Reabsorption Of Light Research Articles

Boosting Photocatalytic Hydrogen Evolution by a Light Coupling and Charge Carrier Confinement Strategy

AbstractHigh‐performance photocatalysis requires efficient light absorption and low charge carrier recombination rates. Herein, a light coupling and charge carrier confinement strategy is demonstrated to simultaneously enhance the light absorption efficiency and suppress the charge carrier recombination for high‐performance photocatalysis. The strategy is achieved by the delicate incorporation of catalysts into a dual‐core‐shell structure (e.g., CdS@SiO2@NaYF4:Yb/Tm), in which CdS (catalyst), SiO2, and NaYF4:Yb/Tm serve as shell, outer core, and inner core, respectively. Interestingly, the absorbed light can be confined within the CdS layer through multiple reflections between the CdS and SiO2 interfaces, achieving light confinement. This confinement endows a longer light residence time, enhanced light reabsorption and reutilization efficacy, and a higher concentration of photogenerated charge carriers per unit of time. Moreover, the insulating SiO2 can confine the photogenerated charge carriers within CdS layer, thus shortening their diffusion length for reduced recombination rates. Notably, when employed as the photocatalyst, this dual‐core‐shell structure showed a superb photocatalytic hydrogen evolution rate up to 74.67 mmol g−1 h−1, which is 11 times higher than that of pristine CdS. This work provides a new strategy for the design and synthesis of high‐performance photocatalysts.

Study on the Optical Coupling Effect of Building-Integrated Photovoltaic Modules Applied with a Shingled Technology

Building-integrated photovoltaics (BIPV) comprise the integration of a solar power generation system into the exterior design and architectural elements of a building to produce electricity, which allows the building itself to generate electricity. By integrating shingled technology into the photovoltaic module with optimization of the optical effect, the output performance of the module can be increased while securing an aesthetic appeal as an architectural exterior material for the building simultaneously. In this research, we studied enhancing the performance of BIPV modules through an analysis of the optical coupling effect for shingled technology using PSpice simulation. Compared to the efficiency of 0.2 cm string spacing, the optical coupling effect was increased by 33.33%, 46.98%, 67.01%, and 193.49% according to the string spacing of 0.5 cm, 1 cm, 2 cm and 4 cm, respectively. To analyze this increase, we focused on studying the increase in current due to the reflection and re-absorption of light in the back layer of the solar cell as the cause of this output enhancement. Additionally, the coupling effect in accord with different layers showed that using white EVA to reflect the incident light from the top layer resulted in 117.14% and 521.90% enhancements in maximum output power (Pm) loss % compared to the conventional and black backsheet applied PV modules, respectively.

Efficient and environmentally friendly white light-emitting diodes with InP-based quantum dots embedded in mesoporous silica

As one of the promising next-generation light conversion materials, indium phosphide quantum dots (InP QDs) deserve much attention due to their great optical performances and environmentally friendly properties in particular. Herein, InP-based QDs are embedded into mesoporous SBA-15 and solid QDs/SBA-15 composites are prepared, which exhibit 1.5 times higher photoluminescence quantum yield (PLQY) and narrower full width of half maximum (FWHM) than traditional QDs powder thanks to the reduction of light reabsorption and the optical waveguide effect of mesoporous structure. These advantages contribute to the performance enhancement of light-emitting diodes (LEDs). The luminous efficacy of the LED with green QDs/SBA-15 is 99.49 lm/W, which is higher than that of QDs powder (66.14 lm/W). In addition, the white LED fabricated with green and red InP-based QDs/SBA-15 shows luminous efficacy of 61.38 lm/W. More importantly, the luminous efficacy of the white LED is improved to 129.62 lm/W when using the K2SiF6:Mn4+ instead of red InP-based QDs/SBA-15, because the K2SiF6:Mn4+ does not absorb the emission from green InP-based QDs/SBA-15. This value is higher than those white LEDs with InP-based QDs reported previously. It is believed that this study demonstrates the promising potential of InP-based QDs for optoelectronic applications.

