Concept: Diffuse reflection
Specular reflection appears as a bright spot or highlight on any smooth glossy convex surface and is caused by a near mirror-like reflectance off the surface. Convex shapes always provide the ideal geometry for highlights, areas of very strong reflectance, regardless of the orientation of the surface or position of the receiver. Despite highlights and glossy appearance being common in chemically defended insects, their potential signalling function is unknown. We tested the role of highlights in warning colouration of a chemically defended, alpine leaf beetle, Oreina cacaliae. We reduced the beetles' glossiness, hence their highlights, by applying a clear matt finish varnish on their elytra. We used blue tits as predators to examine whether the manipulation affected their initial latency to attack, avoidance learning and generalization of warning colouration. The birds learned to avoid both dull and glossy beetles but they initially avoided glossy prey more than dull prey. Interestingly, avoidance learning was generalized asymmetrically: birds that initially learned to avoid dull beetles avoided glossy beetles equally strongly, but not vice versa. We conclude that specular reflectance and glossiness can amplify the warning signal of O. cacaliae, augmenting avoidance learning, even if it is not critical for it.
A galvanic-displacement-reaction-based, room-temperature “dip-and-dry” technique is demonstrated for fabricating selectively solar-absorbing plasmonic-nanoparticle-coated foils (PNFs). The technique, which allows for facile tuning of the PNFs' spectral reflectance to suit different radiative and thermal environments, yields PNFs which exhibit excellent, wide-angle solar absorptance (0.96 at 15°, to 0.97 at 35°, to 0.79 at 80°), and low hemispherical thermal emittance (0.10) without the aid of antireflection coatings. The thermal emittance is on par with those of notable selective solar absorbers (SSAs) in the literature, while the wide-angle solar absorptance surpasses those of previously reported SSAs with comparable optical selectivities. In addition, the PNFs show promising mechanical and thermal stabilities at temperatures of up to 200 °C. Along with the performance of the PNFs, the simplicity, inexpensiveness, and environmental friendliness of the “dip-and-dry” technique makes it an appealing alternative to current methods for fabricating selective solar absorbers.
We demonstrate wide-angle, broadband and efficient reflection holography by utilizing coupled dipole-patch nano-antenna cells to impose an arbitrary phase profile on of the reflected light. High fidelity images were projected at angles of 450 and 200 with respect to the impinging light with efficiencies ranging between 40%-50% over an optical bandwidth exceeding 180nm. Excellent agreement with the theoretical predictions was found at a wide spectral range. The demonstration of such reflectarrays opens new avenues towards expanding the limits of large angle holography.
Perceptual Gloss Parameters Are Encoded by Population Responses in the Monkey Inferior Temporal Cortex
- The Journal of neuroscience : the official journal of the Society for Neuroscience
- Published over 3 years ago
There are neurons localized in the lower bank of the superior temporal sulcus (STS) in the inferior temporal (IT) cortex of the monkey that selectively respond to specific ranges of gloss characterized by combinations of three physical reflectance parameters: specular reflectance (ρs), diffuse reflectance (ρd), and spread of specular reflection (α; Nishio et al., 2012). In the present study, we examined how the activities of these gloss-selective IT neurons are related to perceived gloss. In an earlier psychophysical study, Ferwerda et al. (2001) identified a perceptually uniform gloss space defined by two axes where the c-axis corresponds to a nonlinear combination of ρs and ρd and the d-axis corresponds to 1 - α. In the present study, we tested the responses of gloss-selective neurons to stimuli in the perceptual gloss space defined by the c- and d-axes. We found that gloss-selective neurons systematically changed their responses in the perceptual gloss space, and the distribution of the tuning directions of the population of gloss-selective neurons is biased toward directions in which perceived gloss increases. We also found that a set of perceptual gloss parameters as well as surface albedo can be well explained by the population activities of gloss-selective neurons, and that these parameters are likely encoded by the gloss-selective neurons in this area of the STS to represent various glosses. These results thus provide evidence that the IT cortex represents perceptual gloss space.
Hierarchical micro- and nano-structured surfaces have previously been made using a variety of materials and methods, including particle deposition, polymer molding, and the like. These surfaces have attracted a wide variety of interest for applications including reduced specular reflection and super-hydrophobic surfaces. In this paper we report the first monolithic, hierarchically structured glass surface that combines micron-scale and nano-scale surface features to simultaneously generate antiglare (AG), antireflection (AR), and superhydrophobic properties. The AG microstructure mechanically protects the AR nanostructure during wiping and smudging while the uniform composition of the substrate and the micro-nano-structured surface enables ion exchange through the surface, so that both the substrate and the structured surface can be simultaneously chemically strengthened.
