Color seperation filter used in Drum Scanners

[Peter De Smidt]

Karl Hudson might be a good source.

[calebarchie]

Hi Ted,

Can you elaborate on the purpose or requirement / end goal you are chasing here? That way we can further assist you.

The general tonality differences and similarities you are seeing in that thread are inherent to PMT technology itself. Yes, most of the filters were custom made and possibly tuned to better fit the input of PMT (eg cut off bandwidth with broader spectrum PMT - just remember individual PMTs themselves are identical with adjusted gains). But mainly because they needed custom sized pieces of glass to fit in the entire optical assembly, since they were doing that they would of had to specify coating themselves regardless. The only ones I think of who may not have done this are Isomet, they used most off-the-shelf parts possible to keeps costs down (yet gained rep as best RGB scanners) unfortunately there is also no resources on them in general, much less technical and part numbers.

I guess what I am trying to say is don't think too much into it; most drum scanners if properly calibrated, tuned and maintained will give similar output regarding colour / tonality / resolution of ADC/type / PMT the rest is design, eg optics and mechanics. There are many, many CCD based scanners that used dichroics in similar way or circumstance just due to design of light path / module, a way of turning and filtering the light. Their output still does not match what you see in most PMT based scanners either so it cannot solely be attributed to a mirror or filter.

Perhaps you can find a cheaper, older ccd scanner to pull apart and study instead of potentially wrecking expensive drum scanner. Also, I have seen you interested in cneg conversions so if this is relating to that, just bear in mind at the time these drum scanners were mostly designed for slide and colour separation for prepress.

Bests

[Ted Baker]

Can you elaborate on the purpose or requirement / end goal you are chasing here? That way we can further assist you.

I am back working on my scanning software. What prompted the question was an earlier thread that had a number of drum scans of the same negative, inverted using colorperfect, that seemed to show remarkable similarities at least to me.

In contrast there is a lot of information on DSLR sensors, specifically matrices to convert RAW sensor output to XYZ, so you can work out the spectral characteristics of the sensor.

One of things I am working on involves building a custom light source, for a DSLR. Where the Spectral distribution correctly matches the colour negative materials. Or more correctly the spectral distribution matches the Positive printing materials i.e. (paper/interneg/interpositive).

This removes the need to calibrate the sensor, and at the same I would work out suitable matrices to calibrate other equipment (i.e. a Drum scanner, flatbed etc) to match the ideal measurements.

That is the idea anyway...

I learnt for example many drum scanners use off the shelf PMT's like a 931B Photomultiplier tube. So I just wondered if they used filter equivalent to the recommend Wratten combinations for making colour separations of positive artwork for dye transfer prints or similar. i.e. 47B,99 and 29.

The correct spectral distribution for color negative is different to that of human eye, most sensors in combination with a correcting matrix are design to match the human eye. However if you understand the actual spectral distribution you can better correct it to match the requirements for color negative.

Perhaps that makes some sense

[calebarchie]

Hi,

I cannot confirm that claim but due to nature of the filters needing to transmit and reflect they have still have band range (TR, RF) - look up thorlabs gives good idea. It is not complete monochrome cut off like in those filters, this can be seen by people using discrete RGB sources on drum scanners with improved results. In the end, PMTs cannot see colour only photons > electrons > signal (v) so it is up to dichroics to do so. I am not aware of any drum scanners with this kind of source, the problem back then LEDs were probably not bright or efficient enough despite PMT sensitivity. A monochromatic laser source makes the most sense, if an expert user can confirm - I imagine there would coherence complexities but PMTs still strive with those kind of problems.

I think for your project at hand, you would be better served by looking at minilab print systems and colour management regarding those. It is a complete circle system re calibration and mainly focused on c41 if you have any experience with those. I pulled apart a light module for an old konica machine circa 90s, it used an huge array of early individually controlled R , G & B diodes and one of the largest dichroic mirrors I have ever seen. Just goes to show how far LED tech has come. If you still want to try get a filter from Drum scanner and have a spectro on hand the heidelberg would be your best best - no expense spared, completely custom. Ask Karl!

Feel free to PM regarding your project specifics, although being so complex I am not sure it is practical to attempt with so many sensor types if you are aiming for universal system.

[Ted Baker]

I think for your project at hand, you would be better served by looking at minilab print systems and colour management regarding those

Indeed, along with arriscan, Scanity. (35 mm feature film, no expense spared)

At this stage it is just a line of enquiry. So far I have a sort of prototype that replicates the state of the art which would be the Scanity unit. But it is also worth taking a look back in time...

grateful for the help!

