Hyperspectral imaging system based on a single-pixel camera design for detecting differences in tissue properties.

Optical spectroscopy can be used to distinguish between healthy and diseased tissue. In this study, the design and testing of a single-pixel hyperspectral imaging (HSI) system that uses autofluorescence emission from collagen (400 nm) and nicotinamide adenine dinucleotide phosphate (475 nm) along with differences in the optical reflectance spectra to differentiate between healthy and thermally damaged tissue is discussed. The changes in protein autofluorescence and reflectance due to thermal damage are studied in ex vivo porcine tissue models. Thermal lesions were created in porcine skin (n=12) and liver (n=15) samples using an IR laser. The damaged regions were clearly visible in the hyperspectral images. Sizes of the thermally damaged regions as measured via HSI are compared to sizes of these regions as measured in white-light images and via physical measurement. Good agreement between the sizes measured in the hyperspectral images, white-light imaging, and physical measurements were found. The HSI system can differentiate between healthy and damaged tissue. Possible applications of this imaging system include determination of tumor margins during surgery/biopsy and cancer diagnosis and staging.

Robust sub-micrometer displacement measurement using dual wavelength speckle correlation.

The unique characteristics of speckle correlation techniques including simple setup and fast, non-contact, high resolution measurement capability offer great potential for industrial applications. Robustness is an important requirement for industrial applications, which limits the application of many common techniques such as interferometric or photographic measurements, especially in mechanical workshops. This paper introduces an innovative technique for displacement measurement using speckle photography that is robust to disturbances, imaging errors, and does not require a large number of database patterns for calibration. It uses the relative correlation of the speckle patterns generated by two parallel, overlapping laser beams with an identical spot size and different wavelengths for relative displacement measurement in sub-micrometer order, and requires only one reference pattern that is updated frequently during the measurement process. The method is demonstrated over 200 µm range and is extendable to longer ranges.

Image continuity at different levels of zoom for fringe patterns.

Fringe patterns are raw output data from many measurement systems including laser interferometers and moiré systems. For instruments with a range of zoom levels to measure the object at different scales, a technique (algorithm) is needed to combine and/or compare data to obtain information at different levels of details. A technique to keep the continuity of output images both at different levels of zoom and within the same level of zoom is developed and demonstrated. Image registration is used to correlate images, find relative zoom values, and obtain shift between images in the lateral plane. Fringe patterns from a moiré system and a laser interferometer are used as images to be stitched and demonstrate the technique. Interferomteric fringes are used to find the required parameters to inter-relate locations and scale of the fringe patterns at different levels of zoom. The calculated parameters are scale and translation in both directions; these parameters make it possible to locate the coordinates of the region that the measurement system is zoomed in on, related to the area with lower magnification and relative locations of images within the same level of zoom. Results show that this technique is capable of finding the scale and shift parameters within the resolution of one pixel and therefore can restore continuity between images at different levels of zoom.

Active illumination single-pixel camera based on compressive sensing.

We present an optical imaging system based on compressive sensing (CS) along with its principal mathematical aspects. Although CS is undergoing significant advances and empowering many discussions and applications throughout various fields, this article focuses on the analysis of a single-pixel camera. This work was the core for the development of a single-pixel camera approach based on active illumination. Therefore, the active illumination concept is described along with the experimental results, which were very encouraging toward the development of compressive-sensing-based cameras for various applications, such as pixel-level programmable gain imaging.

Spectral characterization of a photonic bandgap fiber for sensing applications.

We study the measurand-induced spectral shift of the photonic bandgap edge of a hollow-core photonic crystal fiber. The physical measurands considered are strain, temperature, curvature, and twist. A noticeable sensitivity to strain, temperature, and twist is observed, with a blueshift to increase strain and twist. An increase in temperature induces a redshift. On the other hand, curvature has no observable effect on the spectral position of the photonic bandgap edge.

Interferometric technique for faceted microstructure metrology using an index matching liquid.

