<p>What is meant with print quality, what makes people perceive the quality of an image in a certain way? An inquiry was made about what the parameters are that strongly affect the perception of digital printed images. </p><p>A subjective test and some measurements make the basis for the thesis. The goal was to find a tool to predict perceived image quality when investigating the connections between the subjective test and the measurements. </p><p>Some suitable images were chosen, with a variety of motifs. A test panel consisting of people that are used to observe image quality answered questions about the perception of the quality. Measurements were made on a special test form to get information about the six different printers used in the investigation. </p><p>One of the discoveries was made when two images with the same colorful motif were compared. The first image got a much higher grade for general quality than the second image, even though the second image was printed with a printer that had a larger color gamut. The reason of this is that the first image consists of more saturated colors, and the second image has more details. The human eye perceives the more saturated image to be better than the image with more details. Another discovery was the correlation between the perceived general quality of a colored image and the perceived color gamut. One conclusion was that a great difference between two calculated color gamuts resulted in a large difference in perception of the color gamuts. A discovery of an image with very few colors and many glossy surfaces was that print mottle and sharpness are strictly connected to the general quality.</p>

The objective of this thesis is to better understand the impact of paper and printer types on xerographic print quality. To achieve this objective, commercially printed samples comprising of ten different paper substrates printed using three different xerographic printers were examined. The print quality of these samples was assessed in terms of print microgloss and its nonuniformity, print density, print and gloss mottle, print roughness, and visual ranking. This study showed that print mottle conducted by Fast Fourier Transform produced the best correlation with visual ranking at the size range of 0.1 - 1mm, while print gloss mottle was found to affect print quality regardless of the mottle size. Brightness, opacity, basis weight, gloss 75, and roughness of these paper substrates were found to have the most significant effect on print quality. All of the optical properties of paper included in this analysis showed a strong correlation to print quality.

The objective of this thesis is to better understand the impact of paper and printer types on xerographic print quality. To achieve this objective, commercially printed samples comprising of ten different paper substrates printed using three different xerographic printers were examined. The print quality of these samples was assessed in terms of print microgloss and its nonuniformity, print density, print and gloss mottle, print roughness, and visual ranking. This study showed that print mottle conducted by Fast Fourier Transform produced the best correlation with visual ranking at the size range of 0.1 - 1mm, while print gloss mottle was found to affect print quality regardless of the mottle size. Brightness, opacity, basis weight, gloss 75, and roughness of these paper substrates were found to have the most significant effect on print quality. All of the optical properties of paper included in this analysis showed a strong correlation to print quality.

There are high demands on flexographic print quality to be sufficiently high and consistent in order to create a competitive packaging. At the same time the production efficiency need to be high. Printers thus need to achieve the same quality every time and quickly start-up new printing jobs. To accomplish this, one needs to gain a thorough understanding of how the liquid packaging board interacts with the ink and impacts the print quality. This thesis focuses on water-based ink absorption of liquid packaging boards and particularly on a) how uniformity of ink absorption can be measured and b) to what extent the absorption characteristics contribute to print mottle in flexographic printing. The work encompasses two parts. First, an absorption non-uniformity test method has been developed using a staining technique. This method is unique as it measures how unevenly an aqueous solution is absorbed, in a short time period and without impact from surface roughness. Moreover, the contributions from white-top mottle and absorption non-uniformity can be quantified simultaneously from one single measurement. Second, a method to independently study the effects of absorption non-uniformity on print quality has been established. This is achieved by introducing artificial absorption non-uniformities with well-controlled barrier patterns. A barrier pattern may modify local pore structure and/or surface energy, hence lead to lateral absorption variations. By these means, it is possible to produce a substrate property-matrix; encompassing absorption non-uniformity and for example surface roughness. It was demonstrated that non-uniform absorption indeed has a negative impact on print quality, both on smoother and rougher boards. Low absorption made print density decrease and uneven absorption caused print mottle. This was the case when other properties of the samples were kept within a narrow range; otherwise surface roughness appeared to determine print mottle ranking.

What is meant with print quality, what makes people perceive the quality of an image in a certain way? An inquiry was made about what the parameters are that strongly affect the perception of digital printed images. A subjective test and some measurements make the basis for the thesis. The goal was to find a tool to predict perceived image quality when investigating the connections between the subjective test and the measurements. Some suitable images were chosen, with a variety of motifs. A test panel consisting of people that are used to observe image quality answered questions about the perception of the quality. Measurements were made on a special test form to get information about the six different printers used in the investigation. One of the discoveries was made when two images with the same colorful motif were compared. The first image got a much higher grade for general quality than the second image, even though the second image was printed with a printer that had a larger color gamut. The reason of this is that the first image consists of more saturated colors, and the second image has more details. The human eye perceives the more saturated image to be better than the image with more details. Another discovery was the correlation between the perceived general quality of a colored image and the perceived color gamut. One conclusion was that a great difference between two calculated color gamuts resulted in a large difference in perception of the color gamuts. A discovery of an image with very few colors and many glossy surfaces was that print mottle and sharpness are strictly connected to the general quality.

