Performance model for cooled IR filters

A low-temperature model is described for infrared multilayer filters containing PbTe (or other semiconductor) and ZnSe (or other II/VI). The model is based on dielectric dispersion with semiconductor carrier dispersion added. It predicts an improved performance on cooling such as would be useful to avoid erroneous signals from optics in spaceflight radiometers. Agreement with measurement is obtained over the initial temperature range 70-400K and wavelength range 2.5-20µm.

New algorithms for synthesized design of optical filters

Chebyshev optical-filter algorithms for low-cost microcomputers have been improved. An offset ripple is now used for better transmission/matching in low-pass stacks. A prototype for narrowband filters is now more general and nearer practicability.

Exposure to space radiation of high-performance infrared multilayer-filters and materials technology experiments

Infrared multilayer interference filters have been used extensively in satellite radiometers for about 15 years. Filters manufactured by the University of Reading have been used in Nimbus 5, 6, and 7, TIROS N, and the Pioneer Venus orbiter. The ability of the filters to withstand the space environment in these applications is critical; if degradation takes place, the effects would range from worsening of signal-to-noise performance to complete system failure. An experiment on the LDEF will enable the filters, for the first time, to be subjected to authoritative spectral measurements following space exposure to ascertain their suitability for spacecraft use and to permit an understanding of degradation mechanisms.

Measurement of infrared multilayer filters at temperatures down to 4 deg K

Measurement is reported at 4 deg K (and blocked transmission below 10-5) of PbTe/ZnS thin-film filters deposited on Ge substrates. The reduced carrier-absorption which is obtained by cooling these PbTe films is found to accord with simple theory. Advantage for various high-performance multilayers by cooling is significant at the longer wavelengths, and has been verified.

Far infrared filters for the GALILEO-Jupiter and other missions

Extrapolation of PbTe/II-VI multilayer interference-filter technique from 20 to beyond 40µm is described and PbTe transparency reviewed; improvements below 20µm are reported. A composite filter cutting on steeply at 40µm is described that uses absorptive films of ZnS and As2S3, thin Quartz, and supplementary multilayer interference. Absorptive filters are described containing the II-VI compounds since these are found transparent at wavelengths shorter and longer than their reststrahl.

Durability assessment of PbTe/II-IV infrared filters (Space Shuttle 1st LDEF)

Residual stress having been further reduced, selected infrared coatings composed of thin films of (PbTe/ ZnS (or ZnSe) can now be made which comply with the durability requirements of MIL-48616 whilst retaining transparency. Such improved durability is due to the sequence:- i) controlled deposition, followed by ii) immediate exposure to air, followed by iii) annealing in vacuo to relieve stress. (At the time of writing we assume the empiric procedure "exposure to air/annealing in vacuo" acts to relieve the inherent stresses of deposition). As part of their testing, representative sample filters prepared by the procedure are being assembled for the shuttle's 1st Long Duration Exposure Facility (to be placed in earth orbit for a considerable period and then recovered for analysis). The sample filters comprise various narrowband-designs to permit deduction of the constituent thin film optical properties. The Reading assembly also contains representative sample of the infrared crystals, glasses, thin-film absorbers and bulk absorbers, and samples of shorter-wavelength filters prepared similarly but made with Ge/SiO. Findings on durability and transparency after exposure will be reported.

Synthesised design of optical filters assisted by microcomputer

New algorithms and microcomputer-programs for generating original multilayer designs (and printing a spectral graph) from refractive-index input are presented. The programs are characterised TSHEBYSHEV, HERPIN, MULTILAYER-SPECTRUM and have originated new designs of narrow-stopband, non-polarizing edge, and Tshebyshev optical filter. Computation procedure is an exact synthesis (so far that is possible) numerical refinement not having been needed.

Cooled filters 16-42µm for FIR astronomy

The design and manufacture of dielectric-film interference filters for cooled FIR stronmy is described. The bands are 16.5-21.5µm, 17.5-19.5µm, 19.5-21.5µm and 27µm cut on. The films are PbTe/CdSe and the substrates are CdTe (some 1/2 mm thick), without absorption in the region: KRS-6 films are used for antireflection.

