Digital Image Segmentation and On–line Print Quality Diagnostics

During the electrophotographic (EP) process for a modern laser printer, object-oriented halftoning is sometimes used which renders an input raster page with different halftone screen frequencies according to an object map; this approach can reduce the print artifacts for the smooth areas as well as preserve the fine details of a page. Object map can be directly extracted from the page description language (PDL), but most of the time, it is not correctly generated. For the first part of this thesis, we introduce a new object generation algorithm that generates an object map from scratch purely based on a raster image. The algorithm is intended for ASIC application. To achieve hardware friendliness and memory efficiency, the algorithm only buffers two strips of an image at a time for processing. A novel two-pass connected component algorithm is designed that runs through all the pixels in raster order, collect features and classify components on the fly, and recycle unused components to save memories for future strips. The algorithm is finally implemented as a C program. For 10 test pages, with the similar quality of object maps generated, the number of connected components used can be reduced by over 97% on average compared to the classic two-pass connected component which buffers a whole page of pixels. The novelty of the connected component algorithm used here for document segmentation can also be potentially used for wide variety of other applications. The second part of the thesis proposes a new way to diagnose print quality. Compared to the traditional diagnostics of print quality which prints a specially designed test page to be examined by an expert or against a user manual, our proposed system could automatically diagnose a customer’s printer without any human interference. The system relies on scanning printouts from user’s printer. Print defects such as banding, streaking, etc. will be reflected on its scanned page and can be captured by comparing to its master image; the master image is the digitally generated original from which the page is printed. Once the print quality drops below a specified acceptance criteria level, the system can notify a user of the presence of print quality issues. Among so many print defects, color fading – caused by the low toner in the cartridge – is the focus of this work. Our image processing pipeline first uses a feature based image registration algorithm to align the scanned page with the master page spatially and then calculates the color difference of different color clusters between the scanned page and the master page. At last, it will predict which cartridge is depleted

An Analysis of how altering exposure effects color differences in critical color areas on electrophotographic off-press proofs

The purpose of this paper was to quantitatively analyze off-press color proofing methods for advertising material appearing in halftone publication gravure printing. This type of proofing has been an area of debate for the publications industry, especially in reference to the print quality of advertising material. The correlation between dot gain characteristics and the resulting color differences of off-press color proofing methods has been the center of greatest controversy. This paper has tested to determine if a correlation exists between dot gain characteristics and the resulting colorimetric-matching ability for color proofing methods used in halftone publication gravure. The variable dot-gain feature of an electrophotographic proofing system was tested to determine if this feature improved the color matching ability of the off-press proof to the SWOP offset press proof. This system was analyzed by its\u27 ability to objectively match the SWOP offset press proof for dot gain aimpoints. These aimpoints are used as the quality control tool for advertising material appearing in both halftone gravure and web offset publications printing. The experiment concluded that the dot gain characteristic curve of the electrophotographic off-press proof could be altered to quantitatively match closer to the dot gain of an offset press proof printed to SWOP/GAA Specifications. However, the data did not completely support the authors\u27 hypothesis. A direct correlation between dot gain and the resulting delta E values could not be established. None of the data collected from the electrophotographic test sets completely proved that a change in dot gain produced a closer colorimetric match to the SWOP/GAA Offset Press Proof

Development of a Acrylonitrile Butadiene Styrene toner for an electrophotography based additive manufacturing process

Research in to utilising electrophotography for Additive Manufacturing has been under investigation for over a decade. However, the research has been primarily focused on resolving the height limitations caused by charge accumulation and also the development of toners from tough engineering polymers. The aim of this study is to develop a suitable negatively charging engineering polymer for the Selective Laser Printing process and a more efficient method of generating toners with a suitable particle range and to assess the mechanical properties of parts produced using the toner developed through this route. The study is comprised for two parts; Chemical and Mechanical toner production. The chemical production method utilises a process, Evaporative Limited Coalescence, to produce an engineering toner in the desired 20-50μm range. It was found that during the chemical production process the negatively charging polymer was converted to charge positive. Analytical assessment of the toner was carried out to explain the change in polarity. Mechanical milling trials were also conducted under both ambient and cryogenic conditions, as well as a novel method to reduce the feed-stock material to the desired particle size range. The results suggest a significant increase in efficiency compared to prior research. Printing trials were conducted using the mechanically milled toner to assess the mechanical properties for different heating, pressure and stand-off distance conditions. It was found that, due to insufficient heating and applied pressure the final samples suffered from high porosity and poor tensile strength

