Variable transmission filters for UV-IR spectrometry
An Italian Research Centre developed small-dimension ultraviolet-to-infrared transmission filters with variable spatial performance. These optical components were originally manufactured for spaceborne ultra-compact and low-weight spectrometers. The transmission peak varies with the position along the surface of the filter, thus enabling a wide-range spectrometric analysis with a miniaturized and compact instrument. Such methodology can be used to construct portable instruments.
In the context of an ESA project with the aim of studying instruments for the Sentinel mission, an Italian Research Centre developed innovative optical coatings able to give narrow-band transmission filters in which the wavelength of the transmission peak varies linearly along the surface of the filter itself. This effect is obtained by a controlled progressive change in the coating thickness to produce corresponding changes in the coating performance. The main difficulties in making this Linearly Variable Filters (LVFs) for space applications were the wide operating spectrum (400-2500 nm) and the small geometrical dimension (few mm). Both characteristics are, indeed, jointly required for image spectrometry from space. In order to cover the required wavelength range, a dual hybrid detector was used, made of two adjacent CCDs that operate in two contiguous wavelength ranges: 400–930 nm and 930–2500 nm, respectively. The proposed fabrication process is based on magnetron radiofrequency sputtering deposition in which each real-time thickness measurement and analysis cycle during the fabrication adds new data, allowing an improvement of the process accuracy until the end. In order to face the specifications above, the Italian Research Centre developed a dedicated fabrication process for narrow-band transmission filters with a continuous spatial variation of their performance. Both classical all-dielectric transmission filters and metal-dielectric filters (like induced transmission filters) can be offered. An example of variable performance achievable with a 21-layer induced transmission filter is shown in Figure 1 in the range 400–930 nm and 930–2500 nm respectively. All-dielectric filters would require some hundred layers to obtain similar performance. The Linearly Variable Filters presented here were also proposed, by the same Italian Research Centre, as optical components of a miniaturized spectro-photo/radio-metric camera designed for both portable and in-situ applications. An example is the analysis of artworks by a miniaturized camera whose working principle is based on the measurement of the spectral reflectance factor or the spectral radiance of the observed object, without moving the whole instrument. The fundamental difference between a usual scanner and this camera is that in the first case the whole apparatus is moved along a path as long as the scene, whereas in this camera its body is still and the LVF inside of it is the only moving part. Its movement is obtained by a high precision piezo-positioner stage. This solution allows a compact design and easily-portable instrument. The sketch of the whole spectro-photo/radio-metric camera is shown in Figure 2. The objective lens of the camera focuses the image of a scene on a plane that in Figure 2 is called the 1st image plane. The objective lens is designed as an almost telecentric lens on the image side to have light rays crossing the image plane within a narrow angle. A LVF is positioned on the 1st image plane. A shift of the LVF orthogonal to the optical axis of the camera produces an image spectral filtering. Any image strip orthogonal to the shift direction is selected at the wavelength λ, typical of the corresponding line on the LVF. The successive selections of the wavelengths are made through the LVF shifting. Step by step, the measurement of the spectral radiance of all the image strips is fully accomplished.
Innovations & Advantages
Development of dedicated variable filters with the following features:
- Operating wavelength range: a wide range from the ultraviolet to infrared (upon request)
- Spatial profile of the peak transmittance: linear or non-linear distribution over the surface
- Optimized fabrication process: all-dielectric and metal-dielectric design
The Italian Research Centre, in addition to the filter manufacturing, may offer the characterization of the filter performance in terms of optical behavior, and analysis of the filter stability in the conditions of use, including harsh environment
Current and Potential Domains of Application
The proposed technology was originally developed for space exploration applications, namely for Earth Observation instruments. Terrestrial applications can be found for various sectors. In general, the proposed technology can address all areas where real-time non destructive inspections are needed, both for in-situ and portable applications. Areas where this technology could be applied include bio-imaging, diagnostic equipment and process control equipment, for example for industrial manufacturing lines or for cultural heritage applications.