Removing Dust and Scratches with SilverFast SRD
Some scanners come with Digital ICE technology. Digital ICE automatically removes surface defects, such as dust and scratches, from a scanned image. Digital ICE technology works from within the scanner, during the scanning process, but gives users little or no control over the process or the outcome. SilverFast's SRD feature does.
The SRD tool sits in the vertical toolbar at left of the preview window. It is a Red Cross icon that opens a dialogue window when clicked. The SRD process starts with a new prescan of the image. The preview will have an overlay grid in red and a yellow-bordered Navigator area.
For optimum recognition of artifacts SilverFast uses two different methods. The first is the regular dust and scratch removal and the second is the removal of linear artifacts. These work with parameters with similar names, yet have different effects on different artifacts.
There are some interesting buttons in the SRD dialogue window. The arrow pointing to the right turns real-time correction on or off. The three buttons at its right show you the original without correction, the prescan with artifacts removed, and the artifacts highlighted in red.
The SRD Controls
The SRD dialogue has an Auto button, which enables SilverFast to detect scratches and dust automatically, but there is also the opportunity to do it all manually. The latter method might not give the best results unless you are good at masking.
The mask tool in the SRD dialogue serves to protect areas that are not to be touched by the scratch and dust removal algorithm. This ensures that the good parts of the image remain in perfect condition.
Manual corrections start with testing the defect type. You can try different defect types by switching from the current type, another and then see how your setting affects the preview.
The sliders labeled "Defect Size" and "Detection" are preset by the auto mode. For full control over the process, you can change their positions. The Detection slider represents the sensitivity of artifact recognition. Optimum parameters will depend on the type of image, so you will have to experiment to get the best result. With sharp images, Detection will be mostly between 1 and 60.
With blurry images or images that have been smoothed detection will most likely be between 60 and 100. The Defect Size slider sets the size in pixels of the artefact. Values are mostly between 1 and 5.
Always monitor the effect of both sliders in the large preview window, and if necessary check different tiles of the image.
Only when the results of changing the previous two sliders do not yield the desired results, should you change the Intensity slider in small increments to values smaller than 100. This will predominately be the case with detailed images.
SRD Controls in SilverFast Ai Studio
The Environment Size slider is only available in full versions of SilverFast and becomes visible when activating the Expert mode. This slider is used to control the recognition of the defect border. Parameter values are small, usually between 1 and 6.
The SRD dialogue also has a Layer option. Layers can serve to correct increasingly smaller artifacts. You can have up to four of them.
In the full version of SilverFast, there is an extra menu, called Longish Scratches. This menu will only be visible after clicking the Expert button. This is an alternative method of artifact removal which can be applied to 35 mm film where the surface has been scratched by small dust or sand particles, for example.
Many of these scratches proceed across several images, sometimes even across the whole film. They frequently run parallel to the edge of the film. In order to eliminate longish scratches, the following controls are available: Defect Type, Orientation, Length, Width, Contrast and Continuity.
Usually the first three controls (Defect Type, Orientation, Length) are sufficient to get adequate results. Other controls such as Width, Contrast and Continuity can remain at their default settings.
Infrared Dust Removal
If you think this is all SRD is about, you are wrong. LaserSoft Imaging also offers Infrared SRD when your scanner supports infrared scanning. When it does, the artifacts are additionally scanned as infrared blemishes.
Due to the wavelength of infrared light, it can penetrate the colour emulsions of film negatives and slides virtually unhindered. The path of an infrared beam is only hindered when it encounters artifacts such as scratches, dust particles, lint, etc. These will cast shadows in infrared light.
SilverFast's iSRD exploits this characteristic by scanning the image in two passes. The first pass is the normal RGB scan and the second pass the infrared scan. Once the software has completed both scans, an additional image channel created automatically from the infrared image is used for the dust and scratch removal calculation.
iSRD can be used with conventional colour negatives (developed using the C41 process), colour slides (developed using the E6 process) and paper proofs. Due to the silver content in conventional black and white negatives and slides, these can't be retouched using iSRD. However, special black and white negatives which have been developed using the C41 process behave like colour negatives and are iSRD-compatible.