Atmospheric aerosol monitoring by an elastic Scheimpflug lidar system

Broadband continuous-wave differential absorption lidar for atmospheric remote sensing of water vapor

What we believe to be a novel low-cost broadband continuous-wave water vapor differential absorption lidar (CW-DIAL) technique has been proposed and implemented by combing the Scheimpflug principle and the differential absorption method. The broadband CW-DIAL technique utilizes an 830-nm high-power multimode laser diode with 3-W output power as a tunable light source and a CMOS image sensor tilted at 45° as the detector. A retrieval algorithm dedicated for the broadband CW-DIAL technique has been developed to obtain range-resolved water vapor concentration from the DIAL signal. Atmospheric remote sensing of water vapor has been carried out on a near-horizontal water vapor path to validate the performance of the broadband CW-DIAL system. The retrieved water vapor concentration showed a good consistency with those measured by an air quality monitoring station, with a correlation coefficient of 0.9669. The fitting error of the water vapor concentration is found to be less than 10%. Numerical simulation studies have revealed that the aerosol-induced error on the water vapor concentration is below 5% with a background water vapor concentration of 5 g/m3 for most atmospheric conditions. The experimental results have successfully demonstrated the feasibility of the present broadband CW-DIAL technique for range-resolved water vapor remote sensing.

4D hyperspectral surface topography measurement system based on the Scheimpflug principle and hyperspectral imaging

A four-dimensional (4D) hyperspectral surface topography measurement (HSTM) system that can acquire uniform inelastic signals [three-dimensional (3D) spatial data] and reflection/fluorescence spectra of an object is proposed. The key components of the system are a light-sheet profilometer based on the Scheimpflug principle and a hyperspectral imager. Based on the mapping relationships among the image coordinate systems of the two imaging subsystems and the coordinate system of the real space, the spectral data can be assigned to the corresponding 3D point cloud, forming a 4D model. The spectral resolution is better than 4 nm. 700 nm, 546 nm, and 436 nm are selected as the three primary colors of red, green, and blue to restore the color. The 4D hyperspectral surface reconstruction experiments of philodendron and chlorophytum have shown the good performance of the proposed HSTM system and the great application potential for plant phenotype and growth analysis in agriculture.

Development of a multispectral fluorescence LiDAR for point cloud segmentation of plants

The accelerating development of high-throughput plant phenotyping demands a LiDAR system to achieve spectral point cloud, which will significantly improve the accuracy and efficiency of segmentation based on its intrinsic fusion of spectral and spatial data. Meanwhile, a relatively longer detection range is required for platforms e.g., unmanned aerial vehicles (UAV) and poles. Towards the aims above, what we believe to be, a novel multispectral fluorescence LiDAR, featuring compact volume, light weight, and low cost, has been proposed and designed. A 405 nm laser diode was employed to excite the fluorescence of plants, and the point cloud attached with both the elastic and inelastic signal intensities that was obtained through the R-, G-, B-channels of a color image sensor. A new position retrieval method has been developed to evaluate far field echo signals, from which the spectral point cloud can be obtained. Experiments were designed to validate the spectral/spatial accuracy and the segmentation performance. It has been found out that the values obtained through the R-, G-, B-channels are consistent with the emission spectrum measured by a spectrometer, achieving a maximum R² of 0.97. The theoretical spatial resolution can reach up to 47 mm and 0.7 mm in the x- and y-direction at a distance of around 30 m, respectively. The values of recall, precision, and F score for the segmentation of the fluorescence point cloud were all beyond 0.97. Besides, a field test has been carried out on plants at a distance of about 26 m, which further demonstrated that the multispectral fluorescence data can significantly facilitate the segmentation process in a complex scene. These promising results prove that the proposed multispectral fluorescence LiDAR has great potential in applications of digital forestry inventory and intelligent agriculture.

Spatial monitoring of flying insects over a Swedish lake using a continuous-wave lidar system

We have used a continuous-wave bi-static lidar system based on the Scheimpflug principle in measurements on flying insects above, and in the vicinity of, a small lake located in a forested area in Southern Sweden. The system, which operates on triangulation principles, has a high spatial resolution at close distance, followed by a subsequent decline in resolution further from the sensor, related to the compact system design with a separation of transmitter and receiver by only 0.81 m. Our study showed a strong increase in insect abundance especially at dusk, but also at dawn. Insect numbers decreased over water compared to over land, and larger insects were over-represented over water. Further, the average size of the insects increased at night compared to day time.

Remote Nanoscopy with Infrared Elastic Hyperspectral Lidar

Monitoring insects of different species to understand the factors affecting their diversity and decline is a major challenge. Laser remote sensing and spectroscopy offer promising novel solutions to this. Coherent scattering from thin wing membranes also known as wing interference patterns (WIPs) have recently been demonstrated to be species specific. The colors of WIPs arise due to unique fringy spectra, which can be retrieved over long distances. To demonstrate this, a new concept of infrared (950-1650 nm) hyperspectral lidar with 64 spectral bands based on a supercontinuum light source using ray-tracing and 3D printing is developed. A lidar with an unprecedented number of spectral channels, high signal-to-noise ratio, and spatio-temporal resolution enabling detection of free-flying insects and their wingbeats. As proof of principle, coherent scatter from a damselfly wing at 87 m distance without averaging (4 ms recording) is retrieved. The fringed signal properties are used to determine an effective wing membrane thickness of 1412 nm with ±4 nm precision matching laboratory recordings of the same wing. Similar signals from free flying insects (2 ms recording) are later recorded. The accuracy and the method's potential are discussed to discriminate species by capturing coherent features from free-flying insects.

Dual-Band Infrared Scheimpflug Lidar Reveals Insect Activity in a Tropical Cloud Forest

We describe an entomological dual-band 808 and 980 nm lidar system which has been implemented in a tropical cloud forest (Ecuador). The system was successfully tested at a sample rate of 5 kHz in a cloud forest during challenging foggy conditions (extinction coefficients up to 20 km^-1). At times, the backscattered signal could be retrieved from a distance of 2.929 km. We present insect and bat observations up to 200 m during a single night with an emphasis on fog aspects, potentials, and benefits of such dual-band systems. We demonstrate that the modulation contrast between insects and fog is high in the frequency domain compared to intensity in the time domain, thus allowing for better identification and quantification in misty forests. Oscillatory lidar extinction effects are shown in this work for the first time, caused by the combination of dense fog and large moths partially obstructing the beam. We demonstrate here an interesting case of a moth where left- and right-wing movements induced oscillations in both intensity and pixel spread. In addition, we were able to identify the dorsal and ventral sides of the wings by estimating the corresponding melanization with the dual-band lidar. We demonstrate that the wing beat trajectories in the dual-band parameter space are complementary rather than covarying or redundant, thus a dual-band entomological lidar approach to biodiversity studies is feasible in situ and endows species specificity differentiation. Future improvements are discussed. The introduction of these methodologies opens the door to a wealth of possible experiments to monitor, understand, and safeguard the biological resources of one of the most biodiverse countries on Earth.

