There are currently four types of transparent electrically conductive filters available. These are
(1) a free-standing conducting inductive mesh,
(2) a substrate coated with a conducting inductive mesh,
(3) a conducting substrate,
and, (4) a substrate coated with a continuous conducting coating.
Each can be designed to provide high visible, near infrared, and/or far infrared transmittance and high radio frequency (RF) and microwave reflectance. Itek has developed theoretical models to predict the performance of each filter type at optical and very long wavelengths as a function of pertinent design features such as conductivity, electronic mobility, thickness, mesh period and line width. The temperature dependence is explicitly included. This paper presents Itek’s methods for predicting the performance of these four filter types at both optical and RF frequencies.
Coextruded multilayer films consisting of 657 layers of three different polymers have been made which reflect near infrared wavelengths while transmitting visible light. A repeating multilayer stack of the three polymers in layer sequence A/B/C/B with optical thickness ratios of 2/1/2/1 suppress second through fourth order reflectance when refractive indices of the polymers are chosen so that nB = (nAnC)11/2. Broadband reflectance of solar infrared can be obtained by imposing a layer thickness gradient through the crossection of the film. Compared to conventional vacuum deposited multilayer interference stacks, all-polymeric films can be tough and thermoformable. They do not corrode and there is no metal waste. Potential applications include solar energy management. Multilayer film design, polymer specification and coextrusion of initial films to demonstrate feasibility are described.
The design of ideal infrared thin-film polarization preserving reflectors requires the equalization of the p and s polarization reflectances and zero differential phase shift between them. Depending on system design requirements for absolute reflectivity, either non-absorbing substrates such as zinc selcnide, or metallic films such as silver or gold are commonly utilized. In addition, a few dielectric layers are deposited onto the substrate for reflection enhancement and phase correction. This paper will investigate the design of enhancement layers with refractive indices n1 and n2, onto various substrates for a single wavelength and in most cases, 45 degrees angle of incidence. Indices of actual film materials such as thorium fluoride, germanium and zinc sulfide will be utilized to demonstrate actual design performance. Also, an equation is presented that is used to predict the differential phase shift sensitivity to wavelength centering of a quarter wave stack. In this case, the film indices determine the incidence angle sensitivity. Next, the incidence angle sensitivity of some designs is investigated. Some designs act as polarization preserving reflectors from normal incidence to nearly 80 degrees angle of incidence. A brief summary of conventional enhanced metal coatings is presented, along with design methods. Some applications of polarization preserving reflectors are described, especially for C02 lasers.
Far-IR materials and processes used in atmospheric remote sensing are described. Aspects of multilayer design and their manufacture are mentioned, and results of a satisfactory exposure on NASA's shuttle-based LDEF included.
An electrochromic device (ECD) has in its center an electrolyte or ion conductor, which is in contact with films that provide optical modulation, ion storage, and (transparent) electrical conduction. We review designs and properties of ECDs, giving special emphasis to infrared optical characteristics. The ECDs are categorized according to the type of electrolyte or ion conductor: liquid, solid inorganic in bulk- or thin film form, or solid organic (polymer). The electrochromic film is W oxide; this material is used in the majority of all studied ECDs.
Boron phosphide (BP) has outstanding mechanical, optical and thermal properties. It has been prepared as a single crystal and as a coating by various techniques. BP coatings have been shown to combine good broad-band transmission with exceptional durability and erosion protection of all common infrared optical materials. Their protective properties on various substrates have been assessed in detail in whirling arm rain erosion tests and by water jet impact measurements, and the thickness dependences determined. Comparisons have been drawn between BP and other durable coatings. The productionising of BP coating processes is well advanced after overcoming a number of problems, in particular the exceptional hazards presented by the feedstocks.
Ion beam-based thin film deposition processes are reviewed. Results illustrating some of the benefits of ion bombardment to fluoride coatings are presented. Fluoride coatings deposited using ion beam processes have higher packing density, lower stress, improved durability, and better adhesion.
Gain enhancement from IR detectors could be realized if they were antireflection (AR) coated. The feasibility of AR coating silicon surfaces over the 6 to 11 micron and 11 to 17 micron bands has been demonstrated using single and double-layer coatings. The band averaged reflectance values were reduced from about 30% to about 4% with double-layer coatings. The actual spectral reflectance curves agreed very closely to the computer-modelled performances. Temperature and material-pair parameters were established that resulted in thermally durable combinations. The coated silicon was successfully cycled between room temperature and 77 degrees Kelvin. Application of this coating to Si:As Impurity Band Conduction detectors was successfully demonstrated in the 6 to 11 micron spectral band.
The performance of thin film interference bandpass filters is affected by several external parameters. The filters must therefore be characterized under simulated operating conditions. Some of the problems encountered and solutions to these problems are discussed.
Infrared (IR) optical sensor systems use thin film optical coatings to enhance inband transmittance and reject out of band radiation. Contamination degrades spectral performance by scattering, absorbing or reflecting incident radiation. Increasingly stringent requirements dictate that these coated optical surfaces must be maintained clean throughout their lifetime. Carbon dioxide jet spray is a promising method for cleaning delicate surfaces. This paper discusses IR sensor cleanliness requirements, the carbon dioxide (CO2) jet spray cleaning technique, and applications where it is used to clean IR thin film coatings.
BTDF at 3.39 microns was measured on ZnSe substrates and on substrates with 1/2, 2/3, and full multilayer coatings. BRDF at 3.39 microns was also measured on Ge substrates with multilayer coatings. The BTDF and BRDF range for coated samples overlaps the range for the uncoated substrates.
The development of novel, highly durable coatings useful in protecting infrared materials against abrasion and high speed raindrop impact damage is reviewed. Key materials properties including fracture toughness, elastic modulus, hardness as well as internal stress in the film and substrate are discussed. Significant levels of protection have been demonstrated by several investigators.