Main Reflection Peak Research Articles

Prediction of high-temperature infrared radiative properties of nickel foam ligaments

High-temperature radiative properties of open-cell foam ligaments are rather sparse due to measurement limitation. This study aims to numerically predict the infrared radiative properties of nickel foam ligaments at high temperatures. In this work, rough surfaces of ligaments are modeled with SEM scanning pictures, and optical properties of nickel components are simulated using ab initio molecular dynamics method at 298 K, 673 K, 873 K, 1073 K and 1273 K. The reflectivity of smooth surface is determined with empirical formulas at high temperatures. Moreover, the spectral radiative properties of nickel foam ligaments are predicted with finite difference time domain method at the waveband of 0.8–8.5 μm based on the modeled surfaces and high-temperature optical properties of components. It is found high temperature can marginally strengthen radiation scattering of the ligament surfaces, while having little effect on the distribution of radiation scattering. For instance, the directional-hemispherical reflectivity increases approximately linearly with temperature. As the incident wavelength grows, the directional-hemispherical reflectivity and proportion of specular reflection both show a rapid increase in the studied waveband, while the value of reflection main peak of directional-hemispherical reflectivity declines. With the growth of incident angle, the center position of main reflection peak is moving to the direction of increasing angles. Polynomial fit is applied to anticipate the relationship between proportion of specular reflection and incident wavelength with a RSME less than 0.02. This work demonstrates a practical method for obtaining the infrared radiative properties of open-cell metal foam ligaments at high temperatures.

Estimation model and its trade-off strategy of Mangifera persiciforma Colletotrichum gloeosporioides degree based on leaf reflection spectrum.

Colletotrichum gloeosporioides is one of the most common and serious fungal diseases of the tree Mangifera persiciforma. Yet we lack an effective method to evaluate this ecological interaction accurately. Here, we measured the functional traits and leaf reflectance spectrum of the host plants under different disease degrees. The findings provide a fast and efficient method for large-scale and high-precision monitoring of C. gloeosporioides in M. persiciforma stands. Using the collected leaf reflection data, we set up a prediction model of the optimal disease degree. Firstly, we found that leaf functional traits of M. persiciforma generally consisted of low leaf thickness, low relative chlorophyll content, small specific leaf area, high leaf tissue density, high dry matter content, low stomatal density, and large stomatal area. Secondly, leaf reflectivity increases with damage of C. gloeosporioides, which corresponds to five main reflection peaks and five absorption valleys in the spectral reflectance curve of leaves at the same positions (350-1800 nm). Thirdly, with the increase of infection degree, red edge slope and yellow edge slope decrease, while green peak reflectance, red valley reflectance, and blue edge slope all increase. Blue shift was detected in the red edge, green peak, and red valley, while red shift appeared at the blue edge and yellow edge. Finally, the best predictive model was that based on green peak reflectance (y=3.6396-0.0693x, R2=0.5149, RMSE [root-mean-square error] =0.2735), with an R2=0.92 and RMSE=0.0042 between its predicted vs. observed values. Because of its high inversion accuracy, the model can be used to predict the invasion conditions of M. persiciforma by C. gloeosporioides. Our study demonstrated that when plants are infected by C. gloeosporioides, there was a strong trade-off relationship between leaf functional traits. On the global leaf economics spectrum, the leaves tended toward the "slow investment-return" end when infected by C. gloeosporioides.

Response of plant reflectance spectrum to simulated dust deposition and its estimation model

To quantitatively reflect the relationship between dust and plant spectral reflectance. Dust from different sources in the city were selected to simulate the spectral characteristics of leaf dust. Taking Euonymus japonicus as the research object. Prediction model of leaf dust deposition was established based on spectral parameters. Results showed that among the three different dust pollutants, the reflection spectrum has 6 main reflection peaks and 7 main absorption valleys in 350–2500 nm. A steep reflection platform appears in the 692–763 nm band. In 760–1400 nm, the spectral reflectance gradually decreases with the increase of leaf dust coverage, and the variation range was coal dust > cement dust > pure soil dust. The spectral reflectance in 680–740 nm gradually decreases with the increase of leaf dust coverage. In the near infrared band, the fluctuation amplitude and slope of its first derivative spectrum gradually decrease with the increase of leaf dust. The biggest amplitude of variation was cement dust. With the increase of dust retention, the red edge position generally moves towards short wave direction, and the red edge slope generally decreases. The blue edge position moved to the short wave direction first and then to the long side direction, while the blue edge slope generally shows a decreasing trend. The yellow edge position moved to the long wave direction first and then to the short wave direction (coal dust, cement dust), and generally moved to the long side direction (pure soil dust). The yellow edge slope increases first and then decreases. The R2 values of the determination coefficients of the dust deposition prediction model have reached significant levels, which indicated that there was a relatively stable correlation between the spectral reflectance and dust deposition. The best prediction model of leaf dust deposition was leaf water content index model (y = 1.5019x − 1.4791, R2 = 0.7091, RMSE = 0.9725).

