Infrared cleaning

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Infrared cleaning is a technique used by some film scanners and flatbed scanners to reduce or remove the effect of dust and scratches upon the finished scan. It works by collecting an additional infrared channel from the scan at the same position and resolution as the three visible colour channels (red, green, and blue). This infrared channel can be used to automatically remove the appearance of dust and scratches in the visible channels and replace them by inpainting.

[edit] Approaches

Scanners use three different techniques for doing this:

  • Nikon film scanners use four colored light-emitting diodes which are pulsed on and off, one at a time. These LEDs are pulsed on and off at each scan position, the light is gathered by a linear array and then the CCD is moved to the next scan line.
  • Minolta film scanners use a constant visible light source with a pulsed infrared light source. At each scan position, the scanner uses an RGB linear array to scan the film in RGB and RGB+Infrared. This is as fast as the Nikon approach, since only one physical pass over the film needs to be done.
  • Flatbed scanners and some film scanners have two different light sources, an RGB light source and an infrared light source. These scanners make two passes over the film - once for RGB and once for infrared. This is slower than the Nikon or Minolta approach, since two passes need to be made over the film. It also produces lower quality since software methods need to be used to align the two passes. In addition, the two light sources usually have a different focus position and produce images that are stretched in the CCD direction, which results in another source of lower quality.

[edit] Applicability

The three color dyes in modern color film emulsions respond equally to infrared light, so the infrared image looks relatively opaque compared to the RGB image. Any dust spots or scratches appear as dark spots in infrared, making it easy to find and correct for these dust spots.

Kodachrome film dyes are more sensitive to infrared than Ektachrome film dyes, making it more difficult to find dust spots in Kodachrome. Some software algorithms, such as newer ICE implementations and VueScan's Infrared Cleaning can find dust spots even when scanning Kodachrome.

Infrared cleaning doesn't work at all with silver halide black and white film, but does work with chromogenic black and white films This is because silver particles respond equally in visible light and infrared light.

[edit] History

IBM originally developed and patented infrared cleaning, and subsequently licensed this patent to Advanced Science Fiction (ASF). Canon has a patent cross-licensing agreement with IBM and was able to use the IBM's infrared cleaning patent. Canon couldn't use the trademarked Digital ICE name, so instead calls it Film Automatic Retouching and Enhancement (FARE). Another company, Hamrick Software, independently developed an infrared cleaning algorithm that's completely different from IBM's patented algorithm, and has incorporated this algorithm in VueScan.

ASF subsequently went out of business, having spent all their money trying to develop dry film development in a kiosk when film sales significantly dropped. Kodak purchased the assets of ASF, but hasn't done anything with any of ASF's technologies.