As presented in September 2010 in Toulouse CI Systems has been developing the technology to produce Circular
Variable Filters (CVF) production. These filters are used as monochromators of medium spectral resolution in radiometric and spectroradiometric instrumentation for spectroscopic and remote sensing applications in the laboratory and in the field. As mentioned then, OCLI, the original US company that developed the old CVF technology, stopped production in the ‘90’s and CI started to reconstruct the technology in 2009. The first results of CVF performance following this new work were presented in the wavelength region of 1.3 to 2.5 microns in 2010. Since then we made significant progress and completed the development of segments from the visible range up to 14.3 microns, with a spectral resolution in the range of 0.5% to 2% of the peak wavelength. The advantage of the new production method is that both the so called “tooling factor” and the peak wavelength profile along the circumference are controlled by software instead of by fixed hardware gear ratios, yielding higher design flexibility. A new CVF model in the 7.7 to 12.6 micron range with the highest spectral resolution (0.5% FWHM) ever being produced has also been developed.
Up to about a decade or more ago Circular Variable Filters (CVF) were a commercial optical component useful in the
visible and infrared wavelength ranges and were being produced and sold by the US company Optical Coating
Laboratory, Inc. The CVF was used as a monochromator and its advantage was in its ease of use for spectroscopy
especially in remote sensing applications. Based on the idea of multilayer interference coating with linearly varying
thickness on the circumference of a rotating wheel, it worked as a continuously variable center-wavelength narrowbandpass
filter with 1-2% spectral resolution. One spin of the wheel placed on the optical path of a light beam provided a
full scan in the wavelength range and allowed a detector to record a full spectrum in that range. Since then this
component was discontinued and companies used old remaining stocks from previous production batches.
CI is now reconstructing the technology to be able to produce the CVF again and is building new radiometers for new
applications based on it. In this paper we review the history of the CVF and its use by CI and we provide some details on
the characteristics of the CVF as planned and as preliminarily tested.
In this work, physical and optical properties of ZnS films grown at different evaporation conditions have been
studied. ZnS 3000 nm thick films have been deposited on Ge substrates at 200°C, 120°C and without substrate heating.
In addition, evaporation rates of 4, 2 and 1 nm/s have been considered. The structural and morphological properties of
the films have been analysed by XRD and AFM, respectively and the refractive index in the 2.4-11.5 microns range has
been determined from transmittance spectra through reverse synthesis. From this analysis, the most suitable evaporation
conditions for ZnS thin films deposition have been defined in terms of film properties and intended applications on
thermal IR multilayer coatings.
Afterwards, adhesion properties of ZnS films deposited under the optimised conditions have been analysed. ZnS
films deposited at 120°C and 4 nm/s peeled off when subjected to MIL-F-48616 standard surface durability testing. The
use of a MgO bonding layer to enhance the ZnS film adherence to the substrate has been proposed and its effect on the
ZnS film properties has been studied. Finally, the mechanical stability of the ZnS coating under MIL-F-48616 standard
testing has been confirmed for films grown onto MgO coated substrates.
Near-infrared bandpass filters are commonly designed & manufactured using vacuum-evaporated films of Silicon and
Silicon Monoxide. However the transparency of these filters is limited by optical absorption in the films when producing
filters for wavelengths below 1200nm approximately. This work reports improvements in NIR filter transparency
achieved by exploiting recent advances in magnetron sputtering technology.
Sputtered silicon compound films have been used to demonstrate efficient bandpass filters for astronomy applications at
wavelengths below 1000nm. This process technology allows a new selection of film materials to be used in design of
NIR bandpass filters, with transmission and thermal drift characteristics which differ from conventional evaporated
coatings. The spectral location of the bandpass is controlled by a non-optical method, which avoids the complex optical
monitoring configurations normally required. The speed and flexibility of this process also offers a potential solution for
projects which require small batches of custom NIR optical filters.
Highly durable filters are obtained without elevated process temperature, which would otherwise be required in
conventional evaporation processes. This avoids heating filter substrates which may be sensitive to thermal cycling
effects.
Attenuation of sidebands to T<0.0001 is reported across the spectral range of common sensor devices. The thermal
sensitivity for cryostat applications is characterised and compared to conventional evaporated optical coatings. This
method has been applied to 975nm & 985nm bandpass filters for use on VISTA project instrumentation. It also offers
improvements for filters at longer wavelengths in the range 1000nm-5000nm. Some examples are reported in this region.
A recent program at Pilkington Optronics (Barr & Stroud) has resulted in the identification of a nonradioactive thorium fluoride replacement film material with reduced toxicity. As described in this paper, the materials has been incorporated into high-efficiency single and dual-band AR coatings for germanium, zinc selenide, and zinc sulphide, without compromising optical and durability performance.
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