Multi-Exposure is an exposure blending process that can be used when digitizing transparencies such as slides, negatives and film strips with a scanner in order to eliminate image noise and to increase the effective dynamic range of the scanner used. Multi-Exposure uses an auto-alignment technology developed and patented by LaserSoft Imaging for the automatic, exact alignment of multiple scans and for the elimination of image noise.
By using multiple layers with different light sensitivities, a normal slide film achieves a contrast range of eight f-stops; an optimally exposed and developed negative can contain a contrast range of up to twelve f-stops . Scanners cannot fully capture this dynamic range, so that image information would be lost with conventional scanning.
With this technology, which is used in the scanner software SilverFast , the original is scanned several times, but with different exposures. The software calculates the finished HDR scan from the individual images. Multi-Exposure increases the effective dynamic range of the scanner, so that more details are captured, especially in the very light and very dark areas of the image. An overexposed scan illuminates the shadows of the original, the available contrast levels of the scanner are used for the darker areas of the image. An underexposed scan, on the other hand, captures details in the bright areas of the image. A calculation of overexposed and underexposed scans results in a result image that has a greater contrast range than both individual scans.
With repeated scanning, Multi-Exposure also reduces certain forms of image noise ; an overexposed scan helps in particular to remove dark noise. Some similar methods, often called multiple sampling , also scan the original several times, but always under the same conditions. Here, variance-based filters can be used to reduce image noise, but in contrast to the multi-exposure method, there is no increase in the contrast range.
- Multi-Exposure does not work with all scanners, it must be possible to influence the exposure time via software.
- The method is suitable only for the transparency such as slides, but not opaque document , such as photos.
The EDP ( European Digital Press Association ) awarded LaserSoft Imagings SilverFast in 2008 for the increase in the density range of most scanner models through multi-exposure and for the automatic generation of ICC profiles the award "Best color management software of the year 2008" awarded.
- Test report (PDF; 354 kB) about SilverFast - Multi-Exposure (English) by Dipl.-Ing. D. Wüller Image Engineering
- filmscanner.info: Quality improvement with multi-exposure
- EDP (European Digital Press) - Best Product of the Year ( Memento of the original from July 1, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : Best Color Management