Screenprinting Halftones and CMYK

Ever wondered how to print a colour photo with screenprinting???
I've been asked about it a lot lately so thought I'd post some info here.

Firstly, you'll need to know about these 2 important print concepts: halftones and CMYK.
Screens are really only capable of printing the solid colour of the ink,
the hole in the mesh is either open or blocked - "ink or no ink".
Here's a close-up example of a photo-stencil on the screen mesh:

Screen mesh with photo-stencil

Since most images are made up of various tones rather than solid cut-out shapes,
 you need combinations of open and blocked holes to print an image with various shades of grey.
A mechanical method was devised back in 1880s to produce the illusion of tone 
by the placement of dots of varying sizes in a regular pattern, known to us as halftone.

Here's an exaggerated view of halftone dots.
As the tone gets darker the dots get bigger and overlap to create black.

In Adobe Photoshop, the CS4 version, you could easily produce halftones 
with the "Screen" button in the Print Output section.
In the CS5 version, this button disappeared, leaving many screenprinters frustrated.

However, if you want to keep up-to-date with all the features of CS5 (and CS6 etc)
as well as being able to control the dot size and arrangement of your halftone dots,
then try this method below - its almost as straight-forward as the screen button
AND the good news is with this method you do NOT need a postscripted printer to print,
almost any printer will work fine!

Firstly, convert your image to 'greyscale' and make sure it is flattened (or it wont let you do this).
From the Photoshop menu choose Image > Mode > Bitmap,
*regardless of your image resolution make the output 300ppi, and select Halftone Screen.

The following window allows you to select the frequency, angle and shape
the same as you did before with the 'screen' button.
For screens 40-90T I usually use 45lpi, for 100T+ I use 50lpi.
45 degree angle for a single colour. Always select round or ellipse-shaped dots.

Great! Now you have a single-colour image with halftones
so you're ready to make a screen and start printing with this simulated tone image.

You can see the tonal range of the halftones in this close-up of the rose image,
even as a single-colour file you can see how halftones give the illusion of various shades of grey,
more than the bitmapped image on the left which mostly posterises it as black and white:


If you are wanting to go even more advanced and print colour images,
the second important concept you'll need to know about is 'CMYK'.
CMYK is the abbreviation used for 'process colour' used in most commercial printing,
referring to the ink colours: cyan (C), magenta (M), yellow (Y) and key (K), which is black.
When these colours of ink are mixed to the correct transparency and printed over each other
they are able to replicate a fairly good range of the spectrum to print colour images.

Here is a close-up of what the CMYK halftones look like together

Now, if you want to make a colour image for screenprinting,
you'll need to produce CMYK 'seperations' with halftones.
This example shows this CMYK image seperated into is 4 colour files:

Firstly, go to the Channels window and select Split Channels so you get 4 seperate documents,
than proceed with selecting the settings above for frequency and shape,
but using different angles for each colour (see angle suggestions below)...

To prevent moiré patterns (exaggerated grid distortions) appearing in your prints,
a general rule of thumb is to offset each screen angle by 15-30 degrees.
Here's a few combinations from an online discussion that you may want to try:

The CMYK inks are available for paper and fabrics.
Sometimes the inks come ready to print (pre-mixed with transparency), some are concentrated,
so talk to your ink supplier to make sure before you begin printing!

I hope this helps you get started on screenprinting halftones and CMYK!
As we venture into CS6 there may be further advancements on this...
If you have any other solutions or suggestions, please post comments below.