Space-based surveillance sensors are covered by a shroud to protect the delicate optics from adverse environments
(aerothermal heating and contamination) during hypersonic flight through the atmosphere. Once the sensor payload
reaches a safe altitude, the shroud is deployed and then sensor operation begins. When the pyrotechnic actuators are
fired to deploy the shroud or nosecone, large and microscopic particles are dislodged. The source of these particles is the
charred thermal protection insulation material on outer surface of the shroud, and particulate contaminants deposited on
the inside surface of shroud and on sensor components during assembly process. These dislodged particles can end up
within the sensor field of view (FOV), and remain there for extended periods of time, with the duration depending on the
air density and vehicle velocity. These undesirable particles within the sensor FOV can degrade infrared sensor
performance in several ways. These particles can cause obscuration, scattering and produce spurious thermal signature,
thus making it difficult to image the objects of interest. This paper presents the aeromodeling techniques used to estimate
the number and size of particles, and the duration these particles can stay within the sensor FOV. This information can
then be used to predict the resulting degradation in sensor performance.
Particulate contamination scatter is often modeled using Bidirectional Scatter Distribution Functions (BSDFs) based
upon Mie scattering by a distribution of spherical particles. Starting with the basic model described in P. R. Spyak and
W. L. Wolfe [1,2,3,4], we improve upon it by adding multiplicative geometrical form factors. These factors prevent the
Total Integrated Scatter (TIS) from exceeding unity and ensure that reciprocity is always obeyed. Preventing the TIS
from exceeding unity is necessary for energy to be conserved in the raytrace, and obeying reciprocity is necessary to
obtain consistent results between forward and backwards raytraces. As will be shown, this improved model fits
measured data better than the previous model.
It is often necessary in optical analysis software to trace millions of rays in order to determine flux transfer, color
chromaticity, and the distributions of intensity, irradiance, or radiance onto one or more targets. The key question of
"have I traced enough rays?" cannot be answered unless the statistical error in the final simulated result is below some
measurement criteria; for instance, if you are to determine irradiance uniformity to less than 1%, having a 2% statistical
error across the target will wash out what you are trying to analyze. Some optical analysis software packages do not
provide error estimation methods, while others use error estimation algorithms having assumptions that are not valid for
all cases. This paper describes how subdividing and recombining raytraces provides a robust method for estimating
error. We will show that this error estimation technique can be used with most optical analysis packages and we will
compare it with algorithms employed currently. Example systems will be analyzed and presented.
Methods for computing bin-by-bin error estimates of 2-D illumination and chromaticity distributions generated from Monte-Carlo raytracing data will be introduced, as well as algorithms for choosing the optimal number of bins based on the desired accuracy. Methods of improving the accuracy of such distributions will also be discussed, along with methods for smoothing these results for display purposes.
KEYWORDS: Mirrors, Solar concentrators, Sun, Reflectors, Photovoltaics, Solar cells, Solar energy, Detection and tracking algorithms, Collimation, Renewable energy
Advances in Photovoltaic technology using multijunction cells allow sunlight-to-electrical energy conversion efficiencies of 25 percent with the potential of reaching 30 percent. The main drawback with these cells is their high cost. By using a concentrating Photovoltaic (CPV) solar collector, the area/cost of the cells relative to the total system area/cost can be reduced substantially. The design of CPV systems has one constraint not found in standard thermal solar concentrators, namely the target is square and the irradiance uniformity goal is very tight. A novel two-stage solar collector system designed for the National Renewable Energy Laboratory (NREL) is presented here. By tailoring the radial profile of the primary mirror that is slightly non-parabolic and using a straight square tube secondary, designs for concentrations between 100-2,000 suns can achieve uniformity under 3 percent and greater than 95 percent efficiencies. A design using a non-rotationally symmetric primary design is also presented, which reduces the problems with shading by spiders that attach the secondary to the primary mirror.
KEYWORDS: Scattering, Data modeling, Light scattering, Laser scattering, Reflectors, Ray tracing, Palladium, Aluminum, Monte Carlo methods, Illumination software
Many surfaces scatter light in an anisotropic way, that is, for a normally incident beam, the distribution of scattered light varies as a function of the azimuthal angle of the scattered direction. Examples of surfaces with anisotropic scattering characteristics are brushed metal reflectors and certain types of diffusers. A model, based on an anisotropic scatter model proposed by Ward is introduced. The ability to fit this model to various sets of measured BSDF data is investigated. Raytracing simulations are performed using the fitted parameters, and the results are compared with experiments.
Various simulations of volume-based sources are explored, beginning with an overview of optical design software, the industry that utilizes it, and a procedural outline for source simulation. These simulations are explained from the simplest to most complex methodologies to date. Two basic approximations of the volume-emitter, (1) a tubular surface distribution and (2) a cylindrical volume distribution, that cannot model the asymmetry of the original emitting-volume are considered. Simulation methodologies that rely on mathematical tools are investigated. Using a CCD image of the emission and the inverse Abel transform, a 2D irradiance distribution is transformed into a 3D emitting volume. An algorithm developed to handle asymmetric volume-emitters is discussed, and the results of the simulated arc are compared to its original CCD image. In addition, the geometry of the arc source is modeled into a CAD (Computer Aided Design) program, and optical properties are assigned to its components in the optical/illumination design program. Using the most detailed emitter simulation, an assessment of the source geometry's influence on system output is made. The need for a detailed volume-emitter simulation is demonstrated through system output comparison between those utilizing the most complicated simulation and those using basic surface and volume approximations of the actual emitting-volume.
