Spectral Backscatter Research Articles

Absolute spectral backscatter measurements of large-core multimode PMMA polymer optical fibers.

To our knowledge, we are the first to measure the absolute value of the backscattering coefficient of a standard 1 mm core-diameter, multimode (MM) step-index (SI) polymethylmethacrylate (PMMA) polymer optical fiber (POF) for the spectral range of 450 nm to 700 nm. Our optical time domain reflectometer (OTDR) setup consists of a femtosecond supercontinuum laser with an acousto-optical filter as a tunable light source with short pulses and a time-correlated single-photon counting system as a receiver with a high dynamic range. The backscattering coefficient is calculated from the ratio between the energy within the fiber end reflex and the distributed backscattering level. We also measured the spectral attenuation with our OTDR setup and compared it with a standardized measurement method. At the attenuation minima within the measured spectral range the backscattering level of a 1 ns pulse is about -46dB at 520 nm, -48dB at 570 nm, and -51 dB at 650 nm. We were also able to show by the observed wavelength dependence that Rayleigh scattering causes a majority of the scattering.

Estimation of Cortical Bone Microstructure From Ultrasound Backscatter.

Multichannel pulse-echo ultrasound using linear arrays and single-channel data acquisition systems opens new perspectives for the evaluation of cortical bone. In combination with spectral backscatter analysis, it can provide quantitative information about cortical microstructural properties. We present a numerical study, based on the finite-difference time-domain method, to estimate the backscatter cross section of randomly distributed circular pores in a bone matrix. A model that predicts the backscatter coefficient using arbitrary pore diameter distributions was derived. In an ex vivo study on 19 human tibia bones (six males, 13 females, 83.7 ± 8.4 years), multidirectional ultrasound backscatter measurements were performed using an ultrasound scanner equipped with a 6-MHz 128-element linear array with sweep motor control. A normalized depth-dependent spectral analysis was performed to derive backscatter and attenuation coefficients. Site-matched reference values of tissue acoustic impedance Z , cortical thickness (Ct.Th), pore density (Ct.Po.Dn), porosity (Ct.Po), and characteristic parameters of the pore diameter (Ct.Po.Dm) distribution were obtained from 100-MHz scanning-acoustic microscopy images. Proximal femur areal bone mineral density (aBMD), stiffness S , and ultimate force Fu from the same donors were available from a previous study. All pore structure and material properties could be predicted using linear combinations of backscatter parameters with a median to high accuracy (0.28 ≤ adjusted R2 ≤ 0.59). The combination of cortical thickness and backscatter parameter provided similar or better prediction accuracies than aBMD. For the first time, a method for the noninvasive assessment of the pore diameter distribution in cortical bone by ultrasound is proposed. The combined assessment of cortical thickness, sound velocity, and pore size distribution in a mobile, nonionizing measurement system could have a major impact on preventing osteoporotic fractures.

Added Value of Quantitative Ultrasound and Machine Learning in BI-RADS 4–5 Assessment of Solid Breast Lesions

The purpose of this study was to evaluate various combinations of 13 features based on shear wave elasticity (SWE), statistical and spectral backscatter properties of tissues, along with the Breast Imaging Reporting and Data System (BI-RADS), for classification of solid breast lesions at ultrasonography by means of random forests. One hundred and three women with 103 suspicious solid breast lesions (BI-RADS categories 4-5) were enrolled. Before biopsy, additional SWE images and a cine sequence of ultrasound images were obtained. The contours of lesions were delineated, and parametric maps of the homodyned-K distribution were computed on three regions: intra-tumoral, supra-tumoral and infra-tumoral zones. Maximum elasticity and total attenuation coefficient were also extracted. Random forests yielded receiver operating characteristic (ROC) curves for various combinations of features. Adding BI-RADS category improved the classification performance of other features. The best result was an area under the ROC curve of 0.97, with 75.9% specificity at 98% sensitivity.

