|
The diffractive achromat is a computationally optimized
diffractive lens for full visible spectrum imaging, which is used
jointly with a computational image reconstruction algorithm. The
microscope images show a traditional Fresnel diffraction grating
(top) and our diffractive achromat (bottom). In full visible
spectrum illumination, the former can only be focused only at one
specific wavelength (e.g. green here) while all other wavelengths
are out of focus. This results in highly nonuniform spatial and
spectral response (color PSFs) on the image plane coupled with Bayer
filters (top middle). In particular, metamerism introduces a data
dependency in the PSF shape for any kind of broadband image
sensor. Our diffractive achromat is optimized to equalize the
spectral focusing performance within the whole visible
spectrum. Consequently, the PSFs for all wavelengths are nearly
identical to each other (bottom middle). The captured blurry image
shows much higher color fidelity than the conventional diffractive
lens (right). Our diffractive achromat is much thinner and lighter
than an refractive achromatic lens with the same optical power (left-most bottom).
Abstract
Diffractive optical elements (DOEs) have recently drawn great
attention in computational imaging because they can drastically reduce
the size and weight of imaging devices compared to their refractive
counterparts. However, the inherent strong dispersion is a tremendous
obstacle that limits the use of DOEs in full spectrum imaging, causing
unacceptable loss of color fidelity in the images. In particular,
metamerism introduces a data dependency in the image blur, which has
been neglected in computational imaging methods so far. We introduce
both a diffractive achromat based on computational optimization,
as well as a corresponding algorithm for correction of residual
aberrations. Using this approach, we demonstrate high fidelity color
diffractive-only imaging over the full visible spectrum. In the optical
design, the height profile of a diffractive lens is optimized to
balance the focusing contributions of different wavelengths for a
specific focal length. The spectral point spread functions (PSFs)
become nearly identical to each other, creating approximately
spectrally invariant blur kernels. This property guarantees good color
preservation in the captured image and facilitates the correction of
residual aberrations in our fast two-step deconvolution without
additional color priors. We demonstrate our design of diffractive achromat
on a 0.5mm ultrathin substrate by photolithography
techniques. Experimental results show that our achromatic diffractive
lens produces high color fidelity and better image quality in the full
visible spectrum.
|
|
Paper and Video
p.s. The video is with audio.
All images are © ACM, 2016, reproduced here by permission of ACM for your personal use. Not for redistribution. Some of the source graphics on which these works were based are copyrighted by third parties and used with their permission.