Subjective Image Quality Assessment of Digitally Printed Images

Smartphones have become ingrained in our daily activities, driving their cameras to become better with every generation. As more and more images are being taken by cell phones it has become increasingly important to assess the quality of the images taken by different phones. While many cell phone images are only viewed electronically, many images also get transformed into printed images, especially photo-books, as digital printing gets better and cheaper compared to traditional printing processes. The gap between electronic image and printed image in shrinking rapidly and it becomes important to study the transition of images from screen to paper. The main goal of this research was to perform a rank order experiment for assessing cell phone image capture quality that translates to printed images via several different digital printers. It was of interest to investigate whether the overall image quality on displays correlates well with printed image quality. The important aspect was to study was to observe if there is a loss of image quality due to different digital printers

A Survey of digital and offset print quality issues

Using an Internet-based industry survey, the differences and similarities of offset and digital print demerits within the print industry are examined. Running parallel to this examination is a look at the extent to which demerit-based quality assurance procedures exist within the printing industry. The survey findings indicate that offset and digital printing processes share common color-related print demerits. What differentiates offset print demerits from digital print demerits can be attributed to two factors: (1) technology differences and (2) printing standards. Offset printing, given the multitude of material choices, produces more frequent print demerits, relating to non-conforming materials. Digital print demerits more likely result from technical constraints such as addressability and resolution. The other factor associated is a lack of printing standards for digital printers. While both processes show a high frequency of color-related problems, offset print providers have more tools and standards available for addressing these problems. Digital print providers, on the other hand, have few color standards or tools for solving similar problems. When examining procedures for defect prevention, the general lack of digital printing standards makes the use of formal quality assurance procedures difficult. Offset print providers are more likely to have formal quality assurance procedures for dealing with frequent and severe print demerits and formal procedures for how customer quality requirements are communicated

Laser scanner jitter characterization, page content analysis for optimal rendering, and understanding image graininess

In Chapter 1, the electrophotographic (EP) process is widely used in imaging systems such as laser printers and office copiers. In the EP process, laser scanner jitter is a common artifact that mainly appears along the scan direction due to the condition of polygon facets. Prior studies have not focused on the periodic characteristic of laser scanner jitter in terms of the modeling and analysis. This chapter addresses the periodic characteristic of laser scanner jitter in the mathematical model. In the Fourier domain, we derive an analytic expression for laser scanner jitter in general, and extend the expression assuming a sinusoidal displacement. This leads to a simple closed-form expression in terms of Bessel functions of the first kind. We further examine the relationship between the continuous-space halftone image and the periodic laser scanner jitter. The simulation results show that our proposed mathematical model predicts the phenomenon of laser scanner jitter effectively, when compared to the characterization using a test pattern, which consists of a flat field with 25% dot coverage However, there is some mismatches between the analytical spectrum and spectrum of the processed scanned test target. We improve experimental results by directly estimating the displacement instead of assuming a sinusoidal displacement. This gives a better prediction of the phenomenon of laser scanner jitter. ^ In Chapter 2, we describe a segmentation-based object map correction algorithm, which can be integrated in a new imaging pipeline for laser electrophotographic (EP) printers. This new imaging pipeline incorporates the idea of object-oriented halftoning, which applies different halftone screens to different regions of the page, to improve the overall print quality. In particular, smooth areas are halftoned with a low-frequency screen to provide more stable printing; whereas detail areas are halftoned with a high-frequency screen, since this will better reproduce the object detail. In this case, the object detail also serves to mask any print defects that arise from the use of a high frequency screen. These regions are defined by the initial object map, which is translated from the page description language (PDL). However, the information of object type obtained from the PDL may be incorrect. Some smooth areas may be labeled as raster causing them to be halftoned with a high frequency screen, rather than being labeled as vector, which would result in them being rendered with a low frequency screen. To correct the misclassification, we propose an object map correction algorithm that combines information from the incorrect object map with information obtained by segmentation of the continuous-tone RGB rasterized page image. Finally, the rendered image can be halftoned by the object-oriented halftoning approach, based on the corrected object map. Preliminary experimental results indicate the benefits of our algorithm combined with the new imaging pipeline, in terms of correction of misclassification errors. ^ In Chapter 3, we describe a study to understand image graininess. With the emergence of the high-end digital printing technologies, it is of interest to analyze the nature and causes of image graininess in order to understand the factors that prevent high-end digital presses from achieving the same print quality as commercial offset presses. We want to understand how image graininess relates to the halftoning technology and marking technology. This chapter provides three different approaches to understand image graininess. First, we perform a Fourier-based analysis of regular and irregular periodic, clustered-dot halftone textures. With high-end digital printing technology, irregular screens can be considered since they can achieve a better approximation to the screen sets used for commercial offset presses. This is due to the fact that the elements of the periodicity matrix of an irregular screen are rational numbers, rather than integers, which would be the case for a regular screen. From the analytical results, we show that irregular halftone textures generate new frequency components near the spectrum origin; and these frequency components are low enough to be visible to the human viewer. However, regular halftone textures do not have these frequency components. In addition, we provide a metric to measure the nonuniformity of a given halftone texture. The metric indicates that the nonuniformity of irregular halftone textures is higher than the nonuniformity of regular halftone textures. Furthermore, a method to visualize the nonuniformity of given halftone textures is described. The analysis shows that irregular halftone textures are grainier than regular halftone textures. Second, we analyze the regular and irregular periodic, clustered-dot halftone textures by calculating three spatial statistics. First, the disparity between lattice points generated by the periodicity matrix, and centroids of dot clusters are considered. Next, the area of dot clusters in regular and irregular halftone textures is considered. Third, the compactness of dot clusters in the regular and irregular halftone textures is calculated. The disparity of between centroids of irregular dot clusters and lattices points generated by the irregular screen is larger than the disparity of between centroids of regular dot clusters and lattices points generated by the regular screen. Irregular halftone textures have higher variance in the histogram of dot-cluster area. In addition, the compactness measurement shows that irregular dot clusters are less compact than regular dot clusters. But, a clustered-dot halftone algorithm wants to produce clustered-dot as compact as possible. Lastly, we exam the current marking technology by printing the same halftone pattern on different substrates, glossy and polyester media. The experimental results show that the current marking technology provides better print quality on glossy media than on polyester media. With above three different approaches, we conclude that the current halftoning technology introduces image graininess in the spatial domain because of the non-integer elements in the periodicity matrix of the irregular screen and the finite addressability of the marking engine. In addition, the geometric characteristics of irregular dot clusters is more irregular than the geometric characteristics of regular dot clusters. Finally, the marking technology provides inconsistency of print quality between substrates

