Dependence Of Transmission Research Articles

An hourly diffuse fraction model with correction for variability and surface albedo

The paper presents an improved version of a previously published model for the diffuse fraction of hourly global irradiance. In addition to hourly solar elevation and clearness index, an hour-to-hour variability index and regional surface albedo are included among the input parameters. Moreover, to prevent excessively high normal incidence beam irradiances at very low solar elevations, the model does not allow a solar elevation dependent maximum beam transmittance to be exceeded. This new model is tuned to 32 years of data from Bergen, Norway. Moreover, a test against independent data from four European stations showed that the model performs better than the models of Erbs et al. (1982), Maxwell (1987)and Perez et al. (1992).

Design guidelines for a glazing with a seasonally dependent solar transmittance

The basic design guidelines for a window and facade glass which rejects direct solar radiation during the summer period and which transmits direct solar radiation during the remainder of the year are derived. The design employs purely passive control means and is based on the seasonal change of the incident angle of solar rays and on the capability of glass prisms to refract and totally reflect or transmit solar rays dependent on the relevant incident angle. Two new design elements essentially extend the applicability of the glass in accordance with window orientation and season of direct solar light rejection.

Photochromic properties of photoactive yellow protein

The basic intensity dependent optical properties of photoactive yellow protein were studied and shown to resemble to those of bacteriorhodopsin. Using a set of rate equations and a modified photocycle the transmittance behaviour was explained. Fitting the theoretical intensity dependent transmittance curves to the measurements was done by adjusting the photochemical parameters and the relaxation times. The model explains the experiments.

The Influence of Surface Reflectivities on Measurement of the Torsional Anchoring Strength of Nematic Liquid Crystals

The polarisation dependent normal incidence transmissivity of a multilayered system encompassing within it an optically uniaxial layer has been analysed in detail using multilayer optics modelling. It is shown that even for this simple case of normal incidence the orientation of the uniaxial sample and two polarisers needed to give a transmission minimum is significantly influenced by the optical phase shifts caused by reflections at the isotropic boundary layers. We highlight the failure of the simplified approach to this problem which uses the Jones matrix, ignoring multiple reflections within the system. Consequently we show that the standard technique of quantifying the boundary twist offset angles by determining with polarisers the minimum points in the normal incidence transmission for a thin nematic liquid crystal cell may well lead to seriously wrong values for the torsional anchoring strength of aligning surfaces.

Angular selective optical properties of thin films: Measurement of polar and azimuthal transmittance

A new instrument for automatic recordings of angular dependent transmittance through thin film samples is described. The output is a polar plot giving a schematic and convenient overview of the optical performance. The usefulness of the instrument and procedures was documented by measurements on an angular selective Cr film prepared by oblique angle evaporation.

Fabrication and performance of thin amorphous Si subwavelength transmission grating for controlling vertical cavity surface emitting laser polarization

Amorphous Si subwavelength transmission gratings (SWTGs) with a thickness of 240 nm and a period ranging from 50 to 900 nm were fabricated on a fused silica substrate using electron beam lithography, lift-off, and reactive ion etching. At certain grating periods, the SWTGs exhibit polarization dependent reflectance and transmittance due to the polarization dependent coupling of the eigenmodes inside the grating. By putting the SWTG in front of the output window of a vertical cavity surface emitting laser (VCSEL), a polarization dependent optical feedback can be achieved. Using this feedback, we demonstrated the locking, switching, and enhancement of the VCSEL’s polarization. A polarization ratio as high as 200:1 was achieved.

Controlling polarization of vertical-cavity surface-emitting lasers using amorphous silicon subwavelength transmission gratings

Control of the polarization of vertical-cavity surface-emitting lasers (VCSELs) using a 240-nm-thick amorphous silicon subwavelength transmission grating (SWTG) is demonstrated. The grating, which has a strong polarization dependent reflectance and transmittance, was placed in front of the output window of a VCSEL. It was found that the SWTG can fix the VCSEL’s polarization in the direction perpendicular to the grating fingers, switch polarization directions, enhance the polarization ratio from 20:1 to over 200:1, and decrease the threshold current. Integration of these gratings into the VCSEL structure is discussed.

