Color Fidelity and Cross Polarization Copy Photography

Hi D.Fosse,

I am sure you realised this, but I don't think you mentioned it.

it's interesting, I feel, to observe that since the camera captures RAW and the RAW converter output is RGB, an input profile has to be used to translate between that and and the Lab info we see in Lightroom.

So the Lab numbers are not "straight lab as captured" there's no such thing - I thought some users may find it interesting to consider that.

Why?

I've heard input profiling software makers talk about setting camera exposure to give certain Lab values when capturing camera targets [e.g. my favourite Colorchecker SG] - but, meanwhile, those lab values are being created [in a conversion from camera RGB to Lab] courtesy of of the existing input (camera) icc profile [or often the working space profile in the case of photoshop assessment.].

As that icc profile is going to be replaced by a new profile made form the target capture, Lab values for un-profiled target readings re rather irrelevant.

Hi Dfosse, All,

I agree about LED lighting, it doesn’t seem to be a mature technology as far as photographic lighting goes - as yet.

[Maybe its down to cost to manufacture, LED does fine on a small scale, it seems, in internal monitor display illumination, e.g. Eizo Coloredge].

Significant research also suggests that LED light may also contribute to macular degeneration. Not good given its increasing Ubiquity.

Contrary to your own experience, I’ve found that SOME fluorescents can work well. Really well.

I did some extensive testing with a London based freelance fine art photographer and we ended up using Kino Flo lamp units with tubes from a proofing luminaire manufacturer.

I am told that they use Kino’s with Kino tubes at the Van Gogh Museum in Holland.

Long exposures can be a disadvantage with fluorescents though, fine for a studio where all can be locked off and natural light excluded - So he does use flash when working on location.

Again, as you mentioned with regard to fluorescents, all flash is not equal in its ability to make a painting look like it does in daylight.

I hope this helps readers of this resource 


if so, please do mark my reply as "helpful", so others who have similar issues can see the solution


thanks

neil barstow, colourmanagement

Thanks for your reply. I had the impression it wouldn´t be too hard correcting the increased contrast and saturation resulting from Cross-Polarization using Lightroom, I just haven´t figured out how to measure the altered values, so I can subtract the difference. I´ll try your approach and see how it works out for me. Which particular Color Checker card are you using? I´d be very grateful for a short step by step workflow description of your practise using cross-polarisattion applied to an oil painting.

Regarding your advice on light sources. I have visited another photographer, who also recommends tungsten over fluorescent tubes. After switching to mirrorless camera bodies, I have abandoned flash for photographing artefacts that need cross-polarisation, because continuous light enables me to see my output on a monitor while composing the light. I exclude most other light in the studio. I still use flash extensively for portraits or less complicated 3 dimensional artefacts. The photographer I mention using tungsten is specialising in art e.g. oil paintings and dislikes cross polarisation precisely on account of my problem :-D So for him, it´s much simpler to use tungsten bulbs in relatively small, round reflectors with no polarisation involved at all. He has a very special studio, which he has painted in a variation from dark, neutal (matte) grey on the floor to medium light grey ascending to the ceiling. He then points his reflector (often just one) indirectly to the walls/ceiling, and the light is then bounced back onto his canvas. He´s able to modify his angle of light until his reflecting highlights in the oil are practically gone! :-D I´m very envious, but I haven´t got as much space available to create such a specialised studio. I´m forced to work in much less space with no chance of designing such a large, specialised, indirect, neutral bouncing box. So the next best solution is: Fluorescent bulbs. The bulbs used in Kaiser´s repro light banks are specially designed for repro work, and don´t display nasty spikes as regular bulbs do, so I achieve very nice "equivalent color" as you say, but the Kaiser banks are too small for larger canvas photography, so I also use KINOFLO bulbs as recommended below. It would be possible to arrange a vertical array of tungsten reflectors, which might distribute lights as even as the KINO/Kaiser bulbs, but it would practically impossible to apply the Lee polarisation gel I have to use on account of my direct lighting as opposed to the ideal indirect solution I described above. Because of the heat emisson from tungsten, actually some conservators wouldn´t let me get near their arcahaological originals of e.g. textiles, if they were aware of how hot tungsten gets. If you use tungsten for photographing, it´s necessary to have quite long distance to the subject and preferably indirect lighting as described above, but if you work for some hours with tungsten, the heat emission has a consequence on the room tempperature. So I have been left to use fluorescent, continuous light, which is much better than it´s reputation using KINO or Kaiser. I´m sure you have a point about LED´s, but as you said, for most puposes, what´s necessary to achieve is equivalent light, not necessarily dead accurate for all applications.

Thank You very Much for your detailed description of Christies solution, which may be beyond my budget. Unfortunately I don´t have the means of controlling all segments of the variations of print or web use my pictures may be used for, which ofcourse would be an ideal situation. I´d love to read a step by step manual for the work carried out in establishing all relevant input device icc profiles including the second measurement you mention, with the cross-polarisation applied, specifying the measuring hardware/software used in the process. I don´t see any references to equipment used anywhere in the link you have provided, but again thanks for your input. When I figure out which hardware/software I need to create input/output device profiles, I´m going to try this out, and conform my own humble equipment to the ISO 12647-2 (and THEN see if our external printers have any idea of what I´m talking  about :-D - Or maybe initially, I should ask them which ISO they comply to if any). But first I´ll get to next level of actually seeing this approach in function with my own Epson printer and calibrated monitor.