Maximization of FDM-3D-Objects Gonio-Appearance Effects Using PLA and ABS Filaments and Combining Several Printing Parameters: "A Case Study".

In order to consider 3D objects from suitable Fused Deposition Modelling (FDM) printers as prototypes for the automotive sector, this sample must be able to reproduce textural effects (sparkle or graininess) or metallic or gonio-appearance to reinforce the attractive appeal of these materials. This study worked with two different commercial filaments: grey metallic PLA (poly(lactic acid)) and ABS (acrylonitrile-butadiene-styrene copolymer) with diffractive pigments. For both materials, a statistical design of experiments (DoE) was carried out to find the printing parameters effect on the final 3D-objects gonio-appearance. The selected printing parameters were printing speed (2 levels), layer height (2 levels) and sample thickness (3 levels). Twelve smooth square objects were printed from each material. The ABS-diffractive filaments achieved the most significant flop and higher sparkle values than metallic PLA. Graininess was high when working with PLA filaments instead of ABS. Layer height was the most significant parameter to maximize PLA objects' flop or sparkle effects. The best result was found when printing at 0.1 mm. For the ABS samples, the stronger flop and sparkle effects were achieved with the 50 mm/s printing speed, the 0.1 mm layer height and the lowest thickness level. This study shows the methodology to study the printing parameters effects and interactions to maximize the FDM-3D-objects gonio-appearance.

Validation of a gonio-hyperspectral imaging system based on light-emitting diodes for the spectral and colorimetric analysis of automotive coatings.

In this study, a novel gonio-hyperspectral imaging system based on light-emitting diodes for the analysis of automotive coatings was validated colorimetrically and spectrally from 368 to 1309 nm. A total of 30 pearlescent, 30 metallic, and 30 solid real automotive coatings were evaluated with this system, the BYK-mac and X-Rite MA98 gonio-spectrophotometers, and also with the SPECTRO 320 spectrometer for further comparison. The results showed very precise correlations, especially in the visible range. In conclusion, this new system provides a deeper assessment of goniochromatic pigments than current approaches due to the expansion of the spectral range to the infrared.

Improving color reproduction accuracy of a mobile liquid crystal display.

The default method for color representation on displays involves sRGB as device-independent encoding color space. For improving color reproduction accuracy, we develop a device-specific display characterization model for the Apple iPad Air 2. In separate articles, we will evaluate the same method for other devices and for other display technologies. This model is combined with an easy-to-implement new method to account for the influence of illuminance from ambient light. The combination is called the mobile display characterization and illumination model (MDCIM) for the iPad Air 2, representing modern liquid crystal displays. Seven observers performed psychophysical tests at ambient illuminance levels from 600 to 3000 lx. They visually compared colors of calculated images with those of physical samples. The MDCIM model achieves similar color reproduction accuracy as when using the default method involving sRGB encoding at 1000 lx, while considerably improving color accuracy at other illuminance levels. At 600 lx, 98% of the observers prefer images directly generated with the MDCIM model over those created using the default method. The average color reproduction accuracy improves by two categories on a five-point scale. At 3000 lx, the percentage of colors that is represented at least reasonably well, according to the subjective assessment of visual observers, increased from 0% to 60%.

Visual and instrumental correlation of sparkle by the magnitude estimation method.

Most real surfaces and objects show variations in appearance with viewing and illumination directions. Besides angular dependency, they also show spatial variation in color, i.e., they exhibit some sort of texture. Of the surfaces we see, surfaces colored by special-effect pigments produce several complex visual effects, like change in color and lightness with viewing and illumination angles, and effects like sparkle and gloss on other textures. In the last two decades, different commercial devices have appeared to help ensure the proper characterization of materials with special-effect pigments. However, the instrumental characterization of sparkle is currently available only by a commercial device integrated into a multi-angle spectrophotometer. As it is difficult to find complete open original studies about the sparkle effect for designing and calibrating this commercial instrument, the main objective of this work was to check whether a good visual and instrumental correlation exists between the sparkle that the observer perceives and the sparkle value provided by the device using some subsets of goniochromatic samples with different types of special-effect pigments and colors. Visual assessments were made by a conventional magnitude estimation method in a directional lighting booth, which belonged to the same company owner of the sparkle instrument, in different geometries and at distinct illuminance levels. The results revealed that there was a good visual correlation of the sparkle grade value. By separately analyzing the factors used in its instrument algorithm, such as sparkle intensity and sparkle area values, it was clearly shown that the correlation was not good or simply did not exist. Consequently, and perhaps in regards to the choice of new special-effect pigments, such as synthetic mica and other future ones, we generated herein even more questions about current mathematical algorithms, and only recognized calculating this texture effect at the industrial level.

