Currently large volume, high accuracy three-dimensional (3D) metrology is dominated by laser trackers, which typically utilize a laser scanner and cooperative reflector to estimate points on a given surface. The dependency upon the placement of cooperative targets dramatically inhibits the speed at which metrology can be conducted. To increase speed, laser scanners or structured illumination systems can be used directly on the surface of interest. Both approaches are restricted in their axial and lateral resolution at longer stand-off distances due to the diffraction limit of the optics used. Holographic aperture ladar (HAL) and synthetic aperture ladar (SAL) can enhance the lateral resolution of an imaging system by synthesizing much larger apertures by digitally combining measurements from multiple smaller apertures. Both of these approaches only produce two-dimensional imagery and are therefore not suitable for large volume 3D metrology. We combined the SAL and HAL approaches to create a swept frequency digital holographic 3D imaging system that provides rapid measurement speed for surface coverage with unprecedented axial and lateral resolution at longer standoff ranges. The technique yields a “data cube” of Fourier domain data, which can be processed with a 3D Fourier transform to reveal a 3D estimate of the surface. In this paper, we provide the theoretical background for the technique and show experimental results based on an ultra-wideband frequency modulated continuous wave (FMCW) chirped heterodyne ranging system showing ~100 micron lateral and axial precisions at >2 m standoff distances.
Feature specific imaging is a computational imaging technique that minimizes the number of measurements needed to sufficiently reconstruct a scene by using a priori knowledge (e.g., the scene’s second-order statistics) to judiciously, as well as possibly adaptively, choose the projection vectors to be measured. Here, we have developed an approach to three-dimensional adaptive feature specific imaging that takes into account the obstruction of distant objects by closer objects in the adaption of the projections and in the reconstruction algorithm. The developed system reconstructs the cross-range image of the scene at each range bin from a set of range resolved measurements from all the return from the scene at that range using only a single photodetector, while adapting to the obstruction of the scene by closer objects. Simulations and a proof-of-concept demonstration of adaptive three-dimensional feature specific imaging are presented.
Bridger Photonics has been researching and developing a ladar system based on heterodyne detection for imaging through brownout and other DVEs. There are several advantages that an FMCW ladar system provides compared to direct detect pulsed time-of-flight systems including: 1) Higher average powers, 2) Single photon sensitive while remaining tolerant to strong return signals, 3) Doppler sensitivity for clutter removal, and 4) More flexible system for sensing during various stages of flight. In this paper, we provide a review of our sensor, discuss lessons learned during various DVE tests, and show our latest 3D imagery.
We present a miniature non-mechanical zoom camera using deformable MOEMS mirrors. Bridger Photonics, Inc.
(Bridger) in collaboration with Montana State University (MSU), has developed electrostatically actuated deformable
MEMS mirrors for use in compact focus control and zoom imaging systems. Applications including microscopy,
endomicroscopy, robotic surgery and cell-phone cameras. In comparison to conventional systems, our MEMS-based
designs require no mechanically moving parts. Both circular and elliptical membranes are now being manufactured at
the wafer level and possess excellent optical surface quality (membrane flatness < λ/4). The mirror diameters range from
1 - 4 mm. For membranes with a 25 μm air gap, the membrane stroke is 10 μm. In terms of the optical design, the
mirrors are considered variable power optical elements. A device with 2 mm diameter and 10 μm stroke can vary its
optical power over 40 diopters or 0.04mm∧(-1). Equivalently, this corresponds to a focal length ranging from infinity to
25 mm. We have designed and demonstrated a zoom system using two MOEMS elements and exclusively commercial
off-the-shelf optical components to achieve an optical zoom of 1.9x with a 15° full field of view. The total optical track
length of the system is 36 mm. The design is approximately 30 mm x 30 mm x 20 mm including the optomechanical
housing and image sensor. With custom optics, we anticipate achieving form factors that are compatible with
incorporation into cell phones.
A projective compressive sensing system for face recognition is presented. The Fisherfaces method was utilized for
classification of the face images. The system uses a digital micromirror device to project measurement vectors onto the
scene and a single photodetector to collect the backscattered illumination. Experimentally, the system accuracy was
95.5% using only 32 measurements per image; this performance matches the simulation results. The total number of
image pixels was 5,736 (84 × 64) resulting in a compression factor of 168 over a conventional imaging system.
Deformable membrane mirrors are promising MOEMS devices for focus control and aberration correction in vital microscopy, offering high speed focus adjustment in an optical system that can be miniaturized for in vivo use. This paper describes mirrors comprising metalized polymer membranes suspended over three concentric circular electrodes for electrostatic actuation. The membranes are 2-μm thick and 3 mm in diameter, made from the fully cross-linked photoset epoxy SU-8 2002. A layer of SU-8 2025 is used to establish a 30-μm thick air gap between the electrodes and the membrane mirror. The membranes are actuated by applying voltage to each electrode individually to achieve displacement as large as 12 μm while minimizing spherical aberration. Surface deflection is studied using phase-shift interferometry under both static and dynamic excitation. Using the deformable MOEMS mirror for focus control in an optical microscope we demonstrate the ability to adjust the location of the focal plane by 85 μm using an N.A. = 0.75 optical system.
We are developing MEMS deformable mirrors for focus control in miniature optical systems, including endoscopic
microscopes and small form-factor camera lenses. This paper describes a new process to create mirrors made from
the photoset polymer SU-8. The SU-8 also serves as the adhesive layer for wafer bonding, resulting in a simple, low
cost fabrication process. The paper describes the process details and the optical properties of the resulting focus
control mirrors, which have a diameter of 2 mm, a stroke in excess of 8 μm and very low residual aberration.
Multiple actuation electrodes allow control of more than 0.4 μm peak-peak of spherical aberration.
This paper describes deformable membrane mirrors designed for focus control and aberration compensation in vital
microscopy and shows microscope images obtained using these mirrors. The mirrors are metalized polymer membranes
ranging from 1-3 mm in diameter using the photo-cured epoxy SU-8 2002, constructed using a die-bonding process.
They are electrostatically actuated using three concentric electrodes to provide large displacement while minimizing