We describe a method to image objects through scattering media based on single-pixel detection and microstructured illumination. Spatial light modulators are used to project a set of microstructured light patterns onto the sample. The image is retrieved computationally from the photocurrent fluctuations provided by a single-pixel detector. This technique does not require coherent light, raster scanning, time-gated detection or a-priori calibration process. We review several optical setups developed by our research group in the last years with particular emphasis in a new optical system based on a double-pass configuration and in the combination of single-pixel imaging with Fourier filtering.
Precise control of light propagation through highly scattering media is a much desired goal with major technological implications. Since interaction of light with turbid media results in partial or complete depletion of ballistic photons, it is in principle impossible to transmit images through distances longer than the extinction length. In biomedical optics, scattering is the dominant light extinction process accounting almost exclusively for the limited imaging depth range. In addition, most scattering media of interest are dynamic in the sense that the scatter centers continuously change their positions with time. In our work, we employ single-pixel systems, which can overcome the fundamental limitations imposed by multiple scattering even in the dynamically varying case. A sequence of microstructured light patterns codified onto a programmable spatial light modulator are used to sample an object and measurements are captured with a single-pixel detector. Acquisition time is reduced by using compressive sensing techniques. The patterns are used as generalized measurement modes where the object information is expressed. Contrary to the techniques based on the transmission matrix, our approach does not require any a-priori calibration process. The presence of a scattering medium between the object and the detector scrambles the light and mixes the information from all the regions of the sample. However, the object information that can be retrieved from the generalized modes is not destroyed. Furthermore, by using these techniques we have been able to tackle the general problem of imaging objects completely embedded in a scattering medium.
In the last years, many efforts have been devoted to use electrically addressed spatial light modulators (SLMs) in
Adaptative Optics. In this contribution we have optimized a low-cost SLM based on a liquid crystal (LC) device for the
compensation of eye aberrations. This kind of devices is seldom used in ophtalmic applications due to the relatively low
dynamic range of the phase retardation that can be introduced at each pixel. Here, we have optimized the phase
modulation response of a commercial twisted nematic liquid crystal display (TNLCD) by means of a polarimetric
arrangement that includes retarder plates and polarizers. Furthermore, we describe an efficient four-level phase encoding
scheme that allows us to use these conventional SLMs for the compensation of optical aberrations as those typically
found in human eyes. For obtaining experimental compensation results we have used artificial aberrated eyes simulated
with refractive phase plates. This proof-of-concept is the first step to develop a low-cost real-time system for the
correction of eye aberrations.
In this contribution we describe a method for achieving a phase-only modulation regime with an off-the-shelf twisted
nematic liquid crystal display (TNLCD). The keystone of this procedure involves illumination of an addressed TNLCD
with circularly polarized light. The analysis of the distribution of the output polarization states in the S1-S2 plane as the
applied voltage is changed suggests a simple way to optimize the liquid crystal phase response. For this purpose, a
properly oriented quarter-wave plate followed by an analyzer is used behind the TNLCD. Laboratory results for a
commercial display are presented. Our experiments show a phase modulation depth of 240º for a wavelength of 514 nm
with a residual intensity variation lower than 4%.
Off-the-shelf spatial light modulators (SLMs) like those commonly included in video projection devices have been seldom used for the compensation of eye aberrations, mainly due to the relatively low dynamic range of the phase retardation that can be introduced at each pixel. They present, however, some interesting features, such as high spatial resolution, easy handling, wide availability, and low cost. We describe an efficient four-level phase encoding scheme that allows us to use conventional SLMs for compensating optical aberrations as those typically found in human eyes. Experimental results are obtained with artificial eyes aberrated by refractive phase plates introducing either single Zernike terms or complex eye aberration patterns. This proof-of-concept is a step toward the use of low-cost, general purpose SLMs for the compensation of eye aberrations.
An imaging spectrometer covering the 400– to 1000–nm band is conceived and developed. The system is based on an acousto-optic tunable filter (AOTF) attached to a high-performance digital camera. The AOTF enables the selection of spectral bands with an rf signal in the range of 70 to 218 MHz. It includes a telecentric optical system that enhances system efficiency. Additionally, a smart choice of integration time reduces the dependence of the efficiency on the frequency. Calibration includes filter characterization and compensation of crystal nonconstant diffraction efficiency and spatial nonhomogeneity. The system is controlled by a PC application specifically developed for this purpose, providing wide versatility, while enabling transparent and intuitive management to nonexpert users. The spectrometer is validated by estimating the light absorption of leaves and their chlorophyll content.
The equivalence between a twisted-nematic liquid crystal cell and the combination of a retardation wave-plate and a polarization rotator can be used to calibrate a voltage-addressed liquid crystal display. We present a simple polarimetric procedure to determine the two parameters that define the optical properties of the equivalent retarder-rotator system for each value of the applied voltage. Once the calibration procedure is performed, the optical response of the liquid crystal cell can be predicted and optimized. In particular, we demonstrate the generation of a family of equi-azimuth polarization states with a liquid crystal display sandwiched by a polarizer and a quarter-wave plate, whose optimal orientations are evaluated by a numerical simulation. Laboratory results corresponding to a commercial liquid crystal display are pre-sented.
An imaging spectrometer covering the 400-1000 nm band has been conceived and developed. The system is based on an Acousto-Optic Tunable Filter (AOTF) attached to a high performance digital camera. The AOTF permits the selection of spectral bands with an RF signal in the range of 70-210 MHz. The range is covered using two transducers attached to a single crystal. Although the idea is not new it covers a broader spectrum than previous systems. It includes a telecentric optical system that enhances system efficiency, by ensuring that the chief ray of each light cone emerges out of this doublet parallel to the optical axes. Additionally, an smart choice of integration time reduces the dependence of the efficiency on the frequency. Calibration includes filter characterisation and compensation of crystal non-constant diffraction efficiency and spatial non-homogeneity. The system is controlled by a PC application, specifically developed for this purpose, providing wide versatility, while enabling transparent and intuitive management to typical users.
The apodization of diffractive optical elements can be realized by a local change of their diffraction efficiency. In the case of lithographic elements with step-like structure of the period, the variable diffraction efficiency can be achieved by a gradual transformation of the 2m step kinoform into its conjugate counterpart across the apodization region. In the present contribution we show experimental results confirming this idea, which until now was verified only by simulations. The apodized quaternary grating with locally varying diffraction efficiency was obtained on a SLM device as a programmable diffractive optical element by changing gradually the period's profile. Knowledge of the phase heights of the SLM's pixels is required for successful implementation of the apodization function. It was determined from Fresnel images of the binary phase gratings with different phase step height programmed on the SLM. The Fresnel images become then binary and their visibility depends on the phase height of the grating in a known way, what makes possible to calibrate the SLM.
When an axicon is illuminated under an angle different than the assumed one, its focal segment looses quality and broadens significantly. In order to compensate this aberration, harmful in scanning and metrological applications of axicons, a concept of an aberrationless axicon doublet working in the real time is proposed. The constant part of the doublet is an ordinary and conventional linear axicon, whereas the second, variable part, for which the application of spatial light modulator (SLM) device is proposed, introduces a phase shift completing the transmittance of the whole
doublet to that of the corresponding elliptical axicon forming in a given moment a properly inclined focal segment.
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