Giant Exciton Stokes Shift in Interfacially Prepared Cs(Mn/Pb)Cl3 Alloy Nanoplatelets.

Controlling the reabsorption of light by an emitting material is one of the keys to improving the performance of light-emitting devices. We prepare a set of size-dependent Cs(Mn/Pb)Cl3 alloy nanoplatelets (NPls) with substantial enhancement in the exciton Stokes shift, reducing the light-reabsorption significantly. We perform interfacial Mn-alloying using a shuttling ligand that transports MnCl2 from aqueous to nonaqueous phase and delivers it to NPls. While the exciton Stokes shift in 2-5 monolayer (ML) CsPbBr3 NPls rises from 20 to 108 meV, the exciton Stokes shift increases drastically up to 600 meV in 2 ML Cs(Mn/Pb)Cl3 NPls and further reduces upon increasing the thickness. Moreover, the exciton PL peak in the Mn-alloy NPls remains unperturbed by the quantum-confinement effect. A model based on the interplay between Mn2+/Mn3+ during the charge transfer process is proposed, accounting for such a large exciton Stokes shift. Finally, we utilize the large exciton Stokes-shifted alloy NPls for successful demonstration of white-light generation.

Enhanced Photoluminescence and Random Lasing Emission in TiO2-Decorated FAPbBr3 Thin Films.

Herein, titanium-dioxide-decorated organic formamidinium lead bromide perovskite thin films grown by the one-step spin-coating method are studied. TiO2 nanoparticles are widespread in FAPbBr3 thin films, which changes the optical properties of the perovskite thin films effectively. Obvious reductions in the absorption and enhancements in the intensity of the photoluminescence spectra are observed. Over 6 nm, a blueshift of the photoluminescence emission peaks is observed due to 5.0 mg/mL TiO2 nanoparticle decoration in the thin films, which originates from the variation in the grain sizes of the perovskite thin films. Light intensity redistributions in perovskite thin films are measured by using a home-built confocal microscope, and the multiple scattering and weak localization of light are analyzed based on the scattering center of TiO2 nanoparticle clusters. Furthermore, random lasing emission with sharp emission peaks is achieved in the scattering perovskite thin films with a full width at the half maximum of 2.1 nm. The multiple scattering of light, the random reflection and reabsorption of light, and the coherent interaction of light within the TiO2 nanoparticle clusters play important roles in random lasing. This work could be used to improve the efficiency of photoluminescence and random lasing emissions, and it is promising in high-performance optoelectrical devices.

Reply to "Comment on 'Dependence of the Fluorescent Lifetime τ on the Concentration at High Dilution'": Extended Interpretation.

In "Comment on 'Dependence of the Fluorescent Lifetime τ on the Concentration at High Dilution'" by Anne Myers Kelley and David F. Kelley, the newly found concentration dependence of the fluorescence lifetime was attributed to the reabsorption and the delay of re-emission of fluorescence light. Accordingly, a comparably high optical density is required for a damping of the optically exciting light beam, causing a special profile for the re-emitted light with partial multiple reabsorption. However, an extended recalculation and re-examination on the basis of experimental spectra and the initially published data indicated an only static filtering effect by some reabsorption of fluorescent light. The resulting dynamic refluorescence is isotropically emitted in all room directions where only a small share of 0.006-0.6% adds to the measured primary fluorescence so that an interference in the measurement of fluorescent lifetimes becomes unimportant. Thus, the initially published data were further supported. The discrepancy between the two controversial papers could be resolved in terms of the different considered optical densities: a comparably high optical density can explain the interpretation of Kelley and Kelley, whereas low optical densities enabled by the use of the highly fluorescent perylene dye support our interpretation of the concentration dependence of the fluorescent lifetime.

