Pushbroom Imaging Spectrometer Research Articles

Practical recommendations and limitations for pushbroom hyperspectral imaging of tree stems

In this short communication, we present a pilot study testing a new close-range sensing technology – a portable, pushbroom hyperspectral camera – in varying field conditions in forests. We evaluate how measurement conditions affect the in situ collection of stem bark spectra. In situ spectral libraries of woody elements are needed in, e.g., physically-based remote sensing applications, biodiversity mapping, and 3D vegetation modeling. Recent technological advancements bring portable and close-range capable sensors, such as small pushbroom imaging spectrometers, for consumer and research use. However, it is important to investigate the strengths and limitations of sensors utilizing pushbroom technology. Spectral measurements under forest canopies are challenging due to varying illumination conditions, which can have a significant effect on the quality of the data. We acquired hyperspectral reflectance images of Norway spruce (Picea abies (L.) Karst), Scots pine (Pinus sylvestris L.), and silver birch (Betula pendula Roth) stem bark directly in the forest. For each tree we collected reflectance images at 30-minute intervals throughout a day from a fixed view angle. The most significant change in the measured spectra occurred due to spatially varying irradiance between the white reference panel and the bark surface. The spatial variation of irradiance had the largest effect on data quality in visible and red-edge regions, and the smallest in near-infrared. In non-diffuse conditions, changes in irradiance were often unpredictable as clouds or canopy elements moved in and out of the direct solar beams. Diffuse overcast days with clouds can extend the time window for measurements, making it a practical choice for acquiring hyperspectral images of stem bark. We concluded that with a well-planned measurement set-up it is possible to improve the precision of in situ collected spectra of stem bark.

High resolution imaging spectroscopy of the sky

Diffuse sky radiance is a key variable for resolving the global energy budget, weather forecasting, radiative transfer modelling and estimating solar energy potential at the Earth’s surface. This study is the first attempt to directly measure a full-sky view of the sky radiance using a pushbroom imaging spectrometer. We mounted the open-source hyperspectral imager (OpenHSI) on a tripod and rotation stage during a clear day in Rochester, NY, USA, and resampled the swaths to a sphere using the HEALPix projection. We achieved a 0.45∘ angular resolution, which is an order of magnitude better than any previous experimental study. Our results showed agreement with analytic studies using radiative transfer codes, and with photometric studies, demonstrating a strong wavelength dependence for sky radiance distributions. Along the principal plane, we observed a spectral steepening that transitioned from predominantly Mie scattering in the circumsolar region to predominantly Rayleigh scattering. We also compared our results to a CIE standard clear sky. Although all the data were collected within one hour, the movement of the Sun during this time is noticeable in the results. Nevertheless, our study provides confidence in using existing models and introduces a new technique for high resolution mapping of the sky radiance distribution using pushbroom imaging spectrometers.

Signal to Noise Ratio of a Coded Slit Hyperspectral Sensor

In recent years, a wide range of hyperspectral imaging systems using coded apertures have been proposed. Many implement compressive sensing to achieve faster acquisition of a hyperspectral data cube, but it is also potentially beneficial to use coded aperture imaging in sensors that capture full-rank (non-compressive) measurements. In this paper we analyse the signal-to-noise ratio for such a sensor, which uses a Hadamard code pattern of slits instead of the single slit of a typical pushbroom imaging spectrometer. We show that the coded slit sensor may have performance advantages in situations where the dominant noise sources do not depend on the signal level; but that where Shot noise dominates a conventional single-slit sensor would be more effective. These results may also have implications for the utility of compressive sensing systems.

High spectral resolution compact Offner spectrometer based on the aberration-reduced convex holographic gratings recorded by spherical waves under Rowland circle mounting.

