Introduction

When profiling medias one is often faced with challenges in print quality.  The following discussion is written with the intent of helping others to better understand the factors that are involved in print quality so that one can achieve the "best" results within the limitations imposed by the entire printing system.

Print Quality

Print quality is the result of something that is printed and can be assessed by comparing to a known original or reference print.  When evaluating print quality there are at lest three general aspects that are important to consider: color, print artifacts, and smoothness / clarity. 
The Color aspect of print quality is determined by the color gamut (or range of achievable colors), how accurate colors that are within the printable gamut can be duplicated, and how constant the colors appear when viewed by different observers or under different lighting conditions.

Print artifacts are the unintentional aspects of a printed image that are a direct result of the printing process.  Print artifacts are usually undesirable side effects of the processes involved in the reproduction of printed images.  Image reproduction technologies, materials and environment are the largest contributors to print artifacts.

Although smoothness and clarity could possibly be thought of as print artifacts they are usually desirable aspects of print quality.  Smoothness is often also called graininess which is a term that is a holdover from photographic reproduction systems which use grains of silver halide for image reproduction.  The possible sizes of the grains used vary and therefore result in different amounts of graininess.  In digital reproduction systems resolution and dot size are often thought of as the more significant aspects of smoothness and clarity.

The concept of acceptable print quality is subjective and is often based upon the purposes of the print. When evaluating and attempting to select or modify the print quality one should be aware that there are tradeoffs between the aspects of print quality.  The aspects of color, print artifacts, and smoothness/clarity can be thought of as three dimensions that define the print quality of a printing process.  Changing aspects of the printing process (to be discussed below) result in changes in print quality.  The range of print qualities possible can be considered as the print quality gamut of the set of modifiable aspects of the printing process.

In many cases the print quality gamut is limited.  Changing a single aspect of the printing process usually affects more than one aspect of print quality.  Thus the print quality gamut with have limited bounds relative to what can be changed in the printing process.  (For example: Changing maximum ink load can affect both color gamut and banding.  In this case the print quality gamut may not contain a point that has a large color gamut and low adverse print artifacts).

An additional consideration relative to the concept of a print quality gamut is that some modifiable aspects of the printing process are specific to performance and cost. Generally faster printing or using less expensive materials have an impact on print quality (often not an improvement in print quality), but they are desirable aspects of the total system.

Objective Comparison

When trying to make adjustments or determine how print process changes affect print quality it is desirable to try to assess differences between single aspects of print quality at a time.  Here are two recommendations that can help in making more objective print quality comparisons:

1.     For the most part print artifacts and smoothness differences become diminished with viewing distance, but color remains constant.  Because of this color comparisons can generally be made independent of print artifacts and smoothness\clarity by changing observer distance.

2.     Smoothness/clarity and print artifact issues are also usually a factor of color.  Different observed colors will exhibit different aspects of smoothness/clarity and print artifacts.  To ensure that only smoothness/clarity and print artifacts are being compared it is a good idea to first ensure that there is identical color between prints to make objective comparison of smoothness/clarity, and print artifacts.  If the color is not the same the measure of print quality difference becomes much less clear.

Printing System

The object of a printing system is to put colorant onto a media to form an image. Usually the image is formed on the media as a rectangular grid of dots (or pixels).  The resolution of this grid can be independent of the size of dots.  Placement of dots onto a media involves both physical processes (including mechanical, electrical, and chemical processes) and algorithmic processes.  Algorithmic processes rely on the consistency and predictability that the physical processes provide.  Print artifacts are usually the result of cases where the printing processes become inconsistent or unpredictable.  The level to which algorithmic processes can compensate for the inconsistent or unpredictable aspects of the printing processes is often limited.

Physical Processes

An image on a media represents a physical thing and therefore it requires physical processes to achieve.  The most significant aspect of the physical processes involves the physical placement of dots on a media. 

Two of the most common ways of placing dots on a media include: jetting ink onto/into the media, and applying an electrostatic charge to the media where colorant is attracted and fused onto the media.  In either case there are multiple aspects to the physical processes:

1.  Motion

There can be multiple vectors of motion involved in printing systems as follows:

a.       There is the motion of a carriage that carries a jetting mechanism (or print head).   

b.       There is the motion of air as it is pushed around by the carriage or other parts of the printing system. (Note: This can substantially affect the following vectors of motion).

c.       There is the motion of ink as it moves from a storage reservoir to the print head. 

d.       There is the motion of the ink as it jets from the print head towards the media. 

e.       There is motion of the ink comes in contact with the media.

f.        There is motion of the ink as it settles on/in media.

g.       There is motion of the media as it feeds through the printing mechanism.

