Multipurpose spectral imager

A small spectral imaging system is presented that images static or moving objects simultaneously as a function of wavelength. The main physical principle is outlined and demonstrated. The instrument is capable of resolving both spectral and spatial information from targets throughout the entire visible region. The spectral domain has a bandpass of 12 A. One can achieve the spatial domain by rotating the system's front mirror with a high-resolution stepper motor. The spatial resolution range from millimeters to several meters depends mainly on the front optics used and whether the target is fixed (static) or movable relative to the instrument. Different applications and examples are explored, including outdoor landscapes, industrial fish-related targets, and ground-level objects observed in the more traditional way from an airborne carrier (remote sensing). Through the examples, we found that the instrument correctly classifies whether a shrimp is peeled and whether it can disclose the spectral and spatial microcharacteristics of targets such as a fish nematode (parasite). In the macroregime, we were able to distinguish a marine vessel from the surrounding sea and sky. A study of the directional spectral albedo from clouds, mountains, snow cover, and vegetation has also been included. With the airborne experiment, the imager successfully classified snow cover, leads, and new and rafted ice, as seen from 10.000 ft (3.048 m).     

Tunable acousto-optic spectral imager for atmospheric composition measurements in the visible spectral domain

We describe a new spectral imaging instrument using a TeO₂ acousto-optical tunable filter (AOTF) operating in the visible domain (450-900?nm). It allows for fast (～1 second), monochromatic (FWHM ranges from 0.6?nm at 450?nm to 3.5?nm at 800?nm) picture acquisition with good spatial resolution. This instrument was designed as a breadboard of the visible channel of a new satellite-borne atmospheric limb spectral imager, named the Atmospheric Limb Tracker for the Investigation of the Upcoming Stratosphere (ALTIUS), that is currently being developed. We tested its remote sensing capabilities by observing the dense, turbulent plume exhausted by a waste incinerator stack at two wavelengths sensitive to NO₂. An average value of 6.0±0.4×10¹⁷ molecules?cm^-2 has been obtained for the NO₂ slant column density within the plume, close to the stack outlet. Although this result was obtained with a rather low accuracy, it demonstrates the potential of spectral imaging by using AOTFs in remote sensing.     

Total-light imager with flat spectral response for solar photometric measurements

Certain applications in imaging photometry and radiometry require a telescope-detector system with (preferably constant) response over a wide spectral range from the ultraviolet through the infrared. We describe the design and characterization of the Solar Bolometric Imager (SBI), a 30-cm-aperture Dall-Kirkham telescope combined with a gold-blacked, 80, 000-element thermal array detector. Our SBI prototype provides spectrally uniform imaging in total solar light (0.28-2.6 mum) of heat-flow inhomogeneities at the solar photosphere, with better than 5-arc sec angular resolution over a 6.5 x 13 arc min field of view. A balloon-borne SBI would avoid most atmospheric transmission variation over this spectral range, enabling accurate study of the sources of total irradiance variation.     

High-speed fuel tracer fluorescence and OH radical chemiluminescence imaging in a spark-ignition direct-injection engine

An innovative technique has been demonstrated to achieve crank-angle-resolved planar laser-induced fluorescence (PLIF) of fuel followed by OH* chemiluminescence imaging in a firing direct-injected spark-ignition engine. This study used two standard KrF excimer lasers to excite toluene for tracking fuel distribution. The intensified camera system was operated at single crank-angle resolution at 2000 revolutions per minute (RPM) for 500 consecutive cycles. Through this work, it has been demonstrated that toluene and OH* can be imaged through the same optical setup while similar signal levels are obtained from both species, even at these high rates. The technique is useful for studying correlations between fuel distribution and subsequent ignition and flame propagation without the limitations of phase-averaging imaging approaches. This technique is illustrated for the effect of exhaust gas recirculation on combustion and will be useful for studies of misfire causes. Finally, a few general observations are presented as to the effect of preignition fuel distribution on subsequent combustion.     

Sensitivity characterization of a time-domain fluorescence imager: eXplore Optix

A key issue in the practical application of fluorescence imaging is the presence of a background signal detected during data acquisition when no target fluorescent material is present. Regardless of the technology employed, background signals cannot be completely eliminated, which limits the detection sensitivity of fluorescence imaging systems, especially for in vivo applications. We present a methodology to characterize the sensitivity of fluorescence imaging devices by taking the background effect into account through the fluorescent signal-to-background ratio (SBR). In an initial application of the methodology, tissuelike liquid phantoms with Cy5.5 fluorescent inclusions were investigated experimentally over a wide range of varying parameters, such as tissue absorption coefficient, scattering coefficient, fluorophore concentration, and inclusion location. By defining detectable and quantifiable SBR thresholds, empirical relations are established, and the sensitivity performance of Advanced Research Technologies's eXplore Optix using Cy5.5 is characterized.     