High spectral resolution, excellent imaging quality, and compact configuration have become a recent trend in push-broom imaging spectrometers. The concentric Offner imaging spectrometer has become popular due to its high optical performance and compactness. However, astigmatism is the dominant residual aberration in the Offner imaging spectrometer, which makes the meridional and sagittal images unable to be focused well and causes a deterioration in image quality and spectral resolution. In this paper, we present a compact Offner imaging spectrometer with a high resolution based on an aberration-reduced convex holographic grating (ACHG), which is recorded by spherical waves under Rowland circle mounting. The holographic aberration coefficients of ACHG and geometric aberration coefficients of the Offner imaging spectrometer are derived based on the analysis of the light-path function. Furthermore, we analyzed the relationship between holographic aberration coefficients and holographic recording parameters of ACHG under Rowland circle mounting. To balance the geometric aberration of the Offner imaging spectrometer, proper holographic aberration coefficients of the ACHG are achieved through adjusting the holographic recording parameters. The design result indicated that the Offner imaging spectrometer with ACHG provides better images than those with mechanically ruled convex grating (MRCG). Moreover, the spectral resolution is significantly improved. This lays down a theoretical basis for subsequent construction work in the Offner imaging spectrometer with holographic aberration-reduced gratings.

Assessing Grapevine Nutrient Status from Unmanned Aerial System (UAS) Hyperspectral Imagery

This study aimed to identify the optimal sets of spectral bands for monitoring multiple grapevine nutrients in vineyards. We used spectral data spanning 400–2500 nm and leaf samples from 100 Concord grapevine canopies, lab-analyzed for six key nutrient values, to select the optimal bands for the nutrient regression models. The canopy spectral data were obtained with unmanned aerial systems (UAS), using push-broom imaging spectrometers (hyperspectral sensors). The novel use of UAS-based hyperspectral imagery to assess the grapevine nutrient status fills the gap between in situ spectral sampling and UAS-based multispectral imaging, avoiding their inherent trade-offs between spatial and spectral resolution. We found that an ensemble feature ranking method, utilizing six different machine learning feature selection methods, produced similar regression results as the standard PLSR feature selection and regression while generally selecting fewer wavelengths. We identified a set of biochemically consistent bands (606, 641, and 1494 nm) to predict the nitrogen content with an RMSE of 0.17% (using leave-one-out cross-validation) in samples with nitrogen contents ranging between 2.4 and 3.6%. Further studying is needed to confirm the relevance and consistency of the wavelengths selected for each nutrient model, but ensemble feature selection showed promise in identifying stable sets of wavelengths for assessing grapevine nutrient contents from canopy spectra.

Detection of anomalies in the UV–vis reflectances from the Ozone Monitoring Instrument

Abstract. Various instrumental or geophysical artifacts, such as saturation, stray light or obstruction of light (either coming from the instrument or related to solar eclipses), negatively impact satellite measured ultraviolet and visible Earthshine radiance spectra and downstream retrievals of atmospheric and surface properties derived from these spectra. In addition, excessive noise such as from cosmic-ray impacts, prevalent within the South Atlantic Anomaly, can also degrade satellite radiance measurements. Saturation specifically pertains to observations of very bright surfaces such as sunglint over open water or thick clouds. When saturation occurs, additional photoelectric charge generated at the saturated pixel may overflow to pixels adjacent to a saturated area and be reflected as a distorted image in the final sensor output. When these effects cannot be corrected to an acceptable level for science-quality retrievals, flagging of the affected pixels is indicated. Here, we introduce a straightforward detection method that is based on the correlation, r, between the observed Earthshine radiance and solar irradiance spectra over a 10 nm spectral range; our decorrelation index (DI for brevity) is simply defined as a DI of 1−r. DI increases with anomalous additive effects or excessive noise in either radiances, the most likely cause in data from the Ozone Monitoring Instrument (OMI), or irradiances. DI is relatively straightforward to use and interpret and can be computed for different wavelength intervals. We developed a set of DIs for two spectral channels of the OMI, a hyperspectral pushbroom imaging spectrometer. For each OMI spatial measurement, we define 14 wavelength-dependent DIs within the OMI visible channel (350–498 nm) and six DIs in its ultraviolet 2 (UV2) channel (310–370 nm). As defined, DIs reflect a continuous range of deviations of observed spectra from the reference irradiance spectrum that are complementary to the binary saturation possibility warning (SPW) flags currently provided for each individual spectral or spatial pixel in the OMI radiance data set. Smaller values of DI are also caused by a number of geophysical factors; this allows one to obtain interesting physical results on the global distribution of spectral variations.

