Spectral reflectance properties of sand layers covering an underlying target: experimental measurements and mathematical modelling

Abstract

Adapting to natural backgrounds is important, yet very difficult, to achieve effective camouflage. One interesting aspect is the optical depth of biomaterials as a function of both geographical region, in which the biomaterials exist, and wavelength. The effective optical thickness of sand, for instance, will vary from scene to scene, and the amount of light that is reflected and transmitted is lower in the visible wavelength region than in the near-infrared region. Such variations in optical properties should be accounted for to ensure that synthetic camouflage material has high adaption to natural backgrounds over a large set of natural scenes as well as over a large range of wavelengths. The aim with this study has been to study optical reflectance properties of thin sand layers as a function of their thickness, given in weight per area. Sand is a biomaterial abundant in many parts of the world and relevant for many camouflage purposes. In specific, we have studied spectral reflectance (350–2500 nm) of 1–6 layers of sand covering a generic target object with known spectral signature. We also present mathematical models aiming to be able to estimate the reflectance of the sand layer samples, for a given layer thickness. The models are easy to use and we show that the models are able to reproduce the spectral reflectance properties of the sand samples. The model also allows for an estimation of the optical extinction coefficient as a function of wavelength and for a given sand type. This opens for a further estimation of transmittance and absorptance of the sand layers based on the experimental reflectance data, and we explore this in the paper. We also explore how the model is able to capture optical properties of sand and compare with corresponding optical properties of other natural materials, often found in thin layers, such as leaves and snow. We think our results will be a valuable contribution to developing multi-layered camouflage material that accounts for the variation of optical depth.