Nov 15 2016
scyther5/Shutterstock.com
People with an interest in photography will know that a big camera is often necessary to obtain features such as a powerful zoom. This is due to the fact that almost all cameras depend on refraction, where the light passing through lenses slows down and changes direction. A certain amount of space is needed to focus this refracted light.
A promising way to create powerful and smaller cameras for smartphones and other devices is to fabricate optical elements with the ability to control light through diffraction (i.e. bending of light around obstacles or via narrow gaps) rather than refraction.
Wolfgang Heidrich and his collaborators from the Visual Computing Center at King Abdullah University of Science and Technology (KAUST) and from the University of British Columbia (UBC), Canada, are working toward the creation of new diffractive optical elements (DOEs) that can be printed on to thin, small substrates. The group integrates their carefully designed DOEs with advanced computational methods that can significantly improve the images generated by such smaller optical devices.
When at UBC, Heidrich created displays of very high contrast for television sets, and then joined KAUST in 2014.
We developed the first consumer-ready display technology that had a major computational component, in the sense that the hardware itself was not useful without substantial computation. The target image would be sent to the device, and then the device would have to perform some fairly sophisticated algorithms on the image (in real time!) to produce the best image contrast. It really instilled in me the need for hardware-software co-design, where you develop optics, electronics and algorithms at the same time so that they fit together in the best possible way.
Wolfgang Heidrich, KAUST
Recently, Heidrich and his collaborators have used the same technique for computational imaging in cameras. They are currently working to address a major problem known as chromatic aberration, which is where directions of different wavelengths are changed by differing amounts when refracted by lenses, leading to incorrect distribution of colors in images.
Chromatic aberration is more of a problem when light is controlled by diffraction. Therefore, DOEs are affected by a loss of color fidelity as well as blurring that relies on the color distribution of the incident light.
In order to overcome this problem, Heidrich and his collaborators fabricated a light-weight, thin DOE known as diffractive achromat to balance the focusing contributions of different wavelengths. The test results on this new and innovative component were reported in ACM Transactions on Graphics, which is one of the topmost journals for computer graphics studies.
In a regular DOE lens, the focus will be near-perfect for a single design wavelength, and progressively blurred as you move away from that design wavelength. The diffractive achromat sacrifices a little bit of sharpness for the design wavelength in exchange for more sharpness at all other wavelengths. Any remaining blur can then be removed computationally.
Wolfgang Heidrich, KAUST
The same combination of computer algorithms and cutting-edge optics was used by the researchers in a recent research reported in Scientific Reports, which could lead to very small zoom lenses. The researchers designed two DOEs with specific shapes by employing computational algorithms in such a way that the DOEs represent a diffractive lens with a specific focal length upon being positioned one above the other.
Then comes the smartest bit.
As you rotate the two DOEs relative to each other, the focal length, or any other parameter of the optical system, can change smoothly. One obvious application is to produce zoom lenses that do not require the lens barrel to move in and out of the camera for zooming.
Wolfgang Heidrich, KAUST
According to Heidrich, the active research environment at KAUST played a crucial part in accomplishing his research aims. “I have been able to assemble an interdisciplinary team, for more ambitious projects that take our hardware-software co-design to the next level,” he says. “What’s more, all our diffractive optical elements were built in the KAUST Nanofabrication Core Lab, which allowed quick turn-around times for experiments.”
Computational imaging is still in its infancy, and provides many avenues for Heidrich and his collaborators to explore in coming years. Another interesting fact is that the DOEs, being very thin, do not absorb much energy from light when it traverses. This implies that DOEs can, in principle, be used to control any segment of the electromagnetic spectrum, from radio waves to gamma rays.