Image fusion based on median filters and SOFM neural networks: a three-step scheme

This paper presents a new image data fusion scheme by combining median filtering with self-organizing feature map (SOFM) neural networks. The scheme consists of three steps: (1) pre-processing of the images, where weighted median filtering removes part of the noise components corrupting the image, (2) pixel clustering for each image using self-organizing feature map neural networks, and (3) fusion of the images obtained in Step (2), which suppresses the residual noise components and thus further improves the image quality. It proves that such a three-step combination offers an impressive effectiveness and performance improvement, which is confirmed by simulations involving three image sensors (each of which has a different noise structure).

Structure adaptation of linear MMSE adaptive filters

The tap-length, or the number of the taps, is an important structural parameter of the linear MMSE adaptive filter. Although the optimum tap-length that balances performance and complexity varies with scenarios, most current adaptive filters fix the tap-length at some compromise value, making them inefficient to implement especially in time-varying scenarios. A novel gradient search based variable tap-length algorithm is proposed, using the concept of the pseudo-fractional tap-length, and it is shown that the new algorithm can converge to the optimum tap-length in the mean. Results of computer simulations are also provided to verify the analysis.

Advances in sequential data assimilation and numerical weather forecasting: an Ensemble Transform Kalman-Bucy Filter, a study on clustering in deterministic Ensemble Square Root Filters, and a test of a new time stepping scheme in an atmospheric model

This dissertation deals with aspects of sequential data assimilation (in particular ensemble Kalman filtering) and numerical weather forecasting. In the first part, the recently formulated Ensemble Kalman-Bucy (EnKBF) filter is revisited. It is shown that the previously used numerical integration scheme fails when the magnitude of the background error covariance grows beyond that of the observational error covariance in the forecast window. Therefore, we present a suitable integration scheme that handles the stiffening of the differential equations involved and doesn’t represent further computational expense. Moreover, a transform-based alternative to the EnKBF is developed: under this scheme, the operations are performed in the ensemble space instead of in the state space. Advantages of this formulation are explained. For the first time, the EnKBF is implemented in an atmospheric model. The second part of this work deals with ensemble clustering, a phenomenon that arises when performing data assimilation using of deterministic ensemble square root filters in highly nonlinear forecast models. Namely, an M-member ensemble detaches into an outlier and a cluster of M-1 members. Previous works may suggest that this issue represents a failure of EnSRFs; this work dispels that notion. It is shown that ensemble clustering can be reverted also due to nonlinear processes, in particular the alternation between nonlinear expansion and compression of the ensemble for different regions of the attractor. Some EnSRFs that use random rotations have been developed to overcome this issue; these formulations are analyzed and their advantages and disadvantages with respect to common EnSRFs are discussed. The third and last part contains the implementation of the Robert-Asselin-Williams (RAW) filter in an atmospheric model. The RAW filter is an improvement to the widely popular Robert-Asselin filter that successfully suppresses spurious computational waves while avoiding any distortion in the mean value of the function. Using statistical significance tests both at the local and field level, it is shown that the climatology of the SPEEDY model is not modified by the changed time stepping scheme; hence, no retuning of the parameterizations is required. It is found the accuracy of the medium-term forecasts is increased by using the RAW filter.

Ensemble clustering in deterministic ensemble Kalman filters

Ensemble clustering (EC) can arise in data assimilation with ensemble square root filters (EnSRFs) using non-linear models: an M-member ensemble splits into a single outlier and a cluster of M−1 members. The stochastic Ensemble Kalman Filter does not present this problem. Modifications to the EnSRFs by a periodic resampling of the ensemble through random rotations have been proposed to address it. We introduce a metric to quantify the presence of EC and present evidence to dispel the notion that EC leads to filter failure. Starting from a univariate model, we show that EC is not a permanent but transient phenomenon; it occurs intermittently in non-linear models. We perform a series of data assimilation experiments using a standard EnSRF and a modified EnSRF by a resampling though random rotations. The modified EnSRF thus alleviates issues associated with EC at the cost of traceability of individual ensemble trajectories and cannot use some of algorithms that enhance performance of standard EnSRF. In the non-linear regimes of low-dimensional models, the analysis root mean square error of the standard EnSRF slowly grows with ensemble size if the size is larger than the dimension of the model state. However, we do not observe this problem in a more complex model that uses an ensemble size much smaller than the dimension of the model state, along with inflation and localisation. Overall, we find that transient EC does not handicap the performance of the standard EnSRF.