Sensor interpixel correlation (IC), as caused by crosstalk, interpixel capacitance, the brighter–fatter effect, blooming or smear, lowers the sensor MTF and increases color distortion in color filter array sensors. This is generally overlooked, and challenging to mitigate. Recent findings regarding scientific grade (CCD or hybrid CMOS) sensors, describe non-linearities causing pixel charge accumulation dependent IC. This is relevant even for lower grade sensors. High dynamic range imaging (HDRI) is especially susceptible to IC, because the sensor is deliberately operated partly in saturation. Existing HDRI algorithms exclude saturated pixels but generally overlook IC. This review article motivates the need for a generalization of the point spread function (PSF) into an irradiance-exposure dependent (IED) PSF, also considering supersaturation. The IED PSF is challenging to characterize and apply, partly due to its non-linearity. However, doing so can improve the MTF irrespective of image sensor technology, and for conventional as well as HDRI imaging.

High dynamic range imaging (HDRI) by bracketing of low dynamic range (LDR) images is demanding, as the sensor is deliberately operated at saturation. This exacerbates any crosstalk, interpixel capacitance, blooming and smear, all causing interpixel correlations (IC) and a deteriorated modulation transfer function (MTF). Established HDRI algorithms exclude saturated pixels, but generally overlook IC. This work presents a calibration method to estimate the affected region from saturated pixels for a color filter array (CFA) sensor, using the native CFA as a matched filter. The method minimizes color crosstalk given a set of candidates for proximity regions, and requires no special setup. Results are shown for a 21-bit HDR output image with improved color fidelity and reduced noise. The calibration reduces IC in the LDR images and is performed only once for a given sensor. The improvement is applicable to any HDRI algorithm based on CFA image bracketing, irrespective of sensor technology. Generalizations to subsaturated and supersaturated pixels are described, facilitating a suggested irradiance-exposure dependent point spread function charge repatriation strategy.

Documents of high value, such as passports, tickets and banknotes, facilitate means for authentication. Authentication processes aim at mitigating counterfeit “passable products”. The arsenal of “security features” in the business is abundant but an effective and reliable counterfeit mitigating system need an architectural approach rather than either relying on one feature only, or vaguely motivated aggregated security features.

Optically variable device (OVD) is a concept in the industry, including costefficient and unique authentication functionality. OVD based features may serve as the main counterfeit mitigating functionality, as in banknotes. For higher value documents, such as passports, security architectural design may include multimodal (combined) features in which OVD is one characterizing and necessary aspect. Thereby a successful counterfeit need not only to simulate (“hack”) electronic based security features, such as radio frequency based identifier combined with public key infrastructure based cryptography (PKI) but also simulate OVD functionality. Combined feature authentication, based e.g. on PKI and OVD that relies on principally different physics and hence technology competences is of especial interest. Well-architectured and implemented, such multimodal counterfeit mitigating systems are effective to the degree that producing passable products requiring more resources than potentially illegitimately gained by the counterfeiter. Irrespective of level of ambition and efforts spent on counterfeit mitigation, OVD remains critically important as a security concept. One feature of OVD is the possibility to include a human inspector in the authentication procedure. Including such “man-in-the-loop” reduces the risk of successful and unnoticed simulations of algorithms, such as PKI. One challenge of OVD is a lack of standards or even measurements characterizing the significant aspects influencing a human based inspection.

This thesis introduces a system able to measure, characterize and visualize the significant aspects influencing a human based inspection of OVD features. The contribution includes the development of a multidimensional and high-dynamic range (HDR) color measurement system of spatial and angular resolution. The capturing of HDR images is particularly demanding for certain high contrast OVD features and require innovative algorithms to achieve the necessary high contrast sensitivity function of the imaging sensor.

Representing the significant aspects influencing a human based inspection of OVD requires a considerable amount of data. The development of an appropriate information protocol is therefore of importance, to facilitate further analysis, data processing and visualization. The information protocol transforming the measurement data into characterizing information is a second significant achievement of the presented work in this thesis.

To prove the applicability measurements, visualizations and statistically based analyses have been developed for a selection of previously unsolved problems, as defined by senior scientists and representatives of central banks. Characterization and measurements of the degree to which OVD deteriorate with circulation is one such problem. One particular benefit of the implemented suggested solution is the characterization and measurement aim at aspects influencing human based (“first line”) inspection. The principally difference in the problems treated indicates the generality of the system, which is a third significant project achievement.

The system developed achieves the accuracy and precision including a resolution, dynamic range and contrast sensitivity function required for a technology independent standard protocol of “optical document security” OVDs. These abilities facilitate the definition and verification of program of requirements for the development of new security documents. Adding also the capability of interlinking first, second and third line inspection based characterizations may prove a particular valuable combination, which is a fourth significant project achievement.

The information content (Entropy) of characterized OVDs and OVD production limitations in combination opens for OVD based novel applications of “physically unclonable functions” (PUF). This is of significance as it would generalize the established OVDs to facilitate multimodal verification, including PUF verification. The OVDs would thereby transform into a combined PUF first line inspection facilitating security feature.

GonioLabs’ DOVID Reader (GDR) provides a spatially resolved trichromatic goniophotometric characterization. The present work discusses how the GDR quantifies optical features and differences between them (defects), as appreciated by perceptual evaluation. GDR advantages are a) evaluating different DOVID suppliers, b) gaining more detailed understanding of deterioration due to circulation, c) forensic evaluation of different groups of counterfeits potentially originating from the same production equipment

Detailed optical characterization of the relevant features of optically variable devices (OVD) is in general a challenge. Adding to the in general high both spatial and angular resolved characteristics, the high dynamic range of reflection, including high intensity specular reflections and low intensity reflections of other geometries make the metrology even more demanding. A trichromatic reflectance measurement service recently made commercially available, addresses the stated challenges. The resulting data may be described as a spatially resolved trichromatic goniophotometric characterization. The massive data set generated may be approached either by developed visualization tools (static images and dynamic videos) or processed mathematically into higher-order characterizations. The number of potential relevant applications is abundant – one example may be a detailed angular characterization of the iridescent color shifts; how the iridescence function is distorted due to the topography of the underlying paper. The application is demanding in part because the analysis involves spatial and angular dimensions over a high dynamic range of reflectance (specular and non-specular). Besides the OVD, there are other optical document security functions that are challenging to characterize in detail, and where this novel service may prove a useful tool for the professionals in the business.

The advent of digital printing stresses the importance of quality measurement. Print quality measures that give an objective value have long been sought. The aim is 10 be able to dcscrihe objectively the properties of print on paper so that conclusions regarding the apparent quality can be drawn. In this paper, we discuss the measures proposed in the literature and relate them to the subjective quality. Several years ago, the concept of ‘information capacity’ was proposed as such a measure. Some reflections are made in the light of coding. and telecommunication theory. areas in which in information capacity is a vital tool. On the other hand. for the print quality applications. the interest in in formation capacity has in fact been decreasing. An obvious question therefore arises and is treated in this work: Why have we seen so few applications of this theory for practical applications in the area of print? Is the area of print quality so very different from the areas or image compression and telecommunication?