The key performance parameters of a laser imagesetter include recording precision (recording resolution), repeatability (registration error), format size, recording speed, and laser wavelength. Among these, recording precision and repeatability are the two most critical indicators for evaluating performance and classifying the device's grade.
Recording Precision
Recording precision refers to the number of light spots an imagesetter can record per unit of length-known as recording resolution-and is typically expressed in dots per inch (dpi) or dots per centimeter (dpc). Higher recording resolution results in smaller laser spot sizes and greater spot density. Since printed halftone dots are formed by multiple laser spots, a smaller laser spot size means that-at a given screen ruling-more laser spots make up each halftone dot, allowing for a wider range of grayscale levels. Alternatively, maintaining a specific number of grayscale levels while using smaller laser spots enables a finer screen ruling. Within the area of a single printed halftone dot, adding or removing a single laser spot exposure changes the dot size by one step, thereby creating a distinct grayscale level. Consequently, the more laser spots that compose a halftone dot, the greater the number of achievable grayscale levels. In this manner, an imagesetter can produce up to 256 grayscale levels, meaning a single printed halftone dot is composed of a 16×16 grid of laser spots.
Repeatability
Repeatability refers to the accuracy of image positioning across different color separations-a requirement essential for color printing-and is usually measured by the registration error between the first and last color separations. While monochrome printing does not require registration (and thus has low demands for registration accuracy), registration precision is a critical parameter for color printing. Insufficient registration accuracy leads to issues such as color misalignment, gaps (white space) between color blocks, and "ghosting" in small text; therefore, the repeatability of the imagesetter is vital for color printing plate production. **Format Size**
Imagesetters come in various format widths-ranging from standard and large 8-up (octavo) and 4-up (quarto) sizes to half-sheet and full-sheet formats. Typically, an imagesetter allows for the use of different film sizes within its maximum format capacity to meet specific requirements and conserve film. There is a strong correlation between an imagesetter's format size and its recording and repeatability precision; larger formats demand higher precision and present greater manufacturing challenges, resulting in exponentially higher costs.
Imagesetters with internal-drum or external-drum architectures generally achieve high recording precision because the distance the laser travels to reach the film remains constant. In contrast, capstan-style imagesetters use a prism to deflect the laser beam for scanning; consequently, the distance from the laser source to the center of the film differs from the distance to the edges. As the format size increases, the deflection angle widens, increasing the discrepancy in travel distance between the center and the edges of the film. This leads to greater non-linear distortion and, ultimately, higher error rates. Therefore, the format size for capstan-style imagesetters is usually limited to a maximum of 4-up, and measures are taken to minimize the laser beam's deflection angle and reduce spot distortion.
Imaging Speed
Imaging speed is typically measured by the rate at which film advances during recording at a resolution of 1200 dpi. While modern imagesetters can achieve high film-feed speeds, this is not the sole determinant of total output speed, as the process is also constrained by RIP (Raster Image Processor) speeds. The time required to output a page is the sum of the time spent on data transmission, RIP processing, and actual recording; thus, the imaging speed affects only the recording phase.
Laser Wavelength
Common lasers used in imagesetters include Helium-Neon (HeNe) lasers (633 nm wavelength), red semiconductor lasers (650 nm or 670 nm), and infrared semiconductor lasers (780 nm). The laser wavelength dictates the type of film required and can even influence the cost of the film used. For instance, domestically produced helium-neon laser phototypesetting film is the least expensive and is used for almost all text phototypesetting, whereas infrared phototypesetting film is costly and less frequently used.