Print manufacturers include print resolution in their machine specs. It is usually shown as dots per inch (DPI) e.g. 600 x 600 or 1200 x 1200, but you may also see different amounts for the process and cross process directions e.g. 600 x 480, particularly with high speed continuous devices. DPI measures physical jetting capability of the print head configuration and can be called “effective resolution.” In addition, many manufacturers include a description of “perceived resolution” or “apparent resolution.” This spec looks like DPI, but is a higher number that attempts to describe the overall visual appearance created by the print head, ink and RIP. But, unlike DPI which is a fixed measure, the overall visual appearance is completely dependent on the media.
Previously in “Understanding Gray Levels of Inkjet” Elizabeth Gooding discussed the role that binary and grayscale print heads and the RIP plays in creating an apparent resolution that is substantially better than the effective resolution – but we couldn’t put a number to that difference. The apparent resolution specification is the product of a fixed process of ink, hardware and software, but does not take into account the media. Media, generally paper when discussing production inkjet, has a huge impact in how we perceive print quality. Resolution, density and image clarity play a large part in how an image is perceived and all of these measures are affected by the variable of media used.
Media porosity impacts the way a file is processed through the RIP and how many drops are actually jetted. The level of dot gain using a 600 DPI, 10pL binary print head will be much higher than a 600 DPI, grayscale print head with drop sizes of 4, 8 and 10pL, and the volume of ink in a 1 inch square area can be lower. Linearization remaps the dither pattern to adjust the number drops needed for a particular area of a specific media. For a binary print head, it may reduce the number of drops to compensate for dot spread. But the process is more complex when using a grayscale head with multiple drop sizes. In order to manage mid tone and highlight areas which require smaller drops with less ink spread, more drops may be used in these areas. The RIP may reduce the number of drops used in the shadows to compensate for spread. So the actual drops jetted for a particular printed image may be drastically different from a print heads effective resolution depending on the media and this in turn affects the perceived resolution.
Media’s porosity affects ink density. The higher the dot sits on the surface of the media the more dense the color is visually thus, the more we perceive higher resolution.
Media’s drying rate is also a factor. When a drop hits an absorbent media, it will have more or less time to spread and travel depending on the drying rate. A wet drop will want to follow the paper fibers leaving the dot to wick and reducing edge clarity. If not dried quickly, a dot printed on media with low porosity can wick or rewet into previously printed inks degrading edge sharpness and mid tone detail. Every media grade can impact our perception of image clarity.
Media is the differentiator in the process and media’s chemistry can affect the appearance as well as apparent resolution.
What do you think? Is a measure of apparent resolution meaningful to you independent of the media it is printed on? Does it matter to you? Let us know.
It would also be great if you would consider registering (free). Provide some basic information to help us make Inkjet Insight content more relevant. We’re listening and learning.