The compound eye is the most common micro-vision system in nature. It has existed for 500 million years, yet technologies based on its functionality are rare. For large systems, camera-type optical systems still tend to be more practical due to size and resolution. But, for small systems, compound eyes have a much greater field of view, a wrap-around low-volume architecture, and integrated image-processing advantages. Integrating optics and electronics into a very small package in order to simulate the function of a biological compound eye, however, is challenging.
Early artificial compound eyes were primarily used to study visual processing. Some bulky systems were also created for machine vision and robot navigation research. Only in recent years have artificial systems started to reach the size and architecture of their biological counterparts. One such system uses planar array technology to mimic the functionality of a compound eye.1 Another is a hemispherical optical array developed at the University of Berkeley that nearly perfectly matches the optical properties and size of the eye of a honeybee.2 Our approach was to start with planar fabrication techniques to aid alignment and detector integration, and then to form the array into a hemispherical architecture like that of natural compound eyes.
The final shape and function of our system is modeled after the neural superposition eye found on two-winged flies and numerous other insects. In this type of eye, each ommatidia (single optical unit) has an array of rhabdomere (photoreceptors) arranged in a pattern analogous to the ommatidium array (see Figure 1). This configuration results in rhabdomere from seven ommatidia sharing the same field of view.
Figure 1. A fruit fly neural superposition eye is a natural model for artificial compound eyes.
In an insect, a neural network links the signals from these seven rhabdomere to the same lamina (a nerve center located between the rhabdom and brain).3 With this unique arrangement, neural superposition eyes can achieve hyperacuity,4 the ability to see objects and motion at a higher resolution than predicted by the photoreceptor spacing. In an artificial neural superposition system, there are at least seven channels that can access the field of view without sacrificing spatial resolution. This makes it possible to increase sensitivity or gather more information, such as color and polarization. It also allows the system to simultaneously transmit and receive information into the full field of view. In an artificial neural superposition system, optical fibers can carry the signal from the image plane of the lens array to an arrangement of transmitters and detectors (see Figure 2).
Figure 2. In an artificial neural superposition system, optical fibers can be used to carry signals from the image plane to transmitters and detectors.
To fabricate an array, micro lenses are formed onto a thin silicone elastomer substrate that allow it to flex into a spherical shape (see Figure 3). UV light is then projected through the micro lenses into a photosensitive monomer, forming cones that are perfectly aligned behind each lens. (The cones are only partially photopolymerized to allow for diffusion of a second lower index monomer into the tapered elements.) The focal length of the elements is measured in situ during the diffusion process. When the resulting gradient index profile shortens the focus to the back plane of the lens array, the system is heated to fully polymerize the cones. (Using a gradient index lens also shortens the overall system length.) The back substrate can then be removed and photodetectors or optical fibers attached. Finally, the sheet is formed into the desired shape.
Figure 3. Formable compound arrays have been fabricated using this multi-step procedure.
A system created using this technique has seven optical elements connected by a silicone elastomer layer (see Figure 4). The cones are made from CR-39 (diallyl diglycol carbonate) with an added photo initiator (Darocur 1173). Lens diameter has been demonstrated from 200μm to 2.5mm for arrays with up to 19 elements.
Figure 4. The silicone layer holding the polymer optical elements together is very elastic, and can be stretched into the desired form.
Compound lens arrays are a practical alternative to existing camera systems and offer significant advantages for applications in communications, micro-vision systems, motion detectors, rangefinders, and other optical sensor systems. Remaining technical challenges must be overcome before compound arrays can be integrated into everyday technology. For example, a system must be reproducible, easy to manufacture, and integrate optics and electronics in a single package. Our future research will continue to quantify the gradient index profile, control the polymerization process, improve repeatability, and integrate a detector/transmitter scheme.
The authors would like to acknowledge support from the Defense Advanced Research Projects Agency (DARPA) and the Infotonics Technology Center.