Film Positives and Accurate Image Translation
Coudray examine the different types of imagesetters and offers advice for getting consistent, high-quality output.
Obtaining accurate color and image reproduction with screen printing requires careful translation of image information at each step of the design and production process. Today's original artwork is most often created in a digital environment, imaged digitally to film (or similar materials), transferred in an analog manner to the screen, and reproduced onto the final substrate by an analog printing press. For the process to work, the materials and methods used to carry and transfer the image must be stable, predictable, and capable of passing along all of the image information as the image undergoes the digital-to-analog translation.
Each step represents a single translation, bringing with it the potential for gain or loss of image quality. By far, the most common situation is image-quality loss. This month, we'll look at the issues to consider in order to avoid this problem.
Types of imagesetters
Four primary types of digitally imaged film positives are used today. The first group is toner based. Positives created in this manner usually take the form of vellum or polyester substrates imaged on high-resolution laser printers. Image quality on the film is typically enhanced with a separate processing system or solvent-based aerosol fusing agent.
The second type of positive is thermal film. This involves a special polyester-based film that becomes opaque to UV light wherever it's exposed to heat from the thermal imagesetter's heated nibs or lasers.
The third option, which is gaining in popularity, is polyester film imaged with a high-resolution inkjet printer. For this approach, special high-density opaque inks are applied to treated polyester film to create the positive image.
The fourth type of film imaging uses conventional, silver-based, photographic films exposed on high-end digital imagesetters. The resulting films can be used straight from the imagesetter to expose screens or enlarged through a projection system for large-format applications. Because silver-based materials require chemical development after the materials are exposed and reclaiming of metallic silver from the waste stream, their popularity is in decline. Nevertheless, photographic systems offer the best film positives available.
The suitability of any particular film-imaging system depends on a job's resolution and printing requirements. What might be a minimum tolerance for a CD printer would likely be overkill for a textile printer. What may work for a textile printer doing spot colors would be unsatisfactory for process color. Let's look at the capabilities and limitations of the common systems.
The first thing we need to know is how well the base material will support and maintain the desired image size. Polyester, the most common base material, is susceptible to shrinkage and damage from heat. Both toner- and thermal-based film-generation processes subject polyester to substantial heat, and if the substrate is too thin or has not been heat stabilized, it can change size. The more heat the material is subjected to, the greater the chance of distortion. Shrinkage problems occur primarily when very heavy reverse or negative graphics are being imaged.
Problems also occur when imaging a mix of high- and low-coverage areas. In these situations, the material may develop localized misregistration due to uneven shrinkage. Imagesetter manufacturers are very much aware of shrinkage problems, and many have developed their own solutions. Before selecting a system, it's important to make sure it can address this issue.
The second concern is distortion of the substrate or image caused by the film-transport system on the imagesetting device. Two types of transport systems are employed on imagesetters: capstan drives, which use mechanized spindles to move a continuous roll of film past the device's imaging heads, and drums, which hold a sheet of film in a fixed position against the drum surface. The film is imaged in this fixed position as an imaging mirror moves across the film plane.
Capstan-drive systems All four types of imagesetting devices utilize capstan drives (or similar friction-feeding mechanical rollers in the case of laser printers). In fact, capstan-drive imagesetters are, by far, the most common because they are much more economical than drum-based systems. They offer size advantages in both width and length of output.
Many capstan systems are very sophisticated, providing internal feedback loops and the ability to accurately track material position. On large-format imagesetters, manufacturers typically quote the degree of error their devices exhibit over a given length of material.
Older machines, and those requiring minimal investment, are the most prone to drive-based distortion problems. They need to be carefully analyzed before a purchasing decision is made. The most dependable devices have warranties that address this area specifically.
Drum-based systems Only certain models of silver-based film imagesetters use drums. These systems provide both a fixed imaging width and depth of image. They also have the ability to prepunch film before it is imaged. This makes for extremely accurate images than are very easy to register on screens and on the press. The drum-based systems tend to be offered in smaller formats that capstan-drive systems and are usually more expensive. Drum-based imagesetters typically provide a registration tolerance of ±0.0003 in. per 16 x 20-in. sheet of film.
Examine the different types of imagesetters and advice for getting consistent, high-quality output.Mark Coudray
The accuracy of film positives is determined by the resolution of the imagesetter and the optical density or dynamic range it delivers. The higher the resolution, the better formed the dots and the sharper the image.
