Bio-inspired lens gives image sensors night vision capability

March 17, 2016 // By Julien Happich
While low-light imaging typically involves a race for better image sensor pixel design or the search for new photo-conversion materials with better light sensitivity, engineers from the University of Wisconsin–Madison have found their inspiration in nature to design a unique optical lens that dramatically increases the overall light input to a sensor.

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.

Schematic illustrations and images of a natural eye
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.

Fabrication process and micrographs of the artificial eye and BPE. (A–F) Schematic illustration of the fabrication procedures. (G and H) SEM images of a μ-PC. (I and J) SEM of the BPE transferred onto a hemispherical PDMS membrane. (Scale bars: G, 50 μm; H, 1 μm; I, 200 μm, and J, 100 μm.) All images courtesy Hongrui