Image Processing
The Benthos camera places the images on a 100-foot roll of 35 mm film in a non-standard
format. The images are placed along the long-axis of the film (this allows a larger image
size of the bottom than if the image were placed across the film in a standard 35 mm
format). The film is advanced using an O-ring drive; the loose drive and drive-motor
inertia results in the distance between frames varying slightly with each image. A light
emitting diode (LED) data display,
showing a record number and the day, hour, minute and
second of the photo, is placed on each image. The LED clock is a 31-day clock and counts without regard to month of year (the Benthos
camera was originally designed for obtaining photographs from ships over a period of a few
hours and thus the LED day could be reset as necessary). Used in a time-series mode, the 31-day
clock results in a day number on the film that does not always correspond to the day of the month
for deployments that include months with other than 31 days. For this film, the clock day was set
in the lab to day 6 on June 5; thus clock days in June are one day ahead of the true day (the last
day in June (the 30th) is day 31 on the LED clock). Clock days in July, August, and September
correspond to the true month day. A Table showing image number, time, and LED clock time is provided for navigating the jpeg images .
To register each image, the film was scanned and digitized using a scanner developed by
the Woods Hole Oceanographic Institution. The digitizing system adjusts the gain for low
light levels, so the light levels between images are not directly comparable. Especially
during times of sediment resuspension, the digitized images appear bright, where in fact,
the light levels are extremely low due to the high level of suspended sediments. The
imagery from this process was archived as tif images and as an mpeg time-series movie.
The digitized images were registered using a MATLAB
routine that searched the image for the data chamber (that is placed in the same
position on each frame) and adjusted the image position accordingly. This procedure
removed most of the ‘jitter’ resulting from the varying distance between frames
when the images are shown in a time-series mode. The resultant imagery was combined with
ancillary information to produce the Apple quicktime movie.