Environmental CW range-resolved S-lidars with Si/InGaAs arrays: limitations and capabilities under sky background

In this paper, we discuss some features of open-path remote sensing inherent to CW range-resolved S-lidars (S comes from Scheimpflug) as a new, to the best of our knowledge, and promising class of laser instruments for environmental monitoring. In many remote-sensing applications, the accompanying skylight can degrade the sensitivity and overload the photodetectors, which is also very relevant for S-lidars with Si and InGaAs arrays. We paid special attention to the topical problem of predicting the limitations and potential of S-lidars in the VIS and SWIR spectral bands, where the sky background is particularly strongly affected. For this purpose, the index of immunity against external backgrounds as a quantitative indicator of S-lidars' potential insensitivity to the current skylight is introduced. Its evaluation is carried out by comparing the potentially achievable signal-to-noise ratios at the detector output in the presence and absence of external illumination. The detector response to the skylight in the photon-counting mode is normalized to appropriate parameters of the array in order to use dimensionless estimates in describing the variability of conditions. Characteristic spectral and dark-current-related features distinguishing the response of Si and InGaAs array detectors in the presence of background illumination are taken into account. It is then shown how to determine the minimum required full well capacity of the array in order to neglect the skylight contribution and ensure stable operation of S-lidars. The proposed methodology is aimed at providing a rationale for design solutions to expand the applicability of this promising type of remote sensors.

Photon confinement in a silicon cavity of an image sensor by plasmonic diffraction for near-infrared absorption enhancement

Silicon-based image sensors are attractive for applications in the near-infrared (NIR) range owing to their low-cost and high availability. However, novel approaches are required to enhance their light absorption, hindered by the silicon band gap. In this study, we proposed a light trapping strategy in a silicon absorption layer by plasmonic diffraction and reflection within a pixel to improve the sensitivity at a specific NIR wavelength for complementary metal-oxide semiconductor image sensors. The plasmonic grating diffracted light under the quasi-resonant condition of the surface plasmon polaritons. We simulated the silicon absorption efficiency for plasmonic diffraction combined with metal-filled trenches and a pre-metal dielectric (PMD) layer. Backward propagation light in silicon by a total internal reflection at the bottom decoupled with plasmonic grating. A single SiO₂ protrusion was added at the silicon bottom to prevent decoupling by scattering the light in the silicon and trapping it within the pixel. In addition, the light transmitted to the PMD layer is reflected by the wiring layer used as a mirror. The photon confinement in silicon by these constructions improved the absorption by approximately 8.2 times at an NIR wavelength of 940 nm with 3-µm-thick. It is useful for NIR imaging system with active laser illumination.

Visible, near-infrared dual-polarization lidar based on polarization cameras: system design, evaluation and atmospheric measurements

A visible, near-infrared (VIS-NIR) dual-polarization lidar technique employing laser diodes and polarization cameras has been designed and implemented for all-day unattended field measurements of atmospheric aerosols. The linear volume depolarization ratios (LVDR) and the offset angles can be retrieved from four-directional polarized backscattering signals at wavelengths of 458 nm and 808 nm without additional optical components and sophisticated system adjustments. Evaluations on the polarization crosstalk of the polarization camera and the offset angle have been performed in detail. A rotating linear polarizer (RLP) method based on the Stokes-Mueller formalism has been proposed and demonstrated for measuring extinction ratios of the polarization camera, which can be used to eliminate the polarization crosstalk between different polarization signals. The offset angles can be online measured with a precision of 0.1°, leading to negligible measurement errors on the LVDR. One-month statistical analysis revealed a small temporal variation of the offset angles, namely -0.13°±0.07° at 458 nm and 0.33°±0.09° at 808 nm, indicating good system stability for long-term measurement. Atmospheric measurements have been carried out to verify the system performance and investigate aerosol optical properties. The spectral characteristics of the aerosol extinction coefficient, the color ratio, the linear particle polarization ratio (LPDR) and the ratio of LPDR were retrieved and evaluated based on one-month continuous atmospheric measurements, from which different types of aerosols can be classified. The promising results showed great potential of employing the VIS-NIR dual-polarization lidar in characterizing aerosol optical properties, discriminating aerosol types and analyzing long-range aerosol transportation.

Laser triangulation measurement system with Scheimpflug calibration based on the Monte Carlo optimization strategy

We propose a linear laser triangulation measurement system using Scheimpflug calibration based on the Monte Carlo optimization strategy. A Scheimpflug inclination camera calibration model is introduced in the measurement system for improving the image definition in small-range measurements with a large depth-of-field. To address the nonlinear optimization problem between the instrument resolution and measurement range, the Monte Carlo method is adopted to determine the optimal optical parameters (scattering angle, Scheimpflug angle, and focus length) in a practical measurement system. Furthermore, we experimentally constructed the measurement system to demonstrate the measurement precision by measuring a standard step block (measurement range 15 mm). The performance parameters of the maximum measurement error, maximum standard deviation, and linearity are obtained as ±7 μm, 0.225 μm, and 0.046%, respectively. Finally, the proposed measurement system based on the Monte Carlo optimization strategy is promising for high-precision measurements in industrial applications and provides guidance for optimizing the design parameters of ranging measurement sensors.