Tryckflammighet, ojämnheter på tryckta ytor är en icke önskvärd egenskap som länge bekymrat tryckeribranschen. Ojämnheterna har medfört en försämrad kvalité på kartongmaterial som visuellt får materialet att se fläckig och sämre ut. Egenskapen har visat sig vara mer tydlig på ytor som tryckts med färgen cyan, vanligt förekommande i metoderna flexotryck och offsettryck. Detta examensarbete har gjorts hos ett företag inom tryckeribranschen. Företaget har använt två metoder för bedömning av tryckflammighet på tryckta vätske- och förpackningskartong, en Parvis visuell bedömning och en instrumentell mätning med en skanner och programvaran STFI Mottling Expert. Programvaran har utvecklats med syftet att på ett identiskt sätt bedöma bland annat tryckflammighet på kartongmaterialytor så som den upplevs på den visuella bedömningen. Under en lång period har företaget använt instrumentet för att verifiera resultat som erhålls från den visuella metoden. Under de senaste åren har korrelationen mellan dessa metoder blivit sämre, till en punkt där företaget inte litar på instrumentet. Korrelationsanalys har därför utförts för att undersöka sambandet mellan dessa två metoder och orsaken till problemet. I samband med detta har företaget upptäckt en möjlighet, att undersöka topografiska egenskaper på otryckt material från samma produktionstillfälle och dess korrelation med tryckflammigheten. Topografiska värden har därför tagits fram via instrumentet L&amp;W OptiTopo samt programvaran RISE OptiTopo Expert och korrelationsanalys utförts. Resultaten indikerar en fortfarande väldigt hög korrelation mellan de två metoder som används för bedömning av tryckflammighet, dock inom andra våglängder än den rekommenderad av programvaran och orsakerna till tidigare problem har lagts fram. Topografiska värden som ytråhet samt andelen kratrar under en viss tröskel har kunnat kopplas till tryckflammighetsvärdet och sambandet visats. / Print mottle, unevenness on printed surfaces is an undesirable property that has long worried the printing industry. The effect has led to a degraded quality in cartonboard which visually makes the material look stained and worse. The property has been shown to be more evident on surfaces printed with the color cyan, commonly in printing methods such as flexography and offset. This project degree has been conducted at a company in the printing industry. The company has used two methods for assessing print mottle on printed liquid- and packaging board, a Pairwise visual assessment and an instrumental measurement with a scanner and software STFI Mottling Expert. The software has been developed with the purpose of identifying among others print mottle on cartonboard surfaces in an identical way as the visual assessment. For a long period of time, the company has used the instrument to verify the results from the visual assessment. In recent years, the correlation between these methods has gotten worse, to a point where company does not trust the instrument. Correlation analysis has therefore been performed to investigate the relationship between these two methods and the cause of the problem. In connection with this, the company has discovered an opportunity, to investigate topographical properties on unprinted materials from the same production line and its correlation with print mottle. Topographical values have therefore been determined using the L&amp;W OptiTopo instrument and RISE OptiTopo Expert software. The results indicate that there is still a very high correlation between the two methods used to assess print mottle, though within wavelengths other than the one recommended by the software and the causes of previous problems have been presented. Topographical values such as surface roughness and the proportion of craters under a certain threshold have been linked to the print mottle value and the correlation between them shown.

The aim of this work was to develop a solid knowledge on formulation effects controlling offset ink-paper coating adhesion and to identify key factors of the coating and printing process affecting it. Focus lay on comprehending the impact of pigment dispersant on ink-paper coating adhesion and ultimately on the print quality of offset prints. The work covers laboratory studies, a pilot coating trial designed to produce coated material with a span in surface chemistry and structure, and an industrial offset printing trial. The lab scale studies quantified ink-paper coating adhesion failure during ink setting with a developed laboratory procedure based on the Ink-Surface Interaction Tester (ISIT) and image analysis. Additional polyacrylate dispersant resulted in slower ink setting and reduced ink-paper coating adhesion, with a dependence on its state of salt neutralisation and cation exchange, mainly in the presence of moisture/liquid water. The industrial printing trial on pilot coated papers was designed to study how these laboratory findings affected full scale offset print quality. These trials confirmed the dispersant-sensitive effect on ink-paper coating adhesion, especially at high water feeds. Evaluation of prints from the printing trial resulted in two fundamentally different types of ink adhesion failure being identified. The first type being traditional ink refusal, and the second type being a novel mechanism referred to as ink-lift-off adhesion failure. Ink-lift-off adhesion failure occurs when ink is initially deposited on the paper but then lifted off in a subsequent print unit. In this work, ink adhesion failure by this ink-lift-off mechanism was observed to occur more often than failure due to ink refusal. Print quality evaluation of the industrial prints suggested that water induced mottle was caused by a combination of ink-surface adhesion failure, creating white spots on the print, together with variation in ink layer thickness due to emulsified ink. / <p>QC 20161019</p>