Test Targets 6.0: A Collaborative effort exploring the use of scientific methods for color imaging and process control

Test Targets is a collection of scholarly papers contributed by faculty, students, and alumni of Rochester Institute of Technology. We realize the importance of having faculty set examples as authors for students to follow. We have a three-course sequence over a time span of a year to prepare students to publish their first articles when completing Tone and Color Analysis, Printing Process Control, and Advanced Color Management. In this instance, Test Targets 6.0 is a part of the course content in the Advanced Color Management course

Electronic transport properties of stabilized amorphous selenium x-ray photoconductors

Amorphous selenium (a-Se) and its alloys are important photoconductor materials used in direct conversion flat panel digital x-ray detectors. The performance of these detectors is determined, in part, by the electronic transport properties of the a-Se photoconductor layer – namely, the charge carrier mobility m and the deep trapping lifetime t. The product of the mobility and the lifetime mt, referred to as the charge carrier range, determines the average distance that photo-generated charge will travel before being removed from the transport band by deep localized states in the mobility gap of the semiconductor. The loss of carriers to these deep states reduces the amount of charge collected per unit of x-ray exposure, and, hence, limits the x-ray sensitivity of the detector. Two experimental techniques that may be used to measure the transport properties of holes and electrons in high resistivity semiconductors are described in this thesis. The Time-of-Flight (TOF) transient photoconductivity technique is used to evaluate the charge carrier mobility by measuring the time required for the charge carriers to transit a fixed distance under the influence of an applied electric field. The Interrupted-Field Time-of-Flight (IFTOF) technique is used to determine the charge carrier deep trapping time; the drift of the injected carriers is temporarily interrupted at a position in the sample by removing the applied field. When the field is reapplied the number of charge carriers has decreased due to trapping events. The carrier lifetime is determined from the dependence of the fraction of recovered charge carriers before and after the interruption with the interruption time. TOF and IFTOF measurements were carried out on a number of samples of vacuum deposited selenium alloy x-ray photoconductors. Device quality photoconductor films are fabricated by evaporating a-Se source material that has been alloyed with a small quantitiy of As (~0.3 at. %) and doped with a halogen (typically Cl) in the p.p.m. range. The dependence of the carrier range on the composition of the photoreceptor film was accurately measured using both TOF and IFTOF measurements. It was found that the transport properties of the film could be controlled by suitably adjusting the composition of the alloy. Combined IFTOF and TOF measurements were also performed on several samples to examine the effects of trapped electrons on the hole transport properties in a-Se films. It was found that drifting holes recombine with the trapped electrons, and that this process could be described by a Langevin recombination process. This finding is important for the correct modeling of amorphous selenium digital x-ray detector designs. Finally, the effects of x-ray exposure on a-Se films were examined. A temporary reduction in the effective hole lifetime was observed due to an increase in the number of hole capture centers following an x-ray exposure. The capture coefficient between free holes and the x-ray induced hole capture centers was measured using combined TOF and IFTOF measurements. It was shown that this capture process was governed by the Langevin recombination mechanism. From these observations it was concluded that trapped electrons from a previous x-ray exposure act as recombination centers for subsequently generated holes, thereby reducing the effective hole lifetime in the sample