[Drew Wiley]

Ted, although I'm certainly not an authority on scanners, I do have good reason to think you're underestimating the complexity of the whole question, particularly with how color dyes interact in such a wide variety of manners, making any silver bullet unrealistic. And I have a fair amount of personal experience using color separations. That's not to say progress is impossible. But there are already people who have spent staggering amounts of time, money, and custom engineering to facilitate such ideas. And what you're describing is a moving target - films and papers frequently change, certainly enough to skew any kind of generic program or filter set. But I applaud your ambition.

[calebarchie]

Hi Ted,

I didn't want to clip your wings but Drew has been more or less direct to the point. I understand you may have found a way to measure or establish the characteristics of a particular sensor and its digital post processing (hence eliminate) and therefor take the next step in the process eg calibration/light. I believe when there are too many options and (potentially unknown) variables the correct word is 'difficult' rather than 'complex' (I have studied entire subjects between those two words).

But if you wish to follow your current line of enquiry especially focussing on the telecine or film-out part of the spectrum, look at early flying spot scanners which used PMTs and dichroic setup. These were just eventually just replaced with CCDs in similar dichroic set up which you might see in the arri (iirc arrilaser was the film recorder aspect). Again, I encourage you to look a simpler minilab systems first; it is a entire full circle system from scan to print output (especially regarding making profiles from reflective materials)

Also coming back to using lasers in drum scanners it just occurred to me that the gausian profile of beam may be a problem with the spot size of the light source. If you read about optical lab printers (and things like the lambda) you can see they shifted towards led technology because of this problem. This transition can seen in hi-end film-out LVT (Rhino/Kodak) vs laser based film recorders (oce lightjet 5000)

Caleb

[Drew Wiley]

Quite a bit can be learned by searching extant patents. An awful lot of hypothetical ways of doing things turn up, even though, for one reason or another, most of them never get to market. Therefore, an idea which one thinks is new might have actually been around the block several times. Of course, a patent prevents you from copying things for commercial purposes. Tweaking a concept strictly for personal use generally flies under the radar.

[calebarchie]

Quite a bit can be learned by searching extant patents. An awful lot of hypothetical ways of doing things turn up, even though, for one reason or another, most of them never get to market. Therefore, an idea which one thinks is new might have actually been around the block several times. Of course, a patent prevents you from copying things for commercial purposes. Tweaking a concept strictly for personal use generally flies under the radar.

I can attest to this, while back we were researching old ICE patents, had to use WebArchive to retrieve anything of any value but what we found was priceless. We were considering darkslide microscopy approaches as an alternate, the patent had clearly already investigated the potential of this and many others and of course we all know what ICE is now they went with IR approach. But this has directed our thinking to something that wasn't even on that long list of solutions just due to technological advances.

Research papers are very good so are old books, I recall suggesting to Ted to read a few a while ago now. I have read one recently which was pretty interesting let me see if I can find it again..
Found it: https://www.zora.uzh.ch/id/eprint/15...thsepMS0smOFBk

[Ted Baker]

Ted, although I'm certainly not an authority on scanners, I do have good reason to think you're underestimating the complexity of the whole question, particularly with how color dyes interact in such a wide variety of manners, making any silver bullet unrealistic. And I have a fair amount of personal experience using color separations. That's not to say progress is impossible. But there are already people who have spent staggering amounts of time, money, and custom engineering to facilitate such ideas. And what you're describing is a moving target - films and papers frequently change, certainly enough to skew any kind of generic program or filter set. But I applaud your ambition.

Not really IF you isolate the problem. The interaction between color negative and the color positive/inteneg/pos is remarkably consistent. i.e. nearly any color paper will work with color negative. Actually what I am interested is has been done and is being done. I am just interested in how it can be done using the equipment I or others could afford. The correct points to measure color negative are around 690nm, 540Nm and 450nm. There is blind spot around 590Nm which is why you can use a safelight with positive/interneg/positive materials. These wavelengths aren't the same as status-m but not too far either.

Modelling the rest of it, yes is very complex, but there is no need to re-invent it all, nor I am I attempting too.

Research papers are very good so are old books, I recall suggesting to Ted to read a few a while ago now. I have read one recently which was pretty interesting let me see if I can find it again..
Found it: https://www.zora.uzh.ch/id/eprint/15...thsepMS0smOFBk

Indeed! that is a good paper, which I have read BTW. However it is not an old document, it actually is a summary of the state of the art and very current 2018!
My original question is indeed just research into old technologies to consider how certain problems were solved or not solved. This forum has a lot of expertise with these older technologies, by shaking the tree sometimes usefull bits of information fall out...