A BOTTOM STRIP MAP CAMERA 
by DR. ANGELO J. CAMPANELLA, Senior Physicist 
HRB-Singer, Inc. 
State College, Pennsylvania 
ABSTRACT 
A preliminary investigation of photographing 
a lake bottom using a scanning-type system is 
reported, An example of the results of tests 
conducted with a line-scan camera system is 
presented, 
INTRODJCTION 
In ocean and lake bottom photography, large 
area coverage at a suitable illumination level is 
difficult to obtain. To date, successful bottom 
photography has consisted of photographs of local 
coverage on the order of several feet in range in 
relatively clear water, 
The feasibility of a line scan strip map 
technique for underwater bottom photography is 
demonstrated herein, This experimental camera 
system offers the advantage of contrast ene 
hancement, minimum light level requirements, 
and the ability to trade resolution for sensitivity. 
This system was installed in a glass bottomed 
boat and bottom photographs taken of a shallow 
mountain lake are shown, 
CAMERA 
Figure | illustrates the camera's principle. 
A line scan motion is produced by rotating a 
plane mirror on an axis inclined 45° withrespect 
to its surface, This mirror reflects light into 
an objective aperature which focuses the light 
onto a photomultiplier cathode. The area of this 
image impinging onto the cathode is limited by a 
small variable aperture. The image of the 
aperture may be considered as being projected 
onto the ocean bottom. This spot is rapidly 
swept across the bottom surface ina lateral 
direction via the rotational motion of the scanner 
mirror, 
The maximum scan rate used was about 60 
lines per second, The forward motion of the 
vehicle carrying the camera unit provides the 
longitudinal scanning. A contiguous coverage of 
the bottom will occur for scanning speeds as 
fast as or faster than that required to lay one 
scanned strip adjacent to the previous, It is 
conventional to scan at a slightly greater RPM 
than that required for contiguity. This minimizes 
the raster effect in the recorded photograph. 
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The instantaneous signal amplitude received 
at the detector, in this case a photomultiplier, 
is amplified and its dc level carefully controlled 
so as to have a minimum bottom brightness 
encountered correspond to the threshold of the 
recording film, Recording film illumination is 
accomplished by feeding the signal from the 
control box into a glow lamp whose light output is 
a confined spot of brightness proportional to the 
input current, The recording film is wrapped 
around a barrel containing a rotating microscope 
objective, so that the film remains in the focal 
plane of the objective. The glow lamp output is 
projected along the axis of the barrel, folded 
90° by a mirror, and then enters the objective 
lens, The spot is projected on the recording 
film by the microscope objective. A comrnon 
Plus X type film has been found to have ample 
sensitivity. 
The film is moved past the barrel at a speed 
proportional to the velocity of the vehicle and 
inversely proportional to the distance to the 
bottom. The film speed used for these experi- 
ments corresponds to a vehicle velocity of about 
one foot per second and a bottom distance of 
seven feet. Gyro-stabilization of the roll axis 
is provided. 
The camera system can accommodate a 
wide range of illumination levels by the vari- 
ation of the area of the aperture before the 
photomultiplier detector. In particular, reso- 
lution can be exchanged for sensitivity. (This 
is impossibie in a conventional camera.) The 
resolution-sensitivity product is proportional to 
the area of the collecting aperture. Sensitivity 
greater than that of film can be achieved with 
some loss of resolution, 
Figure 2 is an example of a photograph that 
a line-scan system has produced. The resolution 
and dynamic range illustrated here are typical. 
DESCRIPTION OF WATER-BORNE EQUIPMENT 
The vehicle used to carry the camera equip- 
ment was a 12-foot, all-aluminum pram 
modified as shown in Figure 3. 
