Why Does All of This Matter?
Different tasks underwater impose unique constraints and requirements on the system. The balance between lighting properties and practical limits must be based on the intended application. Understanding how to evaluate a light for the mission requirements and how lighting parameters change underwater is critical for system designers.
For instance, when designing illumination for remotely operated vehicle (ROV) piloting, the main objective is to provide the pilot with sufficient visual information about his surroundings to be able to safely maneuver to the work site, avoid hazards, and be alerted to nearby obstacles. This necessitates high contrast and high signal-to-noise ratio in the imaging system as well as broad, diffuse light sources that can be optimized for the local scattering conditions. The parameters with the most importance in this application would be total luminous flux, spectral content, and dimming range. Using higher CCT lighting (>5000K) helps ensure that less of the power emitted by a white source will be attenuated. Selecting a light with a high luminous flux will ensure that sufficient light is available to boost detectability for low contrast subjects or distant objects. Depending on the operating environment, dimming may also be critical in this application. Since conditions at a site can vary drastically, the ability to dim down the light source allows the pilot to compensate for high backscatter.
In a High Definition (HD) imaging application for inspection tasks, biological surveys, or archeological expeditions, the lighting requirements impose a very different set of constraints. For these applications, color accuracy, uniform illumination, and high angular contrast are key. HD imaging particularly benefits from a full, rich color space, which helps differentiate species in biological surveys or distinguish between natural surface variations and corrosion on a subsea structure. Color accuracy is most closely described by CRI parameters, but attenuation causes the spectral content of the light source, even a high CRI source, to change drastically over the distance to and back from the subject. This sets practical limits on imaging distances and the minimum illumination where color accuracy is needed. In some cases, supplemental monochromatic sources are required to boost spectral content in the highly attenuated portions of the visual spectrum. For a given imager size, sensors used in HD imaging have lower sensitivities than their standard definition counterparts and generally require more light. The higher resolution of HD imagers is also more sensitive to scattering from the reflected light off a subject since more subtle angular separation of light is detectable in the captured data. The scattering behavior of light underwater is proportional to the beam angle of the source. Having more tightly collimated light sources helps reduce scatter at longer distances and takes full advantage of the detector resolution.