Radiometric Signal Research Articles

Depth profilometric case studies in caries diagnostics of human teeth using modulated laser radiometry and luminescence

Simultaneous measurements from human teeth of photothermal radiometric (PTR) and luminescence (LM) signals induced by an intensity modulated laser have been performed to assess the feasibility of detecting deep lesions and near-surface cracks, to examine the effects of varying enamel thicknesses, the presence of fillings, and stains on the surface of teeth. A commercial dc luminescence monitoring instrument (DIAGNOdent by KaVo) was also used to examine a set of teeth for comparison purposes with PTR and LM. PTR amplitude signals from carious regions and from thin enamel were higher than those from healthy regions and thicker enamel. A crack produces a peak in the PTR amplitude scan, as well as a sudden change in the luminescence amplitude at the corresponding point. At low frequencies (5 Hz), the PTR amplitude showed high sensitivity to a deep (about 2 mm) lesion, while at high frequencies (700 Hz) it was more sensitive to surface cracks. It was concluded that by selecting proper modulation frequencies of the laser, measurements of PTR and LM signals could be used as a dental diagnostic technique with a small, inexpensive, low-power (<30 mW) semiconductor laser as a light source emitting in the optical window range of hard tissue (650–1000 nm).

Frequency-domain theory of laser infrared photothermal radiometric detection on thermal waves generated by diffuse-photon-density wave fields in turbid media (abstract)

A three-dimensional theory of the frequency-domain thermal-wave field generated inside a turbid medium with optical and thermal properties of human tissue is presented. The optical source is treated as a three-dimensional harmonically modulated diffuse photon-density wave (DPDW) field in the diffusion approximation of the radiative transfer theory. Unlike earlier Green-function-based theoretical models, exact boundary conditions are used based on the requirement that there should be no diffuse photon intensity entering the turbid medium from the outside. Explicit analytical expressions for the DPDW field and for the dependent thermal-wave field are obtained in the spatial Hankel-transform domain. The formalism is further extended to the calculation of the infrared photothermal radiometric signal arising from the nonradiatively generated thermal-wave distribution in turbid media with instantaneous nonradiative de-excitation, as well as in media with non-zero fluorescence relaxation lifetimes. Numerical inversions have been performed and presented as examples of selected special cases of the theory. It is found that the present theory with exact DPDW-field boundary conditions is valid throughout the entire domain of the turbid medium, with the exception of the very-near-surface ballistic photon “skin layer” (7–50 μm). Photothermal radiometric signals were found to be more reliably predicted than DPDW signals within this layer, due to the depth-integration nature of this detection methodology.

Spectroscopic photothermal radiometry as a deep subsurface depth profilometric technique in semiconductors

The theoretical and experimental aspects of spectroscopic photothermal radiometry (PTR) of semiconductors are presented and the potential of the technique for depth profilometry is established. A three-dimensional model of the PTR signal from a semiconductor excited by light of arbitrary optical penetration depth is presented. Numerical simulations of the PTR response to the electronic transport parameters and the optical penetration depth of the excitation source are presented. Intensity-modulated frequency scans and two-dimensional surface scans at fixed frequencies have been performed at several different absorption depths on a Si wafer with various degrees of mechanical damage introduced to either the front or the back surface. The electronic transport parameters obtained from fitting the frequency scans to the theoretical model and analysis of the experimental curves show that while the surface recombination velocities extracted from the fits do not vary significantly with excitation wavelength, the carrier recombination lifetime and the overall sensitivity of the photothermal radiometric signal to spatially localized damage is strongly influenced by the proximity of the injected excess carrier density centroid to the defect location. This correlation between the sensitivity of the PTR signal to a localized defect and the proximity of the injected carriers to the defect demonstrates the potential for spectroscopic PTR as a depth profilometric technique for semiconductors.

Multifrequency radiometric determination of temperature profiles in a lossy homogeneous phantom using a dual-mode antenna with integral water bolus

In the treatment of cancer, microwave hyperthermia has been established as an efficient adjunctive procedure to radiation therapy and chemotherapy. Wider acceptance of this method awaits schemes to measure volumetric temperatures noninvasively in human tissue for control of the heating process. This effort describes the design and performance of a new microstrip applicator intended for homogeneous heating of superficial tissue while at the same time monitoring temperature of the underlying tissue by noninvasive radiometric sensing of black-body radiation from the heated volume. Radiometric capabilities are assessed in terms of accuracy of up to six measured brightness temperatures applied in an inversion algorithm from which one-dimensional depth temperature profiles are generated. Based on radiometric signals recorded over the 1-4-GHz range, the temperature accuracy determined from statistical analysis of 200 realizations of the process is better than /spl plusmn/0.2/spl deg/C to a depth of 5 cm in phantom. Aperture heating uniformity is assessed with electric field scans in a homogeneous muscle phantom. As long as sufficiently thin (< 5 mm) water boli are used, SAR distributions at 1-cm depth in phantom extends effectively just outside the aperture perimeter, making this microstrip antenna an excellent building block element of larger multi-antenna array applicators.

