Absorption type solar blind ultraviolet (SBUV) filters with transmittance levels of 10 - 20%, FWHM band widths of 16 - 20 nm and blocking levels exceeding 12 OD have been developed for use with image intensified CCD (ICCD) cameras with CsTe, RbTe and CsRbTe photocathodes. Solar blind UV ICCD cameras equipped with these filters produce no measurable signals when directly viewing the sun at noon time in a summer day. Images of fire, gunfire flashes and high voltage transmission line corona were obtained in full daylight at high signal to noise ratio, by using these imagers. A solar blind filter was constructed for BCCD camera with UVAR coating. Images of fire and low power light sources can be obtained in full daylight when an appropriate solar blind filter is coupled to a BCCD camera. Results with bispectral imagers, combining solar blind UV images and images of the visible scene, are reported. For visualization of very weak UV sources use of solar blind image intensifiers is of advantage. This advantage can in principle also be achieved by EBCCD technology.
A solar blind UV bandpass filter technology has been developed which is appropriate for use with all types of UV imaging devices under full daylight conditions. This technology is based on dye doped polymer films that provide the critical solar UV blocking and the required sharp rejection slope,due to their intense absorption bands in specific portions of the UV. These films are combined with other components that block visible and near IR light to form selective and very efficient sun blocking UV bandpass filters. This technology is very flexible and permits tailoring of the filter transmission and its blocking characteristics. Solar blind bandpass filters based on this technology have been produced for use with MCP imagers with CsTe and RbTe photocathodes. Filter characteristics include 10-25 percent peak transmission, bandwidths of 16-22 nm and out of band blocking levels in excess of 12 OD. Using these filters, under direct midday summer sun, background signal levels less than 10 photons per second were obtained, when integrate dover the entire field of view of the imager. Under development is a filter for use with back illuminated CCDs. Target specifications include 15 percent peak transmission, 20 nm bandwidth and substantial blocking of solar radiation from 290 to 1100 nm. Feasibility for CCD imaging in the solar blind range was proven with a BCCD camera.
A technology has been developed for producing polymer based UV absorption filters. The filters are fabricated by embedding UV absorbing dyes in polymer matrices. A selection of dyes with different chromophores have been synthesized. Each provides a high level of blocking within a specific band in the UV. Polymer filters are combined with other filtering components that absorb in the visible and IR regions to form selective UV bandpass filters. Typical filter combinations achieve transmission levels of 10% and more in the mid UV and absorption levels reaching up to 12 OD and higher in the remaining UV, visible and near IR regions. An important advantage of these UV bandpass filters is the exclusive use of absorption type components. This provides consistent performance over wide acceptance angles. Filters with spectral transmission profiles optimized for specific detection applications in the UVc and UVb regions have been designed. Optimization takes into consideration the spectral response of the detector to achieve the sensor spectral response required by the application. Solar blind PMT sensors incorporating these files have demonstrated blindness to noontime solar illumination while maintaining good response from 245 to 270 nm in the solar blind range. Complete blocking of solar radiation and good sensitivity in the solar blind range has also been achieved with SiC solid state photodiodes.
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