Epson V750 Colour Profiling and Gamma

[l.j.nash]

I'm new to scanning and am hoping someone can clarify a few things for me.

I have an Epson V750 scanner with Silverfast Ai v6.6.1r1b and Photoshop CS3 running on Windows Vista x64.

I'm interested in genealogy and wish to archive a large number of old positive film slides and also make prints. I have a Monaco EZColor 4x5 positive transparency that came bundled with the Epson V750 scanner. Although I have realised that as I will inevitably have to make colour corrections to the old slides, and hence colour profiling is a bit of a waste of time, I would still like to have a properly profiled set-up.

I have tried various approaches to producing an icc colour profile using EZcolor, scanning using Epson Scan and Silverfast. I have settled on using Silverfast with all colour management disabled and following the advice found on the Silverfast website to scan an image for EZcolor to profile;

When you are using a third party's software to calibrate, you need to make sure that SilverFast scans the image without any color corrections.

In order to do so, please open the SilverFast options dialogue:

* on the "General" tab, check the "for HDR output" box
* make sure the gamma value is set in accordance with your monitor and Photoshop settings

When you do your scan for the third party profiling software, select "48 bit HDR color" scan mode type on the "Frame" panel of the main dialogue.
Save the resulting image to TIFF file format.

I select the profile in the CMS tab of silverfasts configuration window and select ProPhoto RGB as Internal, with gamma graduation set at 2.20 as when profiled. This results in RGB values measured with the densitometer of;

251,252,242 for GS0; 99,101,103 for GS11; and 9,10,11 for GS23, which appears almost consistant with values described at;

http://www.computer-darkroom.com/gamma/gamma_1.htm

The first question; is a white value of 251,252,242 a little high (perhaps a bit too close to 255)?

The second question; Slightly off topic question but why would the silverfast preview be bleached out (very bright) when Internal -> Monitor is selected to be "Image Colour Matching (ICM)"?

The automatic value slightly lightens an image shown with <None> selected and does indeed match the image shown in Photoshop. The image if scanned with this setting looks completely normal in Photoshop!

A question about rendering intent. As I understand Rendering Intent, it describes how colours are mapped between colour spaces. A choice of perceptual will in essence scale colour values to fit within the chosen colour space, whereas relative colorimetric will map colours to equivalent colours within the new colour space as long as they fall within the colour gamut selected, and those outside the colour gamet will be mapped to similar colours with the chosen gamut.

The third question; Would I be right in thinking that silverfast's rendering intent option is mapping from the scanner's colour space to the chosen Internal colour space (ProPhoto RGB in my case)?

If this is correct does this mean that rel. colorimetric would be the best option for accurate colour reproduction?

In a way this is a loaded question, in that if I select rel. colorimetric the RGB values of the grey scale are altered, which I did not expect, with the image overall being darkened;

202,202,194 for GS0; 76,77,79 for GS11; and 6,6,7 for GS23.

Should a near pure white not be represented by a high RGB value irrespective of rendering intent?

Thank you in advance to anyone who is willing to tackle my many questions!

[LSI_Morales]

Dear i.j.

The first question; is a white value of 251,252,242 a little high (perhaps a bit too close to 255)?

No, it is Ok because it' supposed to be white, the closer to 255 the better, it is a very light (almost imperceptible) shade of grey, the problem would be if you have 255 on the three of them, then you would not really know for sure if it is clipping.

why would the silverfast preview be bleached out (very bright) when Internal -> Monitor is selected to be "Image Colour Matching (ICM)"?

Because the system is now using standard color system to interpret the information it has in the monitor.

The automatic value slightly lightens an image shown with <None> selected and does indeed match the image shown in Photoshop. The image if scanned with this setting looks completely normal in Photoshop!

In that case you will have to configure photoshop as well, this is due to inconsistencies in any of the programs, you have to set every imaging software to match the configuration of your system otherwise you will have this kind of inconsistencies.

...whereas relative colorimetric will map colours to equivalent colours within the new colour space as long as they fall within the colour gamut selected, and those outside the colour gamet will be mapped to similar colours with the chosen gamut.

You get it right, but you have to have in mind that it also changes the ration between different colors, hence if you then make a second and third color gamut change, the colors which were changed the first time (those which fell out of the color gamut and were mapped to similar colors) will not keep their relationship to the rest of the colors of your picture.

Would I be right in thinking that silverfast's rendering intent option is mapping from the scanner's colour space to the chosen Internal colour space (ProPhoto RGB in my case)?

Its absolutely right

If this is correct does this mean that rel. colorimetric would be the best option for accurate colour reproduction?

No, because of the same reason I exposed two questions above

In a way this is a loaded question, in that if I select rel. colorimetric the RGB values of the grey scale are altered, which I did not expect, with the image overall being darkened;

202,202,194 for GS0; 76,77,79 for GS11; and 6,6,7 for GS23.

Should a near pure white not be represented by a high RGB value irrespective of rendering intent?

Yes, but up to this point is very hard to tell because you have changed the values in system to monitor as well as rendering intent.

The best thing you can do is to create different profiles and use at will

Cheers