DIY Drum Scanner?

[Ted Baker]

Well, in fact when I suggest "pulsing the laser intensity in two levels, chopping/buffering the signal for the two levels and sending it to two separate A/Ds for each channel." this is a Multi-Exposure (dual) system in a single pass,

You would make two short laser shots, buffering+chopping the signal very fast (two time per pixel) while allowing the converters a full pixel clock time interval to make the conversion.

I guess they are all variations on the same theme. i.e. bringing the voltage variation into the best range of the A/D and ultimately the bit buckets you have available to fill.

Possible problems with a laser I guess would be finding one with a sufficiently broad spectrum, or calibrating it, or maybe not if you carefully calibrated it to match the emulsions, etc. But with the later you would be digitally simulating the color cross talk engineered into the emulsion. So maybe you be better off with just a digital camera...

[interneg]

In fact you need 4 chips, 3 colors + IR for dust correction...

You don't need IR dust removal on a fluid mounted scan... There was one (Dainippon Screen 1030ai - I think) scanner that had a 4th PMT intended for optical unsharp masking, but I can't think of any others that had more than three PMTs

[Pere Casals]

Possible problems with a laser I guess would be finding one with a sufficiently broad spectrum,

Laser light is both monochromatic and highly coherent, this is that even the photons are in phase !

For color scanning, an scanner does not need a broad spectrum in its light components, at the end color dyes in film are a response to silver content in the development, so just reading in the dye peak (for example) it would determine the in film native signal. Add a calibration and you can determine near exact native film spectral result.

What I mean is that for scanning a low CRI light source may be ideal, because at the end scene spectral information is not there anymore, just a tristimulus repersentation of it. This is IMHO...

If we scan Velvia we won't be able to see the same in an sRGB monitor, just the triangles are not the same, nor the possible static contrast. One may say that an scanner delivers nicer colors than another one, but it's about calibration or 3D LUT edition...

[Pere Casals]

You don't need IR dust removal on a fluid mounted scan... There was one (Dainippon Screen 1030ai - I think) scanner that had a 4th PMT intended for optical unsharp masking, but I can't think of any others that had more than three PMTs

true...

[Ted Baker]

Laser light is both monochromatic and highly coherent, this is that even the photons are in phase !

For color scanning, an scanner does not need a broad spectrum in its light components, at the end color dyes in film are a response to silver content in the development, so just reading in the dye peak (for example) it would determine the in film native signal. Add a calibration and you can determine near exact native film spectral result.

That's what I used to think... Until I tried to understand why the gamma curves of color negative film do no match, and there is no such correcting gamma in color print film/paper.

For example the CC graphs are prepared using status-m which is a narrower spectrum than what the print sees. The gammas when measured using a broader spectrum are closer to equal, the gammas are different again when the emulsion is exposed to a narrow spectrum of light that only hits one layer.
This intentional crosstalk designed into the emulsion would be missed or at least you would see a different version of it depending where you take you measurements.

This cross talk is designed to manage saturation, at different densities among other things. I am sure you could model it, but then why not use a digital camera if you going to model nearly everything...

[Pere Casals]

That's what I used to think... Until I tried to understand why the gamma curves of color negative film do no match, and there is no such correcting gamma in color print film/paper.

For example the CC graphs are prepared using status-m which is a narrower spectrum than what the print sees. The gammas when measured using a broader spectrum are closer to equal, the gammas are different again when the emulsion is exposed to a narrow spectrum of light that only hits one layer.
This intentional crosstalk designed into the emulsion would be missed or at least you would see a different version of it depending where you take you measurements.

This cross talk is designed to manage saturation, at different densities among other things. I am sure you could model it, but then why not use a digital camera if you going to model nearly everything...

Gamma, 1D LUTs, etc may work in practice, but the complete spectral characterization of a film would require a 3D LUT.

You can reduce the problem to a 3D LUT, in the technical 3D LUT sense, not in the creative 3D LUT sense (anyway LUT format it's the same in both cases).

In practice you have to create/calibrate a Technical 3D LUT for each film, because every film has an spectral nature. For digital usage a 3D LUT calibration has all. Still you may not be able to show all what's in the film because color spaces are not equal.

If you want to reproduce the spectral footprint of a film (say an slide) then it's also possible in theory, but then for each RGB entry point of the LUT you have to record the spectrum in the original calibration spot, this would be useful to transalate to other color spaces with best matching possible...

In fact it is how it worked color separation in BW films (Kodak 2237 and 2238) for preserving movies. It's the same I mentioned but done with analog tools, when restoring the separation to a color medium a matching spectral footprint medium can nail the original, even spectrally.

[Ted Baker]

Gamma, 1D LUTs, etc may work in practice, but the complete spectral characterization of a film would require a 3D LUT.

I agree to complete an accurate characterization of a film would require a 3D LUT. However it makes sense to have as a starting point a measurement that is closer to physically reality. In particular having the sensor pickup the cross talk in emulsions rather than simulating it through a matrix.

Interesting the color separation film you mentioned are also designed to work with DPX/Cineon which is the scanning technology that is used to capture the film in the first place. DPX is specifically designed to mimic the spectral sensitivity of the print film, as it was originally designed as camera negative to digital then back to print film system. (Or at least to match the Vision 2 stocks of the time).

I am of course making perhaps the erroneous assumption that one of the reasons for using film is to retain its unique characteristics which are not easily replicated. Indeed with a 100% accurate characterisation there is no need for film!

But if you can build your own drum scanner I am sure you can solve this problem too.

[Peter De Smidt]

It is an interesting discussion that would apply to all types of scanners. Do you just need good output at the optimum r, g, b frequencies for the receptor, or is a smoothly continuous spectrum better? With my Dslr scanner, our first light sources went the RGB approach, but currently we are testing very high CRI sources.

[Pere Casals]

It is an interesting discussion that would apply to all types of scanners. Do you just need good output at the optimum r, g, b frequencies for the receptor, or is a smoothly continuous spectrum better? With my Dslr scanner, our first light sources went the RGB approach, but currently we are testing very high CRI sources.

Peter, think that drum scanners illuminate with lasers, and a laser it's pretty monocromatic, so CRI is extremly low, while nobody has ever complained about colors comming from a drum.

IMHO a high CRI illumination only worsens things in a CCD scanner, as we have some "cross talk" or channel mixing from the overlap of CCD channel spectral width , while a per channel monocromatic illumination, placed in the peak, would have low inter channel mixing, this is IMHO, it's what I concluded without having read it, so that thought should be checked, but also IMHO it explains that color purity from drums.

[calebarchie]

Drum scanners do not use lasers.... the later hi end ones used rather costly xenon lamps.