Michele Ann Mossman

Control of reflectance of liquid droplets by means of electrowetting

It has recently been noted that hemispherical structures have useful reflection characteristics. We describe a new application that makes use of these characteristics by controlling the reflectance of a surface composed of an array of hemispherical liquid droplets. In this system the reflectance state is spatially controlled through the use of electrowetting to alter the shape of an array of droplets. This may have an application in the field of electronic image displays.

Tutorial: Color Rendering and Its Applications in Lighting

ABSTRACT This tutorial explains how the human perception of color rendering arises, in terms of the underlying phenomena of light and vision, and using those concepts it presents a clear explanation of the CIE Color Rendering Index. The strengths and weaknesses of the CIE Color Rendering Index are reviewed and some common misunderstandings about color rendering are addressed. It is suitable for self-study, with learning outcomes stated at the beginning and a conceptual summary provided at the end.

39.1: New Reflective Color Display Technique Based on Total Internal Reflection and Subtractive Color Filtering

We present a reflective color display technique using a configuration of subtractive color filtering regions to control total internal reflection (TIR) in prismatic microstructures. The ability of the prismatic surface to efficiently redirect ambient light rays toward the viewer and the use of subtractive color filters result in a bright, easily legible, low power color image which has a four-fold improvement in reflectivity over conventional RGB reflective LCD displays. A key advantage of this technique is the ability to switch between the highly reflective and intensely colored states with only about a half-micron movement of the controllable subtractive color filter.

Cost-effective controlled illumination using daylighting and electric lighting in a dual-function prism light guide

We have developed a system that enables both daylight and electric light to be efficiently delivered to core areas of a building. Daylight is directed into the building by means of a new and cost-effective canopy system which collects sunlight and directs it into the dual-function luminaire. It efficiently transports and distributes the light along its length, and when the amount of daylight is insufficient, auxiliary electric light is automatically provided by fluorescent lamps. This paper focusses on the prototype illumination control system and its performance.

Transparent superhydrophobic surfaces for applications of controlled reflectance

This work involves a new optical application for transparent superhydrophobic materials, which enables low-energy optical contact between a liquid and solid surface. The new technique described here uses this surface property to control the reflectance of a surface using frustration of total internal reflection. Surface chemistry and appropriate micro-scale and nano-scale geometries are combined to produce interfaces with low adhesion to water and the degree to which incident light is reflected at this interface is controlled by the movement of water, thereby modifying the optical characteristics at the interface. The low adhesion of water to superhydrophobic surfaces is particularly advantageous in imaging applications where power use must be minimized. This paper describes the general approach, as well as a proof-of-principle experiment in which the reflectance was controlled by moving a water drop into and out of contact with a superhydrophobic surface by variation of applied electrostatic pressure.

Application of transparent nanostructured electrodes for modulation of total internal reflection

We present a novel method of modulating total internal reflection (TIR) from an optical surface using a solution of dye ions in combination with a nanostructured electrode. Previous work using the electrophoretic movement of pigment particles to modulate TIR was limited by agglomeration of the pigment over time. Dye ions do not suffer from this limitation, but because of their small size they have significantly smaller absorption cross-section per unit charge than pigment particles which are generally two orders of magnitude larger. This significantly limits the maximum absorption caused by electrostatic attraction of the ions to a transparent conductive electrode. This can be overcome by using a transparent conductive nanoporous thin film as the electrode in which the porosity increases the effective surface area, allowing more dye ions to move into the evanescent wave region near the nanoporous transparent electrode and thus substantially increases the amount of absorption. In this paper, we demonstrate the modulation of TIR by observing the time-dependent variation of the reflectance as the dye ions are moved into and out of the evanescent wave region. This approach may have applications in reflective displays and active diffractive devices.

Reflections on Total Internal Reflection

These authors probe some of the most fundamental approximations in optics to explore why total internal reflection is not as straightforward as it might seem.

Control of reflection at an optical interface in the absence of total internal reflection for a retroreflective display application

Reflection at an interface between two materials can be modulated by means of varying the optical properties at the interface. We have studied this modulation of the reflected light with an aim to develop a flashing retroreflector for roadside conspicuity applications. Reflectance modulation has previously been studied under the conditions of total internal reflection (TIR), where a light-absorbing material placed in the associated evanescent wave region can be used to attenuate the intensity of the reflected light. If instead the light rays strike the interface at an angle that is slightly smaller than the critical angle required for TIR, they instead undergo a substantial, but partial, reflection. We have demonstrated that an analogous attenuation effect to the TIR situation is observed, even though there is no evanescent wave present under these circumstances. We have studied this behavior and have developed a model to describe the motion of the absorbing material and the related interference effects that occur.

Jack O’Lanterns and integrating spheres: Halloween physics

Although photometry, the measurement of the intensity and distribution of visible light, is important in many areas, most undergraduate physics courses do not include this topic. We present a simple introduction to key concepts in photometry, and as a fun example, we investigate the optics of a Jack O’Lantern.

Controlled frustration of total internal reflection by electrophoresis of pigment particles

A model based on geometrical optics has been developed to describe the photometric observations associated with a novel method to control the reflectance of a surface. In this new reflectance modulation approach, electrophoresis of pigment particles is used to absorb light reflected by total internal reflection (TIR). The pigment particles are sufficiently small that they substantially do not scatter light, but rather they modify the effective refractive index at the reflection interface. An incident light ray interacting with this modified effective index is attenuated in a spectrally selective manner. Although frustrated TIR has been understood and used in various applications for some time, in this case it is used to substantially modify the color of the reflected light, which to our knowledge has not been previously reported. A numerical model of the pigment particle distribution has been developed to describe the observations.