Halftones are a pattern of tiny dots that can simulate different shades of color using varing percentages of a single ink. Visually, halftones create the illusion of a continuous tone image by using spots of varying size and density to represent darker or lighter color values.

Halftones work by fooling the eye into seeing the combination of the ink color and the color of the shirt they are printed on. When seen from a distance, the colors blend together and the dots merge with the background color of the shirt. If you look closely at or magnify the print, the separate dots are quite clear. You can see good examples of halftones if you magnify a picture in a magazine or a print from a color printer or even if you look closely at your TV screen. All these are made up of tiny dots.

In screen printing we use halftones for three main purposes:

1. To create a tint or lighter shade of a color. This will allow more "colors" in the design without adding more screens.

Example:

	100% Blue	50% Blue	20% Blue
Desired Colors
Halftones
Enlarged Halftones

One color image	Halftones	Enlarged

2. To create a gradient or the appearance of a continuous tone of color:

Example:

1 Color Gradient	Halftones	Enlarged Halftones

3. To create overlapping screens of different ink colors that combine to simulate more colors.

Example:

Simulated Solid Color	Halftones	Enlarged Halftones
2 Color Gradient	Halftones	Enlarged Halftones
4-color process	Halftones	Enlarged Halftones

These terms all refer to the number of vertical lines of halftone spots per inch or lines per inch (lpi). Halftones are created using a grid of cells. Each cell contains one halftone spot. These spots vary in size depending on the shade of color being represented but only one fits in a cell. The size of these cells determines the lpi.

	Individual Cell	Cell Grid ~3 lpi	Cell Grid ~12 lpi
10% Halftone
50% Halftone
90% Halftone

Various lpi numbers are used for different types of printing. Magazine pictures may have 100-130 lpi, newsprint is typically 85 lpi, a 300 dpi laser printer is around 55 lpi, and billboards might be 3-6 lpi. When we are using halftones for screen printing we use 45 lpi for basic designs and 65 lpi for detailed and process type designs.

The correct lpi to use is a function of the detail you want to produce and the distance from which a print will be viewed. When reading a magazine, the 100 lpi halftones will be invisible at 12 ". Likewise, if you view a 45 lpi t-shirt print from 3-4 feet you won't notice the halftone dots.

You must also take into account what screen count (see next question) you plan to use. To calculate the proper LPI for a particular screen count you take the count and you divide by a constant, usually between 3 and 5. So, let's say a 200 mesh screen and the constant we use is 4. That gives us a LPI of 50 (200/4=50). We arrive at this constant through experimentation and it may be different for other printers.

The mesh count is the number of threads per inch (tpi) used to weave the mesh. Typical mesh counts for screen printing t-shirts range from 85 tpi to 355 tpi. The mesh count defines basically two things: The thickness of the ink deposit and the size ( or lpi ) of halftones that can be printed.

The lower mesh counts, 85-110 tpi, have a relatively large thread diameter and more space between the threads ( called "open area" ). This allows for a thicker deposit and more ink to pass through the screen. Typically, lower mesh counts are used for specialty inks ( like glitter ) and when a thick deposit is needed on dark garments.

The high mesh counts, 305-355 tpi, have a very small thread diameter and less open area. These are used when fine detail and high halftone lpi are needed.

In the middle, 200-255 tpi, are general purpose screens for spot color, good detail and acceptable halftones.

Spot-Color is the term used to describe separation and printing with one ink color for every color in the design. Each color that makes up the composite image will be printed using a separate screen.

Example:

	Original	Black	Yellow
Design Colors
Separations

A slightly more advanced type of spot-color uses halftones to create the appearance of a gradient using small dots of the solid ink colors.

Example:

	Original	Black	Red
Design Colors
Separations

This technique can be taken a step farther to combine two gradient screens and produce the appearance of more colors then are actually being printed. The small dots being printed so close together trick the eye into seeing the color they make when mixed.

Example:

	Original	Black	Red	Yellow
Design Colors
Separations

4-color process is a more advanced separation and printing technique that uses 4 colors of transparent ink to produce the colors from the original design. The four colors, Cyan, Magenta, Yellow, and blacK ( CMYK ), are printed as halftones that interact with each other and the white background of the shirt to create color and tonal values. A wide spectrum of colors can be represented but some colors are impossible to produce.

4-color process is used mostly for photographic or digitally created designs because many shades and colors can be created with only four screens. High screen counts and screen frequencies are required to create the tiny halftone dots.

One drawback to 4-color process is that the garment must be white.

	Original	Cyan	Magenta	Yellow	Black
Design Colors
Separations
As Each Color is Printed

Close-up of halftones

Simulated Process is another advanced technique that uses halftones of a few ink colors to represent the colors in the original design. This process differs from 4-color process in that the inks are solid opaque colors usually printed on dark colored shirts. Because the shirt colors are normally dark, simulated process generally requires the use of an underbase.

When working with simulated process, we start with a design created digitaly or one that is scanned into the computer. The image can range from something that is photo-realistic to a line drawing or illustration. Once in the computer at the correct size and resolution, we process the design through a special program that automates the difficult separation process. Currently, we use a plugin for Photoshop called T-seps (formerly FastFilms). We will end up with 7 or 11 color channels to work with. Now, we must adjust these chanels so that the final print on the shirt will match the original artwork. This is the part that takes skill and applied experience from our art department. Once the adjustments are finished, the separations are printed out as usual.

Simulated process requires relatively high mesh counts and screen frequencies. We generally use 305 mesh and 65 lpi seperations. Depending on the design, 3 to 9 or more ink colors may be needed to accurately reproduce all subtle color variations.

Original Design	Underbase White	Yellow
Red	Blue	Highlight White

Index color is another separation and printing technique that uses a few ink colors to produce a full color print. Unlike 4-color and simulated process, index color does not use halftones. Rather, the design is broken up into tiny square areas of solid color. These squares are very small ( 200 per inch ) and when printed they blend together to trick the eye into seeing color shades and variations.

Index color works well on both white and colored garments but generally takes more ink colors to represent a design with lots of color variation. Index color has the added advantage that you can pick the exact ink colors to use. This way you can more accurately represent colors that are difficult to produce using a "process" method. It also requires high mesh counts to print the tiny squares properly.

Original	Black	Yellow	Blue
	Maroon	Green	Tan

Image slice at full size

Image slice at 200% size

Close-up of squares