Visibility of sparkle in metallic paints.

For suitable illumination and observation conditions, sparkles may be observed in metallic coatings. The visibility of these sparkles depends critically on their intensity, and on the paint medium surrounding the metallic flakes. Based on previous perception studies from other disciplines, we derive equations for the threshold for sparkles to be visible. The resulting equations show how the visibility of sparkles varies with the luminosity and distance of the light source, the diameter of the metallic flakes, and the reflection properties of the paint medium. The predictions are confirmed by common observations on metallic sparkle. For example, under appropriate conditions even metallic flakes as small as 1 µm diameter may be visible as sparkle, whereas under intense spot light the finer grades of metallic coatings do not show sparkle. We show that in direct sunlight, dark coarse metallic coatings show sparkles that are brighter than the brightest stars and planets in the night sky. Finally, we give equations to predict the number of visually distinguishable flake intensities, depending on local conditions. These equations are confirmed by previous results. Several practical examples for applying the equations derived in this article are provided.

Measuring color differences in automotive samples with lightness flop: a test of the AUDI2000 color-difference formula.

From a set of gonioapparent automotive samples from different manufacturers we selected 28 low-chroma color pairs with relatively small color differences predominantly in lightness. These color pairs were visually assessed with a gray scale at six different viewing angles by a panel of 10 observers. Using the Standardized Residual Sum of Squares (STRESS) index, the results of our visual experiment were tested against predictions made by 12 modern color-difference formulas. From a weighted STRESS index accounting for the uncertainty in visual assessments, the best prediction of our whole experiment was achieved using AUDI2000, CAM02-SCD, CAM02-UCS and OSA-GP-Euclidean color-difference formulas, which were no statistically significant different among them. A two-step optimization of the original AUDI2000 color-difference formula resulted in a modified AUDI2000 formula which performed both, significantly better than the original formula and below the experimental inter-observer variability. Nevertheless the proposal of a new revised AUDI2000 color-difference formula requires additional experimental data.

Effects of high-color-discrimination capability spectra on color-deficient vision.

Light sources with three spectral bands in specific spectral positions are known to have high-color-discrimination capability. W. A. Thornton hypothesized that they may also enhance color discrimination for color-deficient observers. This hypothesis was tested here by comparing the Rösch-MacAdam color volume for color-deficient observers rendered by three of these singular spectra, two reported previously and one derived in this paper by maximization of the Rösch-MacAdam color solid. It was found that all illuminants tested enhance discriminability for deuteranomalous observers, but their impact on other congenital deficiencies was variable. The best illuminant was the one derived here, as it was clearly advantageous for the two red-green anomalies and for tritanopes and almost neutral for red-green dichromats. We conclude that three-band spectra with high-color-discrimination capability for normal observers do not necessarily produce comparable enhancements for color-deficient observers, but suitable spectral optimization clearly enhances the vision of the color deficient.

Spectral BRDF-based determination of proper measurement geometries to characterize color shift of special effect coatings.

A reduced set of measurement geometries allows the spectral reflectance of special effect coatings to be predicted for any other geometry. A physical model based on flake-related parameters has been used to determine nonredundant measurement geometries for the complete description of the spectral bidirectional reflectance distribution function (BRDF). The analysis of experimental spectral BRDF was carried out by means of principal component analysis. From this analysis, a set of nine measurement geometries was proposed to characterize special effect coatings. It was shown that, for two different special effect coatings, these geometries provide a good prediction of their complete color shift.

Analysis of the colorimetric properties of goniochromatic colors using the MacAdam limits under different light sources.