The radiation transfer through coated and tinted glazing

The incidence angle dependent transmittance and absorptance of special glazing containing coated or tinted glass are conventionally modelled using a simple application of the Fresnel equations, treating the glazing as if it were clear glass. This simplified approach leads to only a rough approximation of the true transmission and can result in an inaccurate assessment of the solar energy gains in highly glazed spaces such as atria and in commercial buildings where special glazing is popular. An improved calculation technique has been developed which is based on the fundamental optical behaviour of radiation in thin films and in substrates. It considers the physical effects of coherent and incoherent layers, interference, and absorption. The main feature of the model is its applicability to any combination of coating and glass, with no limitation on the number or sequence of the layers. This paper shows the theoretical basis of the calculation technique and demonstrates its ability to produce wavelength and incidence angle dependent properties of special glazing. The validity of the predictions is illustrated by comparison with measurements.

CuInSSe-doped glass saturable absorbers for the passive mode-locking of neodymium lasers

By using a CuInS2xSe2(1−x)-doped glass as an intracavity saturable absorber, passive mode-locking of Nd:YAG and Nd:YAlO3 lasers is obtained. Ultrashort laser pulses are achieved with duration of 16 ps (for Nd:YAlO3 at 1.08 μm) and 36 ps (for Nd:YAG at 1.064 μm), respectively. The intensity dependent transmittance and transient absorption change measurements are performed on CuInS2xSe2(1−x)-doped glasses at 1.08 μm with the use of laser pulses of 15 ps width. The bleaching relaxation time is found to be ∼11 ps.

Angular dependent transmittance in multilayer coating

Multilayer stacks with up to 7 layers have been designed to provide high luminous transmittance together with low near-infrared transmittance for unpolarized light incident at oblique angles. A substantial angular dependence in transmission is obtained from both theory and experiment. Six different dielectric materials (MgF2, CaF2, SiO2, ZnS, TiO2 and WO3) and three metals (Ag, Al and Cr) were used in this study.

Transmission switching using micro-encapsulated liquid crystal films

Micro-encapsulated liquid crystal devices have been prepared, which switch the amount of direct-hemispherical transmitted radiation in the solar spectral range by values up to 35%, depending on the film thickness and applied voltage. Examples of spectrally dependent transmittance, voltage dependence and angular distribution of the scattered radiation are presented. Such switchable devices can be considered for nonmechanical variable shading elements to prevent overheating in TIM systems.

Fluctuations and dissipation for a mirror in vacuum

A mirror in the vacuum is submitted to a radiation pressure exerted by scattered fields. It is known that the resulting mean force is zero for a motionless mirror, but not for a mirror moving with a non-uniform acceleration. We show here that this force results from a motional modification of the field scattering while being associated with the fluctuations of the radiation pressure on a motionless mirror. We consider the case of a scalar field in a two-dimensional spacetime and characterize the scattering upon the mirror by frequency dependent transmissivity and reflectivity functions obeying unitarity, causality and high frequency transparency conditions. We derive causal expressions for dissipation and fluctuations and exhibit their relation for any stationary input. We recover the known damping force at the limit of a perfect mirror in vacuum. Finally, we interpret the force as a mechanical signature of the squeezing effect associated with the mirror's motion.

Refractive indices of polymer-dispersed liquid-crystal film materials: Epoxy-based systems

