Corn cobs have night vision prospects

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It could be corn cobs rather than carrots that have the best prospects – thanks to the abundance of lignin and cellulose found in the crop – to improve night vision. Researchers in China have used the materials to produce near-infrared (NIR) transparent optical filters for advanced imaging applications that highlight nature’s growing appeal to device makers.

“Lignocellulose-based bio-sourced materials are abundant, renewable, nontoxic, and mechanically strong candidates for optical materials,” explains the team in a paper published recently in Research – a Science Partner Journal.

NIR filters are crucial to the success of night vision cameras, as we shall soon discuss, and designs are typically made using two approaches. One option requires expensive glass materials that – while effective – can involve complicated fabrication steps. Alternatively, developers can use ultraviolet-visible absorbing dyes, which are easier to work with.

However, neither option has the environmental or economic appeal of using waste corn cobs. Plus, the performance – based on prototypes tested by the group – could be superior to conventional night vision components.

Why night vision cameras use NIR filters

To be effective, night vision cameras need to maximize the available light, which – once the sun has gone down, or is blocked indoors – is to be found in the NIR portion of the electromagnetic spectrum. NIR filters isolate this usable illumination band, while preventing interference from other wavelengths.

Without an NIR filter, night vision cameras would be easy to blind using a torch or a smartphone flash. Imaging systems would be similarly affected by other artificial light sources such as vehicle headlamps.

Regular digital cameras have an NIR blocking filter that stops photos and video from appearing washed out and makes sure that images match the optical response of the human eye.

As an aside, removing the NIR blocking filter from a digital camera can allow users to peer inside electronic devices that otherwise appear to be opaque – for example, if the case material is transparent to NIR light. Vein viewer technology uses near-infrared light to visualize major blood vessels beneath the skin, which also exploits the sensitivity of commercial imaging sensors outside the visible spectrum.

Eye-tracking in the near-infrared

Night vision style cameras are particularly well-suited to eye-tracking, as the NIR images provide strong contrast for algorithms to respond to. And there are a number of applications that exploit this opportunity. For example, several chip designers such as Qualcomm and Analog Devices have offerings that focus on driver and occupant monitoring for automotive applications.

Eye-tracking solutions, which operate at NIR wavelengths, can determine where the driver is looking at any moment in time to ensure that attention is being paid to the road ahead. Systems can also spot if the driver appears sleepy or is using a cell phone while the vehicle is in motion, and issue a safety warning.

Also, eye-tracking persists even if occupants are wearing sunglasses, as regular lenses are designed to block harmful ultraviolet rays and are transparent to NIR light. However, it’s possible to purchase privacy-focused spectacles, such as products sold by Reflectacles, which are fitted with an IR blocker.

In this case, the wearer’s eyes will remain obscured to 3D infrared facial mapping software and 2D facial recognition systems that use infrared light as their illumination source.

Returning to the researchers’ corn cob-derived NIR filter, the combination of cellulose and lignin appears to produce a high-performance and practical film.

“The captured lignin was fused to fill the gaps in a cellulose network, which then held the fibers tightly and created a homogeneous dense structure,” comments the group. “Both the lignin and the dense structure provided the biofilter with unique optical properties, including strong UV-vis light blocking (~100% at 400 nm and 57.58% to 98.59% at 550 nm), low haze (close to 0%), and high NIR transmittance (~90%).”