Expert: LizaL - 7/6/2006

Question
I have read your advice on spot colour sepapration. Will this work for scanned photos? And do I need to do anything to prevent moire when printing (ie. change angles or frequency)?

Answer
Hi Meggan,

Yes, you can set up a scanned photo in Photoshop with its own separations. You need to first make sure it's in CMYK mode, and after you've done your editing and are ready to save, save it as a Photoshop DCS 1 or 2. Both formats will let you save color seperations, but DCS 2 lets you save spot channels. And of course you'll need to print to a Postscript printer.

As far as moiré patterns -- you don't say what equipment you're using for scanning and printing, so I don't have a clear picture of how to advise you. I'd be the first person to tell you that if the job is crucial, and it needs to be printed on a certain paper, take it to a commercial printer. They have far more sophisticated equipment and procedures than the typical desktop user. When taken to a commercial printer, your job will also most likely go through a prepress stage, where its accuracy is checked and corrected prior to going onpress.

But if this isn't what your workflow dictates, and you're doing this yourself, you can perform some checks and preparation to guard against unwanted results. Below are some things you can do yourself:


-- A moiré pattern is going to be determined mostly by several things: the resolution of the file (and thus, the separations), the printer you're using, and your printer drivers.

Your scanner will likely feature a descreening control. Check its help or the manual for this.

In Photoshop, you can use a descreening plug-in to help prevent this from occurring prior to printing, and you should also use as high a resolution as possible for your file (i.e. 300dpi).

Or, if your printed file gives you a pattern you can easily identify, you could try descreening yourself. Here are some steps for doing it yourself -- again, you'll likely not get the results a commercial printer could provide, but you could give it a try:

1. Scan the image at a resolution that's about 1.5 to 2 times higher than what you need for final output. Again, this final output will be determined by the printer you're printing to, and the paper the image will be printed on.

2. Go to Filter > Noise > Median. Use a radius between 1-3. Typically the higher the quality of the source, the lower the radius can be. Use your own judgement, but you will probably find that 3 works well for newspapers, 2 for magazines, and 1 for books.

3. Go to the Image pulldown menu and select the submenu Image Size. Then resample to the desired image size and resolution using the bicubic resampling option. Make sure you keep constrain proportions checked.

4. Zoom in to 100%, and go to the Filter pulldown menu and select Sharpen > Unsharp Mask. The exact settings will depend on the image's resolution, but a good starting point is Amount: 50-100%, Radius 1-3 pixels, and Threshold, 1-5. I mention zooming in, because your eye will be your best judge, at this point.


-- Lines per inch (screen frequency) is determined by your output device, and the paper you'll be printing to. You need to know this ahead of time in order to know how to prevent undesirable resolution effects. For example, newspapers are mostly printed using 85lpi, while magazine pages (uncoated stock) are printed anywhere from 133-150lpi.

Generally speaking, you can use a rough estimate of doubling the lpi and using that number as your scanning resolution. So if you knew you were printing something for a magazine whose lpi is 150, you'd want to scan your image at 300dpi.


But again, I'd take the job to a commercial printer for the best results.

Hope this helps. I've made the assumption here that you're familiar with printing, but if you need clarification, please post back.

Lisa