Backscatter in stratified turbulence

In this paper, kinetic and potential energy transfers around a spectral test filter scale in direct numerical simulations of decaying stratified turbulence are studied in both physical and spectral domains. It is shown that while the domain-averaged effective subgrid scale energy transfer in physical space is a net downscale cascade, it is actually a combination of large values of downscale and upscale transfer, i.e. forward- and backscatter, in which the forward scatter is slightly dominant. Our results suggest that spectral backscatter in stratified turbulence depends on the buoyancy Reynolds number Reb and the filtering scale Δtest. When the test filter scale Δtest is around the dissipation scale Ld, transfer spectra show spectral backscatter from sub-filter to intermediate scales, as reported elsewhere. However, we find that this spectral backscatter is due to viscous effects at vertical scales around the test filter. It is also shown that there is a non-local energy transfer from scales larger than the buoyancy scale Lb to small scales. The effective turbulent Prandtl number spectra demonstrate that the assumption Prt≈1 is reasonable for the local energy transfer.

Spectral Ellipsometry and Electron Backscatter Diffraction Analyses of Silicon Surfaces Implanted with Silver Ions

Amorphous silicon (a-Si) produced on surfaces of single-crystal substrates (c-Si) by low-energy low-dose implantation of silver ions is studied by spectral ellipsometry and electron backscatter diffraction. Implantation was done with an ion energy of 30 keV at a constant ion beam current density of 2 μA/cm2 and doses of 6.24·1012–1.25·1016 ions/cm2 on room temperature substrate targets. Irradiation was carried out with a current density of 0.1–5 μA/cm2 for implantation doses of 6.24·1013 and 1.87·1014 ions/cm2. It was found that spectral ellipsometry is an accurate andreliable method for monitoring low-dose ion implantation processes.

Abstract CT317: Quantitative ultrasound for personalized chemotherapy in locally advanced breast cancer: Clinical trial results

Abstract Many cancer therapies are intended to induce cell death within a target tumour. A substantial body of research using in vitro and in vivo models has demonstrated that cell death can be detected via quantitative ultrasound techniques. This study investigated the potential to quantify tumour responses to therapy in patients, using quantitative spectral and textural biomarkers extracted from low-frequency ultrasound data (4-10 MHz). Results demonstrate for the first time in a large cohort of patients the ability to predict clinical responders from non-responders as early as one week after the start of chemotherapy with over 95% sensitivity and 95% specificity. A clinical study was undertaken investigating the efficacy of ultrasound to quantify cell death in tumor responses with cancer treatment. Patients (n = 100) with locally advanced breast cancer received anthracyline and taxane-based chemotherapy treatments over four to six months. Data collection consisted of acquiring tumor images and radiofrequency data prior to treatment onset and at 4 times during neoadjuvant chemotherapy (weeks 0, 1, 4, 8 and pre-operatively). Data collection was carried out using an Ultrasonix-RP and an L15-5 6cm transducer pulsed at frequencies of ∼5 and ∼7 MHz, respectively. The majority of patients went on to have a modified radical mastectomy and correlative whole mount histopathology. Results obtained from both ∼5 and ∼7 MHz data indicated considerable increases in ultrasound spectral backscatter power in patients who clinically responded to treatment within one week of starting their chemotherapy. This was accompanied by significant increases in quantitative ultrasound spectral parameters such as mid-band-fit (up to 9.1 ± 1.2 dBr) and 0-MHz intercept (up to 10.8 ± 2.4 dBr). Patients categorized as poor responders clinically demonstrated significantly lower increases (1.9 ± 1.1 dBr and 1.4 ± 2.7 dBr for mid-band-fit and 0-MHz intercept, respectively). Textural biomarkers extracted from quantitative ultrasound spectral parametric maps also demonstrated considerable differences in trend between treatment responding and non-responding patients early after the treatment initiation. Statistically significant differences were found between treatment responding and non-responding patient populations using quantitative ultrasound spectral biomarkers 4 and 8 weeks after treatment initiation. Applying quantitative ultrasound textural biomarkers in order to incorporate response heterogeneities resulted in statistically significant differences between these two populations only one week after the start of chemotherapy. There were also associated survival differences between the two groups of patients. Patients with ultrasound-detected responses at week 1 had an over 90+/-5% 3 year survival whereas patients with no ultrasound-detected response at the same time had a 30+/10% survival (p<0.05). This study demonstrates the potential of ultrasound to quantify changes in tumours in response to cancer treatment administration in a clinical setting. The results indicate that such responses can be detected early during a course of chemotherapy. This can potentially permit ineffective treatments to be changed to more efficacious ones potentially leading to improved treatment outcomes within a framework of personalized medicine. Citation Format: Gregory Jan Czarnota, Ali Sadeghi-Naini, Hadi Tadayyon, Lakshmanan Sannachi, Mehrdad Gangeh, William Tran, Frances Wright, Sonal Gandhi, Kathleen Pritchard, Sunil Verma, Maureen Trudeau. Quantitative ultrasound for personalized chemotherapy in locally advanced breast cancer: Clinical trial results. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr CT317. doi:10.1158/1538-7445.AM2015-CT317

