Red Spectral Region Research Articles

Spectral harmonization and red edge prediction of Landsat-8 to Sentinel-2 using land cover optimized multivariate regressors

Multi-sensor remote sensing applications consistently gain importance, boosted by a growing number of freely available earth observation data, increasing computing capacity, and increasingly complex algorithms that need as temporally dense data as possible. Using data provided by different sensors can greatly improve the temporal resolution of time series, fill data gaps and thus improve the quality of land cover monitoring applications. However, multi-sensor approaches are often adversely affected by different spectral characteristics of the sensing instruments, leading to inconsistencies in downstream products. Spectral harmonization, i.e., the transformation of one sensor into the spectral domain of another sensor, may reduce these inconsistencies. It simplifies workflows, increases the reliability of subsequently derived multi-sensor products and may also enable the generation of new products that are not possible with the initial spectral definition. In this paper, we compare the effect of multivariate spectral harmonization techniques on the inter-sensor reflectance consistency and derived products such as spectral indices or land cover classifications. We simulated surface reflectance data of Landsat-8 and Sentinel-2A from airborne hyperspectral data to eliminate any sources of error originating from unequal acquisition geometries, illumination or atmospheric state. We evaluate different methods based on linear, quadratic and random forest regression as well as linear interpolation, and predict not only matching but also unilaterally missing bands (red edge). We additionally consider material-dependent spectral characteristics in the harmonization process by using separate transformation functions for spectral clusters of the input dataset. Our results suggest that spectral harmonization is useful to improve multi-sensor consistency of remote sensing data and subsequently derived products, especially if multiple transformation functions are incorporated. There is a strong dependency between harmonization performance and the similarity of source and target sensor's spectral characteristics. For spectrally transforming Landsat-8 to Sentinel-2A, we achieved the lowest radiometric inter-sensor deviations with 50 spectral clusters and linear regression. Based on simulated data, deviations are below 1.7% reflectance within the red edge spectral region and below 0.3% reflectance for the remaining bands (RMSE). Regarding spectral indices, our results show a reduction of inter-sensor deviation (vegetation pixels only) to 38% of the initial error for NDVI (Normalized Difference Vegetation Index) and to 43% for EVI (Enhanced Vegetation Index). Furthermore, we computed the REIP (Red Edge Inflection Point) with an accuracy of 3.1 nm from Sentinel-2 adapted Landsat-8 data. An exemplary multispectral classification use case revealed an increasing inter-sensor consistency of classification results from 92.3% to 97.3% mean error. Applied to time series of real Landsat-8 and Sentinel-2 data, we observed similar trends, albeit intermingled with non-sensor-induced inconsistencies.

White Light Emission from Pr, Dy Doped ZnSiCa Glass

Background:: Luminescence glass is a potential candidate for developing white light emitting diode (W-LED) due to its good rare earth ion solubility, efficient luminescence, easy fabrication and good mold ability. Pr3+ ion has various visible emission bands from blue to red spectral region, and has attracted considerable attention for potential application to LEDs, ultraviolet laser, and scintillator. The Dy3+ ions can exhibit emission in blue and greenish-yellow (4F9/2→6H15/2, 13/2 transitions of Dy3+) spectral regions under excitation at near UV (ultraviolet). It is possible to obtain white luminescence if Pr3+ ions and Dy3+ ions can be excited simultaneous, due to their multiple luminescence in the visible region. Methods: The Pr, Dy doped ZnSiCa glass samples were prepared by the conventional melting quenching procedure. The XRD, absorption spectra, emission spectra, and ICP-OES measurements were performed to investigate the properties of the materials. Results: The Pr and Dy co-doped ZnSiCa glasses under 443 nm excitation show emission band peaking at about 483 nm, 575 nm and 670 nm / 676 nm. The glass samples exhibit chromaticity coordinates in the white light region in the CIE 1931 diagram, with a Correlated Color Temperature (CCT) at about 7000 K. Conclusion: Pr, Dy codoped ZnSiCa glass samples show chromaticity coordinates in the white light region in the CIE 1931 diagram, with a CCT at about 7000 K. It is suggested that the Pr, Dy codoped ZnSiCa glasses might be considered as promising candidates for white light emitting sources.

