Spectral Light Field Research Articles

Cascaded spectral light field tomography.

We present cascaded spectral light field tomography for multicolor imaging in three dimensions (3D). Building upon light field tomography, our method uses a Dove prism array and a cylindrical lens array to transform a 3D scene into one-dimensional (1D) projections. To further enhance the reconstructed image quality, we incorporate a rotating Dove prism to increase the number of projection angles and a scanning light sheet to sparsify the sample along the depth axis. The resulting 1D projections are then spectrally dispersed for parallel spectral measurements. We demonstrate the effectiveness of our system in both fluorescence and scattering microscopy applications.

Learning based compressive snapshot spectral light field imaging with RGB sensors.

The application of multidimensional optical sensing technologies, such as the spectral light field (SLF) imager, has become increasingly common in recent years. The SLF sensors provide information in the form of one-dimensional spectral data, two-dimensional spatial data, and two-dimensional angular measurements. Spatial-spectral and angular data are essential in a variety of fields, from computer vision to microscopy. Beam-splitters or expensive camera arrays are required for the usage of SLF sensors. The paper describes a low-cost RGB light field camera-based compressed snapshot SLF imaging method. Inspired by the compressive sensing paradigm, the four dimensional SLF can be reconstructed from a measurement of an RGB light field camera via a network which is proposed by utilizing a U-shaped neural network with multi-head self-attention and unparameterized Fourier transform modules. This method is capable of gathering images with a spectral resolution of 10 nm, angular resolution of 9 × 9, and spatial resolution of 622 × 432 within the spectral range of 400 to 700 nm. It provides us an alternative approach to implement the low cost SLF imaging.

Snapshot depth–spectral imaging based on image mapping and light field

Depth–spectral imaging (DSI) is an emerging technology which can obtain and reconstruct the spatial, spectral and depth information of a scene simultaneously. Conventionally, DSI system usually relies on scanning process, multi-sensors or compressed sensing framework to modulate and acquire the entire information. This paper proposes a novel snapshot DSI architecture based on image mapping and light field framework by using a single format detector. Specifically, we acquire the depth – spectral information in two steps. Firstly, an image mapper is utilized to slice and reflect the first image to different directions which is a spatial modulation processing. The modulated light wave is then dispersed by a direct vision prism. After re-collection, the sliced dispersed light wave is recorded by a light field sensor. Complimentary, we also propose a reconstruction strategy to recover the spatial depth – spectral hypercube effectively. We establish a mathematical model to describe the light wave distribution on every optical facet. Through simulations, we generate the aliasing raw spectral light field data. Under the reconstruction strategy, we design an algorithm to recover the hypercube accurately. Also, we make an analysis about the spatial and spectral resolution of the reconstructed data, the evaluation results conform the expectation.

Phytoplankton diversity emerging from chromatic adaptation and competition for light

The phytoplankton absorption cross-section is a fundamental quantity in biogeochemical ocean models that alters the underwater spectral light field and the photosynthetic response of phytoplankton. Phytoplankton taxa are characterized by absorption spectra with defined absorption bands in the visible region of the light spectrum that govern the capability of different taxa of phytoplankton to exploit light as a resource for growth. The interplay between the spatial and temporal gradients of underwater spectral light and the light absorption characteristics of different phytoplankton types contribute to the interactions and selection among groups and hence can be a determinant in shaping phytoplankton diversity and biogeography. In this work, we used a biogeochemical model of the Mediterranean Sea to simulate nine optically different phytoplankton functional types (PFTs) and coupled it to a radiative transfer model to simulate the spectral underwater light field where the PFTs interact. We investigated the competitive advantage provided by the different absorption spectra and the possibilities of coexistence emerging from the availability of different light habitats. By considering non-spectrally resolved optical differences among PFTs, our model results led to the dominance of Synechococcus sp. within picophytoplankton, coccolithophores within nanophytoplankton and dinoflagellates within microphytoplankton. By including the spectral dependency of photosynthesis, we observed how the availability of different spectral light habitats led to the coexistence of picophytoplankton groups in clear waters, whereas for nano- and microphytoplankton PFTs, the extent of coexistence permitted by light fluctuations was minimal. By combining optical and functional differences in the PFT description, we obtained PFT distributions that were in relative agreement with observed phytoplankton distributions in the Mediterranean Sea estimated from satellite and in situ high-performance liquid chromatography sampling. This result suggests that combining spectrally resolved optical traits with functional traits in the description of the autotrophic community can be a valuable tool to improve simulations of the diversity and biogeography of primary producers in the ocean.