Determination of Constituent Composition of Hemoglobin and Structural/Morphological Parameters of Skin Based on Approximating Functions for Radiation Fluxes Backscattered from Skin

We have obtained an analytical expression approximating the dependence of the backscatter signals, detected outside the region where the skin is irradiated, on the optical and structural parameters of the skin. A method is proposed for fast determination of the optical and structural/morphological parameters of skin and also the constituent composition of hemoglobin (including oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, methemoglobin, and sulfhemoglobin), based on measurements of the spatially resolved spectral backscatter signals and their analytical dependence on the unknown parameters. Based on numerical modeling results for the backscatter signal, we studied the effectiveness of the proposed method and have shown that it is possible to improve the diagnostic power as a result of simultaneous processing of the spectral and spatial characteristics of backscatter from skin.

Multilevel Cross-Dependent Binary Longitudinal Data

We provide insights into new methodology for the analysis of multilevel binary data observed longitudinally, when the repeated longitudinal measurements are correlated. The proposed model is logistic functional regression conditioned on three latent processes describing the within- and between-variability, and describing the cross-dependence of the repeated longitudinal measurements. We estimate the model components without employing mixed-effects modeling but assuming an approximation to the logistic link function. The primary objectives of this article are to highlight the challenges in the estimation of the model components, to compare two approximations to the logistic regression function, linear and exponential, and to discuss their advantages and limitations. The linear approximation is computationally efficient whereas the exponential approximation applies for rare events functional data. Our methods are inspired by and applied to a scientific experiment on spectral backscatter from long range infrared light detection and ranging (LIDAR) data. The models are general and relevant to many new binary functional data sets, with or without dependence between repeated functional measurements.

CALIOP observations of the transport of ash from the Eyjafjallajökull volcano in April 2010

The Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) is an elastic backscatter lidar carried on the CALIPSO satellite. CALIOP observations between 15 and 20 April 2010 of volcanic ash plumes following the eruption of the Icelandic Eyjafjallajökull volcano are reported here. While CALIOP has limited spatial coverage due to nadir‐only sampling, ash plumes were observed by CALIOP over a wide region ‐ from the North Atlantic Ocean to eastern Europe ‐ during both day and night. Due to its nadir‐only view, however, CALIOP did not sample the densest ash plumes which were observed by passive satellite sensors and ground‐based lidar. Lidar depolarization and spectral backscatter signatures are used to discriminate ash layers from clouds. Most ash was observed in thin layers of laminar appearance with thicknesses ranging from 0.4 km to a little more than 1 km and at relatively low altitudes, between 1 and 7 km. Most layers reported here produced strong lidar depolarization, indicating a predominance of ash over sulfate aerosol in the plumes. Estimates of lidar extinction‐to‐backscatter ratio allow the retrieval of profiles of ash optical properties. Layers were observed with mean optical extinction ranging from 0.03 km−1 to nearly 1 km−1. Combined with an estimate of mass extinction efficiency, mass concentration profiles can also be estimated. These observations of the three‐dimensional dispersion of the plume are complementary to observations from passive satellite instruments and ground‐based lidars, and can be useful for verification of dispersion models such as those used by the Volcanic Ash Advisory Centers.