Monte Carlo modeling of the sensitivity of x-ray photoconductors

The sensitivity reduction or ghosting mechanism of x-ray photoconductor is studied based on Monte Carlo simulation techniques. We have calculated the sensitivity reduction for different detector operating conditions (applied electric field, x-ray spectrum and photoconductor thickness) and for different levels of carrier trapping. We have analyzed the effect of photoconductor biasing (positive or negative) on ghosting. The following effects are taken into account in modeling the ghosting phenomena: (i) recombination between trapped and oppositely charged drifting carriers, (ii) trap filling, (iii) nonuniform electric field, (iv) detrapping of trapped holes, and (v) x-ray induced trap generation. Our calculation shows that not only the recombination between trapped and oppositely charged drifting carriers but the x-ray induced trap generation is also responsible for ghosting in photoconductor based x-ray image detectors. Moreover not all the trapped carriers take part in recombination; rather only a fraction of the trapped carriers are involved in recombination. Electric field also plays an important role in ghosting calculations via the electron hole pair generation mechanism. Trap filling has also non trivial effects on ghosting. The simulation results show that the amount of ghosting strongly depends on the applied electric field. Ghosting increases with decreasing applied electric field and vice versa. It is observed that ghosting is higher at high carrier trapping level than at low trapping level. Again ghosting is more pronounced in chest radiographic detector than mammographic detector. In chest radiographic detector, carrier trapping is high due to greater thickness hence recombination and electric field effects are prominent in chest radiographic detector. Biasing dependent ghosting depends on the carrier mobility lifetime product. For positively biased detectors, ghosting is less if the mobility lifetime product of hole is higher than that of electron and vice versa for negatively biased detectors. It also appears that the use of only recombination to calculate ghosting, as believed the primary source of ghosting in some literatures, will lead to significant error in the calculation of ghosting

X-RAY DOSE DEPENDENCE OF DARK CURRENT IN AMORPHOUS SELENIUM-ALLOY X-RAY PHOTOCONDUCTORS

The dark current is an important characteristic of a photoconductive X-ray detector, and can impact the dynamic range of the detector and its detective quantum efficiency. It is therefore essential that the dark current and its behavior with time and x-ray irradiation are well characterized and understood in amorphous selenium (a-Se) X-ray detectors for the future enhancement of these detectors. Throughout the course of this work, the dark current in practical a-Se multilayer photoconductors were studied as function of time and x-ray dose delivered to the detector material. The dark current in these multilayer structures has been measured as a function of different rest time periods, sample structure, single X-ray irradiation on the sample and multiple irradiation on the sample. Experiments were performed by resting the sample in dark for a period of time (24 hours) and then samples were exposed to X-ray radiation. It has been observed that most of the trapped charge carriers in the bulk of the material are discharged after resting the sample in dark for 24 hours. It was observed that multilayer sample structures p-i-n and n-i-p exhibit much less dark current compared to other samples with single layer and double layer structures, that is, i-layer only, n-i and p-i structures. The experiments support that the dark current is controlled by injection of charge carriers from contacts. Single X-ray irradiation and multiple irradiation experiments were performed on multilayer a-Se photoconductors at a dose rate of 0.51 Gy s-1 with an exposure duration of 3 s. Samples were exposed to single irradiation at 100 s and 400 s. The dark current following the photocurrent was recorded. Multiple irradiation experiments were also performed on these multilayer samples. With different reverse bias voltages, samples were irradiated 10 times from 200s to 2000s. It was found that the dark current tends to increase with repeated X-ray irradiation but the increase depends on the applied reverse bias; the increase is negligible at a field of 10 V μm-1. After the cessation of the irradiation, the dark current decays and tends to reach a steady state value at t = 4000s. After 24 hr of resting in the dark, the dark current was nearly as low as the original dark current before the X-ray irradiatio