Retrieval of the planetary boundary layer height from lidar measurements by a deep-learning method based on the wavelet covariance transform

Understanding and characterization of the planetary boundary layer (PBL) are of great importance in terms of air pollution management, weather forecasting, modelling of climate change, etc. Although many lidar-based approaches have been proposed for the retrieval of the PBL height (PBLH) in case studies, development of a robust lidar-based algorithm without human intervention is still of great challenging. In this work, we have demonstrated a novel deep-learning method based on the wavelet covariance transform (WCT) for the PBLH evaluation from atmospheric lidar measurements. Lidar profiles are evaluated according to the WCT with a series of dilation values from 200 m to 505 m to generate 2-dimensional wavelet images. A large number of wavelet images and the corresponding PBLH-labelled images are created as the training set for a convolutional neural network (CNN), which is implemented based on a modified VGG16 (VGG - Visual Geometry Group) convolutional neural network. Wavelet images obtained from lidar profiles have also been prepared as the test set to investigate the performance of the CNN. The PBLH is finally retrieved by evaluating the predicted PBLH-labelled image and the wavelet coefficients. Comparison studies with radiosonde data and the Micro-Pulse-Lidar Network (MPLNET) PBLH product have successfully validated the promising performance of the deep-learning method for the PBLH retrieval in practical atmospheric sensing.

Design of a CMOS image sensor pixel with embedded polysilicon nano-grating for near-infrared imaging enhancement

Complementary metal-oxide semiconductor (CMOS) image sensor sensitivity in the near-infrared spectrum is limited by the absorption length in silicon. To deal with that limitation, we evaluate the implementation of a polysilicon nano-grating inside a pixel, at the transistor gate level of a 90 nm standard CMOS process, through opto-electrical simulations. The studied pixel structure involves a polysilicon nano-grating, designed with the fabrication layer of the transistor gate, which does not require any modifications in the process flow. The diffraction effect of the nano-grating increases the length of the light path in the photosensitive area and thus increases the photoelectric conversion efficiency. The nano-grating is integrated in combination with deep trench isolations to reduce cross talk between pixels. Coupled optical and electrical simulations report 33% external quantum efficiency improvement and 7% cross talk reduction at 850 nm.

Small Angle Scattering Intensity Measurement by an Improved Ocean Scheimpflug Lidar System

Quantification of the horizontal patterns of phytoplankton and the distribution of suspended particles across the sea’s surface has been greatly improved by traditional passive oceanic color remote sensing technology. Lidar technology has already been proven to be effective positive remote sensing technology to construct high-resolution bathymetry models. Lidar technology significantly improves our ability to model biogeochemical processes in the upper ocean and provides advanced concepts regarding the vertical distribution of suspended particles and oceanic optical properties. In this paper, we present a novel optical approach to measuring the scattering intensity and characteristics of suspended particles within small angles backwards and distinguish water medium with different attenuation coefficients by a laboratory demonstration of the ocean Scheimpflug lidar system. The approach allows the direct determination of the scattering intensity over a small angle at the backward direction (175.8~178.8°) with an angular resolution of 0.38. Corrections for the effects of refraction at the air-glass-water interface were demonstrated. The data production (initial width and width attenuation rate of the laser beam) of the ocean Scheimpflug lidar system were utilized to distinguish water with different algae concentrations. Application for the measurement of backward scattering intensity and laser beam width were explored in distances up to several meters with spatial resolutions of millimeter precision.

Inelastic hyperspectral Scheimpflug lidar for microalgae classification and quantification

An inelastic hyperspectral Scheimpflug lidar system was developed for microalgae classification and quantification. The correction for the refraction at the air-glass-water interface was established, making our system suitable for aquatic environments. The fluorescence spectrum of microalgae was extracted by principal component analysis, and seven species of microalgae from different phyla have been classified. It was verified that when the cell density of Phaeocystis globosa was in the range of ${{1}}{{{0}}^4}\sim{{1}}{{{0}}^6}\;{\rm{cell}}\;{\rm{m}}{{\rm{L}}^{- 1}}$, the cell density had a linear relationship with the fluorescence intensity. The experimental results show our system can identify and quantify microalgae, with application prospects for microalgae monitoring in the field environment and early warning of red tides or algal blooms.

Real-time dispersal of malaria vectors in rural Africa monitored with lidar

Lack of tools for detailed, real-time observation of mosquito behavior with high spatio-temporal resolution limits progress towards improved malaria vector control. We deployed a high-resolution entomological lidar to monitor a half-kilometer static transect positioned over rice fields outside a Tanzanian village. A quarter of a million in situ insect observations were classified, and several insect taxa were identified based on their modulation signatures. We observed distinct range distributions of male and female mosquitoes in relation to the village periphery, and spatio-temporal behavioral features, such as swarming. Furthermore, we observed that the spatial distributions of males and females change independently of each other during the day, and were able to estimate the daily dispersal of mosquitoes towards and away from the village. The findings of this study demonstrate how lidar-based monitoring could dramatically improve our understanding of malaria vector ecology and control options.

Adaptive digital filter for the processing of atmospheric lidar signals measured by imaging lidar techniques

The lidar signal measured by the atmospheric imaging lidar technique is subject to sunlight background noise, dark current noise, and fixed pattern noise (FPN) of the image sensor, etc., which presents quite different characteristics compared to the lidar signal measured by the pulsed lidar technique based on the time-of-flight principle. Enhancing the signal-to-noise ratio (SNR) of the measured lidar signal is of great importance for improving the performance of imaging lidar techniques. By carefully investigating the signal and noise characteristics of the lidar signal measured by a Scheimpflug lidar (SLidar) based on the Scheimpflug imaging principle, we have demonstrated an adaptive digital filter based on the Savitzky-Golay (S-G) filter and the Fourier analysis. The window length of the polynomial of the S-G filter is automatically optimized by iteratively examining the Fourier domain frequency characteristics of the residual signal between the filtered lidar signal and the raw lidar signal. The performance of the adaptive digital filter has been carefully investigated for lidar signals measured by a SLidar system under various atmospheric conditions. It has been found that the optimal window length for near horizontal measurements is concentrated in the region of 90-150, while it varies mainly in the region of 40-100 for slant measurements due to the frequent presence of the peak echoes from clouds, aerosol layers, etc. The promising result has demonstrated great potential for utilizing the proposed adaptive digital filter for the lidar signal processing of imaging lidar techniques in the future.

