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. On the other hand, dust and scratches absorb in the infrared. 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), VueScan's and SilverFast's, claim to use infrared cleaning to find dust spots even when scanning Kodachrome.
Read more about this topic: Infrared Cleaning
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