High-speed fluorescence detection of explosives-like vapors

In this paper, we report on the preparation of novel cross-reactive optical microsensors for high-speed detection of low-level explosives and explosives-like vapors. Porous silica microspheres with an incorporated environmentally sensitive fluorescent dye are employed in high-density sensor arrays to monitor fluorescence changes during nitroaromatic compound (NAC) vapor exposure. The porous silica-based sensor materials have good adsorption characteristics, high surface areas, and surface functionality to help maximize analyte-dye interactions. These interactions occur immediately upon vapor exposure, i.e., in less than 200 ms and are monitored with a high-speed charge-coupled device camera to produce characteristic and reproducible vapor response profiles for individual sensors within an array. Employing thousands of identical microsensors permits sensor responses to be combined, which significantly reduces sensor noise and enhances detection limits. Normalized response profiles for 1,3-dinitrobenzene (1,3-DNB) are independent of analyte concentration, analyte exposure time, or sensor age for an array of one sensor type. Explosives-like NACs such as 2,4-dinitrotoluene and DNB are detected at low part-per-billion levels in seconds. Sensor-analyte profiles of some sensor types are more sensitive to low-level NAC vapor even when in a higher organic vapor background. We show that single-element arrays permit the detection of low-level nitroaromatic compound vapors because of sensor-to-sensor reproducibility and signal averaging.     

Wide-field fluorescence lifetime imaging with optical sectioning and spectral resolution applied to biological samples

Abstract. Wide-field fluorescence lifetime imaging with spectral resolution and optical sectioning has been performed to achieve five-dimensional fluorescence microscopy. Spectral filtering has been shown to have the potential to provide functional information about biological tissue by simultaneously measuring the spectral/lifetime signature of the sample. The potential to use multispectral imaging to separate cellular components spatially by their different emission wavelengths has also been demonstrated thus reducing artefacts in the calculated lifetime maps. The instrument is based on diode-pumped solid-state laser technology and an ultrafast gated optical image intensifier. Also reported is the use of a picosecond blue laser diode as the excitation source to produce a fluorescence lifetime microscope with a footprint of less than 0.25m².     

Single-molecule kinetics and super-resolution microscopy by fluorescence imaging of transient binding on DNA origami

DNA origami is a powerful method for the programmable assembly of nanoscale molecular structures. For applications of these structures as functional biomaterials, the study of reaction kinetics and dynamic processes in real time and with high spatial resolution becomes increasingly important. We present a single-molecule assay for the study of binding and unbinding kinetics on DNA origami. We find that the kinetics of hybridization to single-stranded extensions on DNA origami is similar to isolated substrate-immobilized DNA with a slight position dependence on the origami. On the basis of the knowledge of the kinetics, we exploit reversible specific binding of labeled oligonucleotides to DNA nanostructures for PAINT (points accumulation for imaging in nanoscale topography) imaging with <30 nm resolution. The method is demonstrated for flat monomeric DNA structures as well as multimeric, ribbon-like DNA structures.     

High-speed two-camera imaging pyrometer for mapping fireball temperatures

A high-speed imaging pyrometer was developed to investigate the behavior of flames and explosive events. The instrument consists of two monochrome high-speed Phantom v7.3 m cameras made by Vision Research Inc. arranged so that one lens assembly collects light for both cameras. The cameras are filtered at 700 or 900 nm with a 10 nm bandpass. The high irradiance produced by blackbody emission combined with variable shutter time and f-stop produces properly exposed images. The wavelengths were chosen with the expected temperatures in mind, and also to avoid any molecular or atomic gas phase emission. Temperatures measured using this pyrometer of exploded TNT charges are presented.     

High-speed digital color imaging pyrometry

Temperature measurements of high-explosive and combustion processes are difficult to obtain due to the speed and environment of the events. To overcome these challenges, we have characterized and calibrated a digital high-speed color camera that may be used to measure the temperature of such events. A two-color ratio method is used to calculate a temperature using the color filter array raw image data and a graybody assumption. If the raw image data are not available, temperatures may be calculated from the processed images or movies, depending on proper analysis of the digital color imaging pipeline. We analyze three transformations within the pipeline (demosaicing, white balance, and gamma correction) to determine their effect on the calculated temperature. Using this technique with a Phantom color camera, we have measured the temperature of exploded C-4 charges. The surface temperature of the resulting fireball was found to rapidly increase after detonation, and subsequently decayed to a constant value of approximately 1980 K.     

