About this guide

Screen printing relies on the photosensitive emulsion within mesh holes to control ink flow. Processing the emulsion through engraving or gasifying facilitates ink penetration, whereas untreated emulsion obstructs ink passage.

This article will show you how to apply a halftone screen to an image, which is a common pre-processing technique for bitmap images, aiming to create a visually rich effect using limited colors of ink.

Things to know beforehand

About halftone

• Halftone is a printing method that uses dots of different sizes and densities or changes their arrangement in a specific way to represent various shades of color. This technique creates the illusion of using multiple-colored inks with a limited number of inks.
• Halftone screen is one of the specific tools or techniques for achieving halftone effects.
• AM (Amplitude Modulation) screening and FM (Frequency Modulation) screening are two different types of halftone screening methods used in printing to reproduce continuous-tone images using dots. Both methods vary in the way they encode tonal information and create halftones.
• AM does this by changing the size of the dots, while PM does it by changing the density of the dots as well as the frequency.

Adjusting dot size and spacing empowers users to simulate diverse colors and shades, creating richer print patterns and effects.

Note: The halftone screen processing of photos in Photoshop applies AM printing.

How does it work

• The visual illusion effect in halftone screening is mainly based on "spatial mixing".
• Spatial mixing is also used in various productions, including displays, textiles, and paintings.
• The principle of this illusion is based on the way the visual system works. Even single-colored ink dots, when properly arranged and spaced, create a mixture of colors that is indistinguishable to the human eye when viewed from a certain distance, resulting in continuous shades and gradations.
• Try taking off your glasses and staring at your screen from a certain distance to observe the following images. They are created using a single color of ink (blue) on a white background to simulate the continuous mixing of colors.

CMYK VS Spot-color printing

CMYK color model

• C: Cyan
• M: Magenta
• Y: Yellow
• K: Key plate (Since the color black is mostly used in practice, it can also be simply regarded as black.)

In theory, for multi-color printing, we only need 3 colors of ink to simulate a wide variety of the desired hues that are commonly seen.

It is common to include black in color printing because the combination of cyan, magenta, and yellow cannot produce pure black. Instead, they can only produce a dark grey or sepia shade. Hence, using black directly instead of relying on color mixing is a more widespread practice.

Spot-color printing

Spot-color printing, also known as Pantone or PMS (Pantone Matching System) printing, involves using pre-mixed and specific ink colors to achieve precise and consistent colors. Each color is printed using a separate ink plate.

Limitations of CMYK in halftone simulation

• The mixed colors produced by CMYK ink, which confuses vision, have limited saturation and brightness.
• Furthermore, when digital images are printed using CMYK, repeated color adjustments are often necessary to remove discrepancies.

Therefore, this guide recommends spot-color printing for halftone screens.

What do LPI, PPI, and DPI stand for?

• DPI: Dots Per Inch
• PPI: Pixels Per Inch
• LPI: Lines Per Inch (Lines/Inch, it is also referred to as frequency in the halftone screening process. )

LPI is crucial in achieving the desired detail in prints. The ideal LPI depends on the detail level and viewing distance.

For example, a halftone with 100 LPI in a magazine is not visible from 30 cm. The dots on the T-shirt with a halftone of 45 LPI will not be noticeable from a height of 1 meter either.

Get a photo halftone screened using Photoshop

Note: The bolded parameters below are for example purposes only, (the value is based on personal theoretical calculations, which have been adjusted using safety coefficients), and can be modified or adjusted according to the actual situation.

#1 Prepare images in Photoshop

1. Open the image in Photoshop.
2. Save as a copy.
3. Open the copy.
4. Click "Image" -> "Image Size".
5. Adjust the resolution to 75 Pixels/Inch (This setting allows 1.3 real-world dots to represent information of 1-pixel size. If set to 100 Pixels/Inch, the image information along each linear inch will be presented at a density of 100 pixels per inch.)

#2 Switch image mode to CMYK

Note: If you don’t want to print color images, skip this step.

1. Click "Image" -> "Mode" -> "CMYK (8-bit/Channels)"
2. Click "Channels" -> "More" (See the picture below.) -> "Split Channels"
3. Four grayscale images will be produced.

#3 Apply "Halftone Screen" to all images

1. Click "Image"-> "Mode" -> "Grayscale" -> "8 Bits/Channel" (See the pictures below)

This is to convert the image into a bitmap.

2. Click "Discard" for "Discard color information".

3. Click "Image" -> "Mode" -> "Bitmap"
4. Set the "Output" to 75 Pixels/Inch (See the picture below.)
5. Select "Halftone Screen" in the "Method" for use.

#4 Configure parameters

• Frequency: 15 Lines/Inch (Depending on your actual need)

It is recommended to set it to 15 Lines/Inch, but finding the ideal frequency may require multiple attempts. Additionally, increasing the frequency will result in smaller dots and vice-versa.

• Shape: Select "Round", or select based on your preference.

• Angle: Depend on your actual need.
• For CMYK overlays, set "Angle" as follows:
  • C (Cyan): 15 degrees
  • M (Magenta): 75 degrees
  • Y (Yellow): 0 degrees
  • K (Black): 45 degrees

Applying certain values can help reduce Moiré patterns in printing. Additionally, the order in which colors are printed can also impact the overall result. For this reason, it's common practice to follow the CMYK sequence (Cyan, Magenta, Yellow, Black) when printing.

• Save the file in PNG format for processing. (Currently, XCS supports the importing of only JPG, JPEG, GIF, PNG, BMP, SVG, DXF, and WEBP files.)

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