A second requirement for underwater photography is some form of artificial 
lighting. A variety of high intensity electronic flash units are available 
from the camera manufacturers which are suitable for submersible camera 
operations. Typically a rating of 150-200 ws ‘! is required. The problems 
of light absorption and scattering in water can be partially overcome by a 
suitable choice of camera and flash position, but even so, photographic 
distances are relatively short and limited to a maximum of about five 
metres. 
Colour film is preferable to monochrome as the photographic emulsion owing 
to-its superior interpretative capability. Suitable high speed emulsions 
are commercially available. 
PHOTOGRAMMETRIC ANALYSIS 
The adoption of a particular method of photogrammetric analysis is 
dictated by several constraints. With respect to the current application, 
both environmental considerations and the method of submersible operation 
are important factors to be considered. A further constraint is the use 
of existing commercially available equipment wherever possible. Any 
method of analysis selected should seek to produce an acceptable and 
consistent level of accuracy whilst retaining both simplicity of operation 
and versatility in its application. 
The underwater environment creates an object space of uncertain and 
variable properties which are radically different from the more 
conventional terrestrial environment. Density, salinity, pressure and 
temperature variations cannot be controlled and may result in Refractive 
Index variations of the order of 2% (McNeil, 1969). Dimensional change 
of the camera housing is another variable. The result is that there is 
no certainty that a camera calibration carried out on the surface in a 
shallow water tank will apply at depth in the marine environment. 
Further, within the marine environment there may well be significant 
changes in camera behaviour from one locality to another. This leads to 
the conclusion that any method of analysis should ideally include some 
form of camera calibration procedure which can be quickly performed 
within the operational environment on the site of the photogrammetric 
survey. 
It is difficult and time consuming to provide photo control for each 
stereopair in the underwater environment. Also it must be appreciated 
that the manned submersible cannot operate as a dedicated camera platform 
as it will have a range of other tasks to perform during the limited 
duration of the dive. Because of this, the dependence of any method of 
analysis on photo control should be minimal. 
The aim of this project is to produce a measurement and analysis system 
which can. be operated offshore on the submersible support ship. Hence all 
equipment must not only be commercially available but also be relatively 
portable and capable of operation on a relatively unstable platform. 
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