Author:

Abstract:

In many applications, customers make purchase decisions based on functional requirements, performance and experiential considerations. The visual appearance - the visual sensation resulting from an interaction between the shape of objects, their optical properties, and the illumination - strongly contributes to this assessment task. In essence, a high quality product must nowadays also look alike, or it will simply not be as attractive as other (cheaper) alternatives with elaborated surface finish or packaging. For this reason, all manufacturing industries are concerned with appearance control: from cars or yachts, to any electronic device or home appliance, paints, cosmetics, flooring, and even food products. Realistic visual appearance is also required for reality perception in today's digital world. In this sense, guiding frameworks for the quantification of visual appearance have become essential for industry to ensure reproducibility and uniformity in appearance descriptions. The optical characterization of visual appearance is however tremendously complex, even though human individuals can easily and continuously asses it in crowded scenes. An international team of scientists dealt with the topic in 2006, introducing four essential attributes: colour, gloss, translucency and texture. This dissertation focuses on the optical characterization of the attribute gloss. Gloss is generally associated with specular reflection (mirror reflection) at the front surface (first interface between the surrounding medium (i.e. air) and an object) of objects. Contributing to the visual appearance, gloss influences our impression of - and interaction with - the surrounding world. In particular, industrial gloss control on visible product surfaces often pursuits a sense of high quality or prestige. A unanimous consensus for the perceptual and optical characterization of gloss is however yet non-existent. In this manuscript, an extensive review of existing research on the visual perception of gloss and its instrumental characterization is provided. A custom framework is suggested for a next generation of gloss meters, consisting of five main attributes: specular gloss, distinctness of the reflected image ('DOI'), haziness of the image ('haze'), contrast and surface-uniformity gloss. Standardized measurement methods for most attributes have been around for a long time. As such, they are mostly based on outdated photo-electric technology and not considering the technological potential of contemporary compact imaging hardware and software. Furthermore, the industrial evaluation of gloss is mainly performed with a simple handheld specular gloss meter, whose evaluations often correlate poorly with the actual gloss appraisal. A dedicated visual inspection thus remains indispensable for a trustful judgement, leading to uncertainties and subjectivity in the process. As a solution, instrument manufacturers developed more modern gloss meters with linear or twodimensional sensors that can combine standard evaluation methods for specular gloss, DOI and haze, with some customized capabilities. Other application-specific and appearance-oriented instruments with camera technology exist, but they are very expensive. Industry would benefit from an affordable gloss meter that could quantify every aspect of surface gloss. In the process towards such a multifunctional and cost-effective gloss meter, two image-based gloss meters (iGMs) have been developed in this project. With CMOS camera sensors, they incorporate arbitrary image-processing algorithms. Their optical geometry is based upon the classical specular gloss meter, providing backward compatibility for industry. The first iGM with a colour camera sensor and two light sources is capable of evaluating image-based metrics for four of the five defined gloss attributes (specular gloss, DOI, haze and contrast). An evaluation of the contrast attribute is a particular novelty of the device. A second extended iGM instrument, additionally equipped with an extra camera sensor and various light sources, was developed to also incorporate the last gloss attribute (surface-uniformity gloss). It includes evaluations of the physical texture, sparkle (glitter), surface defects, material grooves and orange peel (surface waviness). Consisting of compact and cheap components, both instruments are cost-effective and handheld. The applicability and appropriateness of many gloss evaluation algorithms based on measurement image data of both devices was illustrated on a variety of industrially relevant materials. An introductory study was performed on the impact of local surface curvature on specular gloss evaluation. Indeed, despite the complex threedimensional shape of nowadays products, all gloss evaluation standards still assume a flat surface geometry. Based on measurements and ray-tracing simulations, the iGM was adapted to the measurement of specular gloss on non-flat surfaces. The unavailability of the ground truth perception of stimuli complicates a conclusion on the appropriateness of iGM gloss evaluations. As sample haziness has been rarely studied yet, focus was laid on the psychophysical validation of hazy specimen. In a psychophysical experiment, perceptual scales for the glossiness and its two mainly recognized attributes (DOI and contrast) were determined for a batch of hazy surfaces. In the search for a suitable (image-based) representation, the perceptual data was correlated with metrics of the iGM and other optical instruments. In particular, the study indicates the relevance of 'contrast-based' evaluations in describing the perceived glossiness of hazy surfaces. In addition, novel haze and DOI metrics of the iGM provide the best description of the perceived glossiness and DOI, respectively. Future work is required to investigate the generalization of these findings. Finally, the performed research also resulted in the development of a commercialized embodiment of the iGM in collaboration with the company Rhopoint Instruments ltd. Rhopoint is one of the world-wide market leaders in optical instrumentation, with multiple gloss and appearance related devices in their product portfolio. The commercial iGM, with measurement capabilities for each of the five gloss attributes, will be launched in early 2024.