Microstructured optical products are becoming more widespread due to advances in manufacturing. Many of these structures contain faceted surfaces with steep slopes. Adequate metrology for such surfaces is lacking. We describe an interferometric technique that combines plane wave illumination with an index matching liquid to achieve high quality, high speed measurements of such faceted microstructures. We account for refraction at the interfaces, rather than consider only optical path length changes due to the index liquid, and this significantly improves the facet angle measurement. We demonstrate the technique with the measurement of an array of micropyramids and show that our results are in good agreement with measurements taken on a contact profilometer. We also extend the technique to measure opaque microcorner cubes by implementing an intermediate replication step.

Hybrid shear force feedback/scanning quantitative phase microscopy applied to subsurface imaging.

Quantitative phase microscopy allows for the study of the surface morphology and dynamics of transparent biological specimens. Although phase data often contains coupled subsurface information, decoupling the surface and subsurface components is often very difficult or impossible. We hereby present a simple procedure which exploits simultaneous obtained quantitative phase and shear-force feedback topography data to extract subsurface sample information. Our results reveal subsurface features in fabricated samples and fish erythrocytes.

Trimodal imaging system capable of quantitative phase imaging without 2 pi ambiguities.

A neoteric approach to interferometric phase imaging unencumbered by 2 pi phase ambiguities is presented. This technique utilizes an actively controlled angular displacement glass plate positioned in the reference arm of an environmentally stabilized pseudoheterodyne Mach-Zehnder interferometer. The plate is continually adjusted to maintain a constant interferometric output phase, as a phase object in the sample arm is raster scanned. Using a 632.8 nm source, unwrapped phase images of translucent samples ranging from approximately 150 nm to 1.5 microm thick were obtained. This system is incorporated into a conventional near-field scanning optical microscope, which permits simultaneous phase, intensity, and surface morphology studies.

Optical Fiber Sensing Using Quantum Dots.

Recent advances in the application of semiconductor nanocrystals, or quantumdots, as biochemical sensors are reviewed. Quantum dots have unique optical properties thatmake them promising alternatives to traditional dyes in many luminescence basedbioanalytical techniques. An overview of the more relevant progresses in the application ofquantum dots as biochemical probes is addressed. Special focus will be given toconfigurations where the sensing dots are incorporated in solid membranes and immobilizedin optical fibers or planar waveguide platforms.

Effective wavelength calibration for moiré fringe projection.

The fringe patterns seen when using moiré instruments are similar to the patterns seen in traditional interferometry but differ in the spacing between consecutive fringes. In traditional interferometry, the spacing is constant and related to the wavelength of the source. In moiré fringe projection, the spacing (the effective wavelength) may not be constant over the field of view and the spacing depends on the system geometry. In these cases, using a constant effective wavelength over the field of view causes inaccurate surface height measurements. We examine the calibration process of the moiré fringe projection measurement, which takes this varying wavelength into account to produce a pixel-by-pixel wavelength map. The wavelength calibration procedure is to move the object in the out-of-plane direction a known distance until every pixel intensity value goes through at least one cycle. A sinusoidal function is then fit to the data to extract the effective wavelength pixel by pixel, yielding an effective wavelength map. A calibrated step height was used to validate the effective wavelength map with results within 1% of the nominal value of the step height. The error sources that contributed to the uncertainty in determining the height of the artifact are also investigated.

Applications of quantum dots in optical fiber luminescent oxygen sensors.

The potential applications of luminescent semiconductor nanocrystals to optical oxygen sensing are explored. The suitability of quantum dots to provide a reference signal in luminescence-based chemical sensors is addressed. A CdSe-ZnS nanocrystal, with an emission peak at 520 nm, is used to provide a reference signal. Measurements of oxygen concentration, which are based on the dynamic quenching of the luminescence of a ruthenium complex, are performed. Both the dye and the nanocrystal are immobilized in a solgel matrix and are excited by a blue LED. Experimental results show that the ratio between the reference and the sensor signals is highly insensitive to fluctuations of the excitation optical power. The use of CdTe, near-infrared quantum dots with an emission wavelength of 680 nm, in combination with a ruthenium complex to provide a new mechanism for oxygen sensing, is investigated. The possibility of creating oxygen sensitivity in different spectral regions is demonstrated. The results obtained clearly show that this technique can be applied to develop a wavelength division multiplexed system of oxygen sensors.