Technological innovation in all areas has led to the appearance in recent years of new metallic and pearlescent materials, yet no exhaustive studies have been conducted to assess their colorimetric capabilities. The chromatic variability of these special-effect pigments may largely be due to the three-dimensional effect of their curved shapes and orientations when they are directionally or diffusely illuminated. Our study examines goniochromatic colors using the optimal colors (MacAdam limits) associated with normal colors (photometric scale of relative spectral reflectance from 0 to 1) under certain conventional illuminants and other light sources. From a database of 91 metallic and interference samples and using a multi-gonio-spectrophotometer, we analyzed samples with lightness values of more than 100 and others with lightness values of less than 100, but with higher chromaticities than optimal colors, which places them beyond the MacAdam limits. Our study thus demonstrates the existence of chromatic perceptions beyond the normal solid color associated with these materials and independent of the light source. The challenge for future research, therefore, is to replicate and render these color appearances in current and future color reproduction technologies for computer graphics.

Number of discernible colors for color-deficient observers estimated from the MacAdam limits.

We estimated the number of colors perceived by color normal and color-deficient observers when looking at the theoretic limits of object-color stimuli. These limits, the optimal color stimuli, were computed for a color normal observer and CIE standard illuminant D65, and the resultant colors were expressed in the CIELAB and DIN99d color spaces. The corresponding color volumes for abnormal color vision were computed using models simulating for normal trichromatic observers the appearance for dichromats and anomalous trichomats. The number of colors perceived in each case was then computed from the color volumes enclosed by the optimal colors also known as MacAdam limits. It was estimated that dichromats perceive less than 1% of the colors perceived by normal trichromats and that anomalous trichromats perceive 50%-60% for anomalies in the medium-wavelength-sensitive and 60%-70% for anomalies in the long-wavelength-sensitive cones. Complementary estimates obtained similarly for the spectral locus of monochromatic stimuli suggest less impairment for color-deficient observers, a fact that is explained by the two-dimensional nature of the locus.

[Health Surveillance Guide of workers using video display terminals: evaluation from a visual health perspective]. / Protocolo de vigilancia sanitaria de trabajadores con pantallas de visualización de datos: una valoración desde la perspectiva de la salud visual.

BACKGROUND: Visual health surveillance is essential in the protection of workers who use video display terminals (VDT). In Spain, the most used is the Specific Health Surveillance Guide published in 1999 by the Ministry of Health. The increase of the scientific production upon computer related occupational visual hazards and the experience in its applicability during the last decade justify the aim of this work: reviewing the quality of the guide from the point of view of visual health. METHODS: A consensus strategy was used among nine experts by means of a mixed groupal technique in two consecutive stages combining some aspects of the Delphi method and of the nominal group: individual evaluation of the guide using the consensus guide made by the authors based on the AGREE instrument and the subsequent meeting in order to reach an agreement and to fix the final recommendations for improving it. For the analysis the standardized score of the review domains was calculated: scope and purpose, stakeholder involvement, rigour of development, clarity and presentation and applicability. It was also calculated for the items.' Concordance in the answers of the experts was also analyzed. RESULTS: All domains obtained scores under 50%. The applicability, rigour and stakeholder involvement during the development of the guide, where the most deficient domains. Six out of the nine experts would not recommend the guide and think it should be remade. CONCLUSIONS: The guide does not reach the necessary quality for the surveillance of the visual health of the workers who use VDT. Efforts must be focussed to improve the guide.

Computation and visualization of the MacAdam limits for any lightness, hue angle, and light source.

We present a systematic algorithm capable of searching for optimal colors for any lightness L* (between 0 and 100), any illuminant (D65, F2, F7, F11, etc.), and any light source reported by CIE. Color solids are graphed in some color spaces (CIELAB, SVF, DIN99d, and CIECAM02) by horizontal (constant lightness) and transversal (constant hue angle) sections. Color solids plotted in DIN99d and CIECAM02 color spaces look more spherical or homogeneous than the ones plotted in CIELAB and SVF color spaces. Depending on the spectrum of the light source or illuminant, the shape of its color solid and its content (variety of distinguishable colors, with or without color correspondence) change drastically, particularly with sources whose spectrum is discontinuous and/or very peaked, with correlated color temperature lower than 5500 K. This could be used to propose an absolute colorimetric quality index for light sources comparing the volumes of their gamuts, in a uniform color space.