Modeling and Evaluation of the Systematic Errors for the Polarization-Sensitive Imaging Lidar Technique

Polarization lidar plays a significant role in characterizing the properties of cirrus clouds, classifying aerosol types, retrieving aerosol microphysical properties, etc. However, the retrieval reliability and accuracy of the linear volume depolarization ratio (LVDR) of atmospheric particles rely on many system factors, requiring intensive attention and massive efforts on system calibrations and error evaluations, etc. In this work, a theoretical model based on the Stokes–Mueller formalism has been established for the newly developed polarization-sensitive imaging lidar (PSI-Lidar) technique. The systematic errors introduced by the degree of linear polarization (DoLP) of the emitted laser beam, the offset angle, and the quantum efficiencies (QEs) and polarization extinction ratios (PERs) of the polarization-sensitive image sensor, were evaluated in detail for the PSI-Lidar at 450, 520, and 808 nm. Although the DoLP of typical multimode laser diodes is not very high, the influence of non-ideal polarized laser beam on the LVDR can be reduced to less than 1% by employing a high-PER linear polarizer to improve the DoLP of the transmitted laser beam. Laboratory measurements have revealed that the relative QEs of the image sensor with four polarized directions are independent of the total illumination intensity and indicate a good consistency with the factory relative QEs (less than 2% deviation). Meanwhile, the influence of the relative QEs on the LVDR can be well-calibrated from either experimental or factory relative QEs. Owing to the non-ideal PER of the polarization-sensitive image sensor, e.g., ≈74 at 808 nm, ≈470 at 450 nm, the crosstalk between received signals with different polarization states can significantly deteriorate the measurement accuracy for small LVDRs. A relative error of the LVDR less than 4% can be achieved at 450 and 520 nm with the LVDR varying from 0.004 to 0.3 for a PER uncertainty of ± 5%, by taking the polarization crosstalk effect into account. However, in order to achieve a relative error of less than 10% for a small atmospheric LVDR of 0.004 at 808 nm, the uncertainty of the PER should be less than ± 2.5%. The offset angle can be calculated based on the four polarized lidar signals and the PER values at the four polarization angles. It was found out that the retrieval error of the offset angle is less than 0.15° even with a large PER uncertainty (±20%), giving a negligible systematic error on the LVDR (less than 1%).

Evaluation of systematic errors for the continuous-wave NO2 differential absorption lidar employing a multimode laser diode

The NO₂-differential absorption lidar (NO₂-DIAL) technique has been of great interest for atmospheric NO₂ profiling. Comprehensive studies on measurement errors in the NO₂-DIAL technique are vital for the accurate retrieval of the NO₂ concentration. This work investigates the systematic errors of the recently developed continuous-wave (CW) NO₂-DIAL technique based on the Scheimpflug principle and a high-power CW multimode laser diode. Systematic errors introduced by various factors, e.g., uncertainty of the NO₂ differential absorption cross-section, differential absorption due to other gases, spectral drifting of the λ_on and λ_off wavelengths, wavelength-dependent extinction and backscattering effect, have been theoretically and experimentally studied for the CW-DIAL technique. By performing real-time spectral monitoring on the emission spectrum of the laser diode, the effect of spectral drifting on the NO₂ differential absorption cross-section is negligible. The temperature-dependent NO₂ absorption cross-section in the region of 220-294 K can be interpolated by employing a linear fitting method based on high-precision absorption spectra at 220, 240, and 294 K. The relative error for the retrieval of the NO₂ concentration is estimated to be less than 0.34% when employing the interpolated spectrum. The primary interference molecule is found to be the glyoxal (CHOCHO), which should be carefully evaluated according to its relative concentration in respect to NO₂. The systematic error introduced by the backscattering effect is subjected to the spatial variation of the aerosol load, while the extinction-induced systematic error is primarily determined by the difference between the aerosol extinction coefficients at λ_on and λ_off wavelengths. A case study has been carried out to demonstrate the evaluation of systematic errors for practical NO₂ monitoring. The comprehensive investigation on systematic errors in this work can be of great value for future NO₂ monitoring using the DIAL technique.

Mini-Scheimpflug lidar system for all-day atmospheric remote sensing in the boundary layer

Development of a lightweight, low-cost, easy-to-use and low-maintenance lidar technique has been of great interest for atmospheric aerosol remote sensing in recent years and remains a great challenge. In this work, an 808 nm mini-Scheimpflug lidar (SLidar) system with about 450 mm separation between the transmitter and the receiver has been developed by employing a 114 mm aperture Newtonian telescope (F4). System performances, such as the beam characteristic, the range resolution, and the signal-to-noise ratio of the lidar signal, have been carefully investigated. Despite employing a small receiving aperture, all-day measurements were still feasible with about a one-minute signal averaging for both the horizontal urban area monitoring and the slant atmospheric sounding in the boundary layer. The lidar signal in the region of 29-50 m with a scattering angle less than 179.5° could be slightly underestimated due to the variation of the phase function. The extinction coefficient evaluated in the region between 29 and 2000 m according to the Klett method agreed well with the concentrations of particulate matters for both horizontal and slant measurements. The promising result demonstrated in this work has shown great potential to employ the robust mini-SLidar system for atmospheric monitoring in the boundary layer.

Application of Miniaturized Sensors to Unmanned Aerial Systems, A New Pathway for the Survey of Polluted Areas: Preliminary Results

With the aim to have risk mitigation for people and first responders, active remote sensing standoff detection is a fruitful technology, both in case of accidental (natural or incidental) or intentional dispersion in the environment of volatile chemical substances. Nowadays, several laser-based methodologies could be put in place to perform extensive areal monitoring. The present study regards the proposal for a new system architecture derived from the integration of a low-cost laser-based network of detectors for pollutants interfaced with a more sophisticated layout mounted on an unmanned aerial vehicle (UAV) able to identify the nature and the amount of a release. With this system set up, the drone will be activated by the alarm triggered by the laser-based network when anomalies are detected. The area will be explored by the drone with a more accurate set of sensors for identification to validate the detection of the network of Lidar systems and to sample the substance in the focus zone for subsequent analysis. In this work, methodologies and requirements for the standoff detection and the identification features chosen for this integrated system are described. The work aims at the definition of a new approach to the problem through the integration of different technologies and tools in the operative field experiments. Some preliminary results in support of the suitability of the integration hypothesis proposed are presented. This study gives rise to an integrated system to be furtherly tested in a real environment.

Lidar reveals activity anomaly of malaria vectors during pan-African eclipse

Yearly, a quarter billion people are infected and a half a million killed by the mosquito-borne disease malaria. Lack of real-time observational tools for continuously assessing the unperturbed mosquito flight activity in situ limits progress toward improved vector control. We deployed a high-resolution entomological lidar to monitor a half-kilometer static transect adjacent to a Tanzanian village. We evaluated one-third million insect observations during five nights, four days, and one annular solar eclipse. We demonstrate in situ lidar classification of several insect families and their sexes based on their modulation signatures. We were able to compare the fine-scale spatiotemporal activity patterns of malaria vectors during ordinary days and an eclipse to disentangle phototactic activity patterns from the circadian mechanism. We observed an increased insect activity during the eclipse attributable to mosquitoes. These unprecedented findings demonstrate how lidar-based monitoring of distinct mosquito activities could advance our understanding of vector ecology.