Assessment of accuracy of the structure-factor-size-estimator method in determining red blood cell aggregate size from ultrasound spectral backscatter coefficient

A computer simulation study to produce ultrasonic backscatter coefficients (BSCs) from red blood cell (RBC) clusters is discussed. The simulation algorithm is suitable for generating non-overlapping, isotropic, and fairly identical RBC clusters. RBCs were stacked following the hexagonal close packing (HCP) structure to form a compact spherical aggregate. Such an aggregate was repeated and placed randomly under non-overlapping condition in the three-dimensional space to mimic an aggregated blood sample. BSCs were computed between 750 KHz and 200 MHz for samples of various cluster sizes at different hematocrits. Magnitudes of BSCs increased with mean aggregate sizes at low frequencies (<20 MHz). The accuracy of the structure-factor-size-estimator (SFSE) method in determining mean aggregate size and packing factor was also examined. A good correlation (R(2) ≥ 0.94) between the mean size of aggregates predicted by the SFSE and true size was found for each hematocrit. This study shows that for spherical aggregates there exists a region for each hematocrit where SFSE works most accurately. Typically, error of SFSE in estimating mean cluster size was <20% for dimensions between 14 and 17 μm at 40% hematocrit. This study suggests that the theoretical framework of SFSE is valid under the assumption of isotropic aggregates.

Notes on Temperature-Dependent Lidar Equations

AbstractThe temperature dependence of molecular backscatter coefficients must be taken into account when narrowband interference filters are used in lidar measurements. Thus, the spectral backscatter differential cross section of the molecules involved in the backscattering of the radiation has to be calculated or measured and the interference filter transmission efficiency must be known. The present paper is intended to describe in an easily reproducible manner the procedure involved in calculating the temperature-dependent functions introduced in the lidar equations, including the computation of the differential cross sections for air, nitrogen, and water vapor. The temperature-dependent functions are computed for the Howard University Raman lidar (HURL). The interference filter efficiencies are given by the manufacturer. Error estimates in water vapor mixing ratio and aerosol backscatter ratio involved when temperature-dependent functions are omitted are given for measurements taken with HURL. For the data analyzed, it is found that errors in estimating the water vapor mixing ratio are up to ∼6% while in estimating the aerosol backscattering ratio the errors are up to ∼1.3% in the planetary boundary layer and ∼2.2% in cirrus clouds. Theoretical computations are performed to determine temperature-dependent functions for nitrogen, water vapor, and their ratio, using simulated Gaussian-shaped filters. The goal is to find the optimum combination of different filters that will determine the ratio profiles of the temperature-dependent functions that are either the closest to unity or the least variable. The analyses reveal that quite constant profiles can be obtained for several combinations of the filters.

A Spectral Stochastic Kinetic Energy Backscatter Scheme and Its Impact on Flow-Dependent Predictability in the ECMWF Ensemble Prediction System

Abstract Understanding model error in state-of-the-art numerical weather prediction models and representing its impact on flow-dependent predictability remains a complex and mostly unsolved problem. Here, a spectral stochastic kinetic energy backscatter scheme is used to simulate upscale-propagating errors caused by unresolved subgrid-scale processes. For this purpose, stochastic streamfunction perturbations are generated by autoregressive processes in spectral space and injected into regions where numerical integration schemes and parameterizations in the model lead to excessive systematic kinetic energy loss. It is demonstrated how output from coarse-grained high-resolution models can be used to inform the parameters of such a scheme. The performance of the spectral backscatter scheme is evaluated in the ensemble prediction system of the European Centre for Medium-Range Weather Forecasts. Its implementation in conjunction with reduced initial perturbations results in a better spread–error relationship, more realistic kinetic-energy spectra, a better representation of forecast-error growth, improved flow-dependent predictability, improved rainfall forecasts, and better probabilistic skill. The improvement is most pronounced in the tropics and for large-anomaly events. It is found that whereas a simplified scheme assuming a constant dissipation rate already has some positive impact, the best results are obtained for flow-dependent formulations of the unresolved processes.

Simultaneous estimation of aerosol cloud concentration and spectral backscatter from multiple-wavelength lidar data

We present a sequential algorithm for estimating both concentration dependence on range and time and backscatter coefficient spectral dependence of optically thin localized atmospheric aerosols using data from rapidly tuned lidar. The range dependence of the aerosol is modeled as an expansion of the concentration in an orthonormal basis set whose coefficients carry the time dependence. Two estimators are run in parallel: a Kalman filter for the concentration range and time dependence and a maximum-likelihood estimator for the aerosol backscatter wavelength and time dependence. These two estimators exchange information continuously over the data-processing stream. The state model parameters of the Kalman filter are also estimated sequentially together with the concentration and backscatter. Lidar data collected prior to the aerosol release are used to estimate the ambient lidar return. The approach is illustrated on atmospheric backscatter long-wave infrared (CO2) lidar data.

