To capture panoramic views in a single shot, photographers typically use fisheye lenses
— ultra-wide-angle lenses made from multiple pieces of curved glass, which distort incoming
light to produce wide, bubble-like images.
Their spherical, multipiece design makes fisheye lenses inherently bulky and often costly to
produce.
Now engineers at MIT and the University of Massachusetts at Lowell have designed a wide-angle
lens that is completely flat.
It is the first flat fisheye lens to produce crisp, 180-degree panoramic images.
The design is a type of “metalens,” a wafer-thin material patterned with microscopic
features that work together to manipulate light in a specific way.
In this case, the new fisheye lens consists of a single flat, millimeter-thin piece of
glass covered on one side with tiny structures that precisely scatter incoming light to produce
panoramic images, just as a conventional curved, multielement fisheye lens assembly would.
The lens works in the infrared part of the spectrum, but the researchers say it could
be modified to capture images using visible light as well.
The new design could potentially be adapted for a range of applications, with thin, ultra-wide-angle
lenses built directly into smartphones and laptops, rather than physically attached as
bulky add-ons.
The low-profile lenses might also be integrated into medical imaging devices such as endoscopes,
as well as in virtual reality glasses, wearable electronics, and other computer vision devices.
The Researchers have published their results in the journal Nano Letters.
Metalenses, while still largely at an experimental stage, have the potential to significantly
reshape the field of optics.
Previously, scientists have designed metalenses that produce high-resolution and relatively
wide-angle images of up to 60 degrees.
To expand the field of view further would traditionally require additional optical components
to correct for aberrations, or blurriness — a workaround that would add bulk to a
metalens design.
The MIT researchers instead came up with a simple design that does not require additional
components and keeps a minimum element count.
Their new metalens is a single transparent piece made from calcium fluoride with a thin
film of lead telluride deposited on one side.
The team then used lithographic techniques to carve a pattern of optical structures into
the film.
Each structure, or “meta-atom,” as the team refers to them, is shaped into one of
several nanoscale geometries, such as a rectangular or a bone-shaped configuration, that refracts
light in a specific way.
For instance, light may take longer to scatter, or propagate off one shape versus another
— a phenomenon known as phase delay.
In conventional fisheye lenses, the curvature of the glass naturally creates a distribution
of phase delays that ultimately produces a panoramic image.
The team determined the corresponding pattern of meta-atoms and carved this pattern into
the back side of the flat glass.
On the front side, the team placed an optical aperture, or opening for light.
In one demonstration, the new lens is tuned to operate in the mid-infrared region of the
spectrum.
The team used the imaging setup equipped with the metalens to snap pictures of a striped
target.
They then compared the quality of pictures taken at various angles across the scene,
and found the new lens produced images of the stripes that were crisp and clear, even
at the edges of the camera’s view, spanning nearly 180 degrees.
In another study, the team designed the metalens to operate at a near-infrared wavelength using
amorphous silicon nanoposts as the meta-atoms.
They plugged the metalens into a simulation used to test imaging instruments.
Next, they fed the simulation a scene of Paris, composed of black and white images stitched
together to make a panoramic view.
They then ran the simulation to see what kind of image the new lens would produce.
The researchers say that it captures everything across the panorama.
The team says the new lens can be adapted to other wavelengths of light.
To make a similar flat fisheye lens for visible light, for instance, the optical features
may have to be made smaller than they are now, to better refract that particular range
of wavelengths.
The lens material would also have to change.
But the general architecture that the team has designed would remain the same.
The researchers are exploring applications for their new lens, not just as compact fisheye
cameras, but also as panoramic projectors, as well as depth sensors built directly into
smartphones, laptops, and wearable devices.