Field-of-view analysis of a polarization interference Fourier transform imaging spectrometer

The Fourier transform imaging spectrometer (FTIS) is an important tool for the measurement of spectral information in a scene. Advances in electro-optic crystal systems have led to the advent of the FTIS based on polarization interference filters. The operation of these devices as spectrometers has been well characterized, but the imaging capabilities have yet to be thoroughly explored. We explore the field-of-view limitations that occur when using this particular type of FTIS.     

Validation of interferometric imaging from a pupil-masking experiment on a solar telescope

We present the results of a pupil-masking experiment that uses the Sun as the source object. The goal of our experiment was a proof-of-concept validation for a Fizeau (image-plane) interferometric beam combination with a complex source that overfilled the field of view. We employed a phase-diversity technique to measure the optical phases required to recover the instantaneous optical transfer function for the masked pupil. We used a Wiener filter to deconvolve the dirty images.     

Control of chromatic focal shift through wave-front coding

Control of chromatic aberration through purely optical means is well known. We present a novel, to our knowledge, optical-digital method of controlling chromatic aberration. The optical-digital system, which incorporates a cubic phase-modulation (CPM) plate in the optical system and postprocessing of the detected image, effectively reduces a system's sensitivity to misfocus in general or axial (longitudinal) chromatic aberration, in particular. A fully achromatic imaging system (one that is corrected for a continuous range of wavelengths) can be achieved by initial optimization of the optical system for all aberrations except chromatic aberration. The chromatic aberration is corrected by the inclusion of the CPM plate and postprocessing.     

Hyperspectral diffuse reflectance imaging for rapid, noncontact measurement of the optical properties of turbid materials

We present a method and technique of using hyperspectral diffuse reflectance for rapid determination of the optical properties of turbid media. A hyperspectral imaging system in line scanning mode was used to acquire spatial diffuse reflectance profiles from liquid phantoms made up of absorbing dyes and fat emulsion scatterers over the spectral range of 450-1000 nm instantaneously. The hyperspectral reflectance data were analyzed by using a steady-state diffusion approximation model for semi-infinite homogeneous media. A calibration procedure was developed to compensate the nonuniform instrument response of the imaging system, and a curve-fitting algorithm was used to extract absorption and reduced scattering coefficients (mua and mus', respectively) for the phantoms in the wavelength range from 530 to 900 nm. The hyperspectral imaging system gave good measures of mua and mus' for the phantoms with average fitting errors of 12% and 7%, respectively. The hyperspectral imaging technique is fast, noncontact, and easy to use, which makes it especially suitable for measurement of the optical properties of turbid liquid and solid foods.     

N-ocular volume holographic imaging

Volume holographic imaging utilizes Bragg selectivity to optically slice the object space of the imaging system and measure four- (three spatial and one spectral) dimensional object information. The N-ocular version of this method combines multiple-volume holographic sensors and digital postprocessing to yield high-resolution three-dimensional images for broadband objects located at long working distances. We discuss the physical properties of volume holography pertinent to imaging performance and describe two computational algorithms for image inversion based on filtered backprojection and least-squares optimization.     

Measurements of the phase shift on reflection for low-order infrared Fabry-Perot interferometer dielectric stack mirrors

A simple technique based on a Fizeau interferometer to measure the absolute phase shift on reflection for a Fabry-Perot interferometer dielectric stack mirror is described. Excellent agreement between the measured and predicted phase shift on reflection was found. Also described are the salient features of low-order Fabry-Perot interferometers and the demonstration of a near ideal low-order (1-10) Fabry-Perot interferometer through minimizing the phase dispersion on reflection of the dielectric stack. This near ideal performance of a low-order Fabry-Perot interferometer should enable several applications such as compact spectral imagers for solid and gas detection. The large free spectral range of such systems combined with an active control system will also allow simple interactive tuning of wavelength agile laser sources such as CO(2) lasers, external cavity diode lasers, and optical parametric oscillators.     

A local optical probe for measuring motion and stress in a nanoelectromechanical system

Nanoelectromechanical systems can be operated as ultrasensitive mass sensors and ultrahigh-frequency resonators, and can also be used to explore fundamental physical phenomena such as nonlinear damping and quantum effects in macroscopic objects. Various dissipation mechanisms are known to limit the mechanical quality factors of nanoelectromechanical systems and to induce aging due to material degradation, so there is a need for methods that can probe the motion of these systems, and the stresses within them, at the nanoscale. Here, we report a non-invasive local optical probe for the quantitative measurement of motion and stress within a nanoelectromechanical system, based on Fizeau interferometry and Raman spectroscopy. The system consists of a multilayer graphene resonator that is clamped to a gold film on an oxidized silicon surface. The resonator and the surface both act as mirrors and therefore define an optical cavity. Fizeau interferometry provides a calibrated measurement of the motion of the resonator, while Raman spectroscopy can probe the strain within the system and allows a purely spectral detection of mechanical resonance at the nanoscale.     

