Spot Color + Transparency = unreliable PDFs

[Andrew_Mannone@adobeforums.com]

I hope somebody can shed some light on this issue.
Here's the back story: I work for a publisher in Boston and we do a three color mag - Process Black, Pantone 133, and 464 (in our duotones). We have just switched from using clipping paths to using transparency/alpha channel on our product silhouettes. I wasn't untill our mag was on press that the problem was caught.

Here's the problem: The PDF looks fine on screen and prints fine from our ORIS Color Tuner RIP, yet Anytime a spot color is under an area of transparency - be it from a placed photoshop image, or from a drop shaddow effect in InDesign, the spot color gets knocked out. The only way to see this on our end is to open it in Illustrator or Apple Preview.

We're using the latest versions of all the CS2 Apps and Mac OS X 10.4.(latest version)

Our vendor has spent a lot of time working with us and adobe trying to figure out what's going on but, i can't seem to find anybody with answers.

Thanks for any and all help.
-Andy

[Steve_Werner@adobeforums.com]

Whenever you combine spot color and transparency, you need to use overprinting:

<http://indesignsecrets.com/eliminating-the-white-box-effect.php>

[Laura_M.@adobeforums.com]

Adobe also provides additional documentation for print service providers with regard to transparency.

<http://www.adobe.com/designcenter/creativesuite/articles/acs2bgtransparency/acs2bgtransparency.pdf>
<http://partners.adobe.com/public/asn/en/print_resource_center/TXPGuide.pdf>

[peter kleinheider]

Dear Andrew

As Laura and Steve mentioned, it is very important to honor the Overprint Settings of such a PDF file. Your Workflow must not change any ColorSpaces or Overprint Settings, otherwise the visual appearance can be changed - as seen in your case.

The usage of SpotColors in conjunction with Transparency is achieved during the flattening process by using Overprint functions that have to be honored.

Another thing you have to look out: You can not convert SpotColors of such a PDF to CMYK easily. Former transparency effects might get lost.

rgds

Peter

[Andrew_Mannone@adobeforums.com]

Thank you to everybody who responded.
Work has me swamped and now just getting a chance to respond.

We've contacted our vendor and well, they were less then reassuring... "well, that might work, i don't know." So we're going to send test files on Monday and tell our plant to make sure their RIP uses overprint.

When we get the test file output I'll let you all know if it solves our problem.
Agan, thanks

-Andrew