Concentrator PhotoVoltaic (CPV) solar energy systems concentrate the sun 500 - 1,000 times or more, in order to take economic advantage of the most advanced and efficient solar cells. The two prevalent system architectures use either reflective glass optics - such as based on a Cassegrain telescope design - or a refractive plastic system - either an acrylic or silicone-on-glass Fresnel lens - for concentration. Both systems have their advantages in areas of performance and durability. Both system designs manufacture their optics by low-cost processes that are unavailable to the other material system. These contrasts are reviewed. The refractive system embodies a simpler optical concept, requiring a single Fresnel lens rather than two concentrating mirrors. However, the reflective, glass system uses the greater design sophistication to provide a greater acceptance angle, which yields tolerance benefits in both manufacture and installation; and also provides faster optics without suffering the spectral aberrations of the refractive systems. Both glass and plastics are low-cost commodity materials. The long-term durability of optical glass is more firmly established than for optical plastics. And light transmission through optical plastics is attenuated by absorbance in both the UV and IR regions, in regions where such light is harvested by efficient multi-junction solar cells.
A primary failure mode for glass failure in reflective CPV systems is the mechanical stress caused by a
thermal gradient. To establish the necessary reflector specifications, it is essential to have both economic
techniques to measure glass strength and an insight into the failure mechanisms. Due to the highly
stochastic nature of glass fracture, large data sets are necessary for statistical validity and to provide
meaningful estimates of field failure rates. This paper discusses experimental measurement techniques for
both value-added reflectors and for non-value surrogate substrates which are generated as waste during the
manufacturing process. Specialized tooling enables measurement by commercially available stress-strain
equipment (e.g., "Instron" testers). The glass strength is calculated from the force-to-break data, sample
thickness and a substrate shape dependence. These strength data are regressed using a two-parameter
Weibull model, enabling calculation of the Weibull modulus, which is a measure of the distribution of
flaws of a brittle material. Using a finite element analysis model of the thermal-mechanical stress to
determine the critical stress, the Weibull analysis enables extrapolation of the data to predict field failure
rates. The test and regression now comprise an On-going Reliability Test (ORT) that is inherently low-cost
and appropriate for high-volume manufacture. For fracture, the initiating flaws are the result of glass
cutting and trimming operations. There can be low-strength outliers which result from bulk glass defects,
though such flawed product should be culled during the manufacturing and inspection processes. As
expected and commonly known, the glass strength is very sensitive to the cutting method and resulting
The raw materials for optical glasses and optical plastics are very different. The plastic feedstocks are volatile
liquids, petrochemicals, which are highly refined by industrial distillation. The feedstocks for inorganic glasses are
minerals, purified by solid processing methods. The optical plastic resin is always virgin stock; "regrind" is never
used for high-quality optical plastics. In contrast, the inorganic optical glass feedstock is improved by adding
"cullet", which is the recovered waste from breakage and trim during glass part production. This paper discusses the
sources and refinement of feedstocks for both glass and plastic, including consideration of cost, recycle and
ramifications for optical part production, and anticipated future trends. A snapshot summary of current marketplace
conditions is given.
This paper discusses a method to characterize feathering and determine feathering quality.
The characterization is based upon the change of the color coordinate across the transition region.
"Feathering" of an optical coating is the gradual taper, without any discernible boundary, from a
coated to uncoated region. There can be various reasons why a thin film optical coating may not be
applied to the entire surface of a see-through article. Feathering is necessary when the viewer,
looking through a transmissive element, is focused on the far-field and would be distracted by a
coating boundary which redirects focus to the near field. Done incorrectly, feathering may
produce visible artifacts which are ineffective and objectionable to the user. Examples are
Hardcoats are frequently used on polycarbonates and other optical plastics to increase durability by
improving scratch and abrasion resistance. The current study compares several commercially available
hardcoats, both dip-coated and vacuum-deposited materials. Compatibility of the hardcoat with vacuum
deposited optical coats is considered. The successive layers of optical coat and hardcoat on the
polycarbonate result in non-uniform spectral content and visible fringing effects due to thickness variation
and thin-film interference. Experimental results and model correlation are presented.
The HMD (Helmet Mounted Display) visor is a sophisticated article. It is both the optical combiner for the display and
personal protective equipment for the pilot. The visor must have dimensional and optical tolerances commensurate with
precision optics; and mechanical properties sufficient for a ballistic shield. Optimized processes and tooling are
necessary in order to manufacture a functional visor. This paper describes the manufacturing development of the visor
for the Joint Strike Fighter (JSF) HMD. The analytical and experimental basis for the tool and manufacturing process
development are described; as well as the metrological and testing methods to verify the visor design and function.
The requirements for the F-35 JSF visor are a generation beyond those for the HMD visor which currently flies on the
F-15, F-16 and F/A-18. The need for greater precision is manifest in the requirements for the tooling and molding
process for the visor. The visor is injection-molded optical polycarbonate, selected for its combination of optical,
mechanical and environmental properties. Proper design and manufacture of the tool - the mold - is essential. Design
of the manufacturing tooling is an iterative process between visor design, mold design, mechanical modeling and
polymer-flow modeling. Iterative design and manufacture enable the mold designer to define a polymer shrinkage
factor more precise than derived from modeling or recommended by the resin supplier.
The manufacturing design and process development for the Visor for the JHMCS (Joint Helmet Mounted Cueing System) are discussed. The JHMCS system is a Helmet Mounted Display (HMD) system currently flying on the F-15, F-16 and F/A-18 aircraft. The Visor manufacturing processes are essential to both system performance and economy. The Visor functions both as the system optical combiner and personal protective equipment for the pilot. The Visor material is optical polycarbonate. For a military HMD system, the mechanical and environmental properties of the Visor are as necessary as the optical properties. The visor must meet stringent dimensional requirements to assure adequate system optical performance. Injection molding can provide dimensional fidelity to the requirements, if done properly. Concurrent design of the visor and the tool (i.e., the injection mold) is essential. The concurrent design necessarily considers manufacturing operations and the use environment of the Visor. Computer modeling of the molding process is a necessary input to the mold design. With proper attention to product design and tool development, it is possible to improve upon published standard dimensional tolerances for molded polycarbonate articles.
The military environment imposes harsh conditions on adhesives. These conditions differ both qualitatively and quantitatively from typical civilian environments. Military systems must withstand exposure to moisture, temperature extremes, sunlight/ultraviolet radiation and other climatic stresses that are far in excess of what would be expected for commercial applications. Additionally, civilian products rarely consider issues such as fungus susceptibility, resistance to jet fuels and de-icing solvents, or resistance to chemical warfare agents and their decontaminants. The effect of military environments on both the optical and mechanical properties of optical adhesives are discussed for avionic display applications.