the graphic, etc., and even a three-dimensional object such as a coin, a watch or a textile, which is commonly referred to as a scanned original.
When it comes to scanning an original, it is often described by an original indicator. The original index refers to the scanner's ability to adapt to the original, which can be expressed by the original type, original size, and original density range.
The original type refers to a transmissive original, a reflective original, a color negative, a color positive, a color original, a black and white original, and a line original.
The maximum size of the scanned original is the maximum size of the original scanned by the scanner. This indicator is important for reflecting the original. Because some of the reflexes are large in size, the rollers of the drum scanner and the platform of the flatbed scanner have certain limitations. In order to adapt to different sizes of originals, manufacturers have also introduced scanners of different sizes. Commonly used are A4 format, A3 format scanner and A0 format, A1 format engineering drawing scanner.
● scanner speed
The speed of the scanner is closely related to system configuration, scan resolution setting, scan size, magnification, and so on. Under normal circumstances, scanning black and white, grayscale images, scanning speed is 2 ~ 100ms / line; scanning color images, scanning speed is 5 ~ 200ms / line. In general, people always want the scanner to be fast, but the scanner works by scanning the light source, using a color separation method and CCD (charge coupled device) or PMT (photomultiplier tube) to collect and scan. The light information of the object and transfer the light information to a computer image file. The speed of the scanner is of course good, but it does not affect the image quality. Therefore, the scanner does not scan as fast as possible, and scanners with very high scanning speeds may lose some image information during scanning. Some scanners scan at low resolutions, but at high resolutions they don't necessarily scan fast. Therefore, the speed of the scanner must be increased while maintaining the quality.