Screen angles & Moire patterns

In newspaper printing, images are split into their Cyan, Magenta, Yellow and Key separations and the seperate CMYK colours are transered onto plates, which are then overlayed in a lithography similar printing process. To ensure image quality however the separations are also bitmapped, the makes it possible to attain gradient like effects by altering the concentration of bitmapped dots across an image. When the four plates are overlayed with the bitmap, this creates a moire pattern, which is a cluster of cmyk dots which looks like this:

As you can see, the clusters which form the 'Moire" pattern are achieved by rotating the screen angle of each colour separation. The most successful Moire patterns for colour printing often form a 17 point star from the screen rotations. This is done by using specific angles of rotation which are as follows.

Cyan - 15'
Magenta - 75'
Yellow - 0'
Black - 45'

That set of angles seems to be pretty standard across industry, and always yields the same moire pattern. But, this isnt where the process ends pre print. As well as specifying angles, the DPI (dots per inch) and LPI (lines per inch) have to be specified too. I found that through printing out different experiments using different variations and combinations of LPI/DPI, that generally, a high LPI, needs to be balanced with a low DPI - the higher an LPI, the less space there is between dots, so finding the optimum spacing is key. This can vary from image to image, based on the images size, but for a half newsprint size publication like G2, an LPI frequency of 65 combined with a DPI of 600 does the trick. the low LPI means a higher DPI is needed. This is so there are more pixels and therefor more space for the dots to sit in.

These were the settings used to create the final covers of my G2 covers, but the actual process was much larger and consisted of:

1. Isolating the Jpeg of my artwork's channels into CMYK and then altering their image mode to 'Greyscale'. This also prompts you to disregard any other channels (do.)
2. seperately bitmapping each channel with the same DPI and LPI, but then making sure the screen angle of each separation matched the specified number to get the right angle. for example cyan, (Image> Mode> Bitmap> DPI: 600 LPI frequency: 65> Rotation 15 degrees.)
3. After bitmapping, change the images mode to duotone, but only use a monotone of the specific colour, so in this instance cyan (it has to be process cyan for this to work correctly, not any old shade of blue).
4. Save the image out as a jpeg (the file size is huge at this point so a tiff may destroy your computer) Also when saving out make sure you call each separation its colour name to avoid confusion later.
5. Repeat this process using the different screen angles for each separation.
6. Pull all separations back together in photoshop, making sure each layer is on multiply (in the layers panel) and in the order C,M,Y,K. then save the image out as a jpeg - if you want you can then pull the resolution back down to 300 DPI and this should enhance the effect.

As you can see on the covers I have produced this emulates a newsprint effect on screen. In print it also looks convincing, although the moire pattern is slightly large, this is altered if you resize the image or change its DPI down to 300 when saving the final out. The next study I need to take is how this pattern prints on an inkjet printer. because mocks I have done, look a little dull, but this may be down to the bitmapping and the visual pattern it makes. My theory is however, thin newspaper should take this amount of ink, because it has manually taken the print quality down, meaning less ink will actually be transfered onto the stock.

To see the results of this effect, see post named: 'All G2 Covers'