Experimental Calibration of the Overlap Factor for the Pulsed Atmospheric Lidar by Employing a Collocated Scheimpflug Lidar

Lidar techniques have been widely employed for atmospheric remote sensing during past decades. However, an important drawback of the traditional atmospheric pulsed lidar technique is the large blind range, typically hundreds of meters, due to incomplete overlap between the transmitter and the receiver, etc. The large blind range prevents the successful retrieval of the near-ground aerosol profile, which is of great significance for both meteorological studies and environmental monitoring. In this work, we have demonstrated a new experimental approach to calibrate the overlap factor of the Mie-scattering pulsed lidar system by employing a collocated Scheimpflug lidar (SLidar) system. A calibration method of the overlap factor has been proposed and evaluated with lidar data measured in different ranges. The overlap factor, experimentally determined by the collocated SLidar system, has also been validated through horizontal comparison measurements. It has been found out that the median overlap factor evaluated by the proposed method agreed very well with the overlap factor obtained by the linear fitting approach with the assumption of homogeneous atmospheric conditions, and the discrepancy was generally less than 10%. Meanwhile, simultaneous measurements employing the SLidar system and the pulsed lidar system have been carried out to extend the measurement range of lidar techniques by gluing the lidar curves measured by the two systems. The profile of the aerosol extinction coefficient from the near surface at around 90 m up to 28 km can be well resolved in a slant measurement geometry during nighttime. This work has demonstrated a great potential of employing the SLidar technique for the calibration of the overlap factor and the extension of the measurement range for pulsed lidar techniques.

Fluorescence Scheimpflug LiDAR developed for the three-dimension profiling of plants

This work proposes a novel fluorescence Scheimpflug LiDAR (SLiDAR) technique based on the Scheimpflug principle for three-dimension (3D) plant profile measurements. A 405 nm laser diode was employed as the excitation light source to generate a light sheet. Both the elastic and inelastic/fluorescence signals from a target object (e.g., plants) can be simultaneously measured by the fluorescence SLiDAR system employing a color image sensor with blue, green and red detection channels. The 3D profile can be obtained from the elastic signal recorded by blue pixels through elevation scanning measurements, while the fluorescence intensity of the target object is mainly acquired by red and green pixels. The normalized fluorescence intensity of the red channel, related to the chlorophyll distribution of the plant, can be utilized for the classification of leaves, branches and trunks. The promising results demonstrated in this work have shown a great potential of employing the fluorescence SLiDAR technique for 3D fluorescence profiling of plants in agriculture and forestry applications.

Investigation of aerosol absorption with dual-polarization lidar observations

Polarization lidar has been widely used in recent decades to observe the vertical structures of aerosols and clouds in the atmosphere. We developed a dual-polarization lidar system that can detect polarization measurements simultaneously at 355 nm and 532 nm. Dust events and haze episodes over northern China in 2014 were observed by the developed lidar. The results showed that the dust-dominated aerosol depolarization ratios at 532 nm were larger than those at 355 nm, but those of the air pollutants were smaller, indicating that this tool could provide a more accurate classification of aerosols. Moreover, we found a good relationship between the absorption coefficient of aerosols and the ratio of depolarization ratios at 532 nm and 355 nm for dust aerosols. Our results imply that aerosol absorption from polarization measurements may be determined by lidar at the ultraviolet and visible wavelengths.

Underwater spatially, spectrally, and temporally resolved optical monitoring of aquatic fauna

A continuous-wave (CW) Scheimpflug underwater multi-spectral lidar system was constructed to monitor aquatic fauna with spatial, spectral, and temporal resolution. Utilizing a 1 W 414 nm diode laser and a detection set-up with a reflective grating, measurements of shrimp pleopod movements at fixed range, and the swimming of small fish trapped in a clear tube were performed in a 5 m ×0.6 m ×0.6 m water tank. The spatial resolution is about 5 mm, the spectral resolution is 10 nm (from 400 nm to 700 nm), and with proper binning of the CCD, a read-out repetition rate up to 150 Hz can be reached. The experimental results demonstrate that the underwater Scheimpflug lidar system has great potential for detailed monitoring of the small aquatic fauna in oceanic environments.

Feasibility Studies of the Three-Wavelength Mie-Scattering Polarization Scheimpflug Lidar Technique

A three-wavelength Mie-scattering polarization Scheimpflug lidar system, utilizing 808-nm, 520-nm and 405-nm multimode laser diodes as light sources and two CMOS sensors as detectors, is developed for the studies of the aerosol extinction coefficient, depolarization ratio and the Ångström exponent. Atmospheric monitoring has been carried out on a near horizontal path from 23:00 January 14th to 06:00 January 15th, 2019 at Dalian, which is a coast city in Northern China. By studying the depolarization, aerosol extinction coefficient and Ångström exponent, it has been found out that a strong north wind blew away local spherical haze particles and brought external non-spherical large-size particles. The measurement results indicated a promising future of employing the present three-wavelength polarization Scheimpflug lidar system in the applications of atmospheric remote sensing.

Comparison studies of the Scheimpflug lidar technique and the pulsed lidar technique for atmospheric aerosol sensing

The Scheimpflug lidar (SLidar) technique has been recently developed for various remote sensing applications, where the lidar signal is detected by an image sensor according to the Scheimpflug principle instead of the time-of-flight principle. Comparison studies between the SLidar technique and the conventional pulsed lidar technique are crucial for understanding the principle as well as the measurement results of the SLidar technique. In this work, a 520-nm Scheimpflug lidar system and a 532-nm pulsed lidar system have been developed for comparison studies. Atmospheric remote measurements as well as statistical analysis have been carried out on a near-horizontal path and on a slant direction with an elevation angle of 30$^\circ $^∘. The temporal-spatial variations of the atmospheric backscattering maps measured by the 520-nm SLidar system and the 532-nm pulsed lidar system generally agreed well. The median extinction coefficient measured by the SLidar and the pulsed techniques has shown similar temporal evolution during the near-horizontal comparison study, and a correlation coefficient of 0.99 has been achieved through statistical analysis on all lidar measurements. Moreover, the root-mean-square error (RMSE) ratio for each extinction coefficient profile has also been evaluated, and the mean value of the RMSE ratio for all lidar measurements was about 11% in homogeneous atmospheric conditions. During slant comparison studies, the RMSE ratio between the SLidar curve and the pulsed lidar curve was less than 5% in the region of 0.5-2 km, and it generally increased with the increase of measurement distance, primarily due to the decreased range resolution of the SLidar technique. The promising results suggested that the SLidar technique, featuring a short blind range, could be suitable for aerosol sensing, particularly in the planetary boundary layer.

