A photographic image or gradient must be converted to a B&W halftone to be exposed and screen printed. Halftone is the term for tiny dots (or lines) that give the illusion of a tonal range. The image may be used on it's own, or placed back into Illustrator/Photoshop and colorized for the final design.
It is also one of the settings that varies between screen printing studios based on inks, experience, and equipment. For my design projects, it often becomes an aesthetic choice where I go back and forth, save multiple versions, and place into the layout with different lpi/angles and/or dots/lines, before finalizing to print films.
Screen printing halftones does take practice with testing to create and screen print successfully.
A Original 10x10 image.
B Detail showing B&W halftone conversion using 1 screen.
B Detail showing CMYK conversion using 4 screens.
PLEASE FOLLOW THESE STEPS TO CONVERT IMAGES TO HALFTONES
If the film is output as greyscale, the film positive no longer blocks the light 100% and the exposure will be uneven resulting in the image screen printing with unexpected and inconsistent results.
01 Open or place in Photoshop, flatten image.
02 Confirm image is set to 300dpi/Greyscale.
03 Convert to Bitmap and follow the prompts.
PLEASE NOTE These settings may be adjusted based on your screen printing experience.
COMMERCIAL PROJECTS/WORKSHOPS Do not go higher than 35lpi when converting to a bitmap. For the angle, most choose 45º, but for spot colors it generally doesn't matter.
COMMERCIAL CMYK PROJECTS I will do the final conversion.
Bitmap Dialogue Prompt
RESOLUTION Output: 300 Pixels/Inch
METHOD Use: Halftone Screen
Halftone Dialogue Prompt
FREQUENCY 35 Line/Inch (lpi)
ANGLE 45 degrees