Spectral variation of the infrared absorption coefficient in pulsed photothermal profiling of biological samples

Pulsed photothermal radiometry can be used for non-invasive depth profiling of optically scattering samples, including biological tissues such as human skin. Computational reconstruction of the laser-induced temperature profile from recorded radiometric signals is sensitive to the value of the tissue absorption coefficient in the infrared detection band (μIR). While assumed constant in reported reconstruction algorithms, μIR of human skin varies by two orders of magnitude in the commonly used 3−5 μm detection band. We analyse the problem of selecting the effective absorption coefficient value to be used with such algorithms. In a numerical simulation of photothermal profiling we demonstrate that results can be markedly impaired, unless the reconstruction algorithm is augmented by accounting for spectral variation μIR(λ). Alternatively, narrowing the detection band to 4.5–5 μm reduces the spectral variation μIR(λ) to a level that permits the use of the simpler, un-augmented algorithm. Implementation of the latter approach for depth profiling of port wine stain birthmarks in vivo is presented and discussed.

Frequency-domain theory of laser infrared photothermal radiometric detection of thermal waves generated by diffuse-photon-density wave fields in turbid media.

A three-dimensional theory of the frequency-domain thermal-wave field generated inside a turbid medium with optical and thermal properties of human tissue is presented. The optical source is treated as a three-dimensional harmonically modulated diffuse-photon-density wave (DPDW) field in the diffusion approximation of the radiative transfer theory. Unlike earlier Green-function-based theoretical models, exact boundary conditions are used based on the requirement that there should be no diffuse photon intensity entering the turbid medium from the outside. Explicit analytical expressions for the DPDW field and for the dependent thermal-wave field are obtained in the spatial Hankel-transform domain. The formalism is further extended to the calculation of the infrared photothermal radiometric signal arising from the nonradiatively generated thermal-wave distribution in turbid media with instantaneous nonradiative deexcitation as well as in media with nonzero fluorescence relaxation lifetimes. Numerical inversions have been performed and presented as examples of selected special cases of the theory. It is found that the present theory with exact DPDW-field boundary conditions is valid throughout the entire domain of the turbid medium, with the exception of the very near-surface ballistic photon "skin layer" (7-50 microm). Photothermal radiometric signals were found to be more reliably predicted than DPDW signals within this layer, due to the depth-integration nature of this detection methodology.

Dental Dynamic Diagnostics Using Simultaneous Frequency-domain PTR and Laser Luminescence

Frequency-domain infrared photothermal radiometry is introduced as a dynamic dental diagnostic tool complimentary to laser luminescence for quantifying sound and defective enamel or dentin. A high-spatial-resolution dynamic experimental imaging set-up, which can provide simultaneous measurements of laser-induced frequency-domain infrared photothermal radiometric and luminescence signals from defects in teeth, has been developed.. 1 Following optical absorption of laser photons, the new set-up can monitor simultaneously and independently the non-radiative (optical-to-thermal) conversion via infrared photothermal radiometry; and the radiative de-excitation via luminescence emission. In addition, the optical properties of enamel are determined using a three-dimensional luminescence and photothermal model.

Non-invasive temperature profile estimation in a lossy medium based on multi-band radiometric signals sensed by a microwave dual-purpose body-contacting antenna

Microwave radiometry has during the past two decades been investigated as a non-invasive scheme for measurement of subcutaneous tissue temperatures, basically for monitoring and control in hyperthermic treatment of cancer. In this effort, we test a contact-type, dual-purpose antenna with integral water bolus. To overcome conflicting optimization criteria in the integration of this thermometry technique with heat applicators exhibiting a large effective field size during superficial hyperthermia, a stacked configuration design is proposed, where the radiometer receive antenna (Archimedean spiral) is located on the front (skin) surface of the water bolus and the heating antenna (Dual-Concentric Conductor aperture) is placed on the bolus back surface. The motivation is to achieve homogeneous tissue heating simultaneously with non-invasive thermography of the target tissue under the applicator. This paper addresses the feasibility of predicting one-dimensional depth temperature profiles from multi-band brightness temperatures. The performance is investigated statistically by a Monte Carlo technique on both simulated and real heated-phantom data using up to six radiometric bands. Analysis of measured data shows that during the transient heating period in a solid lossy phantom, the inversion technique exhibits a precision (2 † T ) and skewness (bias) of estimated compared to actual temperature profiles of better than - 0.38°C and - 0.55°C, respectively.

