Photonic quasi-crystal LEDs: design, modelling, and optimisation

In this paper we investigate improvement in performance attainable by etching Photonic Crystals and Photonic Quasi-Crystals into the top emitting surface of LEDs. We describe the physical mechanisms of extraction enhancement through ordered surface patterning and investigate benefits in terms of total extraction enhancement, beam directionality, and far field beam quality. Factors such as lattice geometry, etch depth, and epitaxy thickness are investigated. We show that a great variety of far field beam profiles of benefit in applications such as projection TV light engines and direct flat panel display illumination can be obtained simply by adjusting geometric design parameters. Our results show that PCs can provide significant improvement in extraction enhancement for applications requiring non Lambertian beam shapes when etched into standard "production line" epitaxy wafers in comparison to "state of the art" surface roughened thin-GaN LED devices. We investigate PC beam steering effects in these devices confirming that PCs do in fact re-direct light from trapped modes confined within the epi-structure to radiating modes. We also show that by tailoring the thickness of the epi-structure to complement the properties of the photonic crystal, extraction enhancement can be improved by a factor of 9 for some applications

Is countershading camouflage robust to lighting change due to weather?

Countershading is a pattern of coloration thought to have evolved in order to implement camouflage. By adopting a pattern of coloration that makes the surface facing towards the sun darker and the surface facing away from the sun lighter, the overall amount of light reflected off an animal can be made more uniformly bright. Countershading could hence contribute to visual camouflage by increasing background matching or reducing cues to shape. However, the usefulness of countershading is constrained by a particular pattern delivering ‘optimal’ camouflage only for very specific lighting conditions. In this study, we test the robustness of countershading camouflage to lighting change due to weather, using human participants as a ‘generic’ predator. In a simulated three-dimensional environment, we constructed an array of simple leaf-shaped items and a single ellipsoidal target ‘prey’. We set these items in two light environments: strongly directional ‘sunny’ and more diffuse ‘cloudy’. The target object was given the optimal pattern of countershading for one of these two environment types or displayed a uniform pattern. By measuring detection time and accuracy, we explored whether and how target detection depended on the match between the pattern of coloration on the target object and scene lighting. Detection times were longest when the countershading was appropriate to the illumination; incorrectly camouflaged targets were detected with a similar pattern of speed and accuracy to uniformly coloured targets. We conclude that structural changes in light environment, such as caused by differences in weather, do change the effectiveness of countershading camouflage

3.6 and 4.5 μm Phase Curves and Evidence for Non-equilibrium Chemistry in the Atmosphere of Extrasolar Planet HD 189733b

We present new, full-orbit observations of the infrared phase variations of the canonical hot Jupiter HD 189733b obtained in the 3.6 and 4.5 μm bands using the Spitzer Space Telescope. When combined with previous phase curve observations at 8.0 and 24 μm, these data allow us to characterize the exoplanet's emission spectrum as a function of planetary longitude and to search for local variations in its vertical thermal profile and atmospheric composition. We utilize an improved method for removing the effects of intrapixel sensitivity variations and robustly extracting phase curve signals from these data, and we calculate our best-fit parameters and uncertainties using a wavelet-based Markov Chain Monte Carlo analysis that accounts for the presence of time-correlated noise in our data. We measure a phase curve amplitude of 0.1242% ± 0.0061% in the 3.6 μm band and 0.0982% ± 0.0089% in the 4.5 μm band, corresponding to brightness temperature contrasts of 503 ± 21 K and 264 ± 24 K, respectively. We find that the times of minimum and maximum flux occur several hours earlier than predicted for an atmosphere in radiative equilibrium, consistent with the eastward advection of gas by an equatorial super-rotating jet. The locations of the flux minima in our new data differ from our previous observations at 8 μm, and we present new evidence indicating that the flux minimum observed in the 8 μm is likely caused by an overshooting effect in the 8 μm array. We obtain improved estimates for HD 189733b's dayside planet-star flux ratio of 0.1466% ± 0.0040% in the 3.6 μm band and 0.1787% ± 0.0038% in the 4.5 μm band, corresponding to brightness temperatures of 1328 ± 11 K and 1192 ± 9 K, respectively; these are the most accurate secondary eclipse depths obtained to date for an extrasolar planet. We compare our new dayside and nightside spectra for HD 189733b to the predictions of one-dimensional radiative transfer models from Burrows et al. and conclude that fits to this planet's dayside spectrum provide a reasonably accurate estimate of the amount of energy transported to the night side. Our 3.6 and 4.5 μm phase curves are generally in good agreement with the predictions of general circulation models for this planet from Showman et al., although we require either excess drag or slower rotation rates in order to match the locations of the measured maxima and minima in the 4.5, 8.0, and 24 μm bands. We find that HD 189733b's 4.5 μm nightside flux is 3.3σ smaller than predicted by these models, which assume that the chemistry is in local thermal equilibrium. We conclude that this discrepancy is best explained by vertical mixing, which should lead to an excess of CO and correspondingly enhanced 4.5 μm absorption in this region. This result is consistent with our constraints on the planet's transmission spectrum, which also suggest excess absorption in the 4.5 μm band at the day-night terminator

Establishing the behavioural limits for countershaded camouflage

Countershading is a ubiquitous patterning of animals whereby the side that typically faces the highest illumination is darker. When tuned to specific lighting conditions and body orientation with respect to the light field, countershading minimizes the gradient of light the body reflects by counterbalancing shadowing due to illumination, and has therefore classically been thought of as an adaptation for visual camouflage. However, whether and how crypsis degrades when body orientation with respect to the light field is non-optimal has never been studied. We tested the behavioural limits on body orientation for countershading to deliver effective visual camouflage. We asked human participants to detect a countershaded target in a simulated three-dimensional environment. The target was optimally coloured for crypsis in a reference orientation and was displayed at different orientations. Search performance dramatically improved for deviations beyond 15 degrees. Detection time was significantly shorter and accuracy significantly higher than when the target orientation matched the countershading pattern. This work demonstrates the importance of maintaining body orientation appropriate for the displayed camouflage pattern, suggesting a possible selective pressure for animals to orient themselves appropriately to enhance crypsis