Wavelength of infrared channel for scratch/dust detection

[chris]

@Jossie

PIE-scanners (reflecta DigitDia6000).
The IR-LED is centered at a wavelength of about 880nm with a width of about 60nm

This seems to be close enough to the visible band such that optics and sensor may still work good enough to get some picture. I do own a reflecta scanner myself, so I can confirm that dust detection works reasonably well.

Thanks for this information, but now I wonder: How were you able to get it? Typically, LEDs are not marked with type names …

@guille2306

This is of course not true with visible light, you get scattering but also record the image, so it’s difficult to untangle both. In the UV, for a sufficiently short wavelength, it might work if light is absorbed homogeneously on the substrate surface without reaching the silver particles. But it might be difficult to get a UV led with a short enough wavelength, and then the transmission of the scattered radiation through the lens will be horrendously low (I suspect you need something well below the 370nm cut-off of normal glass).

If @Jossie is correct, then it seems that for a decent separation, wavelengths close to the visible spectrum can be used. Looking at the infrared picture from my reflecta scanner, I typically get a ghost image of the actual picture, but I can clearly draw a separation threshold between the ghost image and the dust/scratch data, which is typically much more opaque, see an example image here: Scanned image scratch removal with “ICE” - #77 by chris. Furthermore, the dust image does not need to be perfect, a small amount of false positives will not affect the overall result, given a good inpainting algorithm is used. Here’s a comparison of different inpainting methods: Scanned image scratch removal with “ICE” - #21 by chris with data from my reflecta scanner.

@cedric

the lower the illuminant wavelength, the higher the MTF per Rayleigh et al.

As said, the goal is not a perfect IR image for the mask, and I suspect a little bit of lens blur is even beneficial for a good mask image. 880 nm are not too far off from the visible light anyway. I’ve written a little g’mic script to semi-automate the mask processing and application which you can also find information for in the thread linked above, and what this script does is to increase the detected mask regions a bit anyway such that no nasty edge effects happen.

Typical IR RCs for TVs are operating also between 840 and 940 nm, and typical cameras are able to “see” the light, such that also the IR filter of the cameras may not be an issue.