Scanner Profiling

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This reserved article originally appeared in CHROMiX ColorNews Issue 51 on May 1, 2013.

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Contents

Scanner Profiling

by Patrick Herold


As digital cameras started to develop larger CCD chips, somebody pointed out that cameras were capable of the higher resolutions like flatbed scanners, and scanning was on its way out. There certainly didn't seem to be much of a future in scanning. Scanning targets have been getting harder to find. Two years ago when X-Rite released i1Profiler, their long awaited replacement to their professional profiling engines, it did not even include a module for making scanner profiles. Heck - even CHROMiX doesn't bother to advertise our scanner profiling service on our website.

But, to borrow the colloquialism: the "reports of the death of scanning were greatly exaggerated." Like a lot of predictions about the future, this one has not materialized as anticipated. While high-end digital cameras can capture something like 20 megapixels these days, the ubiquitous cameras that most people use are not up to the quality that we color geeks are expecting.

There is also a great bevy of enthusiasts who continue to shoot medium and large format film. Even in this digital age, large format film can contain the equivalent of hundreds of megapixels. So it is still popular to shoot large format film and scan the film digitally. An 8x10" slide can be scanned and the detail available is astounding. Depending on the use of the final output, these images are oftentimes downsampled before being printed. Devotes of this workflow enjoy smooth images that are sharp and clear. http://www.largeformatphotography.info/

Then again, nowadays it's dead easy to pick up an inexpensive flatbed scanner that just plugs in to a USB port and does what you need without a lot of fuss.

And what do you know? X-Rite's latest update to i1Profiler introduces a new scanner profiling feature.

Long ago, we published an article on Input Profiles and Working Spaces, which briefly touched on scanners. But it's high time we come around again and take another look at scanning from a color perspective. I see this article as a useful companion if you are going to try the i1P scanning module for the first time (to complement the sometimes modest help menus there.) Or for anyone who has a scanner and would like to try getting good, dependable color out of it.

What target to use

example of an IT8

There are many scanning targets available for sale, including these. It's always best to choose a target that is made from the same film type as you'll be scanning most of the time. Different films will have different characteristics, so using the same material will get you more accurate results. For example, if you mainly use Kodak Ektachrome in your large format camera, then try to find a profiling target printed on Kodak Ektachrome.

Reflective and Transmissive

Reflective targets are for those who want to scan a physical print or perhaps an original artwork; something that requires you to scan the face of the media. Reflective scanning targets are usually a 5 x 7 inch layout onto high quality silver halide photographic paper.

If you are scanning transparencies, otherwise known as slide film, or positives, where it is viewed by having light shone through it, or backlit - then you would require a transparency target. For large format you can order a 4x5 inch trans target, or in many cases a 35mm slide version of the target will work. While it is possible to scan negatives, people generally don't pursue profiling for this, because it is inherently very difficult to get consistent color results from the various colors of that dark orange base material from which negatives are made.

Patch arrangement

The most common targets are some flavor of the IT8 patch set. An example is here. This target contains up to 286 colors that do a pretty good job of sampling the entire color range, including neutrals.

a Hutchcolor scanning target

For those who want something better, Hutchcolor provides the HCT targets which contain 528 patches including more saturated colors, more grays and more shadow patches.

All these targets are designed to have very stable color for a long time. Naturally the accuracy depends on the reference data matching the color of the targets. When you first buy a scanning target, it was hopefully not long after the target was measured to create the corresponding reference. Naturally as time goes on the dyes in the target will change - usually not much, but you know - nothing lasts forever. It's a good idea to keep these targets in a cool, dry place (like a refrigerator) and not keep them out in the light any longer than necessary. It is possible for some targets (5x7's and 4x5's) to be remeasured in the future to ensure an accurate and up-to-date reference file.

Note also, that some manufacturers will "batch" measure their targets. They will provide you with a reference file that is a good average for all the targets in a certain batch. In this case you're not getting a reference that goes specifically with your particular target, but one that is "close". The Kodak IT8's (recently discontinued) are among this group.

The more premium targets (such as the custom-measured reflective targets by CHROMIX, and most of the HCT targets) are custom measured, so there is a reference file directly associated with your target.