Snapshot spectral polarimetric light field imaging using a single detector.

In this Letter, we investigate a snapshot spectral-polarimetric-volumetric imaging (SSPVI) system using a single detector. Through compressed acquisition and reconstruction, SSPVI can achieve spectral imaging (x,y,λ), polarization imaging (x,y,ψ,χ), and light field imaging (x,y,θ,φ) simultaneously. The newly discovered performance is showcased by attaining the spectral-polarimetric-volumetric video and different laboratory accuracy experiments. These never-seen-before capacities of the camera open new prospects for many applications, such as biological analysis, object recognition, and remote sensing.

Compressive Spectral Light Field Image Reconstruction via Online Tensor Representation.

In recent years there has been an increasing interest in sensing devices that capture multidimensional information such as the spectral light field (SLF) images, which are 5-dimensional (5D) representations of a scene including 2D spatial, 2D angular and 1D spectral information. Spatio-spectral and angular information plays an important role in modern applications spanning from microscopy to computer vision. However, SLF sensors use expensive beam-splitters or cameras arrays placed in tandem, which split the sensing problem in two time consuming and independent tasks: spectral and light field imaging tasks. This work proposes a compressive spectral light field imaging architecture that builds on the principles of the compressive imaging framework, to capture multiplexed representations of the multidimensional information, so that, less measurements are required to capture the SLF data cube. Alongside, we propose a computational algorithm to recover the 5D information from the compressed measurements, exploiting the inherent high correlations within the SLF by treating them as 3D tensors. Furthermore, exploiting the geometry properties of the proposed optical architecture, the Tucker decomposition is applied to the set of compressed measurements, so that, an ad-hoc dictionary-like image representation basis is calculated online. This in turn, entails a more accurate reconstruction of the SLF since the dictionary fits the specific characteristics of the image itself. We demonstrate through simulations over three SLF datasets captured in our laboratory, and an experimental proof-of-concept implementation, that the proposed compressive imaging device together with the proposed computational algorithm represent an efficient alternative to capture SLF, compared to conventional methods that employ either side-information or multiple sensors. Also, we show that the tensor-based proposed algorithm exhibits a lower computational complexity than the matrix-based state of the art counterparts, thus enabling fast processing of multidimensional images.

Comparison between different strategies for the realization of flashing-light effects – Pneumatic mixing and flashing illumination

A major limitation for the phototrophic cultivation of microalgae in photobioreactors is the low culture density which results in expensive harvesting and downstream processes. It is often mentioned that intensive mixing of the culture can improve the efficiency of light utilization by realizing flashing-light effects which enhance the growth of cell cultures. Alternatively, flashing light sources could provide a way to realize this effect without the need of supplying additional energy for fast mixing. While the effects of mixing have been investigated at various reactor scales, experiments with flashing illumination have been mostly conducted in small geometries. In addition, few studies have been conducted for photobioreactors with larger light paths, being characteristic for the production scale. By means of numerical simulations, we evaluate pneumatic mixing and flashing illumination with regard to their ability to realize flashing-light effects in a 5 cm diameter bubble column filled with a suspension of Chlamydomonas reinhardtii. To the best of our knowledge, a numerical comparison between the effects of flashing illumination and pneumatic mixing on the culture growth has not been reported in literature.A thorough comparison of the two methods requires a robust numerical tool which integrates the computation of the fluid flow and the light field, as well as the growth kinetics of algal cells. In the present work, we compute the three-dimensional flow field in a bubble column photobioreactor tracking also micrometer particles in order to simulate the movement of algae. The spectral light field is computed by solving the three-dimensional Radiation Transfer Equation (RTE) at different wavelengths and biomass concentrations. The coupling of the flow and light fields enables the computation of the spatio-temporal light exposure of individual algae cells, which is used to estimate the respective dynamic photosynthesis reaction rates under different operating conditions of the bubble column reactor.We found, numerically, that the contribution of pneumatic mixing alone is negligible in comparison to flashing-light effects, for the investigated operating conditions. In contrast, illumination with flashing LED leads to an increase of the growth rate up to a factor of 2.5 at flashing frequencies higher than 50 Hz in a PBR with industrially relevant operating conditions. Thereby, a proper selection of the duty cycle is not only needed to prevent photoinhibition but also to maximize the effects of flashing light at the same time. According to these results, the utilization of flashing LED sources can provide a mostly unused way to improve photobioreactor productivity in the future.