Optical Design of a Miniaturized Airborne Push-Broom Spectrometer

Combining the requirements of spectrometers for unmanned aerial vehicle platforms, a miniaturized airborne wide-angle push-broom imaging spectrometer with an Offner configuration is designed. The system comprises an objective lens and an Offner-type spectrometer with a spectral range of 400~1000 nm and a spectral resolution of 15 nm. The objective lens and Offner spectrometer are designed in isolation before integration. The front objective lens is an inverted telephoto with a focal length of 13 mm, a relative aperture of 1/4.5, and a field of view of 54°. The frequency of the convex grating in the Offner configuration is 102 LP/mm, and the dispersion width is 2.6 mm. The modulation transfer function of the integrated system is greater than 0.4 at the Nyquist frequency in all spectral bands. To estimate the volume and weight of the system, a preliminary optical–mechanical design scheme is given in this paper. The entire spectrometer has a volume of 130 × 80 × 120 mm and is less than 3 kg, which realizes the miniaturization design of the imaging spectrometer with a wide field of view for unmanned aerial vehicle platforms.

Optical design and evaluation of airborne prism-grating imaging spectrometer.

We have focused on the optical form that is low cost while maintaining high performance for airborne application. We report the optical design as well as the alignment and test results for a push-broom imaging spectrometer. The smart architecture of the prism-grating based spectrometer ensures high uniformity and image quality. Moreover, an effective method for aligning the spectrometer is also proposed. The results of laboratory-based optical tests and a flight test confirm the easy manufacture and excellent performance. Thus, the proposed system should be suitable for use as a hyperspectral instrument that can be loaded onto airborne and unmanned aerial vehicles.

Comparison and analysis of wavelength calibration methods for prism – Grating imaging spectrometer

Abstract To identify spectral information obtained from the prism - grating imaging spectrometer, it is essential to know the center wavelength for each spectral channel. In this letter, the wavelength calibration principle of push-broom imaging spectrometers is described, and a wavelength calibration device is built. The wavelength calibration of the self-developed prism-grating imaging spectrometer using the peak method and the gravity method is completed, and the center wavelength of each spectral channel is obtained by the two methods respectively. Compared with the peak method, the gravity method can effectively avoid the errors caused by the uneven light spot energy in the process of wavelength calibration, and can more effectively complete wavelength calibration. The two methods can meet the practical application and can be selected according to actual demand.

Real-time hyperspectral anomaly detection system enhanced by graphics processing unit

Hyperspectral images have the advantages of high spectral resolution and abundant spectral information, which helps researchers to analyze, process, and monitor land surface information. However, it also faces problems such as low degree of automatic data processing and poor timeliness of information extraction. This timeliness is due to the high dimensionality when extracting the target information in hyperspectral image. To solve this problem, a practical real-time hyperspectral anomaly detection system enhanced by graphics processing unit (GPU) is designed. The push-broom imaging spectrometers are adopted to obtain the image data line-by-line; then, the real-time anomaly detection algorithm based on QR decomposition is optimized using GPU (i.e., A QR decomposition is a decomposition of a matrix A into a product A = QR of an orthogonal matrix Q and an upper triangular matrix R). The results show that the parallel algorithm based on GPU is up to about 26 times faster than the serial algorithm based on central processing unit. This indicates that the proposed scheme significantly enhances the implementation efficiency while maintaining the accuracy of the results and can meet the application requirements of real-time anomaly detection well.