Toward Hyperspectral Lidar: Measurement of Spectral Backscatter Intensity With a Supercontinuum Laser Source

We have tested the use of a supercontinuum laser source in laser-based spectral backscatter measurement. The calibration and first results with the prototype instrument are presented with a discussion of improvements and applications in laser-based hyperspectral remote sensing and laboratory measurements. This technique enables the spectral study of the backscatter effects and the calibration and test measurements for the purpose of airborne laser measurement. We also explore the prospect of using a supercontinuum laser source in a broadband (hyperspectral) lidar

Influence of the precision of spectral backscatter measurements on the estimation of scatterers size in cancellous bone

The goal of this study is to propose a model for the ultrasonic frequency-dependent backscatter coefficient in femoral cancellous bone. This model has been developed with success to predict backscatter in human calcaneal bone [Jenson, Ultr. Med. Biol. 2003]. A weak scattering model is used and the backscatter coefficient is expressed in terms of a Gaussian autocorrelation function of the medium. The backscatter coefficient is computed and comparison is made with experimental data for 37 specimens and for frequency ranging from 0.4 to 1.2 MHz. An excellent agreement between experimental data and predictions is found for both the magnitude and the frequency-dependence of the backscatter coefficient. Then, a nonlinear regression is performed for each specimen, and the mean trabecular thickness is estimated. Experimental data and theoretical predictions are averaged over the 37 specimens. We also find a close agreement between theoretical predictions obtained using the Gaussian autocorrelation function (scatterer size = 134 ± 15 μm) and the mean trabecular thickness (Tb.Th = 132 ± 12 μm) derived from the analysis of bone 3-D micro-architecture using high-resolution micro-tomography. However, the correlation between individual experimental and estimated Tb.Th values is moderate ( R 2 = 0.44). The performance of the estimator are limited mainly by two factors: interference noise due to random positioning of the scatterers and attenuation. We show that the fundamental limitation of our estimator due to the speckle noise is around 5 μm for trabecular thickness estimation. This limitation is lower than the observed biological variability which is around 30 μm and should not be a limiting factor for individual prediction. A second limitation is the tremendous attenuation encountered in highly scattering media such as cancellous bone, which results in highly damped backscatter signals. The compensation for attenuation is difficult to perform, and it may be a critical point that limits the precision of the estimator.

Influence of the precision of spectral backscatter measurements on the estimation of scatterer size in cancellous bone

Ultrasonic backscatter measurements can be used to characterize trabecular bone structure and to estimate trabecular thickness (Tb.Th, i.e., the size of the scatterers). Our objective was to evaluate the performance of this estimator as well as others spectral estimators, like the frequency dependence and the mid-band amplitude of the backscatter coefficient. The performance of these estimators is degraded mainly by two factors: interference noise due to random positioning of the scatterers and attenuation. We have simulated rf-lines backscattered from trabecular bone assuming a random positioning of the trabeculae (leading to a fully developed speckle), a Gaussian form factor for the scatterers and a linear-frequency dependent attenuation. It is found that the variance in the estimation of the frequency dependence of the backscatter coefficient is as high as the variance due the biological variability in Tb.Th, in agreement with the results showed by K. Wear [J. Acoust. Soc. Am. SA, 110 (2001)]. In contrast, the variances on the mid-band amplitude and on the estimated trabecular thickness are lower than the variance due the biological variability. We also show that the effect of attenuation may be compensated by using an appropriate attenuation-compensation function. These results suggest that the inverse problem can be appropriately addressed.

Variability in spectral backscatter estimated from satellites and its relation to in situ measurements in optically complex coastal waters

A large database of in situ bio-optical measurements was collected at the Long-term Ecosystem Observatory off the southern coast of New Jersey, USA. In part, the research effort focused on reconciling in situ estimates with satellite-derived estimates of the inherent optical properties (IOP). At 442 nm, in situ absorption values ranged from less than 0.2 to over 1.5 inverse metres. Satellite estimates of backscatter ranged from 0.002 to 0.03 inverse metres at 442 nm and showed significant variability in time and space during July 1999, reflecting the recurrent high frequency events that characterize the region—wind-mixing, storms and coastal upwelling. Despite this variability, there was good qualitative agreement between the satellite derived IOP estimates and in situ IOP measurements. Both absorption and backscatter values increased near-shore, reflecting enhanced concentrations of phytoplankton, sediments and dissolved organic matter.

