Discussion on Transmitted Document Scanning Technology (I)

At present, our most common scanning manuscripts are various types of reflected manuscripts: books, magazines, documents, photographs... In fact, there is a very important type of input source that is playing an increasingly important role. This is the transmission of manuscripts. It may be that you are rarely exposed to scans of transmitted documents in your daily work and life, but it does occupy a very important position in scanning applications. In the current booming digital imaging trend, although digital cameras have huge market potential and rapid growth, it is still difficult to shake the foundation of traditional silver salt imaging technology. Before the advent of the “pure digital imaging” digital camera + digital processing + output device, a transitional solution consisting of a conventional camera + film + wash + transmissive document scanner + digital processing + output device was formed. In the field of industrial applications, the transmission of scanned documents also occupies a decisive position. Medical images such as large number of X-ray films taken every year in medical institutions must be digitized because of the requirements of telemedicine and archiving. More and more film films requiring digital special effects must pass through. When the scanner enters the computer, the scanner becomes the primary digitizer, and the reversal or negative scan is much better than the photo.

Transmission features

Compared with the reflection document scanning, the transmission document scanning is rather special, and the requirements for the scanning equipment are more severe. One of the characteristics of the transmitted manuscript is the high resolution of the scanning device. Taking the 35mm reversal film, the most common one in the transmission, as an example, its image area is very small, only 36mm×24mm, so the information density is very high. To print it in A4 size is equivalent to 8x magnification, which requires a very high optical resolution of the scanner. The best photographic film for 35mm film has a resolution of 100 lines/mm or more, and generally has 60 lines/mm or so. The resolution at the edge of the screen is relatively low. When viewed on average, ordinary 35mm film can achieve effective resolution. The rate is about 40 line pairs/mm, and the resolution unit commonly used in scanners is about 2000 dpi. That is, to use a scanner with an optical resolution of 2000 dpi or higher, various contour details in a transmitted document can be recorded.

Another feature of transmissive manuscripts is the large density range, that is, the contrast between bright and dark parts of the picture is very strong, especially the reversal film, which reaches the highest of all types of scanned manuscripts. Relative to the density range of the input document, the corresponding parameter in the scanner is within the dynamic range. This is an important indicator of scanners that we usually do not pay much attention to. It represents the range of brightness and darkness that a scanner can resolve. The difference between the logarithmic values ​​from the lightest point to the darkest point is used as a measure. It is a professional who chooses a scanner. The important basis. For example, a 24-bit scanner theoretically exhibits a grayscale level of 256 with a dynamic range of 2.4. Obviously, the higher the color number of the scanner, the more light and dark levels can be reflected, and the greater the dynamic range. Currently, the 36-bit scanner on the market has a relatively high-quality product dynamic range of 3.4 and a professional 48-bit. The scanner has a dynamic range of up to 4.0. In common transmission manuscripts, the range of color negative film density is relatively small, usually around 2.2, using a 24-bit or 31-bit scanner can handle; and the color reversal film density range is generally 3.2 or more, up to 3.5, To perfectly capture the color and gradation information on the reversal film, scanners with more than 36 bits are the only ones.

In addition, some special transparencies have additional requirements for scanning equipment. Like radiographs and CT scans in medical imaging, they are all grayscale images. The density range is not very large. However, the entire screen is very dark and requires a very bright light source to reflect a good level. Moreover, the area of ​​such transmissive documents is generally large, requiring an A3 format transmissive document scanner.

Transmission manuscript scanning solution

Ordinary flatbed scanners image the light reflected by the surface of the manuscript, and the scan of the transmitted manuscript cannot be completed without adding an attachment to change the light path. From the current products of each scanner manufacturer, there are mainly the following solutions for transmissive document scanning:

1. Special film scanner

This is a class of scanners designed and manufactured for scanning film. It uses the most compact imaging method (see figure) to allow the transmitted light illuminated by the light source to be directly imaged on the CCD sensor through the lens. Because film scanners are generally designed for 35mm film and have a small scan format, the imaging light path is also very short. Instead of using ordinary flatbed scanners, the light path needs to be “folded” to a flat surface with 3 to 4 mirror reflections. In the body. In this way, the film scanner can avoid a large number of interference factors that cause image quality deterioration, and it is easy to obtain high quality scan results.

In addition, the film scanner and the flatbed scanner also use a linear CCD sensor, but the latter forms an image by moving the light source and the imaging section (CCD sensor) along the scanned document, and the film scanning works in a completely opposite manner. - Scanning method using fixed imaging mechanism and moving film. This mode of operation not only brings an improvement in the scanning quality of the film scanner, but also makes them scan a limited width, but with the in and out of the folder, the scan length has been extended, generally can handle more than 20 The centimeter-length film strip (containing 6 frames), even after adding accessories to some professional film scanners, can even process a full roll of 35mm film up to 1.7 meters at a time.

In terms of optical resolution, film scanners are typically more than 2000 dpi, with a typical value of 2700 dpi, sufficient to reproduce sharp objects in the original. Their color number is usually 31 or 36 bits, and the dynamic range is about 3.0. Scanning negatives have no problem. The rich color hierarchy information on the reversal film can also be faithfully recorded. Since the previous film scanners were mainly for professional printing and publishing, most of the products used the classic SCSI interface to ensure high transfer rates and compatibility with the Macintosh.

Domestic common film scanners include EPSON FilmScan 200, Canon CanoScan FS2710, Microtek ScanMaker 35t Plus, Nikon Super CoolScan 2000, Kodak 3570 Plus, and the like.