Nonplanar Mirrors Research Articles

Thermal optics of a collimator radiatively cooled under vacuum

Infrared optoelectronic equipment with cooled optics operating in a background limited mode is tested in a vacuum and a low radiation background. The latter is ensured by cooling the optical elements of the collimator and preventing direct illumination of the entrance pupil of the equipment by warm elements of the vacuum chamber. Subject of study. The thermal optics of a two-mirror collimator are investigated in this study. The mirrors of this collimator are cooled by heat transfer through infrared emission to the cooled cylindrical shields of the thermostat, which was specifically designed for collimator cooling. Method. The collimator cooling modes are calculated considering both thermophysical material properties and structural features of the elements comprised in the thermostat and collimator. The thermophysical model of thermal equilibrium in an isolated system of solid bodies is used in the calculations. The issue of selecting a particular material for the collimator mirrors is considered provisionally. The thermal stability of the mirror material calculated as a ratio of the coefficient of thermal expansion β to its heat conduction coefficient λ is used as a parameter determining the choice. Sitall, silicon, and silicon carbide were considered in this study as materials for infrared mirrors. Main results. The magnitude of thermal deformations is determined by not only the occurring temperature gradients in a plane mirror but also the variation in the curvature of the mirror surface under general cooling of the nonplanar mirrors. The maximum temperature gradient over the surface is observed for the mirrors composed of sitall. The temperature gradient in the mirror composed of silicon carbide is significantly smaller. This is related to the heat conduction coefficient of silicon carbide being 2 orders of magnitude higher than that of sitall. Consequently, the thermal deformations of the mirror surface are also smaller in the plane mirror composed of silicon carbide. These results correspond to the criterial parameter of thermal stability β/λ for each of the discussed materials. However, in the case of cooling of the nonplanar (spherical or aspherical) mirrors, the calculations show that their thermal deformations result from not only the nonuniformity of heat flow distribution over the mirror surface but also the general reduction in mirror temperature. Practical significance. When cooled infrared mirrors are used in optical systems, the appearance of thermal deformation of their mirror surface should be considered. This thermal deformation is determined by not only the occurring temperature gradients in the volume of the mirror material, but also the change in the curvature of the mirror surface. In this regard, the mirrors composed of sitall are preferable for use under thermal vacuum conditions over mirrors composed of silicon carbide.

Virtual image determination for mirrored surfaces.

An object viewed via reflection from a mirrored surface is often perceived by the observer to be located behind the mirror's surface. The image of this object behind the mirror is known as its virtual image. Conventional methods for determining the location and shape of a virtual image for non-planar mirrors are complex and impractical unless both the observer and object are near the optical axis. We have developed a technique designed to be simple and practical for determining the location of a virtual image in a non-planar mirror far from the optical axis. Results using this technique were compared with known results from geometric optics for an object point on the optical axis of a parabola and for an object point imaged off the optical axis of a spherical mirror. These results were also in agreement with experimental measurements for a hemispherical mirror viewed at large angles with respect to its optical axis. This technique has applications for display devices or imaging tools utilizing curved, mirrored surfaces.

The impact of rear-view mirror distance and curvature on judgements relevant to road safety

We report two experiments that investigate the impact of rear-view mirror distance and curvature on distance, spacing, and time-to-contact (TTC) judgements. The variation in mirror distance had a significant effect on TTC judgements, but only marginally influenced distance and spacing estimations. As mirror distance increased, TTC was overestimated, which is potentially dangerous. Control conditions with identical visual angles across different mirror distances revealed that effects were not solely caused by variation in visual angle. The impact of mirror curvature moderated the effect. While observers were unable to compensate for the mirror distance effect, they could do so for the distortions generated by non-planar mirrors, at least up to a certain degree of distortion. Implications for vehicle design and national guidelines are discussed. Practitioner Summary: Regulations regarding rear-view mirrors are vastly different between countries. For instance EU regulations encourage convex driver-side mirrors, whereas US regulations allow them merely on the passenger's side. The use of a dynamic TTC paradigm puts the human factors designer in a position to evaluate the existing regulations and to design safer mirrors.

