Duotones are always going to be considered spot color channels. So, when you print them on your desktop laser, the driver has difficulty converting the spot color channels to whatever the driver reads for output. Even if you assign a Black and a yellow, those are not process colors, but spot color. If your file is heading for offset printing, you have 2 options: option #1 = print a 2 color spot color job consisting of Spot Black and Spot yellow. Option #2 = print a converted duotone to a CMYK image and print your average process color on press. You have to decide. Do I want a 2-color job or do I want a 4-color job? If you add another spot color in Photoshop ( i.e., a tritone ), then you will have to decide on a 3-color job vs. a 4-color job. I've done similar projects where I use Photoshop to create a duotone. I then convert a copy of the duotone to CMYK and adjust the curves to get the look I want and still be able to print process color. This sounds like it will work for you.
Thanks for the helpful answer. But I am still a little confused. My fales will be printed in offset printing. There will be mostly full color images in the book, but I want to include a few duotone images made only from process black and yellow. So can't I just go with option # 1 from your advice? In so doing, I will just send the duotone PSD file to the printer. And (in my understanding) they will prepare their plates. When they prepare their plates, the full color images would be present on each color of CMYK plates. But the duotone image will be present only in Black and Yellow plate, and the Cyan and Magenta plates will be blank on the duotone image space. This way they can still print duotone images at the same time printing 4 process colors. In other words, some images will use only 2 colors while some images will use 4 colors of the CMYK ink. Can't it work this way?
Photoshop creates 2 channels when you create a duotone. Channel #1 = Black. Channel #2 = Yellow. These are probably spot process color channels. Not process color, which is CMYK. Now, I think you'd have to convert a copy of the duotone to CMYK, call it something like DUOc. Name your duotone DUO. What I would do, since you want to manipulate the entire process with these few must-have duotones, is copy the Black channel from your DUO ( duotone ) file and paste it into the Black channel of your DUOc ( process color file ). Then, copy the Yellow channel form your DUO file and paste it into the Yellow channel of your CMYK DUOc file. Delete ( but leave blank ) the M and C channels in your DUOc file. You will then have a CMYK duotone that will fit into your existing CMYK file structure. But, the important distinction here is that you cannot use a duotone in a CMYK file structure without adding 2 colors to the process color which will have an end result of a 6-color job. This is why Photoshop creates just a 2-channel file in their duotone or a 3-channel file in their tritone. Those color channels are considered spot color channels that are always interpreted as separate or additional colors in the process. So, in your case, when the job is RIP'd, the additional duotone channels would output on their own, separate from the rest of the job.
Alternate method to convert a black and yellow duotone to CMYK with no copy/pasting:
1. Set up your duotone as desired.
2. Image > Mode > Multichannel. This gives you separate channels for your yellow and black.
3. Add two blank channels. These can be alpha channels or spot channels, doesn't matter. Just make sure they are blank (filled entirelry with white).
4. Move the two blank channels to the top of the list.
5. Image > Mode > CMYK. When converting multichannel to CMYK Photoshop will assume the top channel to be cyan, the second to be magenta, the third yellow and the fourth black. (Five and up will remain as spot channels).
This is what I would do. This way you're sending the printer CMYK files with no cyan or magenta data. These can be used alongside of the other full-color images in your layout without any futzing on the part of the printer's prepress folks.
I tried to imitate BPYLANT's advice how to create duotone image for offset printing. However, I decided to go just Greyscale on my images. I thought that I better prepare the image in CMYK mode because giving a greyscale image might cause a problem. So I tried to copy the grey channel to the black channel of CMYK document. I make the other three channels white. But when all channels turned on, my Black channel looks dim. But when I turn off the other thre channels, the black gets back to exposure normal look. What should I do in this case? Should I just supply the CMYK document with other three channels turned off? I feel like I am missing something here.
Thank you for support.
As a graphic designer and prepress guy it sounds like your best option is to send it as CMYK and ask the prepress department to only run the three plates. From what I understand you are looking for only Y, M, K? Another alternative and a cheap and dirty trick is to substitute your spot colors with a process color and avoid messing with channels. If the artwork isn't too complicated you can just recolor it for the proof and revert to the process colors for production. The print file will look strange but the final product should look alright.
I use to mess around with dual tones and tri tones in .psd using channels to assign spot colors then saving a Photoshop DCS 2.O. You then link that to a .indd file (the preview always looked strange but once printed it was perfect) that was back a few versions ago perhaps the preview works now for a DCS file but I'm not sure if its the best route anymore. It worked great and the colors separated exactly how I wanted however its a lengthy process, difficult to edit and typically not worth the time. Usually there isn't a huge savings between 3-4 colors.
If you are still trying to figure it out let me know and I can explain the process I would use.