Detection and Correction of Spectral Shift Effects for the Airborne Prism Experiment

Shifts of center wavelengths (CWLs) and related changes of full-widths at half-maximums (FWHMs) occur during in-flight data acquisitions of push-broom imaging spectrometers such as the airborne prism experiment (APEX). Combined with the spectrally changing properties of the dichroic coating that acts as a beam splitter between the visible and near infrared (VNIR) as well as the short-wave infrared (SWIR) channels, these shifts affect both the spectral and radiometric accuracies of the spectrometer data, and hence the accuracy of higher level products. In this paper, two independent standards, i.e., atmospheric absorption features as well as features of the standard reference material filter built in the APEX in-flight characterization facility, are used in a complementary way to improve in-flight spectral calibration. The CWL shift and FWHM change for each detector element are simultaneously detected by using spectrum-matching and surface fitting techniques under constraints from pregenerated shift realizations. Subsequently, the APEX spectroradiometric response model is improved in the aspect of spectral resolution by using performance parameters of optics and detector modules. The radiometric gain and offset for each detector element are corrected according to the detected CWLs and FWHMs, as well as the improved APEX response model. Compared with the spectral and radiometric parameters acquired during laboratory calibration, the detected CWLs and FWHMs promote the accuracy of the atmospheric feature positions in the SWIR channel by 10 nm, whereas the corrected gains and offsets reduce the radiance deviation in the spectral regions 375–550 nm and 950–1080 nm both by 70% on average.

Deconvolution-based restoration of SWIR pushbroom imaging spectrometer images.

Properties of the short wave infrared (SWIR) imaging spectrograph and the front lens along with the misalignment of optical elements contribute to positionally variant displacements and blur that can significantly degrade the overall quality of the acquired images. In this work, we devise a complete routine for simultaneous displacement correction and resolution enhancement of SWIR spectral images along the two spatial and the spectral direction. The proposed restoration routine requires images of widely available and inexpensive calibration targets from which the response function of the imaging spectrometer is extracted. Extensive validation reveals that the displacement error observed in the restored images is reduced to the manufacturing accuracy of the calibration targets. Furthermore, the restored images exhibit up to a two-fold improvement in the spectral and spatial resolution.

How big is an OMI pixel?

Abstract. The Ozone Monitoring Instrument (OMI) is a push-broom imaging spectrometer, observing solar radiation backscattered by the Earth's atmosphere and surface. The incoming radiation is detected using a static imaging CCD (charge-coupled device) detector array with no moving parts, as opposed to most of the previous satellite spectrometers, which used a moving mirror to scan the Earth in the across-track direction. The field of view (FoV) of detector pixels is the solid angle from which radiation is observed, averaged over the integration time of a measurement. The OMI FoV is not quadrangular, which is common for scanning instruments, but rather super-Gaussian shaped and overlapping with the FoV of neighbouring pixels. This has consequences for pixel-area-dependent applications, like cloud fraction products, and visualisation.The shapes and sizes of OMI FoVs were determined pre-flight by theoretical and experimental tests but never verified after launch. In this paper the OMI FoV is characterised using collocated MODerate resolution Imaging Spectroradiometer (MODIS) reflectance measurements. MODIS measurements have a much higher spatial resolution than OMI measurements and spectrally overlap at 469 nm. The OMI FoV was verified by finding the highest correlation between MODIS and OMI reflectances in cloud-free scenes, assuming a 2-D super-Gaussian function with varying size and shape to represent the OMI FoV. Our results show that the OMPIXCOR product 75FoV corner coordinates are accurate as the full width at half maximum (FWHM) of a super-Gaussian FoV model when this function is assumed. The softness of the function edges, modelled by the super-Gaussian exponents, is different in both directions and is view angle dependent.The optimal overlap function between OMI and MODIS reflectances is scene dependent and highly dependent on time differences between overpasses, especially with clouds in the scene. For partially clouded scenes, the optimal overlap function was represented by super-Gaussian exponents around 1 or smaller, which indicates that this function is unsuitable to represent the overlap sensitivity function in these cases. This was especially true for scenes before 2008, when the time differences between Aqua and Aura overpasses was about 15 min, instead of 8 min after 2008. During the time between overpasses, clouds change the scene reflectance, reducing the correlation and influencing the shape of the optimal overlap function.