Effects of titanium dioxide quantum dots on the color deviation and luminous flux of white light-emitting diodes

The application of <span>quantum dots has been considered as a promising approach to the advancement of phosphor-converted light-emitting diodes (pc-LEDs) since they perform an excellent extinction coefficient. Yet, it is challenging to manage their influences on the optical properties of LEDs due to their different nanometers in size. Hence, the object of this research is to analyze the influences of quantum dot (QDs) to figure out the solution to control the enhancement of LED lighting performances. Particularly, the study worked on investigating the scattering and absorption features of titanium dioxide (TiO<sub>2</sub>) QDs. It demonstrated that the radiant efficiency and luminous stability of the TiO<sub>2</sub> QDs-converted LEDs (QC-LEDs) was inferior due to the strong light absorption and reabsorption occurring inside the LED packages. Additionally, it also presented low uniformity of color distribution because the scattering ability of QDs is weak. Therefore, reducing the concentration of QDs when adding to the LED structure seems to be possible to enhance the luminous output of QC-LEDs. We propose 0.05% wt. TiO<sub>2</sub> for white LED to reduce the illumination losing caused by re-absorbent and total internal backscattering, resulting in approximate 31% lumen improvement and high color rendering index (CRI) measured at about 85, at a high color temperature of 7500 K</span>.

Luminescent Lead Halide Ionic Liquids for High-Spatial-Resolution Fast Neutron Imaging.

The fast neutron imaging technique with recoil proton detection harbors significant potential for imaging of thick, large-scale objects containing high-Z elements. However, the challenge to find efficient fast neutron scintillators with high spatial resolution is ongoing. The list of requirements for such scintillators is long and demanding: a proton-rich, scattering-free material combining high light yield with the absence of light reabsorption. To meet these challenges, we look for a suitable material among a rising class of 0D organic–inorganic Pb(II) halide hybrids. The use of large organic cations, e.g., trihexyltetradecylphosphonium, results in room-temperature ionic liquids that combine highly Stokes-shifted (up to 1.7 eV), reabsorption-free, and efficient emission (photoluminescence quantum yield up to 60%) from molecularly small and dense (PbX2 molar fraction up to 0.33) emitting centers. We investigate the optical properties of the resulting ionic liquids and showcase their utility as fast neutron imaging scintillators. Concomitantly with good light yield, such fast-neutron scintillators exhibit both higher spatial resolution and lower γ-ray sensitivity compared with commercial ZnS:Cu-based screens.