Polymer-dispersed liquid crystal (PDLC) films are potentially useful in applications requiring electrically controllable light transmission. In these applications, both a high on-state transmittance and a strong off-state attenuation are often needed over a wide operating temperature range. These transmittance characteristics depend strongly on the refractive indices of the materials in the PDLC films. We have measured the temperature dependent refractive indices of typical PDLC film materials and the temperature dependent electro-optic transmittance of a PDLC film composed of liquid crystal microdroplets dispersed in an epoxy matrix. We show that our refractive index measurements can account for all the features in the measured transmittance characteristics and discuss several methods for controlling refractive indices to optimize electro-optic transmittance over an extended temperature range. We have also measured the room temperature refractive indices of mixtures of epoxy resins and hardeners as a function of composition. We discuss the problems associated with predicting the refractive indices of such mixtures in terms of either the volume fractions or mole fractions of the mixture components. These considerations are important in matching refractive indices of droplets and matrix materials to maximize on-state transmittance. The refractive indices of epoxy matrix materials increase monotonically with time during their chemical cure. The measured time dependence can be described by a simple model in which the concentrations of the reacting resin and hardener each decay exponentially in time with their own characteristic time constants while the concentration of the cured polymer increases. Finally, we relate the measured rates of index change with temperature to the coefficients of volume expansion of PDLC film materials; the results are used to discuss the mechanical stability of PDLC films.

A Light Control Film Composed of Liquid Crystal Droplets Dispersed in a UV-Curable Polymer

Abstract This paper describes a new class of light control films consisting of submicron liquid crystal droplets dispersed in ultraviolct-cured polymer matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electro-optical and thermal properties. This report describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance and light scattering, and calorimetric studies which indicate that microdroplet formation in the films occurs as a result of phase separation which takes place during the cure process.

A Light Control Film Composed of Liquid Crystal Droplets Dispersed in an Epoxy Matrix

Abstract This paper describes light control films consisting of submicron liquid crystal droplets dispersed in epoxy matrices. These films, which can respond optically to both applied electric fields and temperature changes, are potentially useful for displays and light shutters. The optical performance of these films depends on a variety of structural, electrooptical and thermal properties. This paper describes scanning electron microscope studies of film structure, measurements of voltage dependent film transmittance, contrast ratio and light scattering, and calorimetric studies of the cure process which governs microdroplet formation in the films.

Analytical determination of the optical performance of practical parabolic trough collectors from design data

A semifinite analytical formulation is presented to facilitate the efficient numerical computation of the concentrated radiant flux on receiver surfaces in the parabolic trough concentrator. The concentrated flux density is integrated to yield the interceptance ratio for the collector. By including models for the incident-angle dependent envelope transmittance and receiver absorptance, the concentration ratio for absorbed radiation is also calculated and integrated to yield the optical efficiency. Flux distribution results from this detailed analysis are compared with a point source model, and the optical efficiency results are compared with a first-order model. Since this model includes both a realistic nonuniform solar modeling as well as models for imperfect reflection, transmission, and absorption, it is capable of estimating the optical efficiency of practical collectors.

Radiative Transfer With Dependent Scattering by Particles: Part 2—Experimental Investigation

Dependent radiative scattering by particles is experimentally investigated using plane-parallel cells containing latex spheres of 11, 2, and 0.08 μm diameter dispersed in an air or water matrix. The dependent scattering efficiencies and the bidirectional transmittance and reflectance were measured and compared with analytical results. The close-packed 2-μm spheres, which were expected to show dependent scattering from the previous criterion, gave results identical to independent scattering. Measured dependent scattering efficiencies of the small particles tested decrease with increasing particle volume fraction and were compared with those predicted by the theoretical investigation. The bidirectional transmittance and reflectance of dependent scattering were compared with those of independent scattering with the same number of spheres within the test cells. Several different patterns of dependent transmittance and reflectance appeared depending on the optical thickness. Finally, a newly proposed regime map bounding independent and dependent scattering is compared with the present and previous experimental data.

Modeling canopy reflectance and microwave backscattering coefficient

Various approaches to model canopy reflectance (CR) in the visible/infrared region and backscattering coefficient (BSC) in the microwave region are compared and contrasted. It is noted that BSC can be related to CR in the source direction (the “hot spot” direction). By assuming a frequency dependent leaf reflectance and transmittance it is shown that the observed dependence of BSC on leaf area index, leaf angle distribution, angle of incidence, soil moisture content, and frequency can be simulated by a CR model. Thus both BSC and CR can, in principle, be calculated using a single model which has essentially the same parameters as many CR models do.

A Note on the Temperature Dependent Transmittance of Residual Films on Glasses

A Note on the Temperature Dependent Transmittance of Residual Films on Glasses