Spectral Reflectance Characteristics and Chlorophyll Content Estimation Model of Quercus aquifolioides Leaves at Different Altitudes in Sejila Mountain

Quercus aquifolioides is one of the most representative broad-leaved plants in Qinghai-Tibet Plateau with important ecological status. So far, understanding how to quickly estimate the chlorophyll content of plants in plateau areas is still an urgent problem. Field Spec 3 spectrometer was used to measure hyperspectral reflectance data of Quercus aquifolioides leaves at different altitudes, and CCI (chlorophyll relative content) of corresponding leaves was measured by a chlorophyll meter. The correlation and univariate linear fitting analysis techniques were used to establish their relationship models. The results showed that: (1) Chlorophyll relative content of Quercus aquifolioides, under different altitude gradients, were significantly different. From 2905 m to 3500 m, chlorophyll relative content increased first and then decreased. Altitude 3300 m was the most suitable growth area. (2) In 350~550 nm, the spectral reflectance was 3500 m > 3300 m > 2905 m. In 750~1100 nm, the spectral reflectivity was 2905 m > 3500 m > 3300 m. (3) There were 4 main reflection peaks and 5 main absorption valleys in the leaf surface spectral reflection curve. While, 750~1400 nm was the sensitive range of leaf spectral response of Quercus aquifolioides. (4) The red edge position and red valley position moved to short wave direction with the increase of altitude, while the yellow edge position and green peak position moved to long wave direction first and then to short wave direction. (5) The correlation curve between the original spectrum and the CCI value was the best between the wavelengths 509~650 nm. The correlation between the first derivative spectrum and CCI value was the best and most stable at 450~500 nm. The green peak reflectance was most sensitive to the relative chlorophyll content of Quercus aquifolioides. The estimation model R2 of green peak reflectance was the highest (y = 206.98e−10.85x, R2 = 0.8523), and the prediction accuracy was 95.85%. The research results can provide some technical and theoretical support for the protection of natural Quercus aquifolioides forests in Tibet.

Flexible nanofiber-coupled hybrid plasmonic Bragg grating.

We report a hybrid plasmonic Bragg grating composed of a nanofiber coupled with orthogonally oriented metal strips. Numerical simulations are performed to study the transmission and reflection spectra of the grating. It shows that the TM polarization has much stronger Bragg reflection due to the excitation of hybrid plasmonic modes. The dependence of reflection peaks on several key device parameters is analyzed. Light propagation simulation further reveals that both fundamental and first-order TM modes are excited upon Bragg reflection, leading to two separate peaks in the spectrum. We implement the prototype device by attaching a nanofiber onto the surface of an array of sub-micrometer-wide metal strips. The main reflection peak is measured to have a 3-dB bandwidth of 15 nm and out-of-band rejection of more than 30 dB. The effects of nanofiber radius, alignment angle and coupling length on the device performance are also experimentally investigated.

Analysis of the Spectrum Distortions of Weak Fiber Bragg Gratings Fabricated In-Line on a Draw Tower by the Phase Mask Technique

Identical weak optical fiber Bragg grating (FBG) arrays can be fabricated in-line on a draw tower using the phase mask technique. On the FBG arrays, some grating spectrum distortions, for example, asymmetric side lobes or deformed side lobes, low extinction ratio of the main reflection peak and comb filter-type peaks, can be observed. We collected different distorted reflection spectra of the in-line-fabricated weak FBGs and compared them with the simulated spectra of weak FBGs. By comparing the experimental and theoretical weak FBG spectra, it is found that the different kinds of spectrum distortions are caused by phase shifting, asymmetric apodization and nonlinear chirp. This analysis is helpful to improve the FBG array fabricating process by phase mask in-line on a draw tower. And we got good FBG arrays which have good central wavelength consistency, good reflectivity consistency and minor spectral distortion.

Measurement and modelization of silica opal reflection properties: Optical determination of the silica index

Self-assembled artificial opals (in particular silica opals) constitute a model system to study the optical properties of three-dimensional photonic crystals. The silica optical index is a key parameter to correctly describe an opal but is difficult to measure at the submicrometer scale and usually treated as a free parameter. Here, we propose a method to extract the silica index from the opal reflection spectra and we validate it by comparison with two independent methods based on infrared measurements. We show that this index gives a correct description of the opal reflection spectra, either by a band structure or by a Bragg approximation. In particular, we are able to provide explanations in quantitative agreement with the measurements for two features : the observation of a second reflection peak in specular direction, and the quasicollapse of the $p$-polarized main reflection peak at a typical angle of 54${}^{\ensuremath{\circ}}$.

