Slide scan
A film scanner is a special reproduction method , in which a slide by means of a slide scanner digitizes is.
Basic principle
The illuminated slide is scanned line by line. The different colors and brightnesses are converted pixel by pixel into digital signals and saved.
resolution
The resolution of good slide scanners in the amateur sector is 4600 ppi , semi-professional devices reach 7200 ppi. Higher resolutions can often only be achieved with drum scanners .
Since good devices can scan not only with a high point density , but also with a very high density range, they are able to adequately transfer the high contrast range of a slide.
Slides scanned in this way generate very large image files: With a slide of 24 mm × 36 mm, for example, at 3600 ppi there are already approx. 17 megapixels , at 6400 ppi about 54 megapixels. Image files of this size do not make sense in all cases of application, as significantly smaller amounts of data are sufficient for most cases of application ( beamers , small prints , ...).
Color depth
Slides have a special peculiarity. Because hardly any other photo medium delivers such a broad and dynamic color spectrum as the slide. In order to do justice to this special feature, slide scanners must also be designed accordingly. Most slide scanners therefore have a color depth between 24 bit and 48 bit. That corresponds to a room of 16.7 million and 281 billion color tones .
Slide scanner
Slide scanners are specially built scanners to digitize framed slides and in most cases also negative films . The commercially available devices for the amateur and semi-professional sector are usually built for the 35mm format ; however, scanners for medium format originals are also available. Some models can use an adapter to process several slides in batch mode, some from commercially available slide magazines .
The digital output format ranges from JPG to TIFF to RAW .
Depending on the manufacturer, the slide scanners have a USB or Firewire interface.
Some hardware and software developers have developed special technologies that can have a major impact on the quality of the scan result. Relevant are among others:
Dust and scratch correction
Many slides have scratches or fingerprints on them; Dust particles can hardly be completely avoided. A small speck of dust, projected onto a screen, can look like a bird in a blue sky. There are several methods of detecting and eliminating such defects during scanning:
ICE, SRD, iSRD, Infrared Cleaning
ICE is an abbreviation for I mage C orrection and E nhancement , SRD stands for S mart R emoval of D efects , iSRD is as ICE and Infrared Cleaning in addition to the infrared channel of the scanner. The long wavelength of infrared light shines through the color emulsion of negatives and slides almost unhindered; In contrast, scratches, dust particles and lint cast shadows. From the position data determined in this way, the infrared technology can carry out an automatic dust and scratch correction based on the surrounding pixels . In the case of iSRD , the image is scanned in two passes. The infrared scan is carried out in the first pass, the regular RGB scan in the second pass. Dust and scratches are then mathematically removed using an additional image channel that is automatically created from the infrared image. With a few exceptions, iSRD works with all see-through scanners that have an infrared light source and, in contrast to ICE, can also be operated manually. The image quality can be further improved by additional options such as corrections of different strengths on certain parts of the image. ICE is implemented on the hardware side in many current devices. SRD and Infrared Cleaning , on the other hand, are pure software implementations on the PC side that use the raw data of the infrared channel. They therefore work in principle with any scanner.
Almost all methods that use the infrared channel fail with silver- based film material (this applies to the majority of black and white films). Even with Kodachrome slide films, the results are usually unusable because of the special structure (three layers based on silver). The long-wave infrared rays cannot penetrate materials such as silver particles contained in these films. So these particles are treated like defects; H. retouched. The resulting image is extremely blurry. In 2004, Nikon launched the Super Coolscan 9000 ED, a scanner that can remove dust and scratches from Kodachrome films using an improved ICE process ( ICE 4 Professional ) without any additional software. With iSRD and Infrared Cleaning , Kodachrome slides can now also be reliably freed from dust and scratches without losing sharpness. A previous calibration with a special Kodachrome- IT8 target is also helpful .
FARE , Auto Dust
Some scanner manufacturers market other proprietary dust and scratch removal processes under various brand names.
Noise reduction with multi-sampling
The image is scanned several times and the mean value of the pixel values of all individual scans is assigned to each pixel in the image. In theory, a quadruple scan leads to a signal-to-noise ratio that is 2 times better. Some scanners support multiple scans directly, ie the sensor is able to read the same line several times in a row. If this is not possible, several individual complete scans can alternatively be made and processed one after the other. Mechanical tolerances can under certain circumstances mean that the individual scans are not exactly congruent, which can lead to blurred images.
Expansion of the dynamic range with multi-exposure
This process was developed by LaserSoft Imaging . It only works with scanners that allow you to change the exposure intensity. The dynamic range plays a crucial role in the quality of a recording; in general, a larger dynamic range means better results. By using multiple layers with different sensitivities, modern black-and-white films, color negative films and slide films achieve a high dynamic range already during the recording, which can only be captured by very few high-quality scanners through normal (one-time) scanning. With Multi-Exposure , the maximum dynamic range , i.e. that of the original, is retained by multiple scans with different exposures . This procedure differs from classic multi-sampling , in which general noise in the image is reduced by multiple scanning with the same exposure intensity; Multi-Exposure also does this.
microscopy
Some types of scanners are also suitable as a microscope replacement. Instead of a slide, a holder with the specimen to be imaged is used and scanned. For this purpose, some manufacturers offer specially developed accessories, but such a bracket can u. You can also improvise by using a double-walled glass slide frame.
See also
literature
- Sascha Steinhoff: Digitizing slides and negatives . dpunkt.verlag, 3rd expanded edition, 2008, ISBN 978-3-89864-522-5
Web links
References
- ↑ Shutterbug Magazin (issue 06/09 p. 110ff) or online: David Brooks: Better Profiled Kodachrome Scanning (English)
- ↑ proven by independent tests, e.g. test report (PDF; 354 kB) on SilverFast - Multi-Exposure (English) by D. Wüller Image Engineering