Visible and NIR Imagers
We were recently asked to design a very lightweight fisheye imaging systems for airborne use, potentially with an Unmanned Aerial Vehicle (UAV). This work is being done for Sandia National Laboratories, and the ARM UAV program. The instruments are being called the Diffuse Field Camera by the sponsor. The design goals were as follows:
The lens for the NIR camera is a Coastal Optics video lens, with a 185 degree field of view and an image with a 6.8 mm diameter. This lens is directly compatible with many 2/3” format chips. It was custom built with MgF2 coatings, which are compatible with the NIR wavelength. It was also modified with a filter holder, which places the optical filter in the back image plane. The filter target specifications were 1610 peak, 60 nm passband (FWHM). Actual specifications achieved were 1612 peak, 46 nm passband.
The camera is an Alpha NIR, provided by Indigo Systems. It has a 320 x 256 array. It is a InGaAs CMOS device with 12-bit digital output. Its front end was modified by MPL for compatability with the lens and filter. It is temperature stabilized to a temperature of 0 degrees C. It has an electronic shutter, which has the advantage of being more robust for use in flight, but the disadvantage that it is difficult to acquire a dark image. It has sufficient dynamic range, shutter range, absolute sensitivity, readout noise, and dark current specifications for our application.
Calibration of the NIR camera has been quite interesting, because its performance is quite different from Silicon CCD systems we have used before. Although it is considered a linear camera by standard manufacturing definitions, the linearity constraints must be much more rigorous in order to provide radiometric calibrations, and we found that quite significant corrections were necessary for these purposes.
The camera is a DVC 1312M. It has a 1300 x 1030 array, and is a silicon CCD device with a 12-bit digital output. The filter, filter mount, and C-mount were modified at MPL in order to place the image plane on the CCD chip. The camera is cooled, but not temperature stabilized. It has an electronic shutter, as well as sufficient dynamic range, shutter range, absolute sensitivity, readout noise, and dark current specifications for our application.
by the Marine Physical Laboratory, SIO.