Spectrophotometric comparison of translucent composites and natural enamel

Objectives To compare the optical characters of four translucent composites and natural enamel. Methods Thirty natural enamel slabs and 120 composite replicas ( n = 30) using four brands of translucent composites (Polofil Supra, Brilliant Esthetic, Gradia Direct, and Vit-l-escence) were evaluated at the thicknesses of 1.0 mm and 0.8 mm. The colors of the enamel slabs or corresponding composite specimens placed on an A3 shade, white and black backgrounds were measured using a spectrophotometer. Color differences (Δ E*) of the enamel–composite pairs and translucency parameter (TP) of each specimen were calculated. Reflection spectrums were recorded in the wavelength from 380 nm to 780 nm. Paired- t tests were performed to evaluate the differences of color coordinates ( L*, a*, and b*) and TP values between the translucent composites and natural enamel. Results There were significant differences of color coordinates ( L*, a*, and b*) between the enamel and translucent composites ( P < 0.05). Although no statistical difference of TP values were found in the enamel–composite pairs with Polofil Supra and Brilliant Esthetic composites. The main peaks of the reflectance spectrums of the enamel are different from the four brands of the translucent composites. A reddish shifting of the main reflection peaks was observed, while the thickness of the composite specimens decreasing from 1.0 mm to 0.8 mm. Whereas, the main reflection peak was not changed in the teeth enamel. Conclusions The color and the translucency of translucent composites are different from the teeth enamel.

High number of wavelength channels demonstrated by a widely tunable sampled-grating DBR laser

A three-section sampled-grating DBR laser has been realised. It presents a 25 nm tuning range with an SMSR greater than 30 dB. For the first time with sampled reflectors, access to 22 wavelength channels is achieved thanks to discontinuous tuning among the five main reflection peaks of the comb-reflection spectra of the sampled gratings. Operation of this device under amplitude modulation is also discussed: an AM bandwidth greater than 2.5 GHz has been demonstrated.

Optical properties of photosensitive fiber phase gratings

The authors measure and calculate the optical properties of strong ( delta n/n approximately 10/sup -3/) in-core optical fiber phase gratings written by the UV laser sidewriting technique. A pronounced fine structure on the main reflection peak is observed and explained, along with a short wavelength loss associated with radiation mode coupling, modulated by cladding effects. >

Fine structure in Raman spectroscopy and X-ray reflectometry and its uses for characterization of superlattices

Fine structure in the folded longitudinal acoustic (FLA) phonon peaks from high-resolution Raman spectra of Si/Si1−xGex superlattices has been observed. The observed FLA fine structure is attributed to fluctuations in the superlattice periodicity. A 0.2 cm−1 splitting in the FLA peaks is observed in the Raman spectrum of a 15-period superlattice. According to the Rytov theory for the FLA phonons, the peak splitting corresponds to a change in period of about four monolayers (~0.5 nm) during growth. X-ray reflectometry on these samples also shows a similar doublet formation on the main reflection peaks, consistent with a 2–3 monolayer period fluctuation. A test sample with a built-in period variation was also well characterized by both techniques. The good agreement between the two techniques, as well as between the theories and the experiments, demonstrates that both techniques are capable of high precision and allow detailed and quantitative characterization of the superlattice perfection. In addition to determining the phonon velocity, dispersion relation, layer thickness, period, and composition of the superlattices, these results demonstrate the possibility of measuring the fluctuation in the period and the interface quality, as well as the numbers of given periods in some special cases.

Effects of Heat Treatment on Infra-red Reflection Spectra of Phase Separable PbO-B&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-SiO&lt;sub&gt;2&lt;/sub&gt; Frit

Effects of various heat treatments on the peak shift and the change of contour of infra-red spectra of frits in PbO-B2O3-SiO2 system were investigated. The infra-red reflection peak due to Si-O-Si streching vibration in the range of 900-1300cm-1 shifted to higher wave number and the width of it decreased rapidly in the transformation range.When the temperature was elevated to the deformation range the contour of the band became broad again. The peak shift to higher wave number was attributed to the increase of Si-O-Si bond strength in the silica rich phase after phase separation. It was considered that the decrease of peak width in the transformation range was due to the temporary good arrangement of Si-O-Si bond during phase separation. The increase of peak width in the deformation range was dependent on the development of the shoulder at 1050cm-1 and 1150cm-1 of the main reflection peak.

Studies on Structure of Silicate Glaze by Infrared Reflection Method

The structure of silicate glaze was studied by infrared reflection method.The results obtained were as follows:1. The main reflection peak (about 1100cm-1) of silicate glasses shifted to lower wave number with the increse of content of basic and aluminium ions contained in the glass structure.The effect of alkali, alkali earths, and aluminium ions on the Si-O bond shift became more remarkable with the increase of ionic valency and radius.The causes of shift to lower wave number of the infrared reflection peak were considered to be as follows.a. By the electrostatic interaction between cation and oxygen atom, the Si-O bond strength was decreased in the silicate glass structure as compared with silica glass.b. The increase of number of cation in the structure of silicate glass extends the mean Si-O linkage of random network, and lengthens the atomic distance of Si-O bond, therefore weakens the Si-O field strength.2. The cation entered into the structure of silicate glass broaded the peak, because the cation extends the network structure and consequently the field strength of Si-O bond becomes random.That effect increases with the increase of electrostatic interaction between cation and oxygen atom.3. The peak shifts to lower wave number, with the increase of number of cation in the structure of silicate glass.But in the case of precipitating crystal in the glaze, wave number shifted to higher, for the reason that the effect of crystallization is dominant to peak shift and the field strength of Si-O bond increases.