Oscillatory extinction of spherical diffusion flames: Micro-buoyancy experiment and computation

The transient extinction behavior of near-limit spherical diffusion flames under microbuoyancy conditions was experimentally investigated. An inverse flame configuration was employed with the oxidizer being ejected from a porous sphere burner into a fuel (H 2 ) environment in a low-pressure (0.096 atm) chamber to effectively minimize buoyancy. Various inert gasses (N 2 , CO 2 and He) were used to dilute the oxidizer to change the Lewis number and radiative properties of the mixture so that both the transport-induced and the radiative-induced limit could be achieved. Extinction was triggered by gradually decreasing the H 2 mole fraction in the ambient. At the transport-induced limit, extinction is characterized by sudden quenching of the flame as demonstrated by a rapid decrease of the radiometer signal voltage. However, at the radiation-induced limit extinction is preceded by oscillations in the flame luminosity that grows in amplitude before extinction. Computational simulation of the experiment was performed by using a onedimensional spherically symmetric domain with detailed chemistry and transport and radiatively thin heat loss. Oscillations were computationally observed at the radiative limit with approximately the same frequency as the experiment (∼2 Hz). However, the critical ambient H 2 mole fraction that triggered oscillation was much larger in the simulation than in the experiment, possibly due to the neglect of radiation reabsorption in the simulation.

Comparison of measured and modeled stratus cloud infrared spectral signatures

The purpose of the paper is to evaluate our capability to model upwelling radiances at high resolution over broad spectral ranges in clear and cloudy conditions. Spectroradiometric measurements are taken from aircraft flying at low altitude over a clear scene and over a low‐level stratus cloud. It is the ideal case to capture the change in signal due to the presence of a cloud without the interfering effect of the atmosphere above, unavoidable in higher‐altitude measurements. The knowledge of the variability of the radiometric signal over the measurement periods and the availability of in situ measurement from the same aircraft of the meteorological parameters (temperature and humidity profile) and cloud parameters (depth and microphysical parameters of the cloud layer) allow a nearly complete control of the experiment. The results for the clear case show deviations between simulation and measurement generally smaller than 0.5%, while in the cloudy case, differences over most of the spectral range are less than the variability seen during the recording of the measured spectra. The cloud result is not significantly changed when tested with realistic changes in the measured cloud properties. Over cloud an absorbing medium alone is insufficient, and full scattering is required to achieve good agreement. This shows that care is required when assimilating infrared radiances from above such clouds, which can often extend over large areas. The adopted simulation methodology generates spectral radiances in very good agreement with the measurements both in clear air and above the stratus cloud layers.

Cloud detection and clearing for the Earth Observing System Terra satellite Measurements of Pollution in the Troposphere (MOPITT) experiment

The Measurements of Pollution in the Troposphere (MOPITT) instrument, which was launched aboard the Earth Observing System (EOS) Terra spacecraft on 18 December 1999, is designed to measure tropospheric CO and CH(4) by use of a nadir-viewing geometry. The measurements are taken at 4.7 mum in the thermal emission and absorption for the CO mixing ratio profile retrieval and at 2.3 and 2.2 mum in the reflected solar region for the total CO column amount and CH(4) column amount retrieval, respectively. To achieve the required measurement accuracy, it is critical to identify and remove cloud contamination in the radiometric signals. We describe an algorithm to detect cloudy pixels, to reconstruct clear column radiance for pixels with partial cloud covers, and to estimate equivalent cloud top height for overcast conditions to allow CO profile retrievals above clouds. The MOPITT channel radiances, as well as the first-guess calculations, are simulated with a fast forward model with input atmospheric profiles from ancillary data sets. The precision of the retrieved CO profiles and total column amounts in cloudy atmospheres is within the expected ?10% range. Validations of the cloud-detecting thresholds with the moderate-resolution imaging spectroradiometer airborne simulator data and MOPITT airborne test radiometer measurements were performed. The validation results showed that the MOPITT cloud detection thresholds work well for scenes covered with more than 5-10% cloud cover if the uncertainties in the model input profiles are less than 2 K for temperature, 10% for water vapor, and 5% for CO and CH(4).

