Introduction
The basic parameters of print quality are resolution, addressability, gray scale, and dot microstructure. A real device also has intrinsic variability in the printing process, producing visual artifacts, which come under the general heading of noise. Some of the more common manifestations of this are background scatter, dot placement errors, voids (due to nozzle malfunction in ink jet, for example), and banding in images. The significance of any of these aspects of print quality can only be determined by examining them with respect to the properties of the human visual system. The design choices of the basic print quality parameters are, therefore, guided by the properties of the human visual system to determine where improvement needs to be made or where little is to be gained by increasing any one of the specifications.
Resolution and Addressability
Resolution, the most widely used specification to rate print quality, is sometimes confused with the related term addressability. Fundamentally, resolution refers to the ability of the device to render fine detail. This simple definition is complicated by the fact that detail can be regarded as the fineness of the width of a line, the transition between white paper and printed intensity, and/or the smoothness of the edge of a curved line or a line printed at any arbitrary angle. In the simplest case, the resolution of a printer is defined as
the spacing of the dots such that full coverage is obtained, that is, no white paper can be seen. For circular dots placed on a square grid, this number would be calculated by dividing the diameter by the square root of two and taking its inverse. For example, an ideal 300 dots per inch (dpi) printer would produce 120-µm-diam dots at an 85 µm spacing. In practice, the dot would be made somewhat larger to allow for dot placement errors. This definition is best understood in terms of the finest line that can be printed by the device. At 300 dpi the line would exhibit a perceptible edge waviness, especially when printed at certain sensitive angles. This would also be true of curved lines. In addition, the range of lines of increasing thickness would have discontinuities since they would consist of an integral number of the basic line, each spaced at 85 µm. These issues have an important bearing on text print quality, which depends on the ability to render both curved and straight lines at variable widths.
The preceding definition is related to the specification of resolution with respect to the human visual system. In this case resolution is determined by the closeness of spacing between alternate black and white lines of equal width and defined contrast. These are known as line pairs and for a 300-dpi printer it would result in a value of 150 line pairs per inch. This is not strictly correct since the black lines would be wider than the white spaces due to the roundness of the dot. Since the human visual system has a response that approaches zero near 300 line pairs per inch, gains made in text print quality by increasing resolution alone can be expected to diminish above this value. At this point, issues such as print noise and grayscale enter if further improvement in print quality is desired.
To focus only on the resolution as defined in the previous paragraphs ignores the specific needs of the components of the printed material, that is, text and lines vs. images and area fill. Gains in text print quality may be had if the device can space the dots closer than the fundamental resolution. This can result in substantial dot overlap but allows the line width to be varied more continuously. In addition, at the edge of a curved line, the subpixel adjustments of individual dots increase the perception of smoothness commonly known as getting rid of the jaggies. This ultimate dot spacing of the device is called addressability. For example, printers employing this technique are specified as 300 × 600 dpi indicating a native resolution of 300 dpi in the horizontal direction and a vertical addressability of 600 dpi.
Grayscale
The ability of a printing technology to modulate the printed intensity on the page is referred to as its grayscale capability. There are three ways in which this may be accomplished: variation of the dot size, variation of the intensity of the printed dot, and digital halftoning techniques. The first two depend on the intrinsic properties of the technology, whereas digital halftoning can be employed by any printer. A printer that can continuously vary its intensity from white paper through to maximum colorant density is described as having continuous tone capability. Other technologies produce a modest number of intensity levels and make use of digital halftoning techniques to create a continuous tone effect. The manner in which gray scale is achieved is of obvious importance in image printing, particularly in the case of color. In recent years considerable effort has gone into the development of sophisticated digital halftoning algorithms to enable binary (single dot size and no intensity modulation) printers to render images. The resulting image quality depends more strongly on resolution than addressability. But the impact of even a few intrinsic gray levels on the print quality achieved by these algorithms can be dramatic.
An important parameter in grayscale considerations is that of the dynamic range, which is simply called range in the graphic arts. This is measured in terms of optical density, the negative logarithm of the reflectance. An optical density of 1.0 represents 10% of reflected light per instant flux, an optical density of 2.0 corresponds to 1% reflectance, and so on. For printed material the smoothness of the printed surface limits the maximum optical density obtainable. If the surface is smooth and mirrorlike, then the print appears glossy and can have optical densities approaching 2.4. The smooth surface reflects light in a specular manner and, therefore, scatters little stray light from the surface into the eye, and the color intensity is not desaturated. It is most noticeable in the case of photographic paper that has a high gloss finish. If the optical density range of the print is high, it is said to have high dynamic range and a very
pleasing image will result. Not all papers, however, are designed to have a glossy finish. Papers used in the office are also used in copiers and have a surface which produces diffuse reflection at the interface between the air and the paper. For most uncoated, nonglossy papers this will be between 3–4% and limits the maximum optical density to around 1.4. Image quality on these stocks will depend on the fixing of the colorant to the substrate to produce a smooth surface. The potential image quality for a printer is therefore a complex tradeoff involving the design choices of resolution, addressability, grayscale method, digital halftoning algorithm, paper stock, colorant, and fixing technology. Conclusion: for images, resolution alone is not a predictor of print quality.
Dot Microstructure
The microscopic nature of the dot produced by a given technology also has a bearing on final print quality. The most important parameter here relates to the edge gradient of the dot. Known as the normal-edge profile, it characterizes the transition between white paper and maximum colorant intensity, that is, the gradient of optical density that occurs at the edge of the dot and measures the steepness of the transition from white paper to full optical density. Some technologies, such as electrophotography, can vary this profile by adjusting various parameters in the imaging and developing process. For ink jet, various paper types will produce different normal-edge profiles. If the profile is very steep, that is, the transition occurs in a very small distance such as 5 µm, then the dot is described as being a hard dot or having a very sharp edge. This is desirable when printing lines and text that benefit from very sharp transitions between black and white. If this transition is gradual, the dot is described as being soft and produces a blurring of the edge, which can degrade the text quality. In the case of images, where smooth tones and tonal changes are desired, a soft dot can be very beneficial.
Hybrid Methods
From what has been said it should not be inferred that the needs of texts and images are in opposition. In recent years the intrinsic grayscale capability has been used to advantage in improving text print quality. The removal of jaggies can be greatly assisted by the combination of increased addressability and a few gray levels. By the use of gray levels in the region of the jagged stairstep, the transition can be made to take place over several pixels. This is, in essence, blurring the transition to make it less visible to the eye. In the case of certain fonts, there is fine detail requiring resolutions greater than the native resolution of the printer. This fine detail can be rendered through a combination of gray levels and regular pixels. The implementation of these methods requires a complex set of rules to be applied to the data bit stream before it is sent to the marking level of the printer. These rules draw heavily on image processing techniques and a knowledge of the human visual system and are proprietary. Skillfully applied they can have a dramatic effect on the text and line quality. There are a variety of trademarked names for these technologies designed to convey the sense of enhancement of the print quality.
The application of image processing techniques to manipulate the intrinsic properties of electronic printing technologies have made resolution an insufficient measure of print quality. A more comprehensive measure is needed to simplify the identification of the printing technology to serve the design goals for final output quality. Until such a metric is devised, the tradeoff analysis just described, implemented by means of industry standard test charts that separately probe the printer properties, will provide a predictive measure of print quality. Such test charts must also contain test images, which will be subject to the proprietary subjective image enhancement algorithms offered by the manufacturer.