High spatial resolution imaging of methane and other trace gases with the airborne Hyperspectral Thermal Emission Spectrometer (HyTES)

Abstract. Currently large uncertainties exist associated with the attribution and quantification of fugitive emissions of criteria pollutants and greenhouse gases such as methane across large regions and key economic sectors. In this study, data from the airborne Hyperspectral Thermal Emission Spectrometer (HyTES) have been used to develop robust and reliable techniques for the detection and wide-area mapping of emission plumes of methane and other atmospheric trace gas species over challenging and diverse environmental conditions with high spatial resolution that permits direct attribution to sources. HyTES is a pushbroom imaging spectrometer with high spectral resolution (256 bands from 7.5 to 12 µm), wide swath (1–2 km), and high spatial resolution (∼ 2 m at 1 km altitude) that incorporates new thermal infrared (TIR) remote sensing technologies. In this study we introduce a hybrid clutter matched filter (CMF) and plume dilation algorithm applied to HyTES observations to efficiently detect and characterize the spatial structures of individual plumes of CH4, H2S, NH3, NO2, and SO2 emitters. The sensitivity and field of regard of HyTES allows rapid and frequent airborne surveys of large areas including facilities not readily accessible from the surface. The HyTES CMF algorithm produces plume intensity images of methane and other gases from strong emission sources. The combination of high spatial resolution and multi-species imaging capability provides source attribution in complex environments. The CMF-based detection of strong emission sources over large areas is a fast and powerful tool needed to focus on more computationally intensive retrieval algorithms to quantify emissions with error estimates, and is useful for expediting mitigation efforts and addressing critical science questions.

Cooled Dyson long-wave infrared push-broom imaging spectrometer by re-imaging

A cooled long-wave infrared push-broom imaging spectrometer with an F-number of 2 was designed based on the Dyson configuration. A three-mirror off-axis aspherical optical system that provided excellent slit-shaped images was selected as the fore telescope objective. The re-imaging method was applied to obtain a cold stop efficiency of 100%, and the corrector lens in traditional Dyson imaging spectrometers was replaced with re-imaging lenses to correct spherical aberrations. The designed imaging spectrometer provided a spectral resolution of 25nm at a range of 8–12μm and possessed a relatively small volume.

Landsat swath imaging spectrometer design

This paper describes the design of a high-throughput and high-uniformity pushbroom imaging spectrometer and telescope system that is capable of Landsat swath and resolution while providing better than 10 nm per pixel spectral resolution over the full visible to short-wave infrared band. The design is based on a 3200×480 element×18 μm pixel size focal plane array, two of which are utilized to cover the full swath. At an optical speed of F/1.8, the system is the fastest proposed to date to our knowledge. The utilization of only two Dyson-type spectrometer modules fed from the same telescope reduces system complexity while providing a solution within achievable detector technology. Two telescope designs are shown to achieve the required swath and resolution from different altitudes. Predictions of complete system response are shown. Also, it is shown that detailed ghost analysis is a requirement for this type of spectrometer and forms an essential part of a complete design.

Independent Laboratory Characterization of NEO HySpex Imaging Spectrometers VNIR-1600 and SWIR-320m-e

The Remote Sensing Technology Institute (Institut fur Methodik der Fernerkundung) of the German Aerospace Agency (DLR) operates two sensors for airborne hyperspectral imaging, i.e., a Norsk Elektro Optikk A/S (NEO) HySpex VNIR-1600 and a NEO HySpex SWIR-320m-e. Since these sensors are used for the development of physically based inversion algorithms, atmospheric correction algorithms and for calibration/ validation activities, their properties need to be characterized in detail, and an accurate calibration is mandatory. The characterization is performed at the calibration laboratory of DLR for imaging spectrometers in Oberpfaffenhofen. Key results of the characterization are assessments of the radiometric, spectral, and geometric performances, including the typical optical distortions prevalent in pushbroom imaging spectrometers, keystone and smile, and the associated measurement uncertainties. Potential sources of systematic error, the detector nonlinearity and the polarization sensitivity are discussed. The radiometric calibration is traceably performed to the German national metrology institute Physikalisch-Technische Bundesanstalt, whereas the spectral measurements can be traced back to the spectral properties of atomic line lamps. The implemented level 0 to level 1 calibration procedure is presented as well.

