Understanding halftoning in printing
When we discuss printing, it's essential to understand the fundamental building blocks of a printed image: halftone dots. These tiny dots, when placed closely together, create the illusion of continuous tone images, such as photographs. The arrangement, angle, density, and size of these halftone dots significantly impact the quality of the printed output and are crucial for quality control.
Halftoning originates in the prepress stage and is finalized during printing. Any alterations in this process can affect the print quality, emphasizing the importance of understanding these intricate details.
It's important to note that halftone dots are not exclusively circular. They can assume various shapes, each with its own advantages and drawbacks. Professional print inspectors and quality control managers carefully consider the choice of halftone shape, taking into account factors such as graphic design, printing press, and substrate. Understanding the halftoning process from prepress to print is also beneficial for graphic designers.
The Halftoning Process
Once a design file is prepared and preflight checks are completed, it is sent to prepress for platemaking. The prepress operator uses software, often referred to as a RIP (Raster Image Processor), to convert the file into a series of halftone dots. This process of transforming a digital file into tiny dots for printing is known as halftoning.
RIP software, typically provided by reputable companies like Kodak or Heidelberg, separates colors, selects halftone shapes and angles, and applies a suitable color profile to create the final printing plates.
Various errors can compromise the halftone structure, from the prepress stage to the printing process. Common halftone errors include dot gain, incorrect print resolution, color shift, and moiré patterns. These errors can be caused by mistakes made during platemaking or by incorrect RIP settings.
Creating Halftone Dots On Plates
In simple terms, prepress is where printing plates are produced. Plate materials can vary, such as aluminum or metal for offset printing, gelatinous or soft materials for flexographic plates, or durable cylinders for rotogravure printing.
The characteristics of halftone dots, including shape and structure, differ based on the plate type. For instance, halftone dots in flexographic printing are typically circular, while in offset printing, they may be elliptical or square. However, the same shape may cause issues in different areas, such as midtones and shadows.
Therefore, the final halftone shape is usually chosen to minimize problems during production and improve controllability. While it's tempting to standardize halftone shapes for a specific press, a more professional approach involves considering variability to ensure optimal print quality.
Key terms used:
- Halftone: A tiny dot used to create the illusion of continuous tone images in printing.
- Moiré: A wavy interference pattern that can occur when two halftone screens are superimposed at slightly different angles.
- Dot gain: An increase in the size of halftone dots during the printing process.
- Color shift: A change in the perceived color of an image during the printing process.
- Midtone: The middle range of tones between the lightest and darkest areas of an image.
- RIP (Raster Image Processor): Software used to convert a digital image into a halftone image.
