An Upwelling Radiance Distribution Camera System, NURADS

We have built a new instrument to measure the in-water spectral upwelling radiance distribution. This instrument measures the radiance distribution at six wavelengths and obtains a complete suite of measurements (6 spectral data images with associated dark images) in approximately 2 minutes (in clear water). This instrument is much smaller than previous instruments (0.3 m in diameter and 0.3 m long), decreasing the instrument self-shading. It also has improved performance resulting from enhanced sensor sensitivity and a more subtle lens rolloff effect. We describe the instrument, its characterization, and show data examples from both clear and turbid water. ©2004 Optical Society of America OCIS codes: References and Links 1. Tyler, J.E. 1960. Radiance distribution as a function of depth in an underwater environment. Bull. Scripps Inst. Oceanogr. 7: 363-41. 2. Aas, E., and N. K. Hojerslev. 1999. Analysis of underwater radiance distribution observations: apparent optical properties and analytical functions describing the angular radiance distributions. J. Geophys. Res. 104: 8015 8024. 3. Smith, R. C., R. W. Austin and J. E. Tyler. 1970. An oceanographic radiance distribution camera system. Appl. Opt. 9: 2015-2022. 4. Voss, K. J. 1989. Electro-optic camera system for measurement of the underwater radiance distribution. Optical Engineering. 28: 241-247. 5. Voss, K. J. and A. L. Chapin. 1992. Next generation in-water radiance distribution camera system. Proc. Soc. Photo-Optical Instrumentation Engineers, 1759: 384 387. 6. Voss, K. J., C. D. Mobley, L. K. Sundman, J. Ivey, and C. Mazell. 2003. The spectral upwelling radiance distribution in optically shallow waters. Limnol. Oceanogr. 48: 364 373. 7. Voss, K. J. and A. Morel. 2005. Bidirectional reflectance function for oceanic waters with varying chlorophyll concentrations: measurements versus predictions. Limnol. Oceanogr. 50: 698 – 705. 8. Doyle, J. P., and K. J. Voss. 2000. 3D Instrument Self-Shading effects on in-water multi-directional radiance measurements. Ocean Optics XV, Monaco, October 16-20. 9. Helliwell, W. S., G. N. Sullivan, B. Macdonald, and K. J. Voss. 1990. A finite-difference discrete-ordinate solution to the three dimensional radiative transfer equation. Transport Theory and Statistical Physics, 19: 333-356. 10. Voss, K. J., and G. Zibordi. 1989. Radiometric and geometric calibration of a spectral electro-optic "fisheye' camera radiance distribution system. J. Atmosph. and Ocean. Techn., 6: 652-662. Introduction Measurement of the upwelling radiance distribution in the ocean is difficult because of the number of observations required, the variability of the light field due to surface waves, and instrument self-shading, yet the upwelling radiance distribution is vitally important in ocean remote sensing. The radiance distribution is described by the collection of radiance data in all directions coming to a single point. At least two previous instruments have used single radiometers, swept over various directions in the zenith and azimuth directions[1,2]. Tyler’s Pend Oreille data set[1] is still one of the fundamental tabulated data sets in the literature. The Aas and Hojerslev data set[2] was used to develop a model of the angular variation of the radiance distribution. Both of these data sets were collected at a single wavelength. Another method to measure the radiance distribution is the use of a fisheye camera system and a camera. This technique was first developed for in-water use by Smith, Austin, and Tyler[3], using a photographic camera, and a photopic filter. This system allows the radiance distribution to be collected in two images (one upwelling one downwelling), thus the entire radiance distribution can be obtained quickly, allowing rapid profiling if desired. The difficulty in this system was obtaining radiometric data from the photographic images. In addition, with a fisheye system, the lens radiometric effects must be taken into account, the main feature being lens-rolloff effects. More recently a series of instruments have been built, for use in the water, based on this fisheye technique, but using electro-optic cameras and remotely controlled spectral filter changers. The first of these [4] included CID electro-optic camera systems for both the upwelling and downwelling radiance distribution. This system consisted of three cans of instruments, with upwelling and downwelling systems in each can, along with a third can for the control electronics. The filter changer allowed selection of one of 4 spectral filters (25.4 mm interference filters), along with neutral density filters to adjust the overall sensitivity. These cameras were digitized with 8-bit frame grabbers, and did not have an intrinsically high dynamic range. But by coating the dome/window of the downwelling camera for zenith angles less than 45 degrees, and taking into account the lens system rolloff characteristics, the downwelling and upwelling radiance distribution dynamic range could be accommodated. One useful characteristic of the CID architecture was that excess light in one pixel does not spread into neighboring pixels (bloom). The next system in this series, RADS-II [5], was developed to take into account advances in camera CCD development. This system used similar optics and two cooled CCD cameras (512 x 512 pixels). Both cameras, along with the control computer were housed in the same housing. Along with these components, the system had tilt/roll detectors, compass, and a multispectral upand downwelling irradiance system (based on the MER 1032 [Biospherical Instruments] system). The system housing was a cylinder approximately 0.5 m in diameter and 0.75 meter long. The system contained the camera control computer and hard drive, and communication with the system was by an RS-232 link. When on the surface an Ethernet link could be established, which allowed downloading data via an ftp transfer to the host computer on deck. While the instrument was in the water, a remote-control program (PCHost) allowed the