linux/xsane scanned at 300 dpi, 16 bit, 412 * 278

color-3.jpg		original infrared plane
0color-rgb.jpg		original RGB image

dirt removal images/steps
1clean-3.jpg		infrared plane with reduced red component
2yen-threshold-3.jpg	static threshold assuming bimodal histogram
2otsu-threshold-3.jpg	static threshold
	presumed dirt is black, if yen shows less dirt than otsu dirt
	is not clearly separable by static means
2madmean-threshold-3.jpg	adaptive threshold
	is combined with the yen static threshold to
	add large dirty areas
3dilate-rgb.jpg
	dirty pixels are replaced by the nearest clean pixels
	only replaced pixels are mean filtered
4gamma-rgb.jpg		applied gamma 0.4545 for natural appearance