(Invited) The Application of ALD and ALE Passivated MicroLED Display

Micro LED has been engaged to the next generation display technology and evolved for various applications from several electronic manufacturers and institutions. The less than 10×10 μm2 area of micro-LEDs is desired for the high pixel per inch (PPI) of micro display but the sidewall effect from etching process will drop the quantum efficiency. Therefore, a good passivation process is essential to be introduced into micro-LED process to overcome the efficiency droop issue. The passivation technique of atomic layer deposition (ALD) enables to improve the sidewall damages from dry etching and hence the quantum efficiency of micro-LEDs can be enhanced [1, 2]. Furthermore, the improvement can be enhanced within smaller size of micro LEDs. In particular, due to the excellent uniformity and step coverage passivation by ALD Al2O3 thin films, the interest in efficient ALD equipment has gradually increased. The atomic layer etching (ALE) is an advance etching process that promises an excellent depth control in shallow device structures. This paper provides an overview of ALD and ALE process for improving the micro-LED performance.The epitaxial structure of the micro-LED structures was grown on a c-plane sapphire substrate using metal-organic chemical vapor deposition (MOCVD). Following the development of MOCVD, a transparent and ohmic p-contact of indium tin oxide (ITO) with a thickness of 100 nm was deposited by electron-beam evaporation as a transparent and ohmic p-contact. The ITO layer can result in efficient electrical current spreading and provide a transparent emission window for topside emitting devices. N-type GaN, p-type GaN, and the multiple quantum wells (MQWs) active region, which consists of twelve pairs of InGaN wells and a GaN barrier with an emission wavelength of around 445 nm, are the major epitaxial layers. The growth phases of micro-LED processing included mesa etching, ohmic contact metallization, sidewall passivation deposition using Al2O3 of thickness 21.3 nm and SiO2 of thickness 300 nm, and interconnects. The inductively coupled plasma (ICP) dry etching process is used for the mesa etching method, and SiO2 and Al2O3 passivation layers are deposited using PECVD and ALD techniques. Since Al2O3 and SiO2 are well-known dielectric materials for photonic applications, they were selected. Because of the field effect passivation produced by negative fixed charges, Al2O3 has emerged as a unique passivation material for commercial solar cells. The passivation layers tend to prevent surface recombination and reabsorption of emitted light near the etched surface during forward-biased activity. Following the deposition of the dielectric passivation layers, a hole was created using the ICP etching technique for the deposition of metal contacts. The electrode metal consisted of 50/300 nm of Ti/Au for p- and n- metal contacts deposited using electron-beam evaporation followed by rapid thermal annealing. The process flow of ALD passivated micro-LED is shown in Fig. 1(a).Fig. 1(b) showed the effect of the ALD-Al2O3 passivation technique on the EQE enhancement of µ-LEDs. According to the literature, the peak EQE of a small LED is considerably small due to the surface and nonradiative recombination of the mesa layer caused by plasma-assisted dry etching. ALD sidewall passivation is an important method for reducing plasma damage in LEDs. We can observe the 21.3 nm Al2O3 passivation on micro-LED in Fig.1(a). Therefore, Fig. 1 (b) presents the EQEs of 10 × 10-µm2 µ-LEDs with and without an Al2O3 passivation layer deposited using the ALD technology for reducing plasma damage. Compared with the device not subjected to ALD-Al2O3 passivation, the 10 × 10-µm2 array subjected to ALD-Al2O3 passivation exhibited an EQE enhancement of 37.5%. This result proves that ALD sidewall passivation can effectively reduce the sidewall damage caused by SRH nonradiative recombination and thus increase the EQE of small LEDs.

Canopy active fluorescence spectrum tracks ANPP changes upon irrigation treatments in soybean crop

Accurate estimation of aerial net primary production (ANPP) using remotely acquired data is one of the main challenges in both environmental monitoring and precision agriculture. Reflectance-based techniques have been widely used for decades, but detection of fluorescence emission by chlorophyll has emerged as a promising alternative in recent years. Although passive sun-induced fluorescence (SIF) monitoring has shown interesting results, the information it provides is limited to few wavelengths (Fraunhofer and telluric lines). On the other hand, active measurements of steady-state fluorescence and its spectral distribution cover the full-emission spectrum but have not been fully explored due to obvious experimental limitations. In this work we develop a novel active fluorescence measurement procedure, based on lamps and sensors mounted on a field tractor. This technique allowed the detection of the full spectrum of fluorescence emission of a plant crop for the first time in the literature.The main objective of this work was to analyze how the information based on reflectance and fluorescence, recorded by the new proposed methodology, tracks the differences caused by different irrigation treatments in the ANPP of three soybean varieties. We observed that reflectance-based vegetation indices showed limited sensitivity to these cumulative differences, as only EVI2, NDWI and SRWI were able to distinguish between rainfed and irrigation treatments in some few cases. Passive, irradiance-normalised SIF showed this same trend, but active fluorescence peak ratio (FRed/FFar-red) revealed statistically significant differences for the three cultivars studied. In addition, the latter showed a significant correlation with ANPP for two soybean varieties after correction for light re-absorption and scattering (p < 0.05, R2 > 0.5), which was observed for only EVI and foliar water status VIs among passive indicators. Active fluorescence measurements at leaf level by PAM fluorometry did not show differences between treatments in the upper part of the canopy but revealed a biomass-dependent decrease in PSII yield along the vertical axis. Our study demonstrated that fluorescence emission spectrum holds highly valuable information that might allow monitoring ANPP changes upon irrigation from remote sensing applications, and therefore should be carefully studied. Lastly, it highlights the potential of SIF retrieval at both O2-A and O2-B lines.