<p>Print mottle is a problem that has been hassling the printing business for a long time. Along with sharpness and correct colour reproduction, absence of print mottle is one of the most important factors of print quality. The possibility to measure the amount of print mottle (reflectance variation) may in many ways facilitate the development of printing methods. Such a measurement model should preferably follow the functions and abilities of the Human Visual System (HVS). </p><p>The traditional model that STFI-Packforsk has developed to measure print mottle uses frequency analysis to find variations in reflectance. However, this model suffers some limitations since is does not perfectly agree with the functions of the HVS and does only measure variations in lightness. A new model that better follows the functions of the HVS has thus been developed. The new model does not only consider variations in lightness (monochromatic) but also variations in colour (chromatic). The new model also puts a higher weight on systematic variations than on random variations since the human eye is more sensitive to ordered structures. Furthermore, the new model uses a contrast sensitivity function that weights the importance of variations in different frequencies. </p><p>To compare the new model with the traditional STFI model, two tests were carried out. Each test consisted of a group of test patches that were evaluated by the traditional STFI model and the new model. The first test consisted of 15 greyscale test patches that originated from conventional flexo and offset presses. The second test consisted of 24 digitally simulated test patches containing colour mottle and systematic mottle. </p><p>The evaluation results in both the traditional and the new model were compared to the results of a visual evaluation carried out using a panel of test persons. The new model produced a result that correlated considerably better with the visual estimation than what the traditional model did.</p>

Print mottle is a problem that has been hassling the printing business for a long time. Along with sharpness and correct colour reproduction, absence of print mottle is one of the most important factors of print quality. The possibility to measure the amount of print mottle (reflectance variation) may in many ways facilitate the development of printing methods. Such a measurement model should preferably follow the functions and abilities of the Human Visual System (HVS). The traditional model that STFI-Packforsk has developed to measure print mottle uses frequency analysis to find variations in reflectance. However, this model suffers some limitations since is does not perfectly agree with the functions of the HVS and does only measure variations in lightness. A new model that better follows the functions of the HVS has thus been developed. The new model does not only consider variations in lightness (monochromatic) but also variations in colour (chromatic). The new model also puts a higher weight on systematic variations than on random variations since the human eye is more sensitive to ordered structures. Furthermore, the new model uses a contrast sensitivity function that weights the importance of variations in different frequencies. To compare the new model with the traditional STFI model, two tests were carried out. Each test consisted of a group of test patches that were evaluated by the traditional STFI model and the new model. The first test consisted of 15 greyscale test patches that originated from conventional flexo and offset presses. The second test consisted of 24 digitally simulated test patches containing colour mottle and systematic mottle. The evaluation results in both the traditional and the new model were compared to the results of a visual evaluation carried out using a panel of test persons. The new model produced a result that correlated considerably better with the visual estimation than what the traditional model did.

Starch in paper coatings is known to increase the risk of print mottle in lithographic offset printing. The objective of this study was to increase the understanding of this behaviour. Four phenomena that could lead to print mottle, where the presence of starch might be important, have been identified: uneven binder migration, uneven coating mass distribution, uneven deformation during calendering and differential shrinkage. The latter three were investigated in this project. Starch-containing coating colours often have high water retention. A relationship between the water retention of the coating colours and the distribution of coating thickness was found in a pilot trial. A theory is proposed, where the surface profile of the base paper beneath the blade, that governs the coat weight distribution in blade coating, is affected by moisture from the dewatering coating colours and the compressive force exerted by the blade. Drying strategies were studied to see whether they would induce porosity variations in the coating layers. There is a strong connection between the rate of evaporation and the shrinkage of the coating layer, but no porosity variations due to the choice of drying strategy were found. Shrinkage is governed by the capillary forces. At the same capillary pressure, the coating shrinks more for some binder systems, which is suggested to be due to a weaker chemical interaction between the binder and the pigment. Oxidized starch/latex coatings, stained with a fluorescent marker, had a greater standard deviation in fluorescence intensity than CMC/latex coatings caused by a difference in either porosity or latex distribution. It was shown that calendering introduces porosity variations into the coating layer that are larger for starch-containing coatings. The drying strategies appeared to have a significant effect on these porosity variations and they correlated positively with print mottle in some cases and in another case negatively. In the case of the negatively correlated, the mottle was probably caused by variations in surface porosity existing prior to the calendering.