Deceptive Iridescence

Iridescence is a property of materials both widespread in nature and of growing use in a range of industries. We aim to determine whether it can function as a form of deceptive camouflage and, if so, how. Understanding the costs and benefits of having such colours will not only help explain their widespread occurrence in nature, but also help direct their efficient use in commercial, security and defence applications.
Colour is an integral, and striking, feature of the natural world and has diverse functions in both plant and animal kingdoms - from pigments that enhance the photosynthetic capacity of plants to the vivid plumage of birds. However, the function of one sort of colour, iridescence, is not yet fully understood. Iridescence is termed a "structural" colour because the hue of the reflected light is due to the structural rather than chemical properties of a material. The defining feature of iridescence is that the colour of the object changes with changing angle of viewing.
In both animals and plants there are multiple ways of producing and modifying iridescence, and these have different visual effects. Many of these biological mechanisms have been characterised and can be reproduced synthetically, and these methods of producing colour are increasingly used in various industries, from car paints where their role is to create a striking visual impact, to the production of bank notes, where the complex colour variation is hard to forge. The iridescent colours of Christmas wrapping paper and peacocks' trains are clearly designed to attract attention, but not all iridescence can be explained this way. A fundamental property of iridescence is that it changes colour when either the object or observer moves. This means it could act as effective camouflage through several mechanisms - the shifting shades could disrupt object outline, by distracting the visual system from other identifying features, and/or by acting as a dynamic 'dazzle coloration' to confuse the viewer. Pilot data collected by the PI suggests at least one of these possibilities is effective, as iridescence makes it hard for bees to discriminate between the shapes of objects. Our proposed research aims to test these possibilities.
We will focus on iridescence in land animals. We will investigate its impact on the ability to identify, discriminate and track objects using three model animals with very different visual systems - bee, bird and human. Standard iridescent 'target' materials, or different types, will be generated in collaboration with commercial companies. Their colours will be characterised using a specialised camera able to record specific wavelengths from the ultraviolet to the infra-red, attached to robot arms that can move the camera and light source to any relative angle. The spectral information will be combined with knowledge of bee, bird and human vision to model the colours and shapes as seen by each of our model species. This will be followed by a series of behavioural experiments to determine the impact of iridescence on object detection, object identification, and accurate evaluation of object speed and trajectory.
Ultimately, we aim to answer the question - can iridescence interfere with visual systems to the extent that it could function as a hitherto unsuspected, but extremely widespread, form of camouflage? As well as understanding the function and diversity of iridescence in nature, this research could also have important societal impacts. Interference with object identification is relevant in both military and security (document fraud prevention) applications. Iridescence is also increasingly being used in many industries without a clear understanding of how the beneficial visual impact (conspicuousness at some viewing angles, attraction of attention) are offset by costs (interference with recognition and localisation). Understanding how evolution has 'solved' these trade-offs will help us use iridescence more strategically.

Iridescence is a form of structural coloration in which hue changes with viewing angle. It is widespread in nature and of growing use in industry, from car paint, where its role is to create a striking visual impact, to the production of bank notes, where the complex colour variation is hard to forge. The iridescent colours of wrapping paper and peacocks' trains are designed to attract attention, but not all iridescence can be explained this way. We will test whether it can function as deceptive camouflage and, if so, how. Understanding the costs and benefits of having such colours will not only help explain their widespread occurrence in nature, but also help direct their efficient use in commercial, security and defence applications.
We will focus on terrestrial iridescence. We propose that the shifting spatio-chromatic signal from iridescent objects could act as effective camouflage through several mechanisms: the shifting shades could disrupt object outline, distract the visual system from other identifying features, or act as a dynamic 'dazzle' coloration to confuse. We will investigate its impact on the ability to identify, discriminate and track objects using 3 model species with different visual systems: bee, bird and human. Biomimetic iridescent and non-iridescent targets of different types, with different visual effects, will be used. Their angle-specific reflectance (i.e. BRDF) will be characterised across the UV to nIR using a hyperspectral camera attached to robot arms that can move the camera and light source to any relative angle. The spectral information will be combined with data, from stereophotogrammetry and motion capture, of how animals move past targets, and models of bee, bird and human vision, to model the colours and shapes as seen by each of our species. This will be combined with behavioural experiments to determine the impact of iridescence on object detection, identification, and accurate evaluation of speed and trajectory.

As this project will evaluate an unstudied property of materials that are both widespread in nature and of growing use in a range of industries, there are a range of beneficiaries in both academia and industry. Due to the charismatic and appealing nature of both iridescence and many of the species that produce it, this research will also have strong possibilities for public engagement activities.
Main non-academic beneficiaries from the project:
1) Industrial third parties (packaging, vehicle paint manufacturers, security, military)
2) Public policymakers (Highways Agency)
3) General public
4) PDRA, PI and Co-Is will also benefit in ways distinct from science per se.

1) Industrial Third Parties. Establishing the visual impact of iridescence could have particular benefits for several industrial areas.
a) Packaging. Many products have packaging that contains iridescent material.
b) Vehicle paint manufacturers. There is an increasing market for various forms of iridescent vehicle paint (e.g. chameleon, color-flip or prism paints).
c) Security. Iridescent materials are a key component of the portfolio of measures used in the production of security documents such as passports and national currency.
d) Military. Iridescent materials that were able to confuse object identity, speed or trajectory would be of use in protecting moving vehicles.
Three of these industries (packaging, vehicle paint manufacturers, security) have been using iridescent materials as part of their portfolio for the last decade. Our hypotheses state that; iridescence (in certain forms) can result in the disruption of object identity (resulting in potential confusion of brand/security feature identity); iridescence can obscure accurate assessment of object speed and trajectory, (safety implications for iridescent paint on vehicles). This research would highlight which iridescence generating structures would result in increased detectability but minimal object disruption - ensuring that packaging or security features could draw the eye without confusing it, or enhancing vehicle appearance without compromising safety. The reverse of this - materials that optimised the visually disruptive effects of iridescence while minimising those of detectability - would be of potential benefit to the military.
Currently, we have established links with three companies that have expressed an interest in this research and its outcomes in two of these areas (security and military): De La Rue (the world's largest integrated banknote printer), Malvern Optical and QinetiQ (world leading defence technology research and innovation). Through these established interactions, and by establishing additional interactions, this project will not only assist the maintenance of the UK's pre-eminence in sensory biology, and develop new methods to understand a well used but poorly understood visual phenomena, but will also assist with the translation of that research into insights for several world leading UK industries.

2) Public Policy Makers. As mentioned above, to increase vehicle attractiveness, car paint manufactures are increasingly using iridescent materials. This may have safety implications, assessment of which could be of benefit to public policy makers such as the Highways Agency.

3) General Public. This topic would provide an excellent vehicle to provide activities that would engage the public on a range of topics including visual ecology, camouflage, evolution, optics, biomimetics and nanotechnology.

4) PDRA, PI and Co-Is. The three PDRAs employed on this grant would gain extensive training experience in an interdisciplinary range of novel techniques and experimental methods that have both academic and industrial relevance. The PI and Co-I will also benefit through the development of novel techniques (such as the hyperspectral-goniometer) which we predict will be of interest to many in both academia and industry, leading to additional future collaborations.