Experimental evaluation of new thermal inversionapproach in correlation microwave thermometry

Thermal inversion, using correlation microwave thermometry, consists in estimating subsurface temperature gradients existing in a lossy material. An experimental evaluation is presented of a new thermal inversion approach, based on applying the Kalman algorithm to radiometric signals. These signals are surface measured using an experimental bench piloted by a computer.

Dual-mode antenna design for microwave heating and noninvasive thermometry of superficial tissue disease.

Hyperthermia therapy of superficial skin disease has proven clinically useful, but current heating equipment is somewhat clumsy and technically inadequate for many patients. The present effort describes a dual-purpose, conformal microwave applicator that is fabricated from thin, flexible, multilayer printed circuit board (PCB) material to facilitate heating of surface areas overlaying contoured anatomy. Preliminary studies document the feasibility of combining Archimedean spiral microstrip antennas, located concentrically within the central region of square dual concentric conductor (DCC) annular slot antennas. The motivation is to achieve homogeneous tissue heating simultaneously with noninvasive thermometry by radiometric sensing of blackbody radiation from the target tissue under the applicator. Results demonstrate that the two antennas have complimentary regions of influence. The DCC ring antenna structure produces a peripherally enhanced power deposition pattern with peaks in the outer corners of the aperture and a broad minimum around 50% of maximum centrally. In contrast, the Archimedean spiral radiates (or receives) energy predominantly along the boresight axis of the spiral, thus confining the region of influence to tissue located within the central broad minimum of the DCC pattern. Analysis of the temperature-dependent radiometer signal (brightness temperature) showed linear correlation of radiometer output with test load temperature using either the spiral or DCC structure as the receive antenna. The radiometric performance of the broadband Archimedean antenna was superior compared to the DCC, providing improved temperature resolution (0.1 degree C-0.2 degree C) and signal sensitivity (0.3 degree C-0.8 degree C/degree C) at all four 500 MHz integration bandwidths tested within the frequency range from 1.2 to 3.0 GHz.

Theoretical and experimental aspects of three-dimensional infrared photothermal radiometry of semiconductors

A general theoretical model for the infrared photothermal radiometric (PTR) signal from a semiconductor wafer is developed for the case of three-dimensional sample geometry with finite thickness. Carrier diffusion and heat conduction along the radial direction of the sample as well as along the thickness coordinate are taken into account. The simulated results for the modulation frequency dependence of the PTR signal amplitude and phase are applied to experimental data from Si wafers. Good agreement between the theoretical and experimental curves is obtained and several electronic and thermophysical parameters are estimated. This indicates that the three-dimensional PTR measurement is useful to remotely characterize semiconductor wafers patterned for large scale integrated circuits.

Absolute Wighting Functions for Near-Field Microwave Radiometric Applications

Microwave radiometry is being used in order to measure the subsurface temperature in lossy materials—such as living tissues—from the near-field thermal noise emitted in the microwave frequency range. Efforts have been ongoing during the past years to well determine and to increase the volume of the material coupled to the probe. This article discusses a new definition of the near-field weighting functions for microwave radiometric signals and examines its impact on diagnostic performance of several applications. A method is also proposed to improve the knowledge of the weighting functions from an estimation of the backward signals in the air surrounding the probe in active process.

Study of the stability of the ScaRaB shortwave channel. Application: Determination of uniform desert zones

The first flight model of ScaRaB (Scanner for Earth Radiation Budget) on board the Russian Meteor-3/07 weather satellite (altitude 1200 km; inclination, 82.56 ) transmitted data over a period of 1 year (24 February 1994-5 March 1995). We use solar channel (shortwave radiation, 0.2 mu m-4.0 mu m) data over desert regions (clear-sky scenes) to study the stability of the radiometer signal. We consider 1.25 1.25 areas in desert regions and we compute bi-directional reflectance for these areas. These values, depending on the three angles defined by the Sun-target-satellite geometry, are grouped by a similar angular configuration for each area. In this way it is possible to compare reflectance, with the same illumination and observation geometry, for different months, for each area. The results show great stability of the reflectance values, demonstrating the stability of the radiometer signal. For computing variability of the reflectance for each area (the relative standard deviation over an area) we study uniform desert zones. The low-variability zones agree very well with uniform zones selected by Cosnefro' for CNES (Centre National d'Etudes Spatiales, France) in the Saharan and Arabian deserts, from Meteosat-4 data. This demonstrates the high quality of the ScaRaB shortwave radiometer which scans with a pixel size 540 times greater than Meteostat-4.