Reconciliation of dipole emission with detailed balance rates for the simulation of luminescence and photon recycling in perovskite solar cells.

A theoretical description of light emission, propagation and re-absorption in semiconductor multilayer stacks is derived based on the transverse Green's function of the electromagnetic field in the presence of a complex dielectric. The canonical dipole emission model is parametrized in terms of the local optical material constants and the local quasi-Fermi level splitting using the detailed balance relation between local absorption and emission rates. The framework obtained in this way is shown to reproduce the generalized Kirchhoff relations between the luminescent emission from metal halide perovskite slabs under uniform excitation and the slab absorptance of light with arbitrary angle of incidence. Use of the proper local density of transverse photon states in the local emission rate includes cavity effects in the generalized Planck law for internal spontaneous emission, which are neglected in the conventional Van Roosbroeck-Shockley formalism and avoids spurious divergencies due to non-radiative energy transfer via longitudinal modes. Finally, a consistent treatment of re-absorption provides the local rate of secondary photogeneration required for the consideration of photon recycling in an opto-electronic device simulator that includes the effects of charge transport.

Effects of sub-optimal illumination in plants. Comprehensive chlorophyll fluorescence analysis

The fluorescence signals emitted by chlorophyll molecules of plants is a promising non-destructive indicator of plant physiology due to its close link to photosynthesis. In this work, a deep photophysical study of chlorophyll fluorescence was provided, to assess the sub-optimal illumination effects on three plant species: L. sativa, A. hybridus and S. dendroideum. In all the cases, low light (LL) treatment induced an increase in pigment content. Fluorescence ratios – corrected by light reabsorption processes – remained constant, which suggested that photosystems stoichiometry was conserved. For all species and treatments, quantum yields of photophysical decay remained around 0.2, which meant that the maximum possible photosynthesis efficiency was about 0.8. L. sativa (C3) acclimated to low light illumination, displayed a strong increase in the LHC size and a net decrease in the photosynthetic efficiency. A. hybridus (C4) was not appreciably stressed by the low light availability whereas S. dendroideum (CAM), decreased its antenna and augmented the quantum yield of primary photochemistry. A novel approach to describe NPQ relaxation kinetics was also presented here and used to calculate typical deactivation times and amplitudes for NPQ components. LL acclimated L. sativa presented a much larger deactivation time for its state-transition-related quenching than the other species. Comprehensive fluorescence analysis allowed a deep study of the changes in the light-dependent reactions of photosynthesis upon low light illumination treatment.

Luminescent films employing quantum dot-cellulose nanocrystal hybrid nanomaterials

Colloidal quantum dot (QD) materials are promising candidates for luminescent solar concentrator (LSC) technology thanks to their broadband absorption above semiconductor bandgap and tunable Stokes-shift for minimizing the light reabsorption. Previously reported QDs in LSC contain non-polar organic ligands that can be easily incorporated into common host matrix/waveguide materials – polyacrylates or polycarbonates, however, these organic ligands on QD surfaces introduce problems when some new matrix materials are employed. For example, cellulose nanocrystal (CNC), a wood-derived eco-friendly material, has the potential for LSC, but no solvents are available to dissolve both QD and CNC simultaneously. Here we report high-quality luminescent films employing QD-CNC hybrid materials. Such films are produced from stable QD-CNC suspension by applying solution-phase exchange to replace non-polar organic ligands on QDs to the metal halide complex, as well as performing acetylation on CNC surfaces. The fabricated QD-CNC hybrid film keeps similar photoluminescence quantum yield and lifetime compared to as-synthesized QDs in non-polar organic solvents. The developed method in this work has the potential to be widely used to mix QD with other materials having polar functional groups.

