Measurement system for marine animal reflectance functions

Abstract

Photonic structures in the skin of pelagic fishes and squids evolved specifically for hiding in the complex light field of the open ocean. To understand the principles under which these structures operate, a detailed characterization of their optical properties is required. An optical scatterometer has been developed to measure one important property, the bidirectional reflectance distribution function (BRDF). The instrument was used to collect reflectance functions from the squid Pterygioteuthis microlampas and fish Sternoptyx sp. Although the animals appear very different to a casual observer, the results reveal interesting similarities in their scattering patterns, suggesting a similar optical strategy for hiding in open water.

Three-dimensional midwater camouflage from a novel two-component photonic structure in hatchetfish skin

“…When radiance is incident on the lateral flank of the fish, the fish skin exhibits diffuse, broadband backscattering that is markedly concentrated in an arc parallel to the anterior–posterior axis of the animal, independent of the angle at which a ray of light strikes the skin ([14], figure 2). The same composite skin structures also scatter light ventrally, such that, when the lateral side of the animal receives normally incident illumination, some of this light exits the fish body through the ventral counterilluminating photophores.…”

“…For single-cell models with grid dimensions greater than 50 mm (figure 2 e ), we used the near2far function in MEEP to find scattering in the far field from these structures covering the same angular range probed by the BRDF instrument [14]. Otherwise, we had sufficient computational resources to reach the far-field regime for scattering in all directions.…”

“…We used these relationships between the number of photons in a bioluminescent flash and sighting distances r to calculate sighting distances (figure 5 b ). To scale between the reflective behaviour of a metallic mirror and the more complex scattering behaviour of the hatchetfish flank, we used the observed decrease in intensity of the scattered peak in the hatchetfish at the specular angle compared with that of a mirror as measured previously by the marine reflectance apparatus [14]. These two cases, of an isotropic searchlight and a completely collimated one, will then place bounds on the biologically realistic case in which a predatory searchlight is perhaps partially focused by a transparent lens-like layer over the surface of the isotropic bioluminescent tissue.…”

“…( a ) Bidirectional reflectance distribution function (BRDF) of hatchetfish skin and definition of incident and scattering angles, as measured by Haag et al . [14]. Units along the intensity axis are arbitrary, and remain arbitrary in all panels in this figure.…”

A contemporary perspective on underwater optical imaging

“…Man-made targets can be of special interest, especially when considering both underwater shipwrecks and explosive mines that were placed on the sea floor. The physical properties of objects such as spectral dependence, the wavelength dependence of the IOPs, and the directional dependence of the reflectance function, the Bidirectional Reflectance Distribution Function (BRDF), are necessary to achieve a complete understanding [4].…”