Software Compatibility

I1P icon.jpg

You must also choose a target that will work with your profiling software. Some scanners come with their own profiling targets & software. If you happen to have a scanning target lying around, check to see if it is compatible with your scanning software. Interestingly, while the HCT targets are of the highest quality, they are currently not supported in the most recent version of X-Rite's i1Profiler. I understand that there are discussions between X-Rite and Hutchcolor concerning this, so maybe we'll see the software support these targets in the near future.

Custom-made reflective targets

If you are scanning reflective material (rather than transparency material), and if you have software that allows for customized targets, you can create your own reflective target just by printing a series of patches on high quality paper. I know we used to do this using the ProfileMaker software by X-Rite.

Here's what you need:

- high quality printer

- a target containing patches that can be read by a spectrophotometer, and will fit on the scanner.

- spectrophotometer that can read the patches,

- software that allows for the use of customized targets

This printed target is scanned by the scanner, and is also read by the spectrophotometer. This measurement will become the reference for the scanner target. The scanned image along with the custom-measured reference is used to make the scanner profile.

Note that this method has the benefit of being custom-measured, so the reference data should be very accurate. On the other hand, this kind of profile will be somewhat limited by the gamut of the printer you've printed the target on. This is not generally a problem, but it is something to be aware of.

Scanning the target on your scanner

Once you have a target, it's time to create the actual scan. The scanner settings must be set so that any automatic image adjustment is off, any color management (any use of profiles) in the scanner is off, sharpening is off, and the scanner must be clean, warmed up and in good working order.

Since the scanner profile will handle all the color correcting, you want to bypass all of the extra color correction bells and whistles that the scanner is capable of. Adjust the gamma, white point and black point settings in the scanning software so that it will scan the target in a well-rounded way. Specifically, you would want the middle 50% gray patch of the target to be scanned at as close to 127 127 127 gray as is reasonable (as sampled in Photoshop in RGB mode). Also, the white patch of the target should read about 240-250 or so… leaving room above the maximum. Similarly, the black point should scan at about 5-15… not all the way to zero. You might very well someday be scanning something lighter or darker than the extremes of your scanning target, so you want to "leave room" so that your scanner can define those colors too. The scanned image does not have to be particularly large. We're just interested in color here, and a great big megapixel file is not going to tell us anything more about the color, or make the colors more accurate.

Check the scan

If your scanned target image has any blotches due to dirt or dust spots, you can clean them up with a clone tool in Photoshop. Naturally, be very careful with this and do not copy color from a neighboring patch!


Create the profile

example of a scanner profile gamut

Once the image has been scanned, bring it along with your reference file into your profiling software to make a profile. The software allows you to precisely crop and line up the scanned target so that it can identify the patches.

The actual creation of the profile is very simple, and only takes a few seconds. In fact, this kind of service can be done remotely. Once you have a scanned target, that image file can be sent to anyone who has the software, and the completed profile can be simply emailed back. There's no need to ship a hardcopy print or anything. For example, it just takes a few minutes for us to create a scanner profile for customers anywhere in the world.

Scanner profiles have a different shape than printer profiles or monitor profiles. They are usually a lot larger when you view them in a 3D Grapher like ColorThink. In fact, a scanner profile will generally to be larger and more encompassing than the colors of the actual target that was used to make it. How can this be? When the profile is created, the profiling software will extrapolate beyond the target image colors and make a profile that sort of "guesses" how to handle those extra-saturated colors. This makes sense when you realize that you will potentially be scanning all different colors from different materials, and will want the profile to be able to handle colors that are beyond those few represented in the target image.

Note that all settings in the scanner must remain the way they were when the target image was created. If anything is changed that can affect color, the profile will become obsolete and will likely give you wrong colors.

Using the profile

Once you have the scanner profile, install it in your system where your imaging programs can access it.

To use the scanner profile, you would leave your scanner exactly as it was when you scanned the target. Scan your images on the scanner, bring your scanned files into Photoshop and assign this new scanner profile to these images. Assigning the profile will "correct" the color of the image back to what it should look like. When you go to save this image, the profile will be embedded into it as you save. If you are going to print this image, the profile will be converted to the profile of the printer automatically, the corrected image will print with the proper colors, and everything will look great!

There is much more information that can be learned about this whole topic, that we just don't have the space for here. The Hutchcolor.com website has extensive instructions for setting up scanners and how to make the most of a scanner profile.


Thanks for reading,

Patrick Herold

CHROMiX


(Thanks to Makoto Takada for inspiration for this topic, and to Ken Lee for his help.)

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