Enhancement in greenish-white photoluminescence of Zn3(VO4)2 phosphor by Bi3+ doping

In this paper, we have explored the photoluminescence properties of rare-earth free Zn(3-x)Bix(VO4)2 (x = 0, 0.01, 0.03, 0.05) phosphors, synthesized by solution combustion method. The Rietveld refinement confirms that the prepared phosphors have an orthorhombic phase. The Bi3+ doping in the host supports the crystallite growth and reduction of microstrain. The absorption study ascertains the shift in the absorption edge with Bi3+ doping. The lowering of Urbach energy with Bi3+ doping in the host confirms the improvement of crystallinity. The prepared phosphors have near-Ultraviolet (UV) excitation of 350 nm, which reduces visible light reabsorption. The host has broadband emission centered on 540 nm and is enhanced by the factor of 11 after 1% doping of Bi3+ ions. The emission enhancement is attributed to the improved crystallinity and reduced microstrain. Thus, Bi3+ doped Zn3(VO4)2 phosphor can be used as a greenish-white emission source for optical devices.

Direct evaluation of self-quenching behavior of fluorophores at high concentrations using an evanescent field.

The quantum yield of a fluorophore is reduced when two or more identical fluorophores are in close proximity to each other. The study of protein folding or particle aggregation is can be done based on this above-mentioned phenomenon-called self-quenching. However, it is challenging to characterize the self-quenching of a fluorophore at high concentrations because of the inner filter effect, which involves depletion of excitation light and re-absorption of emission light. Herein, a novel method to directly evaluate the self-quenching behavior of fluorophores was developed. The evanescent field from an objective-type total internal reflection fluorescence (TIRF) microscope was used to reduce the path length of the excitation and emission light to ~100 nm, thereby supressing the inner filter effect. Fluorescence intensities of sulforhodamine B, fluorescein isothiocyanate (FITC), and calcein solutions with concentrations ranging from 1 μM to 50 mM were directly measured to evaluate the concentration required for 1000-fold degree of self-quenching and to examine the different mechanisms through which the fluorophores undergo self-quenching.

High-Performance Perovskite Solar Cells Based on NaCsWO3@ NaYF4@NaYF4:Yb,Er Upconversion Nanoparticles.

Extending photoelectric response to the near-infrared (NIR) region using upconversion luminescent (UCL) materials is one promising approach to obtain high-efficiency perovskite solar cells (PSCs). However, challenges remain due to the shortage of highly efficient UCL materials and device structure. NaCsWO3 nanocrystals exhibit near-infrared absorption arising from the local surface plasmon resonance (LSPR) effect, which can be used to boost the UCL of rare-earth-doped upconversion nanoparticles (UCNPs). In this study, using NaCsWO3 as the LSPR center, NaCsWO3@NaYF4@NaYF4:Yb,Er nanoparticles were synthesized and the UCL intensity could be enhanced by more than 124 times when the amount of NaCsWO3 was 2.8 mmol %. Then, such efficient UCNPs were not only doped into the hole transport layer but also used to modify the perovskite film in PSCs, resulting in the highest power conversion efficiency (PCE) reaching 18.89% (that of the control device was 16.01% and the PCE improvement was 17.99%). Possible factors for the improvement of PSCs were studied and analyzed. It is found that UCNPs can broaden the response range of PSCs to the NIR region due to the LSPR-enhanced UCL and increase the visible light reabsorption of PSCs due to the scattering and reflection effect, which generate more photocurrent in PSCs. In addition, UCNPs modify the perovskite film by effectively filling the holes and gaps at the grain boundary and eliminating the perovskite surface defects, which lead to less carrier recombination and then effectively improve the performance of PSC devices.

Output facet heating mechanism for uncoated high power long wave infrared quantum cascade lasers

Output facet temperatures of an uncoated high power continuous-wave quantum cascade laser (QCL) emitting at 8.5 μm were measured by using micro-Raman thermometry. The rate of the measured temperature changes with the injected electrical power increased from 6.5 K/W below the laser threshold to 12.3 K/W above the threshold. In addition, the measured temperature rise exceeded 220 K at an optical power of 0.9 W, well above the model projections based only on Joule heating. Facet oxidation was characterized via x-ray photoelectron spectroscopy measurements at incremental etch depths. While the oxidation reactions of InP and Ga were observed only at the surface level, the measured penetration of native Al2O3 was ∼24 nm. COMSOL thermal modeling demonstrated that light reabsorption by the native Al2O3 layer could well explain the additional temperature rise above the threshold. These results suggest that facet oxidation must be addressed to ensure the reliability of high-power long wave infrared QCLs.