Theoretical and experimental evaluation of the modal field shift and the associated transition loss in perturbed index-profile single-mode optical fiber with application of Fizeau interferometry

Fizeau micro-interferometry is applied to evaluate some parameters of a curved single-mode optical fiber. The field shift of the fundamental mode and the associated transition loss in a perturbed index-profile fiber due to bending are determined. The preceding fiber parameters are determined as a function of the shift of multiple-beam Fizeau fringes. For a curvature range between 0.13 and 0.053 mm(-1), a range of field shift between 0.44 and 0.21 microm is determined. A fraction of the transition loss ranging between 0.0056 and 0.028 is calculated within the same curvature range. Because our method has high index resolution and spatial resolution, it shows good agreement with theory. The results and the agreement with theory indicate that the use of multiple-beam Fizeau fringes is a promising technique that is capable of determining with high accuracy some guidance parameters of the optical fibers.     

Optical Bilinear Transformations: General Properties

Because of the quadratic relation between the optical field and intensity, an inherent non-linearity exists in almost all optical systems. A class of non-linear transformations which we call bilinear (quadratic with memory) is defined; its properties and relevance to optical imaging systems are discussed. For space-invariant systems, a generalized transfer function is defined which characterizes the bilinear system completely. We also examine the approximate linearization of bilinear systems for low-contrast images, and the propagation of noise through such systems. Special emphasis is given to the partially coherent system which is but a special case of this general bilinear system.     

Validation of the Infrared Emittance Characterization of Materials Through Intercomparison of Direct and Indirect Methods

A comparison of the spectral directional emittance of samples as a function of wavelength was performed at the Fourier Transform Infrared Spectrophotometry (FTIS) and the Advanced Infrared Radiometry and Imaging (AIRI) facilities at NIST. At the FTIS, the emittance is obtained indirectly through the measurement of near-normal directional-hemispherical reflectance (DHR) using an infrared integrating sphere. At the AIRI, the normal directional emittance is obtained directly through the measurement of the sample spectral radiance referenced to that from blackbody sources, while the sample is located behind a black plate of known temperature and emittance. On the same setup at the AIRI, the normal emittance at near ambient temperatures is also measured indirectly by a “two-temperature” method in which the sample spectral radiance is measured while the background temperature is controlled and varied. The sample emittance measurements on the comparison samples are presented over a wavelength range of 3.4 μm to 13.5 μm at several near-ambient temperatures and for near-normal incidence. The results obtained validate the two independent capabilities and demonstrate the potential of the controlled background methods for measurements of the radiative properties of IR materials.     

High-speed spectral imager for imaging transient fluorescence phenomena

We describe fluorescence spectral imaging results with the microscope computed-tomography imaging spectrometer (muCTIS). This imaging spectrometer is capable of recording spatial and spectral data simultaneously. Consequently, muCTIS can be used to image dynamic phenomena. The results presented consist of proof-of-concept imaging results with static targets composed of 6-mum fluorescing microspheres. Image data were collected with integration times of 16 ms, comparable with video-frame-rate integration times. Conversion of raw data acquired by the muCTIS to spatial and spectral data requires postprocessing. The emission spectra were sampled at 10-nm intervals between 420 and 710 nm. The smallest spatial sampling interval presented is 1.7 mum.     

Phase plate to extend the depth of field of incoherent hybrid imaging systems

A hybrid imaging system combines a modified optical imaging system and a digital postprocessing step. We describe a spatial-domain method for designing a pupil phase plate to extend the depth of field of an incoherent hybrid imaging system with a rectangular aperture. We use this method to obtain a pupil phase plate to extend the depth of field, which we refer to as a logarithmic phase plate. Introducing a logarithmic phase plate at the exit pupil of a simulated diffraction-limited system and digitally processing the detector's output extend the depth of field by an order of magnitude more than the Hopkins defocus criterion. We also examine the effect of using a charge-coupled device optical detector, instead of an ideal optical detector, on the extension of the depth of field. Finally, we compare the performance of the logarithmic phase plate with that of a cubic phase plate in extending the depth of field of a hybrid imaging system with a rectangular aperture.     

