What's "pure" black

Hi Ariel,

Thanks for the explanations.

I have two more questions:
1. My printer person asks for R,G,B images -- mostly because he may use either an ink jet or offset press to print the brochures depending on how many copies there are and how busy his machines are.
If he uses the ink jet, will I get pure black or rich black.

2. My prints from COSTCO are made photographically for small sizes and ink jet for larger sizes.
Is there any way of knowing whether pure black or rich black will be used.

I can't seem to tell the difference by looking at the prints. Is that typical or do I have poor tone resolution.

Phil

For desktop printers or digital printers, RGB will produce a darker gray.
The answer is also relative if you send a CorelDRAW file or just images (TIF, JPG, etc) for print.
If you send CorelDRAW files, you can have all vectors on CMYK, then use pure black on vector and rich black on images. Moreover, you can maintain images as RGB on the document, send as PDF as their native color profile (images as RGB or gryscale and vectors as CMYK) and convert to CMYK according a color profile before to print.
Since RGB images allows a widest rage of colors, most companies try to use RGB for digital, laser or inkjet. And if you send to offset printing, they will convert images to CMYK using their color profile. If the client doesn't know how to apply the right color profile when convert to CMYK, most of the times it's better to say "don't worry, send me the RGB image, I'll do it for you". Since you're not the only one who send files, maybe they use a general rule for prevent different profiles. But you can be sure that they will convert to CMYK according a color profile before to print, or as a part of their workflow. Sometimes, the RIP contains convertion rules that overrides the information of the files.
The most important point is: don't use pure black on images. Never. The result could be a disaster, specially if the images has a lot of dark areas. Pure black is important for vectors and text, but it's a bad choice for convert an image from RGB to CMYK

Phil your RGB images that print to CMYK devices will be converted to the device media profile with that profiles predetermined TIC limit, they may assume a source RGB space or honor the embedded RGB profile. In short no rich black.

The photographic reproductions will have one of two processes applied. They will assume a specific (generally sRGB) RGB source space and then convert to the device space. In general with photographic device it's another RGB color space. Or they will honor your embedded RGB ICC profile and convert to the device RGB profile, again no rich black as it does not exist in the RGB world.

In practice rich black is never used for photographic reproduction. However a designer my decide that they need to artistically enhance an image.