operator to control the embedded computer functions. The data from the cameras was digitized with a 16 bit digitizer, however system noise reduced overall dynamic range to approximately 12 bits. This instrument was used during several experiments, in particular to gather surface upwelling radiance distribution data [6,7]. Because of the interest in the surface upwelling radiance we found that we were only using the upwelling radiance distribution from the system. To get accurate radiance distributions, without the effect of ship shadow, necessitated floating the instrument away from the ship, which made it impossible to take upwelling/downwelling profiles, or collect useful downwelling information. The size of the instrument, including the buoy to float it, caused a noticeable shadow effect in the images even in clear water [8]. This effect had been seen with the earlier instrument [9] and was smaller than before, however it was still a noticeable perturbation. Because of instrument self-shading, and the desire to collect only upwelling radiance distribution data, the NURADS instrument was developed. The system still has a filter changer, embedded computer and hard drive, CCD camera, compass/tilt/roll sensor. However, by concentrating on only the upwelling radiance distribution and with advances in technology, this whole instrument package is only 0.3 m in diameter and 0.3 m long. With the reduction in size, the flotation can be reduced further reducing shadowing issues. The reduced self-shading and several other improvements have helped the instrument provide more accurate radiance distribution data. This paper will describe this instrument, and provide some sample data from the instrument. 2) Fundamental NURADS instrument description The instrument is based on an Apogee CCD array camera system (AP 260Ep) that uses the KAF-2610E CCD array. The camera housing includes the frame grabber electronics and interfaces directly with the embedded computer (Versalogic Panther, EPM-CPU-6) via the standard computer ECP parallel interface. The frame grabber digitizes the images at 16 bit resolution, the final system dynamic range will be discussed below. Included in the instrument is a 20 Gbyte 2.5” hard drive on which the camera images and auxiliary data is stored. There is an embedded EZ-Compass-3 (Advanced Orientation Systems, Inc.) that collects the tilt/roll/compass information and transfers it to the computer over a serial interface. This information is logged to the hard drive continuously in the background during camera operation using the Windmill program (Windmill Software, Ltd.). The system uses a fisheye adapter lens, designed for a Nikon Coolpix 950 camera (FC-E8), a custom lens relay system, and a MFW filter changer (Homeyer) (also controlled over a serial interface) to form the fisheye camera image and provide spectral filtering. The filter changer uses 25.4 mm interference filters, and allows the selection of one of 6 spectral filters for the image. The embedded computer is controlled by a surface laptop computer by use of the Timbuktu (Netopia) remote control software over an Ethernet link. While the data is stored on the embedded computer hard drive, once data collection is completed the data on the hard drive can be extracted via an ftp transfer. Because of speed advances in the camera frame grabbing technology, enhanced sensitivity of the CCD array, and faster optics, in clear water a complete set of data (6 light images, 6 dark images, one with each spectral filter) can be obtained in 2 minutes. The embedded computer controls the camera with a combination of MaxIM DL (Diffraction Limited), Microsoft Excel, and Microsoft Basic programs. The typical operation of the camera is to run the system continually acquiring data during the data collection period. By taking these multiple data sets, data can be excluded where the camera tilt/roll exceeds some threshold (usually 5 degrees), clouds are determined to be causing a problem with the incident light field, or some other temporary artifact is in the image. Multiple images are also averaged to reduce the effect of bright light rays due to waves at the air-sea interface. In 30 minutes, 15 complete sets of data (approximately 90 Mbytes of data) can be acquired. Figure 1) Picture of NuRADS system (fore ground) with the older RADS-II system (background). The new system is significantly more compact than the older system. 3) Characterization and Calibration The method to characterize and calibrate the fish eye systems has been described earlier for the first RADS system [10]. We will concentrate on the specific results for this system. The basic characteristics of the system relevant to the radiance distribution measurements are the lens system rolloff function, camera linearity, camera noise characteristics (dark noise and readout noise), and spectral calibration of filters/system. We will start with camera noise characteristics. A) Camera noise characteristics To see the background readout noise, and an indication of the dark noise level of the camera system, we averaged dark images (shutter closed) at several different integration times. The images were averaged, pixel-by-pixel to see if the variation was correlated with individual pixels, or was uniform across the array. Figure 3 shows a histogram of this pixel average (left) and the histogram for the standard deviation of the individual pixel averages (right). The figure below shows that the spread in integration times is very small for less than 2 sec, at 5 sec there is more variation. In terms of the standard deviation of the individual pixel averages, the average spread does not change significantly even at 5 sec integration. These two graphs imply that at longer integration times, there is some pixel-to-pixel variation in dark counts, but the noise does not increase. Thus subtracting a dark image from the data image is a better strategy then subtracting one overall average number. In our data collection we take a dark image for each data image, and subtract (pixel-by-pixel) this image from the data image.