Rawtherapee: infrared cleaning for scanned slides using Dark Frame?


(srowed) #1

A significant part of my photography involves digitising slides and other film in high resolution. In the past i used various film scanners but now use DSLRs, shooting in raw and processing in Rawtherapee. It’s much faster and gives generally better results than even the best film scanners. There is, however, one feature of film scanners that would be a time saver - infrared cleaning such as ICE that many film scanners have.
The infrared light identifies scratches and dirt, then the scanning software covers up the affected pixels with the surrounding pixels, much as a healing brush does, if I understand it correctly.
My question is, would it be possible to shoot an extra frame of the slide using infrared light, then use this frame to simulate the infrared channel from scanners? Perhaps Rawtherapee’s Dark Frame feature could be used for this?
Thoughts?


#2

The problem with using your camera for this is that you’d need to refocus your lens for infrared, and you’d need an infrared-capable camera.


(srowed) #3

CarVac, thanks for that. Doesn’t seem feasible given the focus and camera issues you mentioned. Another approach that might be worth looking at is to use visible light at an angle. With no back lighting on the film dust shows up as white specs against the dark film. I’ll give it a try.


#4

Or you could get the Coastal Optics UV-VIS-IR 60mm Apo Macro lens.


(srowed) #5

Haha! I can do a lot of manually cloning out scratches for the price of one of those lenses!


#6

I wonder what the frequency of the infrared light in the scanners is and if the difference in diffraction would really require refocusing. Usually, you would lowpass filter the infrared channel anyway in one or the other way (see e.g. the thread you linked to). Do you or somebody else have any source? I heard (don’t know if here or somewhere else) of a test of this method in a magazine (was it the German “C’t Digitale Fotografie”?), does anybody know some details?


#7

Actually, if that’s the case, then you could certainly make use of the tendency of the focus to shift and you wouldn’t have to refocus.


#8

It would depend on the amount I guess. The closer the frequency of the infrared channel is to the visible light, the less problem I see. But one would require either the data (frequencies etc.) to calculate the feasibility or evidence by testing.
Probably, it could be enough to add an infrared filter after your light source for the second picture (and probably some more stops of flash), and use the resulting picture already. But this may be totally wrong, so is there any experience? Has somebody read the article I mentioned?


(Hermann-Josef) #9

@chris
Just came across this thread and would like to give some comments.

As far as I know, DSLRs have an IR-filter incorporated. You need to remove that. My Sony F828 has the option to remove this filter and shoot at night in IR-light. But the S/N is lousy and would not be suitable for “ICE”.

The IR-lamp in my slide scanner peaks at 870nm with a width of about 50nm.

If all else settled, the problem I would see with a second IR-exposure is the alignment of the images. Scanners like my flatbed scanner CanoScan9950F do the IR-scan in a second pass and there are in some scanners mechanical problems repositioning the scan head with sufficient accuracy (works nicely for my flatbed, but not for my slide scanner). Thus now the driver for my slide scanner has been upgraded and does the IR-scan at the same position of the stepper motor just after the RGB-scan for each scan line.

SilverFast still aligns the RGB- and the IR-image by software and this does not always work out, rendering the correction impossible. Thus I would see problems with aligning RGB- and IR image.

Hermann-Josef