Advanced radiometry measurements and Earth science applications with the Airborne Prism Experiment (APEX)

We present the Airborne Prism Experiment (APEX), its calibration and subsequent radiometric measurements as well as Earth science applications derived from this data. APEX is a dispersive pushbroom imaging spectrometer covering the solar reflected wavelength range between 372 and 2540nm with nominal 312 (max. 532) spectral bands. APEX is calibrated using a combination of laboratory, in-flight and vicarious calibration approaches. These are complemented by using a forward and inverse radiative transfer modeling approach, suitable to further validate APEX data. We establish traceability of APEX radiances to a primary calibration standard, including uncertainty analysis. We also discuss the instrument simulation process ranging from initial specifications to performance validation. In a second part, we present Earth science applications using APEX. They include geometric and atmospheric compensated as well as reflectance anisotropy minimized Level 2 data. Further, we discuss retrieval of aerosol optical depth as well as vertical column density of NOx, a radiance data-based coupled canopy–atmosphere model, and finally measuring sun-induced chlorophyll fluorescence (Fs) and infer plant pigment content. The results report on all APEX specifications including validation. APEX radiances are traceable to a primary standard with <4% uncertainty and with an average SNR of >625 for all spectral bands. Radiance based vicarious calibration is traceable to a secondary standard with ≤6.5% uncertainty. Except for inferring plant pigment content, all applications are validated using in-situ measurement approaches and modeling. Even relatively broad APEX bands (FWHM of 6nm at 760nm) can assess Fs with modeling agreements as high as R2=0.87 (relative RMSE=27.76%). We conclude on the use of high resolution imaging spectrometers and suggest further development of imaging spectrometers supporting science grade spectroscopy measurements.

Wavelet-based threshold denoising for imaging hyperspectral data

Imaging spectroradiometer is highly susceptible to noise. Accurately quantitative processing with higher quality is obligatory before any derivative analysis, especially for precision agricultural application. Using the self-developed Pushbroom Imaging Spectrometer (PIS), a wavelet-based threshold (WT) denoising method was proposed for the PIS imaging hyperspectral data. The WT with PIS was evaluated by comparing with other popular denoising methods in pixel scale and in regional scale. Furthermore, WT was validated by chlorophyll concentration retrieval based on red-edge position extraction. The result indicated that the determination coefficient R2 of the chlorophyll concentration inversion model of winter wheat leaves was improved from 0.586 to 0.811. It showed that the developed denoising method allowed effective denoising while maintaining image quality, and presented significant advantages over conventional methods. Keywords: wavelet, denoising, spectral domain, Pushbroom Imaging Spectrometer, red edge position DOI: 10.3965/j.ijabe.20140703.005 Citation: Yang H, Zhang D Y, Huang L S, Zhao J L. Wavelet-based threshold denoising for imaging hyperspectral data. Int J Agric & Biol Eng, 2014; 7(3): 36－42.

Research Vertical Distribution of Chlorophyll Content of Wheat Leaves Using Imaging Hyperspectra

Abstract Chlorophyll content is an important indicator for judging crop photosynthesis ability and monitoring growth status. Hyperspectral imaging is one of the hot spots in quantitative remote sensing research. As an image-spectrum merging technology, it could be used to explore and develop new methods for diagnosing of crop nutrition, diseases and insect pests. In this study, an auto-development pushbroom imaging spectrometer (PIS) was applied to measure the chlorophyll content of wheat leaves. The tested sites of spectrum and the chlorophyll content measured positions were on the same area of single leaf. Partial least square (PLS) regression was used to establish prediction models of chlorophyll content. The model accuracy of single leaf with values from different positions was evaluated; and the model accuracy of leaves from different layers was also studied. The results showed that the model of the leaf with 2, 4, 6 sites was better than that of 1, 3, 5 sites; those models of leaves from vertical le...