Near-field dynamics at a metallic transmission grating with femtosecond illumination: A theoretical study

This paper reports on the temporal and spectral light field evolution for two different types of metallic gratings after illumination with a near-infrared TM-polarized femtosecond pulse: a freestanding grating in air and a grating on a dielectric substrate. The electromagnetic field components are calculated numerically at various transverse locations of the gratings by use of a finite-difference time-domain model. The spectral dynamics are explained by the dynamics of surface plasmon polaritons (SPPs). Fano-like profiles are observed close to an SPP resonance in the calculated spectral curves, indicating a connection between the Rayleigh anomaly and a nearby SPP resonance. For the freestanding grating, only a single Fano-like profile close to the air/metal SPP resonance and, with the dielectric support, two Fano-type profiles are observed, one in the proximity of the air/metal SPP resonance, the other in the proximity of the substrate/metal resonance. Investigating the changes in the spectra for an increasing distance to the grating, it is found that with increasing distance, the spectral curves related to different transverse locations coincide almost completely, providing evidence that the near-field contributions disappeared.

Spectral light fields for improved three-dimensional profilometry

Light fields are four-dimensional parametrizations of rays, extensively used for the representation of the rays emitted from three-dimensional objects. With the increasing availability of spectral imagers, conventional light fields can be augmented by the additional spectral information, yielding a five-dimensional ray parametrization referred to as a spectral light field. We demonstrate that utilization of the spectral channel information can improve the profilometric performance of light-field cameras.

Spectral effects on Symbiodinium photobiology studied with a programmable light engine.

The spectral light field of Symbiodinium within the tissue of the coral animal host can deviate strongly from the ambient light field on a coral reef and that of artificial light sources used in lab studies on coral photobiology. Here, we used a novel approach involving light microsensor measurements and a programmable light engine to reconstruct the spectral light field that Symbiodinium is exposed to inside the coral host and the light field of a conventional halogen lamp in a comparative study of Symbiodinium photobiology. We found that extracellular gross photosynthetic O2 evolution was unchanged under different spectral illumination, while the more red-weighted halogen lamp spectrum decreased PSII electron transport rates and there was a trend towards increased light-enhanced dark respiration rates under excess irradiance. The approach provided here allows for reconstructing and comparing intra-tissue coral light fields and other complex spectral compositions of incident irradiance. This novel combination of sensor technologies provides a framework to studying the influence of macro- and microscale optics on Symbiodinium photobiology with unprecedented spectral resolution.

3D-Tailored Gold Nanoparticles for Light Field Enhancement and Harvesting over Visible-IR Spectral Range

A method for practical area upscaling of nanopatterning for light-harvesting and photocatalytic applications is presented. Large area electron beam lithography is used to design patterns of simple-shape nanoparticles. After evaporation of gold, ion beam lithography is used to slice nanoparticles with grooves as narrow as 17 ± 3 nm in width for the required spectral performance and light field enhancement. It is demonstrated by systematic numerical simulations that cutting grooves into the Si and SiO2 substrates up to a ∼10 nm depth augments the volume where the light-field enhancement occurs. The dominant component of the field enhancement in the groove is |Ez|2, perpendicular to the substrate's surface. The application potential of 3D-tailored nanoparticles in light harvesting applications is discussed.

