Single-pixel imaging of the retina through scattering media

Imaging the retina of cataractous patients is useful to detect pathologies before the cataract surgery is performed. However, for conventional ophthalmoscopes, opacifications convert the lens into a scattering medium that may greatly deteriorate the retinal image. In this paper we show, as a proof of concept, that it is possible to surpass the limitations imposed by scattering applying to both, a model and a healthy eye, a newly developed ophthalmoscope based on single-pixel imaging. To this end, an instrument was built that incorporates two imaging modalities: conventional flood illumination and single-pixel based. Images of the retina were acquired firstly in an artificial eye and later in healthy living eyes with different elements which replicate the scattering produced by cataractous lenses. Comparison between both types of imaging modalities shows that, under high levels of scattering, the single-pixel ophthalmoscope outperforms standard imaging methods

Tunable telephoto: governable Fourier spectrum anamorphic scaling

We discuss the Gaussian design of a device that controls the scale, in an anamorphic fashion, of Fraunhofer diffraction patterns. The device uses two pairs of varifocal cylindrical lenses. For spherical lenses, the optical powers can be predicted by using a previously known high-level solution. We emphasize the anamorphic capabilities, by considering the case My = 1/ Mx. The proposed device does not introduce vignetting, and it does not alter the axial location of the Fraunhofer diffraction patterns. Since the composing elements work at fixed inter-lens separation, the device does not require mechanical compensation

Photon Counting 3-D Object Recognition Using Digital Holography

We present an analysis of the recognition performance of 3-D objects reconstructed from digital holograms recorded under photon counting conditions. The digital holograms are computed by applying four-step phase-shifting techniques to interferograms recorded with weak coherent light. Recognition capability is analyzed as a function of the total number of photons by using a maximum-likelihood approach adapted to one-class classification problems. The likelihood is modeled assuming a Gaussian distribution, whose centroid corresponds to the highest value in a mixture of two Gaussian values. The recognition capability is studied both in terms of the axial distance and the lateral position of the reconstructed 3-D object

Diffractive digital lensless holographic microscopy with fine spectral tuning

We experimentally demonstrate an all-diffractive optical setup for digital lensless holographic microscopy with easy wavelength line selection and micrometric resolution. In the proposed system, an ultrashort laser pulse is focused with a diffractive lens (DL) onto a pinhole of diameter close to its central wavelength to achieve a highly spatially coherent illumination cone as well as a spectral line with narrow width. To scan the complete spectrum of the light source the DL is displaced with respect to the pinhole plane. The proposed microscopy setup allows us to spectrally separate contributions from different sections of a sample, which may be attractive for several applications in life sciences

Single-pixel digital "ghost" holography

Since its discovery, the "ghost" diffraction phenomenon has emerged as a non-conventional technique for optical imaging with very promising advantages. However, extracting intensity and phase information of a structured and realistic object remains a challenge. Here, we show that a "ghost" hologram can be recorded with a single-pixel configuration by adapting concepts from standard digital holography. The presented homodyne scheme enables phase imaging with nanometric depth resolution, three-dimensional focusing ability, and shows high signal-to-noise ratio

Signal-to-noise ratio of single-pixel cameras based on photodiodes

Single-pixel cameras have been successfully used in different imaging applications in the last years. One of the key elements affecting the quality of these cameras is the photodetector. Here, we develop a numerical model of a single-pixel camera, which takes into account not only the characteristics of the incident light but also the physical properties of the detector. In particular, our model considers the photocurrent, the dark current, the photocurrent shot noise, the dark-current shot noise, and the Johnson–Nyquist (thermal) noise of the photodiode used as a light detector. The model establishes a clear relationship between the electric signal and the quality of the final image. This allows us to perform a systematic study of the quality of the image obtained with single-pixel cameras in different contexts. In particular, we study the signal-to-noise ratio as a function of the optical power of the incident light, the wavelength, and the photodiode temperature. The results of the model are compared with those obtained experimentally with a single-pixel camera

Full-Color Stereoscopic Imaging With a Single-Pixel Photodetector

We present an optical system for stereoscopic color imaging by using a single-pixel detector. The system works by illuminating the input scene with a sequence of microstructured light patterns generated by a color digital light projector (DLP). A single monochromatic photodiode, synchronized with the DLP, measures the light scattered by the object for each pattern. The image is recovered computationally by applying compressive sensing techniques. The RGB chromatic components of the image are discriminated by exploiting the time-multiplexed color codification of the DLP. The stereoscopic pair is obtained by splitting the light field generated by the DLP and projecting microstructured light patterns onto the sample from two different directions. The experimental setup is configured by simple optical components, a commercial photodiode and an off-the-shelf DLP projector. Color stereoscopic images of a 3D scene obtained with this system are shown.This work was supported in part by MINECO under Grant FIS2013-40666-P, Generalitat Valenciana under Grant PROMETEO2012-021 and Grant ISIC 2012/013, and Universitat Jaume I under Grant P1-1B2012-55

Imaging the optical properties of turbid media with single-pixel detection based on the Kubelka–Munk model

We present a diffuse optical imaging system with structured illumination and integrated detection based on the Kubelka– Munk light propagation model for the spatial characterization of scattering and absorption properties of turbid media. The proposed system is based on the application of singlepixel imaging techniques. Our strategy allows us to retrieve images of the absorption and scattering properties of a turbid media slab by using integrating spheres with photodiodes as bucket detectors. We validate our idea by imaging the absorption and scattering coefficients of a spatially heterogeneous phantom

Spectral analysis of femtosecond pulse diffraction through binary diffractive optical elements: theory and experiment

We report on the changes in the spectrum of a femtosecond pulse originated by diffraction of the ultrashort waveform through a circularly symmetric binary diffractive optical element. The analysis is performed in the framework of the Rayleigh-Sommerfeld formulation of the diffraction, where an analytical expression for the monochromatic amplitude distribution close to the optical axis is obtained. To corroborate our results, we experimentally measure the variations of the pulse spectrum within the collecting area of a spectrometer located at the output plane. Multiple splitting of the pulse spectrum in the vicinity of a focal position and a phase singularity are show

Phase imaging by spatial wavefront sampling

Phase-imaging techniques extract the optical path length information of a scene, whereas wavefront sensors provide the shape of an optical wavefront. Since these two applications have different technical requirements, they have developed their own specific technologies. Here we show how to perform phase imaging combining wavefront sampling using a reconfigurable spatial light modulator with a beam position detector. The result is a time-multiplexed detection scheme, capable of being shortened considerably by compressive sensing. This robust referenceless method does not require the phase-unwrapping algorithms demanded by conventional interferometry, and its lenslet-free nature removes trade-offs usually found in Shack–Hartmann sensors