Investigation on optical band gap, photoluminescence properties and concentration quenching mechanism of Pb1-x Tb(3+)xWO4 green-emitting phosphors
- Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy
- Published 15 days ago
A series of monophasic Tb(3+) (2, 5, 7, 10 and 15at%) doped PbWO4 phosphors were successfully prepared via hydrothermal method. X-ray diffraction patterns revealed that the prepared samples possess a high crystallinity with tetragonal scheelite-type structure. FT-IR and Raman analysis exhibited a WO stretching peak of WO4(2-) group, which is also related to the scheelite structure. UV-visible diffuse reflectance spectra indicated a reduction in the optical band gap with the replacement of Pb(2+) by Tb(3+) ions. The presence of strong and intense emission peaks characteristic of Tb(3+) with the dominant peak at 545nm (green, (5)D4→(7)F5 transition) under UV irradiation at 320nm demonstrated an efficient energy transfer from the host to Tb(3+) ions. Using Van Uitert’s model, the concentration quenching mechanism between Tb(3+) ions in PbWO4:Tb(3+) phosphor was attributed to a dipole-dipole interaction and the critical distance was determined to be ~12Å. The decay lifetimes and CIE chromaticity co-ordinates of PbWO4:Tb(3+) phosphors were also investigated in detail. These prepared materials might serve as a potential phosphor for LED applications.
Although metal-catalysts are commonly used to create nanoscale materials at surfaces, little is quantitatively known or understood about the depth distribution profile of the catalyst during the growth process. Using X-ray reflectivity, we report the first quantitative investigation, with nanoscale resolution, of the Ag metal-catalyst depth distribution profile during metal-assisted chemical etch (MACE) growth of Si nanowire (SiNW) arrays on Si(100). Given the very low optical reflectivity of these nanowire arrays, specular reflection from these materials in the X-ray region is extremely challenging to measure because it probes interfaces on the nanoscale. Nevertheless, we demonstrate that with suitable investigation, X-ray specular reflection can be measured and utilized to obtain unique structural information about the composition profile of both Ag and Si. The measurements, which also include X-ray diffraction and complementary electron microscopy, reveal that the Ag nanoparticles distribute along the length of the nanowires upon etching with a Ag density that increases towards the etch front. The Ag nanoparticles coarsen with etch time, indicating a high mobility of Ag ions even though we also find that the Ag does not migrate from the SiNW region into the etch bath during etching. The Ag density gradient and the Ag mobility suggest the existence of a strong chemical force that attracts Ag towards the etch front. These results provide unique and important new insight into the growth process for creating SiNWs from wet chemical etching using metal-catalysts.
Intravenous (IV) procedures are often difficult due to the poor visualization of subcutaneous veins. Because existing vein locators lack the ability to assess depth, and also because mis-punctures and poor vascular access remain problematic, we propose an imaging system that employs diffuse reflectance images at three isosbestic wavelengths to measure both the depth and thickness of subcutaneous veins. This paper describes the proposed system as well as proof-of-principle experimental demonstrations. We initially introduce the working principle and structure of the system. All measurements were based on the Monte Carlo (MC) method and accomplished by referring an optical density (OD) ratio to a multi-layer diffuse reflectance model. Results were all validated by comparative ultrasound measurements. Experimental trials included 11 volunteers who were subjected to both ultrasound measurements and the proposed optical process to validate the system’s applicability. However, the unreliability of the “thickness” measurement of the vein may be due to the fact that the veins have collapsible walls - so excess pressure by the transducer will give a false thickness.
The photocatalytic performance of graphitic carbon nitride (g-C3N4) has been limited to low efficiency due to the fast charge recombination. Here, we constructed g-C3N4 nanosheets/TiO2 mesocrystals metal-free composite (g-C3N4 NS/TMC) to promote the efficiency of charge separation. The photocatalytic H2 evolution experiments indicate that coupling g-C3N4 NS with TMC increases photogenerated charge carriers in g-C3N4 NS/TMC composite due to efficient charge separation. g-C3N4 NS (31 wt%)/TMC shows the highest photocatalytic activity and the corresponding H2 evolution rate is 3.6 µmol h(-1). This value is 20 times larger than that of g-C3N4 NS without any noble metal cocatalyst under visible light irradiation (λ> 420 nm). The photocatalytic activity of g-C3N4 NS/TMC (3.6 µmol h(-1)) is 7 times higher than that of g-C3N4 NS/P25 (0.5 µmol h(-1)), confirming the importance of strong interface interaction between two dimensional g-C3N4 NS and plate-shape TMC. Femtosecond time-resolved diffuse reflectance (fs-TDR) was employed to study the fundamental photophysical processes of bulk g-C3N4, g-C3N4 NS, and g-C3N4/TMC composite which are essential to explain the photocatalytic activity. Using fs-TDR, we demonstrate that the photocatalytic activity depends on increased driving force for photoelectron transfer and a higher percentage of photogenerated charges.
A metal-lustrous self-standing film, named “porphyrin foil”, was formed from a glass-forming polymeric porphyrin. The amorphous glass nature of the porphyrin foil played a key role in spontaneously producing a smooth surface. Its sharp contrast in intense absorption and specular reflection of light at each wavelength provided a brilliant metallic lustre.