Optical closure for an aerosol column: Method, accuracy, and inferable properties applied to a biomass‐burning aerosol and its radiative forcing

During the Lindenberg Aerosol Characterization Experiment (LACE 98), airborne measurements of aerosol size distribution, fine‐particle concentration, particle absorption coefficient, backscatter coefficient, depolarization, and chemical composition as well as ground‐based measurements of spectral particle optical depth and of spectral backscatter and extinction coefficients were performed in the aerosol column above Lindenberg, Germany. We compare the measured optical parameters with calculations from the size distributions, which assume the aerosol to consist of sulfuric acid near the tropopause and mixtures of ammonium sulfate and soot in the remaining column. We obtain closure to within 25% for the optical depth of a column, which includes a biomass‐burning aerosol of North American origin, and infer a soot volume fraction of 35% for this aerosol. Assuming spheroidal particles of prolate shape and the average aspect ratio of the particles to be 1.3 in the biomass‐burning aerosol layer, the calculated depolarization agrees with the lidar measurement, whereas comparing the spectral backscatter coefficient shows the soot to be externally mixed with the nonabsorbing particles. With the two‐stream approximation, we estimate the local, instantaneous, cloud‐free radiative forcing of the biomass‐burning aerosol at the tropopause to −5.8 W/m2 with a corresponding optical depth of 0.09 at 710 nm wavelength and solar zenith angle of 56°. The radiative forcing for the biomass‐burning aerosol is as sensitive to a change in state of mixture, either external or internal, as to a change in surface albedo, ocean to coniferous forest.

Spaceborne lidar calibration from cirrus and molecular backscatter returns

In order to make optimal quantitative use of multiwavelength spaceborne lidar data, it is essential that the lidar be well calibrated. Due to system gain/efficiency changes that can be expected to occur during the course of a shuttle or satellite mission, it is essential to employ a calibration approach that can be implemented on-orbit, preferably repeatable at least a few times per orbit. For wavelengths less than about 550 nm, in situ calibration can be accomplished via normalization to high-altitude nearly molecular scattering regions. However, for longer wavelengths beyond about 800 nm, particularly the popular Nd: YAG fundamental wavelength at 1064 nm, the Rayleigh normalization approach becomes questionable due to both an inherently weaker signal and a stronger, variable, and somewhat unknown aerosol scattering contribution. For lidars operating at both longer and shorter wavelengths, a viable approach is to retrieve the longer wavelength calibrations ratioed to the shorter wavelength calibrations via comparisons of spectral backscatter from known/quantifiable scatterers. Cirrus clouds are good for this purpose because they occur at high altitudes with significant frequency and provide strong nearly spectrally flat backscatter. This paper presents both the molecular normalization and cirrus spectral backscatter ratio calibration approaches, including results obtained from case studies of lidar data collected during the LITE shuttle mission. Attention is focused on developing a simple autonomous approach applicable to satellite lidar missions such as Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) and the Geoscience Laser Altimeter System (GLAS).

Multiwavelength lidar observation of a strange noctilucent cloud at Kühlungsborn, Germany (54°N)

On July 6/7, 1997, we observed a noctilucent cloud (NLC) by lidar at Kühlungsborn, Germany (54°N, 12°E) using four laser wavelengths (393, 423, 532, and 770 nm). While at the near‐ultraviolet and visible wavelengths clear backscatter signals were detected, the NLC did not produce significant backscattering at the near‐infrared wavelength 770 nm. The latter signature can not be explained by backscattering on any size distribution of homogeneous water ice spheres. In this work we discuss the spectral backscatter signatures of particles with various shapes and compositions. We found three different particle types matching the experimental results, but there are physical arguments excluding these candidates as well. Our lidar observations of this strange NLC event appear to contradict the common understanding of NLC particle formation and sublimation. We emphasize the importance of multiwavelength observations for an in‐depth interpretation of noctilucent cloud lidar backscatter data, including also the infrared part of the spectrum.