Three-wavelength polarization Scheimpflug lidar system developed for remote sensing of atmospheric aerosols

Multiple-wavelength polarization lidar techniques have been of great interest for the studies of aerosol backscattering color ratio, Ångström exponent, particle size distribution, hygroscopic growth, etc. Conventional lidar techniques are mainly based on the time-of-flight principle. In this paper, a three-wavelength polarization Scheimpflug lidar (SLidar) system, based on the Scheimpflug imaging principle, has been developed for studying optical properties of atmospheric aerosols. The SLidar system utilizes low-cost, compact, multimode laser diodes as light sources and two complementary metal oxide semiconductor (CMOS) sensors as detectors. The depolarization ratio was measured at the 808 nm band by successively detecting atmospheric backscattering signals from two orthogonally polarized laser beams with a polarization CMOS camera, while the 520 nm and the 405 nm backscattering signals were recorded by a second CMOS camera based on the time-multiplexing scheme. Atmospheric remote measurements were carried out in May and July 2019 on a near-horizontal path. The aerosol extinction coefficient, linear volume depolarization ratio, and the Ångström exponent have been retrieved and evaluated to study aerosol properties during different atmospheric conditions, which were in good agreement with optical properties reported by previous studies.

Detection of the planetary boundary layer height by employing the Scheimpflug lidar technique and the covariance wavelet transform method

A portable Scheimpflug lidar system has been employed for atmospheric boundary layer studies. Atmospheric backscattering signals were continuously recorded from 21 August to 28 August 2018. The covariance wavelet transform (CWT) method was utilized to identify the maximum gradient of recorded lidar curves as the planetary boundary layer (PBL) height. As the directly retrieved PBL height could be underestimated or overestimated due to the presence of residual layers and thin clouds, localized atmospheric turbulence, aerosol stratification, etc., a CWT-based quality-control algorithm has also been developed to improve the reliability of the PBL height retrieval. The temporal distribution of the final PBL height has shown a clear diurnal variation as the ambient temperature changed due to the increasing and decreasing of surface heating during a one-week continuous measurement campaign. The promising results have shown great potential in employing the Scheimpflug lidar technique and the CWT-based method in the determination of the PBL height.

Overwater light-sheet Scheimpflug lidar system for an underwater three-dimensional profile bathymetry

The feasibility of an overwater light-sheet Scheimpflug lidar system for underwater three-dimensional (3D) profiling is considered. Corrections for the refraction at the air-water interface are proposed. Applications for profiling marine ecosystems are explored in distance up to several meters, and millimeter precision is accomplished. The experimental results demonstrate that our system is suitable for underwater 3D profiling and has great potential in marine biological detection.

Atmospheric CO2 sensing using Scheimpflug-lidar based on a 1.57-µm fiber source

A molecular laser-radar system, based on the Scheimpflug principle, has been constructed and demonstrated for remote sensing of atmospheric CO₂ concentrations using Differential Absorption Lidar (DIAL) in the (30012←00001) absorption band. The laser source is a Continues Wave (CW) Distributed-FeedBack (DFB) diode laser seeding an Erbium-doped fiber amplifier, emitting narrowband (3 MHz) tunable radiation with an output power of 1.3 W at 1.57 µm. The laser beam is expanded and transmitted to the atmosphere. The atmospheric backscattered signal is collected with a Newtonian telescope and detected with a linear InGaAs array detector satisfying the Scheimpflug condition. We present range-resolved measurements of atmospheric CO₂ concentration from a test range of 2 km located in the city of Lund, Sweden. We discuss and provide scalable results for CO₂ profiling with the Scheimpflug-lidar method.

Imaging enhancement based on stimulated Brillouin amplification in optical fiber

The detection of weak optical signals embedded in strong background illumination has broad application prospects. We propose an imaging enhancement method based on stimulated Brillouin scattering (SBS) in a single-mode fiber, which is capable of amplifying the weak optical signal while neglecting the broadband background noise because of its narrow gain bandwidth. In experiment, a high gain of 60 dB was achieved. An imaging enhancement experiment was carried out, where a target which cannot be seen because of transmission loss could be clearly captured with the amplification of SBS in the fiber. Because of the employment of continuous pump rather than a pulsed pump, this system has wide application in the monitoring of non-cooperative targets.

Preliminary Studies on Atmospheric Monitoring by Employing a Portable Unmanned Mie-Scattering Scheimpflug Lidar System

A portable unmanned Mie-scattering Scheimpflug lidar system has been designed and implemented for atmospheric remote sensing. The Scheimpflug lidar system employs a continuous-wave high-power 808 nm laser diode as the light source and the emitted laser beam is collimated by an F6 lens with a 100 mm aperture. Atmospheric backscattering light is collected by a F5 lens with a 150 mm aperture and then detected by a 45° tilted image sensor. The separation between the transmitting and the receiving optics is about 756 mm to satisfy the Scheimpflug principle. Unmanned outdoor atmospheric measurements were performed in an urban area to investigate system performance. Localized emissions can be identified by performing horizontal scanning measurements over the urban atmosphere for 107° approximately every 17 min. The temporal variation of the vertical aerosol structure in the boundary layer has also been studied through zenith scanning measurements. The promising result shows great potential of the present portable lidar system for unmanned atmospheric pollution monitoring in urban areas.

Lidar thermometry using two-line atomic fluorescence

In this work, Scheimpflug lidar has been combined with the thermometric technique two-line atomic fluorescence, to carry out stand-off, spatially resolved temperature measurements. Indium atoms were seeded into a modified Perkin-Elmer-burner and two tunable single-mode diode lasers with their wavelengths tuned to 410.17 and 451.12 nm were used to excite the seeded atoms. The fluorescence signal was collected using both a line-scan detector and a two-dimensional intensified CCD camera. One-dimensional flame temperature profiles were measured at different heights above a porous-plug burner, located at a distance of 1.5 m from the lidar system. The technique was also used to demonstrate two-dimensional temperature measurements in the same flame. The accuracy of the measured temperature was found to be limited mainly by uncertainty in the spectral overlap between the laser emission and the indium atom absorption spectrum as well as uncertainty in laser power measurements. With the constraint that indium can be introduced into the measurement volume, it is anticipated that the developed measurement concept could constitute a valuable tool, allowing in situ spatially resolved thermometry in intractable industrial applications, sufferings from limited optical access, thus requiring remote single-optical-port sensing.

