I want to use a color image, of which most parts are black, as a background. But in the color separation it is shown as a full CMYK color. Using a CMYK rich black is not convenient for me, because it overlaps acute design while printing. I just want that black areas are made from just the black plate.
What you're looking to achieve really cannot be done in Illustrator alone. The parts of the image that appear black, are not really pure black. There is cyan, magenta, and yellow information in the black areas also. This can be visually seen in this photograph where I separated the cyan magenta yellow and black channels.
Take notice of the info panel and where my cursor is. Notice the K value is only showing 89%. That means nothing in the black channel is actually 100% Black. This will result in a separation of the black channel being only half tone with nothing solid.
With 30 years experience in color separations, it is my opinion, that to achieve what you are asking, would be this:
- In Photoshop, convert your image from CMYK to multichannel mode
- While in multichannel mode, open your channels panel and select your black channel while holding the command key. This will load the channel as a selection. We will use this selection to subtract what is in the black channel from all of the other channels.
- without deselecting, just select the yellow channel in the panel then go to menu item Edit/Fill/ then click the drop-down menu and choose white. Repeat this process for the cyan and magenta channels also. This next image shows the results
Keep in mind, in my images I extended the canvas and moved each channel so that we can see all four at once. In your file, all the channels will be directly on top of each other visually.
The next thing I did was chose my black channel in the channels panel and adjusted the levels so that what is White in the image is actually pure white and the darkest blacks in the image are actually 100% Black as you can see in the info panel where the K value is 100%
Finally, save this file as a Photoshop DCS 2.0 and make sure the spot colors option is selected.
Next create a new illustrator document then go to menu item File/Place and choose that DCS file you just created. This will load your separations with all color information removed from the black as you requested
There is also a second option. If color accuracy is not extremely important, this image can be converted to index mode in Photoshop, using only two colors: red and black. Therefore CMYK (4 separations) would not be necessary. Once in index mode, we can convert the image back to RGB and create two new spot channels. One for the red and one for the black. Eventually, we will place this file into illustrator and print our separations.
This next image shows a comparison. The top image is the original CMYK and the bottom image is indexed into two spot colors. The red and the black.
Here are the steps I took to create two spot channels from the image converted to index mode in Photoshop
- Clear out all swatches from your swatches panel
- Using your eyedropper tool, select the brightest red from the image and add that color to your swatch panel and do the same for black
- Now we convert the image to index mode, selecting the "custom" option. One at a time add both colors from your swatches panel. Be sure to select diffusion dither at 100%. Click "OK"
Now your document is indexed into two colors.
- Convert your image back to RGB color mode by selecting menu item Image/Mode/RGB
NOTE: the image will look a bit funky with weird pixelation. Zooming in on the image can give you better visuals. Either way, your separations will be correct.
- Now it's time to create 2 new spot channels for our separations for Illustrator. Because we added those red and black swatches to our swatch panel, it will be easy to separate our colors. Go to menu item Select/Color Range
- With your channels panel visible, go to menu item Select/Color Range. With Fuzziness at 0, click on the red swatch in the swatches panel then click OK.
- Now with all of your red in the image selected, in your channels panel drop-down menu, select the option "New Spot Channel"
This will be your result for your new separation for the red:
- Repeat steps 6 and 7 but this time with the color range selecting the black swatch.
This will be your result for your new separation for the Black:
I'm not sure if this is exactly what you are looking for, but there is a quick (and slightly hacky) way to strip one or more separations out of a CMYK image...
Create a box whose colour is made up of the colour(s) that you want to KEEP. In this case that would be 100K (solid black). Place this box over the image that you wish to strip colours out of and set its transparency mode to COLOUR DODGE. In the following screenshot I have only partly covered the image to show the effect...
As you can see, in my example this causes the image to look somewhat lighter and lacking detail. The extent of this problem and whether it is an issue will depend on the colour makeup of your images and what you think of the results.
wch1zpink aproach is interesting.
My alternative aproach.
Your image is not in CMYK, so use it as RGB.
1) Click on the red channel, select it and copy it.
2) Paste it as new file. This wil be a grayscale one. Now you have almost all the black information.
You can go 2 diferent paths here.
3a) Change the mode to multichannel, and add a new channel and fill it with a flat red color (the red is a little bit orange)
I like more this option so you have flexibility to modify the curves of the two channels as needed.
(3b) You could convert it into a duotone, add a red color and move the graph so the red chanel is mainly a flat uniform red. The result is simmilar to the previous one.)
I might suggest an alternate route. Instead of manipulating the blacks in the image, which will change the character of it, consider instead focusing on what you're placing over the image. If you're worried about misaligned registration, say with some delicate reversed-out text over the background, stroke the text with 1pt black, making sure the stroke isn't overprinted, so that the dots in the rich black don't touch the letters, in essence trapping letters within 0C 0M 0Y 100K. Just a thought.