Casimir energy and geometry: beyond the proximity force approximation

We review the relation between the Casimir effect and geometry, emphasizing deviations from the commonly used proximity force approximation (PFA). We use, to this aim, the scattering formalism which is nowadays the best tool available for accurate and reliable theory–experiment comparisons. We first recall the main lines of this formalism when the mirrors can be considered to obey specular reflection. We then discuss the more general case where non-planar mirrors give rise to non-specular reflection with wavevectors and field polarizations mixed. The general formalism has already been fruitfully used for evaluating the effect of roughness on the Casimir force as well as the lateral Casimir force or Casimir torque appearing between corrugated surfaces. In this paper, we focus our attention to the case of the lateral force which should make possible in the future an experimental demonstration of the nontrivial (i.e. beyond PFA) interplay of the geometry and Casimir effect.

Visual Characteristics of Elderly Drivers – Aging Effects on Visual Acuity and Glare Feeling

The objective of the study was to investigate age related effects on visual acuity and reflected-glare associated with daytime and nighttime driving conditions. Fifty-six participants (28 young, 28 old) recruited from Virginia Tech student population and Blacksburg Community were tested in the study. Landolt's Circle Method was adopted to assess visual acuity under simulated daytime and nighttime conditions. Although, the age and distance main effects were found to be significant, no differences were reported among all the distances within the young group. The study also examined the effects of reflected headlamp glare on age and mirror types on the basis of angle of incidence, illumination-front-of-eyes and rating scores. The results indicated that with the same glare level (as measured by angle and illumination in front of the eye), elderly adults reported lower glare rating scores (i.e. worse feelings of glare). Furthermore, the young and elderly both reported lower De Boer's rating scores for planar driver-side mirrors than non-planar driver-side mirrors. This result can have practical implications in reducing nighttime discomfort glare for both young and elderly.

Effects of Nonplanar Driver-Side Mirrors on Lane-Change Crashes

In this quasi-experiment, we investigate the effects on lane-change crashes of nonplanar (spherical convex and multiradius) driver-side mirrors compared to planar mirrors. The analysis was based on 1,062 crashes reported from 1987 to 1998 to Finnish insurance companies for vehicles with passenger-side spherical convex mirrors and one of three types of driver-side mirror (planar, spherical convex, or multiradius). The results show that the mean effect of nonplanar mirrors compared to planar mirrors was a statistically significant decrease of 22.9% in lane-change crashes to the driver side. The effects of spherical convex and multiradius mirrors were not statistically different from each other. The nonplanar mirrors were beneficial especially for the high-risk driver groups, as well as for the lane-change situations and environmental conditions in which most lane-change crashes take place in the United States. These findings support the use of nonplanar driver-side mirrors. If drivers have problems with jud...

Unstable resonator semiconductor lasers. Part 2: Experiment

We review the technology involved in the fabrication of semiconductor lasers with non-planar mirrors. The symmetric unstable resonator semiconductor laser, in a planar (gain guided) configuration is shown to effectively suppress filamentation, and to emit a laterally coherent output up to approximately 350 mW. We show how the geometry of this laser can be changed slightly to produce different devices with improved output characteristics. As in the case of macroscopic resonators, the laser cavity can be designed in such a way that the output beam is optimised to certain specific applications.

Unstable resonator semiconductor lasers. Part 1: Theory

We review the theory necessary to describe the class of semiconductor lasers with nonplanar mirrors whose resonator geometry is of the unstable resonator configuration. We present formulas from geometrical unstable resonator theory and calculate quantities relevant to semiconductor lasers, including quantum efficiency and the width limitation due to total internal reflection. We describe the mirror coupled mode formalism for analysing the modes of unstable res onator semiconductor lasers and present near-field patterns and calculations of loss as a function of length, width, and lateral index step. We show that the periodicity of losses as a function of length is determined by a quantity analogous to the equivalent Fresnel number of macroscopic unstable resonators, which is determined by the lateral index step within the laser.

Modal analysis of semiconductor lasers with nonplanar mirrors

We present a formalism for analyzing laser resonators which possess nonplanar mirrors and lateral waveguiding [e.g., an unstable resonator semiconductor laser (URSL)]. The electric field is expanded in lateral modes of the complex-index waveguide and is required to reproduce itself after, one roundtrip of the cavity. We show how the waveguide modes, their gain and loss, and hence the criterion for truncation of the infinite set of modes can be derived from the Green's function of the one-dimensional eigenvalue equation for the waveguide. Examples are presented for three cases of interest-a purely gain-guided URSL, an index-guided URSL, and a gain-guided tilted-mirror resonator. We compare theoretical calculations to previous experiments.