Software packages capable of simulating complex optical systems have the power to shorten the design process for non-imaging illumination, projection display, and other imaging illumination systems, Breault Research Organization's Advanced Systems Analysis Program (ASAP) and Robert McNeel and Associates' Rhinoceros computer aided design software, together, allow complicated optical systems to be simulated and analyzed. Through the use of Rhinoceros, an optical system can be accurately modeled in a 3D design environment. ASAP is then used to assign optical properties to the Rhinoceros CAD model. After the optical system has been characterized, it can be analyzed and optimized, by way of features specific to the ASAP optical analysis engine. Using this simulation technique, an HID arc source manufactured by Ushio America, Inc. is accurately represented. 2D CCD images are gathered for the source's emitting-volume across its spectral bandwidth. The images are processed within ASAP, via the inverse Abel command, to produce a 3D emitting-volume. This emitting-volume is combined with an accurate model of the source geometry and its optical properties, to finalize a functioning virtual source model. The characterized source is then joined with a simulated optical system for detailed performance analysis: namely, a projection display system.
With the development of faster computers, the ability to design and optimize complex optical systems has been dramatically improved. This directly translates into faster product development cycles with less need to build costly prototypes. Systems using light pipes, faceted Fresnel lenses, and nonimaging optics demand non-sequential raytracing, generalized surface modeling, and scattering and/or ray-splitting off of surfaces. Addressing these issues slows computation, resulting in time constraints that, in the past, prevented the use of software codes to do much more than analyze complex systems. Now, a system’s radiometric performance can be evaluated in minutes instead of hours, allowing more exotic computer aided design and optimization techniques to be used. We present rules-of-thumb on how to design, optimize, and tolerance illumination systems. Examples of systems include faceted light pipes and Tailored Edge-Ray Concentrators that create uniform illuminance. Applications for such systems are broad and include automotive, appliance, and room lighting.
We present a general design method of nonimaging optics that obtains the highest possible concentration for a given absorber shape. This technique, which uses a complimentary edge ray to simplify the geometrical formulism, recovers familiar designs for flat phase space distributions, such as trumpets, and (theta) 1-(theta) 2 concentrators. This method is easy to use and handles diverse boundary conditions, such as reflection, satisfying total internal reflection or design within a material of graded index. Presented is a novel two-stage 2D solar collector with a fixed circular primary mirror and nonimaging secondary. This newly developed secondary gives a 25% improvement over conventional nonimaging concentrators.
We have designed a Nd:YAG laser to be pumped by the High-Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory. Based on the unique features of the HFSF, the design objectives are high brightness and superior efficiency in primary mirror area utilization. The HFSF has a primary mirror of 11.5 m2 and a 1.85 f-number. With such a high f-number, the target is set off-axis and does not block incoming solar flux. Moreover, large f-number enables concentration which approaches the theoretical limit, and a two- dimensional non-imaging concentrator deposits the solar flux onto the internal part of a 10 mm diameter laser rod. For high brightness, we plan a wide low-loss fundamental mode and a laser rod aperture that suppresses high order modes. To get a fundamental mode, of up to a 2.5 mm waist, we have designed a convex-concave resonator, following well-known g1g2 equals 0.5 design for resonators with internal beam focusing. We have used the edge ray principle to design the concentrator, and ray traced the deposited power inside the laser rod. A 1.3% Nd doping level supports a maximal power deposition inside a 5 mm diameter.
A class of instruments designed for remote sensing of space plasmas by measuring energetic neutral atoms (ENA) uses a thin foil as both a signal generator and a light shield. An ENA imager must look directly at the ENA source region, which is also usually in intense source of H Ly(alpha) (1216 angstroms) photons. ENA are produced by charge exchange between energetic ions and the ambient neutrals, and both charge exchange cross section s and ion populations decrease with increasing ion energy. Therefore it is desirable to minimize the energy threshold for ENA detectors, at the same time maximizing the blocking of H Ly(alpha) . Optimizing filter design to meet these two contrary requirements has led us to measure the transmittance of thin C, Si/C, and Al/C foils at H Ly(alpha) . Our results indicate that (1) transmittance of < 7 X 10-4 can be achieved with (mu) g/cm2 Si on 1.7 (mu) g/cm2 C; (2) an Si/C composite foil with a thin carbon layer is more effective in blocking UV radiation while having the lowest energy threshold of all the foils measured; and (3) transmittance of Si/C foils of known Si and C thicknesses cannot be accurately predicted, but must be measured.
Nonimaging Optics and Efficient Illumination Systems II
31 July 2005 | San Diego, California, United States
Nonimaging Optics and Efficient Illumination Systems
2 August 2004 | Denver, Colorado, United States
Design of Efficient Illumination Systems
4 August 2003 | San Diego, California, United States
Course Instructor
SC389: Design of Non-Imaging Illumination Systems
This course covers the basics of illumination system design with emphasis on computer aided techniques. It begins by exploring the fundamental principles of illumination systems. An overview of various types of systems and light sources are presented and issues with Light Emitting Diodes (LEDs) are covered extensively. Detailed instruction on the tailored edge ray design (TED) method of nonimaging optics and algorithms for segmented reflector design of automotive headlamps will be covered. Design of lightpipes and other precision plastic-molded optics are covered along with mathematical optimization procedures. Scattering of light can be very useful in the design of illumination systems, particularly in backlighting applications. Techniques to handle these effects are discussed and the design of painted LCD backlights are covered in detail.
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