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 color channels (red, green, and blue). The infrared channel, in combination with the other channels, is used to detect the location of scratches and dust. Once located, those defects can be corrected by scaling or replaced by inpainting.

Method

The three color dyes in typical color film emulsions are transparent to infrared light, so the infrared image looks relatively clear compared to the RGB image.[1] On the other hand, dust and scratches absorb in the infrared.[2] Any dust spots or scratches appear as dark spots in infrared, making it easy to find and correct for these defects.

Pixels that are partially occluded (for example, the dust only obscures a small portion of the pixel) may be corrected by scaling. The infrared pixel value indicates the fractional amount of the occlusion, and RGB values can be scaled appropriately.

If most or all of the pixel is occluded, then scaling is not feasible. Instead, the pixel value may be interpolated from nearby good pixels (inpainting).

Not all films are transparent to infrared, so infrared cleaning may not be feasible. Infrared cleaning doesn't work with silver halide black-and-white film because silver particles respond equally in visible light and infrared light. Consequently, the infrared channel cannot distinguish between dark pixels and dust. Infrared cleaning does work with chromogenic black-and-white films.

Infrared cleaning works with Ektachrome slide film. However, Kodachrome film dyes block infrared, so it more difficult to find dust spots in Kodachrome. Some software algorithms, such as the latest ICE implementation (Nikon Coolscan LS 9000 with ICE professional[3]), VueScan's[4] and SilverFast's,[5] claim to use infrared cleaning to find dust spots even when scanning Kodachrome.

Infrared scanning

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.

History

IBM originally developed and patented infrared cleaning, and subsequently licensed this patent to Applied 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). Other companies, like Hamrick Software and LaserSoft Imaging, independently developed infrared cleaning algorithms which are completely different from IBM's patented algorithm.

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.

References

  1. ↑ Edgar 1993, col. 4
  2. ↑ Edgar 1993, Sheet 4
  3. ↑ CreativePro - Bit by Bit: New Scanner Gives Life to Old Film by Brian P. Lawler (Photographer)
  4. ↑ http://www.hamrick.com/vuescan/vuescan.htm
  5. ↑ http://www.silverfast.com/highlights/isrd/en.html
  • US 5266805, Edgar, Albert D., "System and method for image recovery", published May 5, 1992, issued November 30, 1993 . IR dust removal patent. Column 3–4: "Accordingly, the infrared image may serve [a]s an indicator or map of the spatial position of these non-image imperfections on and in the media, thereby allowing recovery of the underlying desired image."

External links

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