Bio-inspired lens gives image sensors night vision capability
The inspiration came from a combination of the Lobster’s superposition compound eyes and the retinal structure of the small elephantnose fish, the later featuring thousands of crystalline cups covering its inner retina.
of elephantnose fish and an artificial eye.
In a paper titled “Artificial eye for scotopic vision with bioinspired all-optical photosensitivity enhancer”, the researchers unveil an optical lens made out of thousands of micro-photocollectors (μ-PCs), each consisting of a tiny glass pillar with parabolic reflective sidewalls that focus the faint incoming light through a tiny output port.
The micro-photocollectors are arranged on a dome-shaped structure, so as to mimic the lobster’s superposition compound eye where multiple light input ports concentrate the incoming light onto individual sensor pixels.
Using this unique lens, the researchers reported a four-fold improvement in light sensitivity when imaging objects in what could be described as pitch-black darkness.
To manufacture the minuscule parabolic side-walled μ-PCs only about 120μm tall, the engineers relied on a hybrid laser ablation process.
“First, we use laser ablation to form the parabolic micro-cup structures in glass. Then we smoothen the sidewall surface by reflowing Su-8 photoresist, followed by coating aluminium as the reflective (mirror) layer”, explained Hongrui Jiang, professor of electrical and computer and biomedical engineering at UW–Madison and the corresponding author on the study.
These micro-cups are then transferred to a 300μm thick PDMS hemispheric membrane to create the so-called bioinspired photosensitivity enhancer (BPE), in effect a fish-lens which can be used to boost just any imaging system, regardless of the imaging sensors in use.
As demonstrated through ray-tracing models, each μ-PCs collects the incoming light and concentrates the rays through reflection on the four parabolic sidewalls, to the narrower output port (going from a 77μm input port to a 20μm output port).
Because the closely packed μ-PCs are omnidirectionally arranged on the hemispheric lens, the whole structure functions as a superposition compound eye with multiple incoming light ports for each pixel on the imager.
To validate the concept, the researchers designed a full fish-eye featuring a ball lens at the centre of a 8mm diameter iris, to generate a hemispherical image plane on a 48×48 array of μ-PCs laid out on the inside of a 25mm diameter dome (the PDMS hemispheric membrane).
They used multiple step image capture and super-resolution image reconstruction algorithms to compensate for the superposition compound eye’s intrinsic blur to produce crisp, clear pictures.
Would Jiang and his team expect further light concentration by simply scaling down these micro-mirror patterns?
“We studied the geometry effect of the cup structures. The dimension is not totally optimized, but it did consider these geometry effect. Too small a structure would have diffraction, though, so it might not work better” answered Jiang who hopes to further develop the technology for its commercialization, most probably through IP licensing.
As for mass manufacture, “The molding process might work well. That is, using laser ablation to create a master and then use that to mold the micro-cup array. Nano imprint might not work, though, as we are talking about 10’s of micron in width and about 100 micron in depth”, clarified the researcher.
On top of this all-optical light concentration, would it be conceivable to include some form of light amplification through the use of dopants within the glass of these micro-structures?
“That’s an interesting idea. I haven’t thought through about it yet, though. Pretty challenging”, says Jiang.
“Whether doped glass could help is questionable. Our mechanism is based on reflection and concentration. Unless doped glass can reduce scattering, it should not matter. Our mechanism is not pumping up light intensity like laser. The incoming light would not trigger that. What you described is more like PMT, which is a different mechanism. That said, it would be really interesting to combine both”.
Through further process and geometry optimization, the researchers hope to improve the artificial eye by at least an order of magnitude.
Visit the University of Wisconsin-Madison at www.wisc.edu
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