Visible imaging characteristics of the space target based on bidirectional reflection distribution function

A modelling method for visible imaging characteristics of a space target is presented. Background radiation of a space target consists of direct solar radiation, reflected radiation from the earth and the moon, and that from other stars. The target surface was divided into grids and the light reflection properties of each grid are described by introducing a bidirectional reflection distribution function (BRDF) model obtained in advance. Then a mathematical model for the visible imaging properties of the space target was built using given parameters of the optical detection system. Visual surfaces of the target to detection system were determined by a vector coordinate method. Simulation of the target optical imaging characteristics in orbit was achieved according to its given physical dimensions and parameters. The results show the method is feasible and robust for optical characteristics of the space target. It can provide a facility for real-time analysis of optical imaging characteristics of space targets.     

Comparative signal-to-noise analysis of fibre-optic based optical coherence tomography systems

Several optical coherence tomography (OCT) systems are proposed using optical-fibre components and based around Fizeau sensing interferometers. The theoretical signal-to-noise ratio (SNR) is calculated for each of the proposed configurations, using a constant set of assumed values for illumination and detection parameters. The SNR values obtained are compared with values calculated for typical existing configurations based around Michelson interferometers. Fizeau-based systems incorporating a secondary processing interferometer offer the advantage over current interferometer configurations of down-lead insensitivity, which prevents signal fading and reduces thermal fringe drift. The most basic form of the Fizeau system makes inefficient use of optical power, and has a low SNR compared with the widely used Michelson configuration. However, the results of the analysis described in this paper show that the SNR for more sophisticated Fizeau configurations, incorporating optical circulators and balanced detection systems, can be as high as the value for the most sensitive existing fibre-based OCT systems. Fizeau configurations therefore offer the combined advantages of optimized SNR and down-lead insensitivity, indicating their suitability for use in relatively poorly controlled environments such as in-vivo measurements.     

Reduced depth of field in incoherent hybrid imaging systems

A hybrid imaging system combines a modified optical imaging module and a digital postprocessing step. We define what to our knowledge is a new metric to quantify the blurring of a defocused image that is more suitable than the defocus parameter for describing defocused hybrid imaging systems. We use this metric to design a pupil phase grating to reduce the depth of field, thereby increasing the axial resolution, of an incoherent hybrid imaging system using quasi-monochromatic illumination. By introducing this grating at the exit pupil and digitally processing the output of the detector, we reduce the depth of field by more than a factor of 2. Finally, we examine the effect of using a CCD optical detector, instead of an ideal optical detector, on the reduction of the depth of field.     

Phase-shifting technique for improving the imaging capacity of sparse-aperture optical interferometers

We describe the principle of a multiaperture interferometer that uses a phase-shifting technique and is suitable for quick snapshot imaging of astrophysical objects at extreme angular resolution through Fourier inversion. A few advantages of the proposed design are highlighted, among which are radiometric efficiency, field of view equivalent to those of Fizeau interferometers, and a preliminary calibration procedure allowing characterization of instrumental errors. For large telescope numbers, the proposed design also results in considerable simplification of the optical and mechanical design. Numerical simulations suggest that it should be possible to couple hundreds of telescopes on a single 4K × 4K detector array, using only conventional optical components or emerging technologies.     

Asphere testing with a Fizeau interferometer based on a combined computer-generated hologram

Fizeau interferometers with an additional diffractive optical element are frequently used for measuring spherical and aspherical surfaces. We present a new (to our knowledge) optical test method, in which the Fizeau principle is now perfectly fulfilled by generating reference and measuring wavefront on the last optical surface, which carries a diffractive optical element. This method has been examined experimentally by testing a reference f/0.68 spherical mirror and can be applied identically for testing aspheres. Several advantages of this method are discussed and proved experimentally.     

Radiative transfer. II. Impact of meteorological variables and surface albedo on atmospheric optical properties retrieved from ground-based multispectral measurements

In a companion paper we describe a radiative transfer model and a consequent algorithm for retrieving atmospheric variables from ground-based multispectral measurements of direct solar irradiance. The accuracy of retrieved data depends on measured spectral irradiance as well as surface meteorological variables. Here we analyze the impact of the surface albedo on diffuse scattered solar irradiance in the Sun-sensor direction. We also investigate the impact of visibility on the retrieved spectral transmission function and optical thickness. We discuss the application of a spectrometric system, the passive pyrheliometric scanner (PPS), for the estimation of atmospheric turbidity and visibility. The spectral transmission of the atmosphere derived with the PPS for the Athens atmosphere and for different zenith angles is given. We present results of retrieved aerosol optical properties using as atmospheric turbidity those values estimated from the ground-based measurements of direct solar radiation with the aid of the PPS. It is shown that another application of the PPS may be the estimation of horizontal visibility.