A three-dimensional biophysical model of Karenia brevis dynamics on the west Florida shelf: A look at physical transport and potential zooplankton grazing controls

The development of accurate predictive models of toxic dinoflagellate blooms is of great ecological importance, particularly in regions that are most susceptible to their detrimental effects. This is especially true along the west Florida shelf (WFS) and coast, where episodic bloom events of the toxic dinoflagellate Karenia brevis often wreak havoc on the valuable commercial fisheries and tourism industries of west Florida. In an effort to explain the dynamics at work within the maintenance and termination phases of a red tide, a simple three-dimensional coupled biophysical model was used in the analysis of the October 1999 red tide offshore Sarasota, Florida. Results of the numerical experiments indicate that: (1) measured and modeled flowfields were capable of transporting the observed offshore inoculum of K. brevis to within 16 km of the coastal boundary; (2) background concentrations (1000 cells L −1) of K. brevis could grow to a red tide of over 2×10 6 cells L −1 in little more than a month, assuming an estuarine initiation site with negligible offshore advection, no grazing losses, negligible competition from other phytoplankton groups, and no nutrient limitation; (3) maximal grazing pressure could not prevent the initiation of a red tide or cause its termination, assuming no other losses to algal biomass and a zooplankton community ingestion rate similar to that of Acartia tonsa; and (4) the light-cued ascent behavior of K. brevis served as an aggregational mechanism, concentrating K. brevis at the 55 μE m −2 s −1 isolume when mean concentrations of K. brevis exceeded 100,000 cells L −1. Further improvements in model fidelity will be accomplished by the future inclusion of phytoplankton competitors, disparate nutrient availability and limitation schemes, a more realistic rendering of the spectral light field and the attendant effects of photo-inhibition and compensation, and a mixed community of vertically-migrating proto- and metazoan grazers. These model refinements are currently under development and shall be used to aid progress toward an operational model of red tide forecasting along the WFS.

Diel migration of phytoplankton and spectral light field in the Ría de Vigo (NW Spain)

Diel migration of Mesodinium rubrum, Eutreptiella sp., Scrippsiella trochoidea, Dinophysis acuminata and Ceratium furca throughout a 24 h cycle is described for a stable, well-stratified estuary (Ria de Vigo, NW Spain). Daily changes in light quantity and in spectral light ratios i.e. red:far-red, blue:red, green:red and blue:green have been analysed. The spectral light ratios changed at twilight and around noon at various depths. Some of the downward migrations were well predicted by Stokes' law, while other migrations were faster and deeper than calculated. The coincidence of these movements with abrupt changes in red:far-red, green:red and blue:green light ratios is discussed. Some species are able to migrate through the pycnocline, whereas others do not seem to be able to do so. Several species are present in maximum numbers at depth at night, while others display upward migration independent of light, suggesting the existence of endogenous rhythms. Upward migration at dusk began with dispersal of populations, with renewed aggregation at the sea surface coincident with an increase in the red:far-red ratio at 6 m and the green:red ratio at 6 and 10 m. Based on direct evidence for the control of flagellar mobility by light quality reported by other authors from laboratory studies, it is suggested that, together with other cues, spectral light ratios of different light qualities modulate vertical phytoplanktonic migration.

Spectral light attenuation and phytoplankton distribution during a daily cycle in the reservoir of La Concepción, Southern Spain

The spectral light field and the concentration of phytoplankton, total particulate material larger than 0.7 μm and chlorophyll were analyzed during a daily cycle in a stratified reservoir (La Concepcion, Southern Spain). Vertical attenuation coefficient (K d ) at different wavelengths depends on solar elevation and both phytoplankton and total particle concentration. Kd at blue and red wavebands but no in green, yellow-orange increased between 11:00 to 15:00 and it was related to the aggregation of phytoplankton at 12m depth. The product of K d and Secchi Disk (SD) depth was inversely correlated with chlorophyll. Chlorophyll per cell presented a daily cycle of acclimation to the changes in the irradiance with maximum level around noon at the maximum phytoplankton density (8-12 m depth). This pattern was similar to that of the spectral attenuation coefficient between 400-575 for the water column. Two phytoplanktonic aggregations were found at 10 % (SD) and 1% (2 SD) depending on the incident light. A chlorophyll maximum was located at 1% of incident light. The chlorophyll maximum at daylight was caused by motile dinoflagellates (Peridium and Ceratium) and diatoms (Fragilaria), but during the night only dino-flagellates remained at this depth and they were the only ones responsible for the chlorophyll maximum as well as for the particulate material concentration. The two main phytoplanktonic groups, large dinoflagellates and diatoms affected the light attenuation in different ways. In the samples dominated by dinoflagellates, with high chlorophyll content per cell, the increase in chlorophyll concentration produced a small increase in the light attenuation due to the package effect. However, in the samples dominated by diatoms, a good positive linear correlation between chlorophyll concentration and attenuation coefficient was found. The ecological implications of the short-term variations in the bio-optic properties in the reservoir are discussed.