The resolution needed is determined by the applications that will be printed. Most textile printers will only need 600 dpi. Graphics printers may need 1200 dpi or higher. For graphics work, the minimum resolution should be 16 x the halftone line count that will be printed. So, for a 65 line/in. halftone, the minimum film positive resolution would be 1040 dpi. Anything less than this, and you will lose tonal steps.
Be careful of interpolated resolution, which you'll find in devices with reported resolutions of 400 x 800, 600 x 1200, 720 x 1440, and so on. This means that the resolution of the head is fixed in the "X" direction, and half stepped in the "Y." In other words, the resolution is artificially increased by overlapping the dots in one direction. The resulting positive may be fine, but this quality depends largely on the RIP software that drives the imagesetter. Every RIP is different, and you need to do the verification, rather than rely on a salesman's promise.
Image resolution also depends on the minimum "spot" size the device images on the film (multiple printed "spots" comprise a single halftone dot). Most systems have a fixed spot size and change the size of imaged halftone dots by adjusting the spacing and overlapping of spots.
Optical characteristics of the positive
The optical properties of the film carrier and imaged area also determine the quality of the film positive. The points to consider here include Dmin, Dmax, and dynamic range.
Dmin is the minimum density of the film itself and refers to its transparency to UV light. Vellums and frosted polyesters used for toner-based systems have a very high Dmin. This means that screen-exposure time must be increased in order to burn through this fog or haze. This, in itself, is not necessarily bad. But Dmin has to be considered in relation to Dmax.
Dmax refers to the maximum density of imaged areas and represents their ability to block UV light in the 365-420 nanometer range. It's important to note that imaged areas only need to fall within this particular range of wavelengths. An image that is somewhat transparent in the visible spectrum can be completely opaque to UV light, which means it would be suitable for screen exposure. For those of you who remember ruby masking films, they're a good example of a visibly transparent but UV-opaque material.
The ideal Dmax density is 4.0 or higher. This value is logarithmic, meaning that a density of 4.0 transmits only 10-4 or 1/10,000 of the light hitting the surface. Lower-end imaging devices will generate Dmax densities in the order of 2.0 -2.5, which may work for large areas of solid art, but are too low for good halftone work. A transmission densitometer is used to determine the Dmin and Dmax values.
The difference between the two Dmax and Dmin values represents the dynamic range of the positive. The minimum dynamic range required for half tones is 2.5 but ideally should be greater than 3.0. To achieve this with most toner positives, the film must be treated with a fusing agent. All of the other imaging system types will give you enough density if they're functioning correctly and have been properly calibrated.
Silver-based positives have the highest dynamic range--I've seen examples with a range in excess of 5.0. But this is overkill. It's better to adjust the laser or thermal heads to provide a dynamic range of 4.0. This will greatly extend imaging head life.
No imaging device, including a camera, will provide satisfactory halftone positives until it has been linearized. This means adjusting the machine or the RIP (via a transfer curve) to assure that each halftone dot percentage will image at the expected value.
Screen printers do not realize how far off target their positives can be. I have seen brand new imagesetters generate halftones on which the 10% dot averaged 37-41% and the 50% dot imaged around 85-88%. If the user was expecting 10%, he was in for a big surprise. Once the machines were calibrated to produce an ideal transfer curve, none of the tonal values in the 5-99% range were off more than 1%.
It is extremely common for imagesetters to be this far out of calibration, and simply spending money on an expensive imagesetter does not guarantee success. The calibration process itself involves making a test scale or running a premade scale, measuring the results, and adjusting the unit until it provides correct values. For a thorough description of the procedure, refer to the "Calibration" section of the Adobe Photoshop User Manual.
The first step in accurate halftone reproduction is finding the right imagesetting system, one that ensures accurate registration from film to film, delivers ample resolution for your applications, uses stable media, and provides proper optical characteristics for film exposure. Before settling on a particular system, ask the manufacturer to run test samples of images you typically print. This is the only way to be certain that the device is appropriate.
The second step is to calibrate the system. This way, the imagesetter will provide the tonal values you expect for halftone work. The next step is to determine the quality of the stencil on the exposed screen itself, which is a topic for another day.