Absorption-Enhanced Ultra-Thin Solar Cells Based on Horizontally Aligned p-i-n Nanowire Arrays.

A horizontally aligned GaAs p–i–n nanowire array solar cell is proposed and studied via coupled three-dimensional optoelectronic simulations. Benefiting from light-concentrating and light-trapping properties, the horizontal nanowire array yields a remarkable efficiency of 10.8% with a radius of 90 nm and a period of 5 radius, more than twice that of its thin-film counterpart with the same thickness. To further enhance the absorption, the nanowire array is placed on a low-refractive-index MgF2 substrate and capsulated in SiO2, which enables multiple reflection and reabsorption of light due to the refractive index difference between air/SiO2 and SiO2/MgF2. The absorption-enhancement structure increases the absorption over a broad wavelength range, resulting in a maximum conversion efficiency of 18%, 3.7 times higher than that of the thin-film counterpart, which is 3 times larger in GaAs material volume. This work may pave the way for the development of ultra-thin high-efficiency solar cells with very low material cost.

Influence of Surface Structure, Pigmentation and Particulate Matter on Plant Reflectance and Fluorescence.

Optical properties of plant leaves are relevant to evaluate their physiological state and stress effect. The main objective of this work was to study how variegation, pigment composition or reflective features modifies leaves' photophysical behavior. For this purpose, green leaves (Ficus benjamina), purple leaves (Tradescantia pallida), green leaves covered by white trichomes (Cineraria maritima) and variegated leaves (Codiaeum aucubifolium) were analyzed. Firstly, foliar surface morphology was evaluated by scanning electron microscopy. UV-vis and near-IR reflectance and transmittance spectra were obtained to calculate absorption (k) and scattering (s) coefficients. The theoretical approaches of Pile of Plates and Kubelka-Munk's theory resulted still valid for nonstandard leaves with differing surface conditions. However, frequently used spectral indices were not reliable for predicting water content, when leaves differed from conventional ones. The proportionality between the absorption factor and chromophore/pigment concentration was also lost for hairy leaves. A simplified model to describe these facts was presented here. Fluorescence spectra were recorded and corrected, due to light re-absorption. Water-optical parameter connection and pigment-optical parameter connection were thoroughly discussed. Leaf surface morphology and pigmentation have not only influenced the optical features of leaves but also played a role in the effect that particulate matter could cause on leaf photosynthesis.

Re-absorption and scattering of chlorophyll fluorescence in canopies: A revised approach

The measurement of chlorophyll fluorescence in remote way represents a tool that is becoming increasingly important in relation to the diagnosis of plant health and carbon budget on the planet. However, the detection of this emission is severely affected by distortions, due to processes of light re-absorption both in the leaf and in the canopy. Even though some advances have been made to correct the signal in the far-red, the whole spectral range needs to be addressed, in order to accurately assess plant physiological state. In 2018, we introduced a model to obtain fluorescence spectra at leaf level, from what was observed at canopy level. In this present work, we publish a revision of that physical model, with a more rigorous and exact mathematical treatment. In addition, multiple scattering between the soil and the canopy, and the fraction of land covered by vegetation have also been taken into consideration. We validate this model upon experimental measures, in three types of crops of agronomic interest (Pea, Rye grass and Maize) with different architecture. Our model accurately predicts both the shape of fluorescence spectra at leaf level from that measured at canopy level and the fluorescence ratio. Furthermore, not only do we eliminate artifacts affecting the spectral shape, but we are also able to calculate the quantum yield of fluorescence corrected for re-absorption, from the experimental quantum yield at canopy level. This represents an advance in the study of these systems because it offers the opportunity to make corrections for both the fluorescence ratio and the intensity of the observed fluorescence.