WAVELENGTH DEPENDENCY OF THE MAXIMUM QUANTUM YIELD OF CARBON FIXATION FOR TWO RED TIDE DINOFLAGELLATES, HETEROCAPSA PYGMAEA AND PROROCENTRUM MINIMUM (PYRROPHYTA): IMPLICATIONS FOR MEASURING PHOTOSYNTHETIC RATES1

ABSTRACTThe influence of photoadaptive state on the spectral dependency of the maximum quantum yield for carbon fixation was determined for two red tide dinoflagellates, Heterocapsa pygmaea Loeblich, Schmidt, et Sherley and Prorocentrum minimum Pavillard. Cultures were acclimated to green, blue, red, and white light. The spectral dependency in the light‐limited slope of the photosynthesis–irradiance curves (α) was measured with carbon action spectra that, when divided by the spectrally weighted absorption coefficient, provided estimates of the maximum quantum yield (φmax) for carbon fixation. Values of φmax varied with wavelength within each culture condition as well as between different culture conditions. The degree to which the spectral dependency in φmax was influenced by the presence of photoprotective carotenoids and/or energy imbalances between photosystems I and II was assessed for both dinoflagellates. The impact of photoprotective pigmentation on the spectral dependency of φmax was most significant for cells grown under high light conditions reflecting the enrichment of diadinoxanthin. Energy imbalances between the photosystems was assessed by quantifying enhancement effects on spectral φmax in the presence of background illumination. Under our experimental conditions, enhancement effects on carbon action spectra were evident for H. pygmaea under nearly all growth conditions but were not detectable for P. minimum under any growth condition. We hypothesize that sensitivity to enhancement effects reflected differences in the structure of the photosynthetic machinery of these two peridinin‐containing dinoflagellates. While measurements of φmax are sensitive to the color of the light within an incubator, the relative impact on the spectral dependency of a was less than the wavelength dependency associated with the cellular absorption properties. Finally we used our data to validate an approach proposed by others to aid in the correction of photosynthetic measurements where the in situ spectral light field cannot be easily mimicked. The average error using this approach was 8%, which was significantly less than the error associated with ignoring the spectral dependency in α.

Influences of Canopy Cover on Spectral Irradiance and Periphyton Assemblages in a Prairie Stream

Influences of the transmission properties of the riparian canopy and the water column on spectral irradiance and periphyton composition were examined at four sites in Otter Creek, a small spring-fed stream in the Sand Hills region of western Nebraska. Values for per cent sky in the canopy at the four sites were: 65.1% (open site), 31.9% (partial shade), 19.0% (shade), and 15.4% (heavy shade). Above canopy, below canopy, and underwater irradiahces were measured at 2-nm intervals from 300-800 nm in late morning, mid-day, early afternoon, and late afternoon at each site on a clear day (13 June 1991). Photosynthetically active radiation (PAR) incident at the stream surface did not consistently correspond to per cent sky values at the sites because of temporal changes in the penetration of direct sun through small openings in the canopy (sunflecks). The relationship of solar path to canopy structure and orientation was the primary determinant of sunfleck activity. There was very little absorbance of specific wavelengths by the canopy at the open site, but the proportion of blue light (400-500 nm) increased at all sites by late afternoon. At the three shaded sites, the amount of red light (600-700 nm) was proportionally lower underneath the canopy than above it. Underwater PAR was always greater at the open site than at the more shaded sites, but the relationship between canopy cover and underwater irradiance for the more shaded sites varied temporally. The spectral distribution of PAR at the mean depth in Otter Creek (39.7 cm) was similar to that found at shallow depths in clear, oligotrophic lakes, in that there was only a slight maximum transmission in green wavelengths (-550 nm). The degree of temporal variability in the underwater spectral distribution at a site was often greater than differences between sites because of the effects of sunflecks and surface reflection by ripples. Algal chlorophyll a concentrations at the sites were significantly correlated with per cent sky for gravel substrata but not for sand substrata. The flora on both substrata was dominated by diatoms, and the distributions of several species were related to the degree of canopy cover at the sites. The relative importance of the spectral light field versus total PAR in structuring benthic lotic assemblages warrants further study.