Optical Brighteners 2 - X-Rite OBC
This Month's Feature Article:
Optical Brighteners Part 2 - X-Rite's OBC
by CHROMiX's Patrick Herold
X-Rite's OBC 1.0
In the last edition of ColorNews, we delved into the world of Optical Brightening Agents in paper (OBAs), and how their presence can cause problems with creating accurate profiles. Today's article talks about a software module made by X-Rite called the "Optical Brightener Compensation Module" or OBC, that compensates for, and neutralizes the undesirable effects of OBAs in pressroom situations.
- ColorNews articles are specifically designed to be informative rather than commercial, so we don't normally publish reviews of software or hardware here. (We publish them on the www.colorwiki.com website). However, there hasn't been very much said about this software and how it works, so we thought you'd find it interesting.
The OBC system utilizes a special measurement capability of the iSis instrument, and also make use of your eyes to take into account how much optical brightener is being used in the different papers and how the OBA's react in different viewing environments. Keep in mind that OBAs require ultraviolet light shining on them in order to fluoresce, so a particular paper may look brighter or not, depending on the light source. The OBC module interfaces with X-rite's ProfileMaker and Monaco Profiler software, and requires using the iSis automated high-speed chart reader.
When I first heard about this product, I had a hard time understanding what exactly it was. Is it a software program? Is it some kind of hardware? It works with Monaco Profiler and GretagMacbeth ProfileMaker, but how?
The package itself does not have much to it. It is essentially a software disc and a small envelope full of gray cards in various shades with a small rectangular window cut out of the middle of each of them.
When the software is installed, you'll see the OBC program, instructions on how to use it, and a collection of profiling target images. They provide a 918-patch target image for RGB profiling, and a 1734-patch target image for CMYK profiles in various paper sizes. These are adequate for most needs. If you are working with ProfileMaker, there is a way to create your own targets and have them be used in this process.
- Print the appropriate target.
- Once your printed profiling target has fully dried and the inks have cured, follow four steps using the OBC software:
Read the target with an Isis spectrophotometer using the OBC program. The program automatically checks to see if you have a ProfileMaker or Monaco Profiler dongle attached.
The iSis will measure with both UV light included and filtered out (UVi and UVx). These two measurements are used by the software to make a preliminary correction for optical brighteners. (To fine tune things, step three has you comparing a print from your printer in your actual viewing area, in your own real-world lighting.)
The program creates a sheet (TIFF) of gray color bars using the measurements. Save the Tiff and print it using the same settings you used for printing the target.
The program warns that this "process may take a couple of minutes." That estimate is on the optimistic side. Depending on the speed of your computer, this can take 5 - 10 minutes or more, and during this time, there is no indication of any progress being made. You may think the software is hung. Don't get impatient and kill the program. This is a great time to go have a coffee break or discuss how Gonzaga is going to do in the tournament next year.
The sheet of gray bars is printed and cured for a reasonable period of time.
Bring it into your viewing area. Here's where the envelope with gray cards comes in. The window cut out of each of the gray cards is the same size as each of the gray samples on the sheet you just printed out. The idea is to hold each gray card over the corresponding column of gray samples, and slide it up and down until you find the one that matches the cover gray card. (You will take the letter that corresponds to that match and plug it back into the software.) There are light gray, medium light gray, medium dark gray and dark gray cards. There is a complete set for matte finishes and for glossy.
Depending on how much optical brightener is in your paper, you may see a big difference between the gray bars on your printed sheet, or they may look almost identical. If you can't see much of a difference, that's an indication that there's not much correction needed, so don't worry about it too much. This is especially true for the darker gray columns. It is not surprising if you can't see much of a difference with optical brighteners when the paper is mostly covered up by ink!
Take the four letters that correspond most closely to the samples, and enter those into the "Select Gray Patches" step back in the OBC program. There is also an option for "NA" if you don't see any difference.
The OBC program rolls this whole brightener correction into a new measurement file, which can then be plugged into your ProfileMaker or Monaco Profiler application, and the profile is built as usual.
This really works! When viewing our final test images in our custom viewing environment, the X-Rite OBC prints were clearly the best match, the neutrals the more truly gray. Profiles made without a software correction to the OBA, and profiles made using UV filtering alone, did not look as good as the X-Rite OBC prints.
We tested this with RGB and CMYK profiles, matte paper and glossy, and the results were equally good with each variable. Naturally, the benefit of this module will go up with the amount of OBA in the paper.
Also keep in mind that X-Rite's OBC module is intended for press room and other color-critical environments where a light booth is used. It is not all that applicable to the average photographer printing to an inkjet sitting beside their desk. Photographers are going to want their prints to be optimized to be viewed best under normal daylight as the illuminant, rather than a specific lighting condition.
A few problems
As with any 1.0 program, there are a few issues, but nothing that is insurmountable.
- The software has a clever knack of remembering where you were in the process the last time you used it. This is handy, since it does not allow you to save the process at certain points. However, it also means that on subsequent trips through the process, it might assume you are doing the same target as before. So, if you first made a profile for your CMYK press using a tabloid-sized target, and now you are making an RGB profile using a letter-sized target, when you open the program it will start at Step 3 of the process, still using the measurements of your tabloid, and will tell you to go on.
A little bit of back stepping through the wizard and you can get back to step one, click the Clear button to eliminate your previous measurement, and proceed with your new target. You can even override this condition by merely inserting a new target into the iSis. The bar code information on the target tells the software which target it is scanning, and you are asked to confirm this change with a dialog box.
- When using ProfileMaker, making CMYK profiles, there is a known bug that forces the total ink limit to 320. The manual offers a workaround, and there are other easy steps available to be allowed to adjust up to 400 if you wish. Not a big problem.
- The process of matching the gray printed bars with the gray standards is a bit difficult if the two paper types are significantly different. For example, I tested this process using office bond paper, and the lack of good ink coverage made the bond paper look lighter compared to the gray standards. If you stick with standard prepress media, the supplied matte and glossy cards should match well.
Finally, I have just received the results of an unscientific survey of ColorNews subscribers which shows that a significant number of you have not even tried my apple cider vinegar tip from last month. You don't know what you're missing. Just the right amount (not too much) will add zest to an otherwise boring smoothie. Good for other drinks too!
Thanks for reading,
Patrick Herold CHROMiX Tech Support / Lab Operations
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