Camera Filter Study
The NOAA NGS Remote Sensing Division carries out congressionally mandated shoreline mapping. The production technique used for decades has been aerial photogrammetry using large format metric cameras (e.g., the Wild / Leica RC series) and two different film emulsions: color-negative and black & white infrared (B&WIR). The B&WIR imagery collection is carried out at the MLLW tide state. Because near infra-red (NIR) radiation is strongly absorbed by water, B&WIR emulsion yields very high contrast between land and water, facilitating identification of the land-water interface. The NIR imagery is especially critical for quality assurance purposes as very clear shallow water is not always distinguishable from land in panchromatic or color imagery. The NOAA NGS Remote Sensing Division has conducted research on multiple techniques for shoreline mapping over the years. Some of these include topographic lidar, bathymetric lidar, hyperspectral spectroscopy, thermal imaging, and recently medium format digital frame photogrammetry from an Applanix DSS.
The NIR characteristics of the NOAA NOS owned DSS aerial digital camera had not been investigated. Based on sensor specifications published by the sensor element manufacturer (Kodak KAF-22000), it appeared that although it was not designed for it, black & white NIR imagery collection may be possible using the NOAA owned DSS camera. The chief concern was that the sensor utilized a CCD with a Bayer filter array which results in different sensitivities to visible & NIR bands from pixel to pixel. In addition, the quantum efficiency of the sensor decreases rapidly in the Red/NIR region of the spectrum. A field test was devised to examine the radiometric characteristics of the sensor using multiple long-pass filters (Phase I).
In May of 2006, the NOAA Citation II was conducting shoreline mapping work in the Great Lakes region. Because of the extremely clear water in the vicinity of Thunder Bay National Marine Sanctuary, this was identified as an excellent opportunity for the radiometric evaluation. Imagery was collected using two different long-pass filters (Wratten 87 & 87C), and one band-pass filter (Schott RG-9). For this trial, an altitude of 10,500' AGL (approx 3,500m) was flown which resulted in a 0.5 m GSD. High contrast images were collected using all three filters; however there was an unexpected problem with image focus. Light in the NIR region of the spectrum focused differently enough that the fixed-focus lens used for visible (RGB) image collection was not suitable. Based on the radiometric qualities of images taken using the different filters, it was determined that imagery further into the IR region could be collected. A Schott RG-850 filter was procured and working with the camera manufacturer (Applanix), a 60mm lens was modified to focus correctly in the NIR region.
Results from the refocused 60mm lens using the RG-850 filter have been extremely promising. High contrast imagery has been acquired and the image sharpness is sufficient for mapping purposes. RSD continues to evaluate the 850 nm filter as well as filters that reach further into the NIR.
