Halftone and Simulated Process: Printing a Gradient

Screen printing cannot reproduce continuous tone the way a photograph does. Instead, it uses halftone dots—tiny ink spots arranged in a grid—where dot size and spacing create the illusion of tonal variation. Simulated process printing applies this principle to full-color images by layering CMYK halftone screens over a white or colored underbase, letting screen printers reproduce photographic artwork on garments without needing a separate screen for every color.

Halftone quality depends on three variables: line screen frequency (LPI), dot angle, and dot gain. Get these wrong and you see visible moiré patterns, loss of detail, or colors that print darker than the proof. Understanding how each parameter affects the final print lets you optimize separations for your press, mesh count, and substrate.

How Halftone Dots Create Tone

A halftone screen divides an image into a grid of cells. Each cell contains one dot. In light areas, the dot is small; in dark areas, the dot is large. Your eye blends the white space between dots with the ink color, perceiving a continuous gradient. This technique dates to 19th-century printing but remains the only way to print grayscale or color photos with spot-color inks.

Line screen frequency, measured in lines per inch (LPI), determines how fine or coarse the halftone appears. A 45 LPI screen has 45 rows of dots per inch, producing large, visible dots. An 85 LPI screen has 85 rows per inch, producing smaller, finer dots. Higher LPI gives smoother gradients but requires finer mesh screens and more precise press registration.

For screen printing on fabric, LPI typically ranges from 35 (coarse, for rough textiles) to 65 (fine, for smooth garments). Offset printing uses 150–300 LPI because paper holds finer detail than fabric. If you apply a 150 LPI halftone to a 110-mesh screen, the mesh cannot resolve the dots—ink bleeds through multiple openings, and the image turns muddy.

Simulated Process vs. CMYK Process

Simulated process is not true CMYK. True process printing (like offset or digital press) uses transparent cyan, magenta, yellow, and black inks that blend optically where dots overlap, producing a full spectrum of colors. Screen print inks are opaque, so overlapping dots do not blend—they stack. A cyan dot over a magenta dot does not produce blue; it produces whichever ink layer is on top.

To work around this, simulated process adds a white underbase (or a highlight color underbase, like red or yellow) printed first, then layers CMYK halftone screens on top. The white base gives the CMYK inks something bright to sit on, preventing the dark fabric from showing through and dulling colors. Some printers add a fifth or sixth screen (spot red, spot blue) to boost colors that CMYK cannot hit accurately—this is called index separation or UltraSep.

Index separations use spot colors for key elements (skin tones, brand colors) and halftone screens for less critical areas (backgrounds, shadows). This hybrid approach gives better color accuracy than pure CMYK halftones but requires more screens. A six-color simulated process job might use white underbase, cyan, magenta, yellow, black, and spot red.

Mesh Count and LPI Relationship

Mesh count is the number of threads per inch in the screen fabric. A 110-mesh screen has 110 threads per inch; a 230-mesh screen has 230. Higher mesh counts allow finer detail and smaller halftone dots, but they also reduce ink deposit, which can cause incomplete coverage on dark fabrics.

The rule of thumb: mesh count should be at least 4× the LPI. For a 55 LPI halftone, use a 220-mesh screen minimum. For a 35 LPI halftone, 140-mesh is sufficient. If you try to print a 65 LPI halftone on a 110-mesh screen, the mesh openings are too large relative to the dot size—ink floods through, dots merge, and fine details disappear.

White underbase screens use lower mesh counts (110–156) because you need thick ink deposit to block the fabric color. CMYK halftone screens use higher mesh counts (200–230) because you want thin, precise dots that do not obscure each other. This is why simulated process requires multiple screens with different mesh specifications—you cannot use the same mesh for every color.

Dot Angle and Moiré Prevention

Each halftone screen has an angle—the direction the grid of dots tilts. If you stack two halftone screens at the same angle, the grids align and create a visible pattern called moiré. Moiré looks like wavy lines or rosettes and ruins the print. To prevent it, rotate each color screen to a different angle, offset by at least 30°.

The standard angles: cyan at 15°, magenta at 75°, yellow at 0°, and black at 45°. Yellow is least visible, so it tolerates a 0° angle without causing moiré. Black is most visible, so it gets the 45° angle—the eye perceives 45° diagonals as softer than vertical or horizontal lines. These angles are not arbitrary; they are mathematically spaced to minimize interference between overlapping grids.

Some RIP software (AccuRIP, Separation Studio) applies these angles automatically. If you are generating halftones manually in Photoshop, you must set the angle for each channel individually. In Photoshop, convert the image to Grayscale, then Image → Mode → Bitmap, choose Halftone Screen, and enter the LPI and angle for each separation.

Stochastic (random dot) halftones eliminate moiré entirely by randomizing dot placement instead of using a fixed grid. This works well for fine-detail images, but stochastic screens are harder to burn (they require higher-resolution films or direct-to-screen imaging) and are less common in garment printing. Most screen printers stick with angled halftones.

Dot Gain Compensation

Dot gain is the tendency for halftone dots to spread during printing. A 50% dot in your file may print as a 60% dot on fabric because ink soaks into the weave and expands beyond the mesh opening. This makes the entire image darker and can fill in highlight details (light areas where dots should be small).

Compensate for dot gain by reducing dot size in the prepress file. If you expect 10% dot gain, reduce all dot values by 10% before burning screens. Most RIP software has a dot gain compensation setting—enter the expected gain percentage, and the software shrinks dots proportionally. Alternatively, underexpose screens slightly during burning, which produces smaller mesh openings and smaller dots.

Dot gain varies by substrate. Smooth, tightly woven fabrics (performance polyester, dress shirts) have less dot gain (5–10%) because ink sits on the surface. Coarse, open-weave fabrics (canvas, fleece) have more dot gain (15–25%) because ink soaks in. Run test prints on your target fabric and measure dot gain with a densitometer or by visual comparison to the proof, then adjust compensation settings for that substrate.

Underbase Strategy for Dark Garments

On white or light-colored garments, you can skip the underbase and print CMYK halftones directly—the fabric itself acts as the white base. On dark garments (black, navy, dark gray), you must print a white underbase first, or the CMYK inks will not be visible. The underbase blocks the fabric color and gives the halftones a bright foundation.

Two underbase approaches: full-coverage (print white everywhere the design appears) or highlight-only (print white only in light areas, let the fabric color serve as the black in shadows). Highlight-only underbases save ink and reduce hand (stiffness), but they require precise registration—if the white underbase shifts, you see white edges around dark areas.

Some printers use a colored underbase instead of white. For a sunset image, a yellow or orange underbase printed first can warm the entire palette, reducing the amount of yellow and magenta halftone ink needed on top. This technique is common in high-end simulated process and requires custom color profiling to predict how the underbase affects the CMYK layers.

For general screen print file preparation, see screen print color separations. For other apparel printing processes, compare sublimation, DTF, and UV DTF. If you are preparing vector artwork for screen printing, use PNG to SVG conversion or logo vectorization to ensure clean edges before applying halftones.

When to Use Simulated Process

Simulated process suits photographic images, designs with gradients or shading, and artwork with more colors than you want to print as spot colors. It is the standard for full-color T-shirt printing on dark garments. If the design has ten distinct colors, simulated process (six screens) is cheaper and faster than printing ten spot color screens.

Simulated process does not suit logos or text that require exact Pantone matching, because halftone dots introduce visible texture and color shifts. For brand-critical color matching, use spot color separations (covered in manual and automatic separation workflows). For print-ready file requirements across all processes, see print-ready artwork checklists.