Linear polarization of light emanating from bioluminescence has not been reported (but see [29
]), although several types of photophores possess light-reflecting platelets that help broadcast the light from such organs [30
]. Since reflected light generally becomes partially linearly polarized (depending upon platelet structure or any depolarizing structures that may be associated with photophores), the possibility exists that some bioluminescence is partly polarized. Thus, polarization sensitivity may play a role in detecting bioluminescent sources, yet this possibility awaits empirical examination.
In dim waters, detecting a flash of light at near horizontal lines of sight is mainly a question of contrast detection ([31
], reviewed in [34
]). However, estimating the distance to this point source is much more challenging, especially if no other distance markers are available. Although binocular vision is the predominant way for depth perception, such light flashes can often be detected only by a single eye or they appear at an orientation where binocular vision is not reliable. Polarization sensitivity may play a role in estimating the range to bioluminescing objects. In the open mesopelagic region, where light is coming down in a fairly regular and predictable manner [36
], it is possible to estimate the distance to an unpolarized source by examining the polarization characteristics of the light arriving from it. Veiling light, meaning downwelling light that is scattered between the object and the viewer, will be partially polarized. The amount of this partial polarization is positively correlated with the amount of veiling light, i.e. the distance between the object and the viewer. This relationship holds up to a given maximum of polarization that is characteristic of the water properties and the orientation of the line of sight. This maximal value can be seen at the open waters near by, yet unobscured by the examined light source [10
]. In a terrestrial setting, Namer et al.
] have been able to use such levels of linear polarization to estimate distances to objects located several kilometres away from a camera. In an aquatic setting, this range is naturally much shorter.
Assuming a non-polarized bioluminescing source, one can calculate the distance to it (d
) by examining the level of polarization arriving in the beam.
is the beam attenuation coefficient of the water, L
is the radiance of the bioluminescence viewed at distance d
the radiance of the background, P∞
the degree of background polarization and P
the degree of polarization of light arriving from the bioluminescent source (see appendix A for derivation). Since c
is relatively stable at these depths [36
], one can assume that animals have a sense of the attenuation of light in their environment.
Several species of mesopelagic squid possess polarization sensitivity [38
]. Such range-detection capabilities can be an important function of this sensitivity. It is unlikely that squid perform such calculations in each case, but it can be expected that a distance estimation system is hard-wired in their nervous system or acquired over time and experience. Such a neurological approach for polarization calculations can be found in compact nervous systems such as those of insects [39
Behavioural experiments as well as physical modelling are required to examine the actual use of this polarization-based target ranging. Anatomical studies of photophore structure correlated with polarization measurements from individual photophores are also needed to determine if/how certain types of photophores produce polarized light.