Agile dual-channel spectral imaging with spectral zooming

A dual-channel spectral imaging system with agile spectral band access and spectral bandwidth tuning capability is presented. A diffractive grating and an acousto-optic tunable filter (AOTF) are respectively used as spectral dispersion and spectral filtering elements for the two channels. A 4f spectral filtering channel using an adjustable slit is set up at the first diffraction order of the grating to realize coarse spectral band selection. The AOTF selectively filters the spectrum of the nondispersed zero order to realize fine spectral imaging. The spectral zooming function is achieved without increasing spectral frame number facilitating real-time spectral imaging operation. Feasibility of the spectral imaging has been demonstrated through preliminary experiments. Minimum 6 nm spectral resolution and 1.2 degrees field of view have been achieved. The real-time spectral imaging capable of wide spectral band operation without loosing desired fine spectral capability is particularly useful for a variety of defense, medical, and environmental monitoring applications.     

Compact imaging spectrograph for broadband spectral simultaneity

We report on the design of a small spectrograph that is capable of imaging several thousand angstrom simultaneously at a moderate spectral resolution. The prototype instrument included a number of developmental items that were used to assess their utility in this and other applications. Some we would recommend using again, some we would not. In the configuration that was built and tested, the instantaneous wavelength range was chosen to be 3700-11,700 ?. However, the wavelength range could be selected for a lower wavelength, as low as ~ 1200 ?. The spectral imaging was achieved with an intensified-CCD focal-plane detector. The broad wavelength coverage was achieved with a matrix of four diffraction gratings and a custom-designed photocathode system. The photocathode was specially built to provide a response over the chosen broad wavelength range by use of a single image intensifier. The theoretical spectral resolution of the instrument varied from 12 to 20 ? depending on waveleng th segment. A higher spectral resolution can be selected at the expense of total wavelength coverage. The optical system was designed to be moderately fast (f/6) when considered at the level of each of the four optical subchannels and suitable for use on relatively weak airglow signals. The instrument was designed to be readily portable, weighing 15 kg, with an envelope of 37 cm × 37 cm × 48 cm. The advantages and weaknesses of such an instrument are discussed, and improvements are suggested for specific applications. This study represents a stepping stone in the evolution of electronic spectrographs and leads to later designs that are currently being evaluated.     

Color ellipsoscope for real-time imaging of nanometer-scale surface phenomena

An imaging method was developed based on null ellipsometry with a white-light source. It is useful for visualizing the kinetics of fast surface phenomena on the nanometer scale.     

High-speed, whole-column fluorescence imaging detection for isoelectric focusing on a microchip using an organic light emitting diode as light source

An integrated and simplified microfluidic device using a 250 microm x 1-4 cm of organic light emitting diode (OLED) array as a two-dimensional light source for single-channel and multichannel whole-column imaging detection was developed. This fluorescence detection system was used for isoelectric focusing (IEF) of R-phycoerythrin in a microchip. The IEF conditions were optimized, and the total analysis time was extremely reduced to 30 s for 2-cm-long microchannels at 700 V/cm of electric field strength without the presence of electroosmotic flow. The compression of pH gradient caused by electrolytes drawing into the microchannels was efficiently restrained when 1% hydroxylpropylmethyl cellulose in 2% ampholyte was used as the carrier for IEF. Under optimized IEF conditions, the detection limit of this system was approximately 0.6 microg/mL or 45 pg at 75 nL/column injection of R-phycoerythrin. This OLED-induced fluorescence detection system for WCID provides a high-speed IEF technique with quantitative ability and the potential for high integration and throughput microchip systems.     

Exciplex fluorescence imaging for liquid mixing studies

A diagnostic technique based on excited-state complex (exciplex) fluorescence is reported for visualization of diffusion layers formed between mixing liquids. High-spatial-resolution instantaneous images and time-sequenced images are presented. Single drops and jets are visualized mixing in a liquid pool. Time-resolved images of exciplex fluorescence have also been obtained that provide information on the duration of the mixing event and the length and time scales of the diffusion process. The results demonstrate that the technique has the potential to provide new information concerning the physics of liquid mixing processes.     

High-resolution two-grating spectrometer for dual wavelength spectral imaging

A two-grating high-resolution spectrometer for dual wavelength imaging is demonstrated based on the standard Czerny-Turner mounting with an auxiliary grating and a mirror. A two-dimensional charge-coupled device (CCD) detector in the spectrometer focal plane allows simultaneous detection of two spectral intervals. Each spectrometer grating is driven by a high-precision stepper motor interfaced to a computer via home-made software. The software allows fast tuning of the gratings to a desirable spectral interval anywhere between 200 nm and 800 nm. The spectral interval widths are 2-3 nm for a 'high-resolution' (2400 grooves/mm) grating and 4-5 nm for a 'low-resolution' (1200 grooves/mm) grating. The resolution varies between 0.01 nm and 0.02 nm depending on the grating used. The performance of the spectrometer is demonstrated by detecting spectrally resolved images from a back-illuminated template and from a laser-induced plasma. The spectrometer can be useful for two-line spectroscopic diagnostics or can be expanded for multi-element spectral analysis.