Digitization of the hottest image manuscript

Digitization of image originals

3 scanning principle and scanner performance index

1) scanning principle

the steps of platform scanner scanning image are: first, lay the front of the original to be scanned face down in the scanning area on the scanner glass plate. Start the scanner drive, otherwise the lens surface will be scratched. After the moving program, the scanning part will not move. The movable light source installed in the scanner will move horizontally through the transmission mechanism of the scanner, and the emitted light will be projected onto the original. After reflection (positive scanning) or transmission (negative scanning), the light will be focused by the optical lens and enter the spectroscope, The three RGB color light bands obtained through the prism and the red, green and blue color filter shine on their respective CCD. The CCD converts the RGB light band into an analog electronic signal, which is converted into a digital electronic signal by the A/D converter, and then sent to the software resident in the computer content

from the perspective of image information processing, the scanning process principle of the scanner is shown in Figure 4, which is essentially the digital acquisition process of image information, that is, the sampling and quantization process of the original image information

2) scanner performance index

scanning accuracy

scanning accuracy is usually expressed by scanner resolution, which is one of the important parameters to measure scanner performance index. It is directly related to the quality of the scanned image. It is expressed by the number of scanner sampling points in the unit length. The usual unit is DPI (dot per inch). High resolution means that the scanner has more samples per unit length and more information to obtain images. Generally, what we call scanner resolution includes optical resolution, maximum resolution, etc. Optical resolution is the actual resolution that the scanner hardware can achieve, also known as physical resolution or real resolution. The optical resolution can also be subdivided into horizontal resolution and vertical resolution. The horizontal resolution is the real resolution of the CCD element, while the vertical resolution is determined by how many steps the stepper motor can move per inch. The maximum resolution is also called interpolation resolution. It is a method to calculate the average value of color or gray between adjacent pixels, so as to increase the number of pixels. The interpolation algorithm increases the number of pixels, but it cannot add real image details. The optical resolution of the drum scanner is determined by the rotation speed, the brightness of the light source, the function of the stepping motor, the size of the lens aperture, etc. it is generally composed of the resolution in the main scanning direction along the axial direction of the drum and the resolution in the sub scanning direction along the transverse direction of the drum

photoelectric conversion accuracy and color accuracy

an important technical index of the scanner is the gray level. The gray level of monochrome scanner refers to the ability to recognize and reflect the brightness of pixels. Color accuracy can also be called color resolution, color bit depth, or color scale. It is an indicator of the fineness of the scanner in distinguishing color or gray. The unit is bit. With high bit depth conversion accuracy, the scanner can capture more color details, enrich the level of the scanned image, and the image can be clearly reproduced. The essence of color accuracy is the depth of each primary color (R, G, b) of the original, which is also expressed by gray level. The scanner with photoelectric conversion accuracy of 8bit means that each scanned pixel is encoded with 8bit, which reflects 256 gray levels. A scanner with a color accuracy of 36bit means that the scanner can reproduce 236 colors, and each color channel can provide 212 levels of brightness difference (i.e. hierarchy)

density and dynamic range

density is an important index used to measure the printed original. It refers to that the light emitted by the scanner light source will reflect (or transmit) when it shines on the original. The value of the ratio of incident light to reflected light or transmitted light after taking logarithm is the density value. The darker the original, the higher the density. On the contrary, the smaller the density. The difference between the maximum density and the minimum density on the original is called the density range of the original, also known as the dynamic range. The dynamic range of the scanner is the difference between the maximum density and the minimum density that the scanner can measure, that is, the scanner's ability to recognize the density of the original, especially the ability to express the dark level of the original. The scanner with good quality can read the density of more than 2.5D at the darkest place. The scanner with a large dynamic range will absorb more layers, and the image will contain more details; The scanner with small dynamic range cannot detect a wide enough gradient range. At this time, the obtained image either compresses the bright tone, or the dark tone part is merged, which cannot correctly reflect the level and color of the original. 8. Its fasteners should be locked regularly: color

① scanner's color restoration ability

color restoration ability refers to the scanner's ability to restore various colors in color image information, which specifically includes two aspects: the scanner's overall color restoration ability and the scanner's ability to restore standard colors. The overall color restoration ability of the scanner is usually evaluated by the color condition after reproducing the standard gray scale, that is, the scanner scans the standard gray scale under the specified color mode, obtains the neutral gray balance parameters and their distribution characteristics of the standard gray scale, and compares them with the neutral gray balance parameters of the standard experimental samples, so as to comprehensively analyze the overall color restoration ability of the scanner. The scanner's ability to restore standard colors refers to the scanner's ability to reproduce each standard color block (C, y, m, R, G, B, y+m+c, BK, w) in the standard color code

② scanning repetition accuracy of the scanner

scanning repetition accuracy of the scanner refers to that the scanner scans the same original for many times, and inspects the error and distribution law of the coordinates of a certain control point. If the overall repetition error of the scanner is less than 50 μ M, the image scanned by the scanner can meet the requirements of image replication. The smaller the scanning repetition error of the scanner, the better the corresponding accuracy

③ scanner interface mode

interface mode refers to the interface type used between the scanner and the computer. USB interface and SCSI interface are commonly used. Attention must be paid to keep the sample in the plumb line and the middle position and parallel interface. Because the SCSI interface has fast transmission speed, very stable performance and very little CPU resources, professional scanners usually use SCSI interface. Ordinary scanners often use parallel interfaces and USB interfaces. The USB interface is the most convenient to use and can be directly hot unplugged without restarting. At present, USB interfaces are also divided into USB1.1 and USB2.0. The transmission rates between them are 12mbps/s and 480mbps/s respectively, with a difference of 40 times. The parallel interface has been basically eliminated because of its slow speed, and only the old style scanners are used

④ scanning speed of the scanner

the scanning speed of the scanner is usually calculated by how many milliseconds it takes to scan each line. The speed of the scanner is closely related to the configuration of the computer system, scanning resolution setting, scanning size, magnification, etc. Generally, the scanning speed is 2 ~ 100ms/L when scanning gray-scale images; When scanning color images, the scanning speed is 5 ~ 200ms/L. Scanning according to the project leader, the scanning speed of the instrument is not as fast as possible. The scanner with very high scanning speed may lose some image information during the scanning process

(to be continued)