Particle profiling and classification by a dual-band continuous-wave lidar system

A dual-band continuous-wave (CW) light detection and ranging (lidar) system has been developed for particle classification. In this lidar system, the range-resolved atmospheric backscattering signal is recorded by an optical imaging system satisfying the Scheimpflug principle instead of the conventional time-of-flight approach. It is thus possible to employ low-cost and compact CW diode lasers, facilitating the development of a robust multiple-wavelength atmospheric lidar system that can attain high accuracy of the retrieved parameters of atmospheric particles. The present work demonstrates a dual-band Scheimpflug lidar system employing two diode lasers at 405 nm (0.5 W) and 808 nm (3.2 W). Exposures are milliseconds apart and interpolated. Measurements of various types of particles and smoke have been performed to verify the feasibility of using the present system for improved particle classification and sizing, for the situation when plumes were dilute and no significant opacity was detected.

Dual-wavelength Mie-scattering Scheimpflug lidar system developed for the studies of the aerosol extinction coefficient and the Ångström exponent

A dual-wavelength Scheimpflug lidar system, utilizing a 4-W 808-nm and 1-W 407-nm multimode laser diodes as light sources and two CMOS sensors as detectors, is developed for the studies of the aerosol extinction coefficient and the Ångström exponent. The system performance has been successfully validated by a two-week continuous measurement campaign on a near horizontal path in May 2018 at Dalian, which is a coastal city in Northern China. The aerosol extinction coefficients retrieved by the Fernald method show good correlations with particle concentrations and relative humidities (RHs). It has been found that the enhancement factor of the backscattering coefficient at the short wavelength due to hygroscopic growth is larger than that at the long wavelength for the aerosol particles off the coast of the Yellow Sea. The Ångström exponent obtains from the aerosol extinction coefficients at the two wavelengths, varies between 0 and 2, and is found to relate with the mass concentration fraction of fine mode particles, specifically PM2.5 particles. Moreover, the Ångström exponent has a positive correlation with the RH, implying a bimodal or multimodal size distribution of aerosol particles in the measurement season.

Light-sheet based two-dimensional Scheimpflug lidar system for profile measurements

This work presents a novel concept for 2D Scheimpflug lidar. A light-sheet based 2D Scheimpflug lidar system is developed and realized for surface profile measurements. The theory of a geometrical relationship underlying the system is developed, and the possibility of 3D profile measurements for a plastic bowl, a rhombic carton box and a manikin are presented. The sizes of reconstructed images are consistent with respective physical objects with small (~mm) errors at close range. Experimental results show that the 2D Scheimpflug lidar system performs well for 3D surface profiling and has great potential for close-range applications in other fields.

Scheimpflug Lidar for combustion diagnostics

A portable Lidar system developed for large-scale (~1-20 m) combustion diagnostics is described and demonstrated. The system is able to perform remote backscattering measurements with range and temporal resolution. The range resolution is obtained by sharply imaging a part of the laser beam onto a CMOS-array or ICCD detector. The large focal depth required to do this is attained by placing the laser beam, the collection optics and the detector in a so-called Scheimpflug configuration. Results from simulations of the range capabilities and range resolution of the system are presented and its temporal resolution is also discussed. Various applications, important for combustion diagnostics, are also demonstrated, including Rayleigh scattering thermometry, aerosol detection and laser-induced fluorescence measurements. These measurements have been carried out using various continuous-wave GaN diode lasers, emitting in the violet-blue (405 - 450 nm) wavelength regime. It is anticipated that Scheimpflug Lidar will provide a useful and versatile diagnostic tool for combustion research, not only for fundamental studies, but in particular for applications at industrial sites.

Atmospheric Pollution Monitoring in Urban Area by Employing a 450-nm Lidar System

In past decades, lidar techniques have become main tools for atmospheric remote sensing. However, traditional pulsed lidar systems are relatively expensive and require considerable maintenance. These shortcomings may be overcome by the development of a blue band Scheimpflug lidar system in Dalian, Northern China. Atmospheric remote measurements were carried out for 10 days in an urban area to validate the feasibility and performance of a 450-nm Scheimpflug lidar system. A 24-h continuous measurement was achieved in winter on a near horizontal path with an elevation angle of about 6.4°. The aerosol extinction coefficient retrieved by the Fernald-inversion algorithm shows good agreement with the variation of PM10/PM2.5 concentrations recorded by a national pollution monitoring station. The experimental result reveals that the linear ratio between the aerosol extinction coefficient and the PM10 concentration under high relative humidity (75–90%) is about two-times that in low relative humidity (≤75%) when the PM10 concentrations are less than 100 µg/m³.

Short-Wave infrared atmospheric scheimpflug lidar

Atmospheric dual-band Scheimpflug lidar is demonstrated at 980 and 1550 nm. Signals are compared during three weather conditions, and the spatio-temporal resolution of the atmospheric structure is considered. The potential for aerosol classification is evaluated, and future directions are discussed.

The bat-bird-bug battle: daily flight activity of insects and their predators over a rice field revealed by high-resolution Scheimpflug Lidar

We present the results of, to our knowledge, the first Lidar study applied to continuous and simultaneous monitoring of aerial insects, bats and birds. It illustrates how common patterns of flight activity, e.g. insect swarming around twilight, depend on predation risk and other constraints acting on the faunal components. Flight activity was monitored over a rice field in China during one week in July 2016, using a high-resolution Scheimpflug Lidar system. The monitored Lidar transect was about 520 m long and covered approximately 2.5 m³. The observed biomass spectrum was bimodal, and targets were separated into insects and vertebrates in a categorization supported by visual observations. Peak flight activity occurred at dusk and dawn, with a 37 min time difference between the bat and insect peaks. Hence, bats started to feed in declining insect activity after dusk and stopped before the rise in activity before dawn. A similar time difference between insects and birds may have occurred, but it was not obvious, perhaps because birds were relatively scarce. Our observations are consistent with the hypothesis that flight activity of bats is constrained by predation in bright light, and that crepuscular insects exploit this constraint by swarming near to sunset/sunrise to minimize predation from bats.