Usually motion involves moving parts.  The precision to which parts are manufactured and the maintenance of those parts become significant concerns in getting consistent predictable results.  Defects in moving parts can lead to vibrations which may lead to disturbances in dot placement.  Defects in ink supply and/or jetting mechanisms can also lead to differences in ink drop formulation.

In many cases movement starts and stops.  Inertia and momentum become factors as movement changes.

Additionally, since multiple motions are involved, the accurate placement of dots requires the precise timing of events in the printing system.  This means that registration and alignment procedures may need to be in place, and maintenance needs to be performed as changes to the printing system occur. 

2.  Thermodynamics

Physical and chemical properties of matter are affected by heat.  Some of these properties include physical size, viscosity, friction, reaction time, and drying time.  Therefore it is important that the temperature of the operating environment be stable because the behavior or the printing system can change as the environment changes over time. Additionally, heat can be generated by the printing system can which can change the operating environment.

 In other words - the predictability in the operating environment can be very important to the predictability of the printing system.

The temperature within a printing system can be affected by some of the following:

a.      Heat generated within a print head in an jetted ink system.  Heat can either be generated by heating elements that rapidly expand the ink as a gas, or it can be generated by the flexing of channels within the head to jet the ink. As more or fewer dots are placed by individual nozzles in the head the temperature within the print head can change. This can affect how future dots are formed by the nozzles.

b.     As parts move they undergo friction with leads to heat.  As they change temperature the coefficient of friction can also change which then lead to a can change the way that they move.  Additionally changes in a parts temperature can also change its size which also can affect how it moves.

c.      As electricity is used by motors and electrical components they release heat.  Changes in temperature can change their efficiency and/or electrical properties thus changing how they behave.

d.     In some cases heating elements and controls are applied directly to elements of the printing system to intentionally affect physical and chemical properties of the media and/or ink like absorption or drying.

The accurate and consistent control of heat is important since changes in temperature can lead to inconsistent and/or undesirable behavior of a printing system.

1. Physical state changes (gas, liquid, solid)
2. Absorption / adhesion
3. Chemical interactions between colorants and/or media
4. Static buildup and electrostatic forces
5. Light modifying properties of materials
   • Spectral reflectance
   • Fluorescence
   • Gloss
6. Stability of materials
   • Structural stability
   • Chemical stability

Algorithmic Processes

A requirement of the printing system is that dots or pixels are placed onto a media.  Physical processes provide the mechanism for doing this.  Algorithmic processes provide the decision making involved I determining where and what kid of dots to place on the media.

• Algorithmic Processes
  • Modeling of physical processes
  • Modeling of observer
  • Colorant usage/separation
  • Relative placement of colorants
  • Order of placement

Controlling Print Quality

"Knobs" that can affect Print Quality 

    1.  Physical System Maintenance

• Parts functioning correctly and properly aligned

    2.  Print Settings

• Print head
  • Speed
  • Direction
  • Order of colorant application
• Media advance
  • Number of passes
  • Interleave controls
• Temperature control

     3. Profiling choices

• Resolution (addressability of dots)
• Ink/Colorant primaries use
• Limits on Ink/Colorant usage
• How and where to place dots (Halftoning algorithm)
• Amount of colorant per location (example dot size)
• Ink usage calibration
• Select amounts of colorants to reproduce each achievable color
  1. Black Generation
  2. Effective use of colorants
  3. Eliminate redundancies
• Selection of alternates for colors that are not achievable (Gamut Mapping)

Print Quality Issues

1. Color

• Gamut
• Accuracy
• Constancy

2. Print Artifacts

• Media/Ink interaction
  • Cockling of media
  • Head Strikes
  • Poor absorption of ink in media
    • Haziness
    • Ink Bleeding
    • Blooming of drops
  • Dark drops placed in a pool of lighter ink
  • Coalescing of ink
  • Cracking of ink
• Banding
  • Feed Alignment
  • Nozzle Failure
  • Directional
  • Drop consistency
    • Placement registration
    • Dot shape
  • Ink Absorption/Dry Time
  • Colorant Adhesion/Layering
  • Pattern repetition

3. Smoothness/Clarity

• Resolution
• Graininess/Mottling

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