A detailed laboratory experiment has been completed which models a simultaneous multiple beam Fourier telescopy
(FT) technique capable of imaging rapidly changing objects. Fourier telescopy uses multiple beams that illuminate the
target with a complex fringe pattern that sweeps across it due to frequency differences between beams. Using this
method, the target spatial frequency components are encoded in the temporal signal that is reflected from the target.
Previous work has concentrated on system designs where the target is illuminated with 3 individual beams in order to use
a standard phase closure process. Data processing and image reconstruction for this laboratory experiment invoked a
novel reconstruction algorithm that has been previously developed. The algorithm compensates for atmospheric phase
fluctuations affecting the large number of beams transmitted simultaneously and includes a new type of global phase
closure which allows image reconstruction from the time history of measured total reflected intensity from the target.
The reconstruction algorithm also solves for hundreds of image Fourier components simultaneously, permitting rapid
reconstruction of the image. This multiple beam laboratory experiment includes effects from realistic photon and speckle noise. Additional effects have been expanded to include uplink turbulence, piston jitter, and beam scintillation on the target, which will be encountered in an actual FT imaging system. Experimental results have obtained reconstructed image Strehl values which are greater than 0.9 under scaled system conditions.
Fourier telescopy (FT) is an active imaging technique that is a candidate for high resolution imaging systems which can be used to obtain satellite images out to geosynchronous target ranges. Fourier telescopy uses multiple beams that illuminate the target with a fringe pattern that sweeps across it due to a set frequency difference between beams. In this way the target spatial frequency components are encoded in the temporal signal that is reflected from the target. The FT receiver can then be composed of a large area "light bucket" collector, since only the integrated temporal signal is necessary to reconstruct the target image. The GEO Light Imaging National Testbed (GLINT) system was previously designed to obtain satellite images at geosynchronous ranges by using this technique. The "light bucket" receiver was designed use forty heliostats, each having a collection area of ten meters square, and composed of a 16 x 16 grid of two foot square mirrors. The heliostats would redirect the return light from the target onto a large spherical concentrator array composed of hexagonal mirror segments. This concentrator would then focus the return light onto a photomultiplier tube (PMT) detector. The FT Field experiment presented in this paper uses one 10-meter square heliostat and a single PMT, plus a scaled down secondary array to provide the optical elements of the receiver for the FT field experiment. In this paper, we will describe the performance characteristics of the heliostat, secondary, and PMT detector. Performance characteristics include optical wavefront, alignment, and alignment stability of the optical elements. Finally, results will be presented after the receiver was integrated with a transmitter system that provided the modulated FT signal from various targets. Image reconstructions will show that even using low quality "Light bucket" receiver optics and a 1.5 km horizontal path through the atmosphere, the modulated signal can still produce good image quality of the targets. Image reconstruction will also be presented for different SNR values in the received signal.
This paper reviews work done at Trex Enterprises Corporation over the past 18 years on electro-optic surveillance and tracking systems. The range of objects that can be detected and tracked cover awide range of brightness and velocities, from slower moving mortars to fast moving bullets.
GLINT (Geo Light Imaging National Testbed) is a program to image geo-synchronous satellites using Fourier telescopy. The design of the GLINT system requires knowledge of the reflectance properties of the satellites in certain specific wavelength ranges. Calibrated measurements of satellite brightness due to solar illumination can be made with a telescope. This report details such measurements and the data processing necessary to yield curves of normalized satellite return versus phase angle in given wavelength ranges. These measurements can be used to check the accuracy of satellite reflectivity models.
For many skin treatments with light, it is important to have deep photon penetration into the skin. Because of absorption and scattering of photons by skin tissue, both the color and the diameter of the incident beam affect the penetration depth of photons. In this study, the dependence of light transmission through human skin tissues (ear lobs and between the fingers) has been measured in-vivo at six wavelengths (532 nm, 632 nm, 675 nm, 810 nm, 911 nm, and 1064 nm). The same measurement was also made on pig skin in-vitro for comparison. It was observed that (1) the photons at 1064 nm penetrate deeper than the other colors studied for a given incident beam diameter; and (2) the transmittance at a particular wavelength increases asymptotically with incident beam diameter. For some skin tissues, the transmittance flattens at about 8 mm for 532 nm photons and approaches saturation at about 12 mm for all other colors. The results on pig skin is similar.
The resolution achievable in imaging objects in space from ground-based telescopes is limited by atmospheric turbulence. If enough naturally occurring illumination is available then speckle imaging techniques can be used to recover the original object phase using short exposure images. Analogous techniques exist for recovering the phase of a laser illuminated object from measurements of either the incoherent Fourier modulus or coherent Fourier modulus. In both cases many exposures are required to accumulate sufficient statistics. In the case of coherent illumination lack of a priori information concerning the object makes image reconstruction very difficult. In this paper we discuss one approach to circumventing these difficulties, in which multiple modulated laser beams are broadcast off of an object and the relative phase between the beams is measured at a simple light-bucket receiver. The original object phase is recovered from the phase differences using an iterative reconstructor.
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