Small-scale Scheimpflug lidar for aerosol extinction coefficient and vertical atmospheric transmittance detection

In this paper, a new prototypical Scheimpflug lidar capable of detecting the aerosol extinction coefficient and vertical atmospheric transmittance at 1 km above the ground is described. The lidar system operates at 532 nm and can be used to detect aerosol extinction coefficients throughout an entire day. Then, the vertical atmospheric transmittance can be determined from the extinction coefficients with the equation of numerical integration in this area. CCD flat fielding of the image data is used to mitigate the effects of pixel sensitivity variation. An efficient method of two-dimensional wavelet transform according to a local threshold value has been proposed to reduce the Gaussian white noise in the lidar signal. Furthermore, a new iteration method of backscattering ratio based on genetic algorithm is presented to calculate the aerosol extinction coefficient and vertical atmospheric transmittance. Some simulations are performed to reduce the different levels of noise in the simulated signal in order to test the precision of the de-noising method and inversion algorithm. The simulation result shows that the root-mean-square errors of extinction coefficients are all less than 0.02 km^-1, and that the relative errors of the atmospheric transmittance between the model and inversion data are below 0.56% for all cases. The feasibility of the instrument and the inversion algorithm have also been verified by an optical experiment. The average relative errors of aerosol extinction coefficients between the Scheimpflug lidar and the conventional backscattering elastic lidar are 3.54% and 2.79% in the full overlap heights of two time points, respectively. This work opens up new possibilities of using a small-scale Scheimpflug lidar system for the remote sensing of atmospheric aerosols.

Implementation of a violet Scheimpflug lidar system for atmospheric aerosol studies

A violet Scheimpflug lidar system employing a 1-W 407-nm multimode laser diode is developed for remote sensing of atmospheric aerosols. The laser beam of the laser diode that is transmitted into atmosphere has been investigated in detail and a cylindrical lens pair is designed to improve the geometrical transmission efficiency. A measurement scheme with automatic exposure as well as a generalized signal processing method are established to optimize the signal-to-noise ratio of lidar signals. The performance of the violet Scheimpflug lidar system has been evaluated during a six-day continuous measurement campaign on a near horizontal path. The maximum measurement distance can reach up to 7 km in sunny clean weathers and to about 2 km during haze with an aerosol extinction coefficient of about 0.9 km. The aerosol extinction coefficient retrieved by the Fernald method is promising and shows good correlation with particle concentrations measured by a local national pollution monitoring station. This work promotes the development of all-time Scheimpflug lidar systems operating at other wavelengths or multiple wavelengths for various atmospheric applications.

Oil pollution discrimination by an inelastic hyperspectral Scheimpflug lidar system

An inelastic hyperspectral Scheimpflug lidar system is developed for range-resolved oil pollution detection and discrimination. A theory of system parametric design is built for aquatic circumstances, and laser-induced fluorescence spectra with an excitation wavelength of 446 nm are employed to detect oil pollution. Seven kinds of typical oil samples are tested and well distinguished using the principal component analysis (PCA) and linear discriminant analysis (LDA) methods. It has been shown that blue laser diodes (LD) have great potential for oil pollution detection, and our system could be further utilized for more applications in both marine and terrestrial environments.

Remote sensing of atmospheric NO2 by employing the continuous-wave differential absorption lidar technique

Differential absorption lidar (DIAL) technique employed for remote sensing has been so far based on the sophisticated narrow-band pulsed laser sources, which require intensive maintenance during operation. In this work, a continuous-wave (CW) NO₂ DIAL system based on the Scheimpflug principle has been developed by employing a compact high-power CW multimode 450 nm laser diode as the light source. Laser emissions at the on-line and off-line wavelengths of the NO₂ absorption spectrum are implemented by tuning the injection current of the laser diode. Lidar signals are detected by a 45° tilted area CCD image sensor satisfying the Scheimpflug principle. Range-resolved NO₂ concentrations on a near-horizontal path are obtained by the NO₂ DIAL system in the range of 0.3-3 km and show good agreement with those measured by a conventional air pollution monitoring station. A detection sensitivity of ± 0.9 ppbv at 95% confidence level in the region of 0.3-1 km is achieved with 15-minute averaging and 700 m range resolution during hours of darkness, which allows accurate concentration measurement of ambient NO₂. The low-cost and robust DIAL system demonstrated in this work opens up many possibilities for field NO₂ remote sensing applications.

Atmospheric extinction coefficient retrieval and validation for the single-band Mie-scattering Scheimpflug lidar technique

An 808 nm single-band Mie scattering Scheimpflug lidar system is developed in Dalian, Northern China, for real-time, large-area atmospheric aerosol/particle remote sensing. Atmospheric measurement has been performed in urban area during a typical haze weather condition, and time-range distribution of atmospheric backscattering signal is recorded from March 18th to 22nd, 2017, by employing the Scheimpflug lidar system. Atmospheric extinction coefficient is then retrieved according to the Klett-inversion algorithm, while the boundary value is obtained by the slope-method in the far end where the atmosphere is homogeneous in a subinterval region. The correlation between the extinction coefficients retrieved from the Scheimpflug lidar technique and the PM10/PM2.5 concentrations measured by a conventional air pollution monitoring station is also studied. The good agreement between the measurement results, i.e., a correlation coefficient of 0.85, successfully demonstrates the feasibility and great potential of the Scheimpflug lidar technique for atmospheric studies and applications.

Observations of movement dynamics of flying insects using high resolution lidar

Insects are fundamental to ecosystem functioning and biodiversity, yet the study of insect movement, dispersal and activity patterns remains a challenge. Here we present results from a novel high resolution laser-radar (lidar) system for quantifying flying insect abundance recorded during one summer night in Sweden. We compare lidar recordings with data from a light trap deployed alongside the lidar. A total of 22808 insect were recorded, and the relative temporal quantities measured matched the quantities recorded with the light trap within a radius of 5 m. Lidar records showed that small insects (wing size <2.5 mm(2) in cross-section) moved across the field and clustered near the light trap around 22:00 local time, while larger insects (wing size >2.5 mm(2) in cross-section) were most abundant near the lidar beam before 22:00 and then moved towards the light trap between 22:00 and 23:30. We could distinguish three insect clusters based on morphology and found that two contained insects predominantly recorded above the field in the evening, whereas the third was formed by insects near the forest at around 21:30. Together our results demonstrate the capability of lidar for distinguishing different types of insect during flight and quantifying their movements.