Active cavity absolute radiometer based on high-Tc superconductors

To implement the detector-based radiometric scale in the new Medium Background Infrared (MBIR) facility at the National Institute of Standards and Technology (NIST), we have developed an electrical-substitution cavity radiometer that can be operated just above liquid-nitrogen temperature. This MBIR active cavity radiometer (ACR) utilizes a temperature-controlled receiver cone and an independently temperature-controlled heat sink. Being a thermal-type detector, low noise and drift of the radiometer signal depends mainly on low-noise temperature control of the receiver and heat sink. Using high critical-temperature (Tc) superconducting thin-film temperature sensors in the active control loops, we have achieved closed-loop temperature controllability of better than 10 µK at 89 K for a receiver having an open-loop thermal time constant of about 75 s. For a flux level of 1 µW to 10 µW, the rms noise floor over a measurement cycle time is below 20 nW. This is the lowest noise level yet reported for a liquid-nitrogen-cooled electrical-substitution radiometer, and it is the first demonstration of the use of high-Tc superconductors in such a radiometer. Potential uses for this ACR in the MBIR facility include absolute measurement of the broadband radiance of large-area 300 K cryogenic black-body sources, and absolute measurement of the spectral radiance of laser-illuminated integrating spheres for improved spectral responsivity measurements of infrared transfer standard radiometers.

Detecting vegetation change in the southern Kalahari using Landsat TM data

Using Landsat TM data we explored the magnitude and direction of change in reflectance from two study areas in the southern Kalahari Desert from 1989 to 1994. The study areas comprise three land use types: nature conservation, commercial game ranching and commercial pastoralism. The presence of these land use types provides the opportunity to explore the impacts of land management policies on vegetation structure, cover, species composition and the radiometric signal. In other semi-arid savannas provision of surface water and the associated increase in herbivory by domestic stock have resulted in a shift from grassy arid savanna to a shrubland with low perennial grass cover and high annual grass cover. We wished to ascertain whether these changes around water points and across fence lines could be detected using satellite-derived imagery. Using change vector analysis (CVA), we explored three bands (visible, red, near-infrared) of Landsat TM data recorded in April 1994. Change vector analysis shows the direction and magnitude of change between two anniversary data images. This study found that near-infrared activity has increased on active dunes near water points on commercial farms since 1989. We attribute this increase to two major changes to the ecosystem: (a) an increase in the exposed soil surface area; and (b) an increase in active greenness of woody shrubs. A previously identified (before 1989) site of invadingRhigozum trichotomumin the dune swales did not significantly change between the 2 years. Reflectances in the conservation area did not significantly change in magnitude or direction between 1898 and 1994.

Buried thermoplastic layer diagnostics by the use of combined frequency-domain and impulse response photo-thermo-mechanical radiometry

A photothermal approach to the problem of characterizing the thermoplastic layer thickness sandwiched between two metal foils used in heat-sealed food containers is described. Real-time signal acquisition instrumentation for the photothermal radiometric signal based on impulse-response fast Fourier transform (FFT) processing via chirped laser-beam modulation and cross-correlation spectral analysis, instead of the conventional point-by-point discrete frequency scans with a lock-in amplifier has been introduced. A theoretical frequency-domain model for the signal generation due to laser heating, which contains both a thermal component and a mechanical component due to the thermal expansion of the thermoplastic layer, is presented. The time domain impulse response theoretical data have been obtained by a numerical FFT of the frequency-domain theoretical data. The total signal was measured via radiometric detection in both domains and was fitted to the theory to obtain thermal transport properties of the three-layered system. The thickness of the thermoplastic layer has been extracted with better than 5% precision.

Two layer model for photothermal radiometry applied on semiconducting thin films

A two layer model has been developed in order to study the dependence of the photothermal radiometric signal on the optoelectronic and thermophysical properties of a two layer semiconducting sample. In the following theoretical analysis, both thermal and electronic contributions have been used in order to interpret the infrared emission of a semiconducting sample under the influence of a pump modulated laser beam. The potential of the above model towards a quantitative analysis of photothermal radiometric experimental data, for the extraction of thermal and electronic parameters of the semiconducting thin film and substrate, has been examined. Experimental evidence of the quality of this model has also been presented.