Asymmetrical nebula of the M33 variable GR290 (WR/LBV)

Context. GR 290 (M 33 V0532 = Romano’s star) is a luminous M 33 object undergoing photometric variability typical for luminous blue variable stars. It lies inside the Wolf-Rayet region in the Hertzsprung-Russell diagram and possesses a WN8-type spectrum at the light minima. Analysis of Gran Telescopio Canarias spectra obtained in 2016 led to the conclusion that it is surrounded by an unresolved H II region formed mostly of ejected material from the central star and revealed the presence of a second, more extended asymmetrical emission region. Aims. The aim of this paper is to further explore the structure of the nearby environment of GR 290. Methods. Long-slit spectra of GR 290 were obtained with three slit orientations in the visual and red spectral regions. The emission-line distribution for each slit was analyzed. Results. We confirm the presence of an asymmetric H II region that extends ~50 pc to the south; ~30 pc to the north, and southeast; ~20 pc to the east and northwest; and ~10 pc to the west. We also present the first spectrum to be acquired of a star belonging to the neighboring OB 88 association, J013501.87+304157.3, which we classify as a B-type supergiant with a possible binary companion.

Hot-Band Anti-Stokes Fluorescence Properties of Alexa Fluor 568

Hot-band absorption and anti-Stokes emission properties of an organic fluorescent dye, Alexa Fluor 568, were characterized and compared with those of Rhodamine 101. The comparison of the properties (e.g., quantum efficiency, spectral distribution, thermal properties, and fluorescence lifetime) between the two dyes confirms that both dyes undergo the same process when excited in the red spectral region. Possible undesirable crosstalk effects and applications in dSTORM microscopy were demonstrated and discussed.

Satellite Ocean Color Based Harmful Algal Bloom Indicators for Aquaculture Decision Support in the Southern Benguela

The aquaculture industry of southern Africa faces environmental threats from harmful algal blooms (HABs), which have the potential to cause devastating economic losses (Pitcher et al 2019). Satellite earth observation offers a systematic and cost effective method for operational monitoring of HABs over large areas. Whilst the chlorophyll-a concentration ([Chl-a]) product, often used as a proxy for phytoplankton biomass, can be used to indicate high biomass blooms (elevated biomass against a background signal of 5 - 10 mgChl m$^{-3}$), there is a clear need for value-added products that not only alert on bloom presence, but also on the bloom type and persistence. This study demonstrates the identification of different phytoplankton communities that can feasibly be identified in bloom concentrations from space, relevant to the aquaculture industry of South Africa. In terms of water-leaving reflectance, 76 % of the variance in the red and NIR spectral region is significantly positively correlated to phytoplankton abundance, [Chl-a], and the maximum line height (MLH) (defined as the height of the maximum reflectance peak above a baseline between 665 and 753 nm). The MLH is related to dominant phytoplankton types derived from phytoplankton count data, in order to identify thresholds which represent blooms that pose a high hypoxia and/or toxicity risk; whilst 0.00160.003 has a strong likelihood of indicating high biomass dinoflagellate or Pseudo-nitzschia blooms. These techniques are routinely used by the aquaculture industry in South Africa for decision support and risk mitigation. The high biomass nature of the South African regional waters provide strong assemblage-related spectral variability, which can be exploited with the spectral bands of OLCI and MERIS. Application to these sensors not only ensures future monitoring capability, but also allows the creation of a historical risk climatology that can guide the site selection of industries sensitive to the presence of HABs, such as aquaculture facilities and desalination plants.

Radiation-induced processes in alkali-alumina-borate glass-ceramics doped with Cr3+ ions

Alkali-alumina-borate glass-ceramics doped with Cr3+ ions are successfully synthesized by the melt quenching technique. LiAl7B4O17:Cr3+ nanocrystals are formed in the glass host during the subsequent one- and two-stage isothermal treatments above the crystallization temperature of the nanophase. For the samples of the initial and heat-treated glass, absorption spectra are obtained containing two absorption bands that are shifted to the short-wave region after the heat treatment. The impact of the electron flux (absorbed dose of 300 kGy) leads to an increase in the optical density and color center formation. There are a wide emission band appears in the blue-green region of the spectrum (λ = 463 nm), the nature of which is possibly due to the luminescence of the borate host and series of R lines in the chromium ions in the cathodoluminescence (CL) spectra of heat-treated glass-ceramics. The optimal annealing time is determined at which the maximum value of the absolute quantum yield reaches of ≈ 40%. The luminescence decay kinetics at 463 nm is fitted by the sum of the exponential and the hyperbolic approximation. The decay time for all samples is about 200 µs. In the red spectral region (λmax = 699 nm), the luminescence decay kinetics is complex and is described by the sum of 3 exponentials and a second-order hyperbola with decay times of 0.3; 5 ± 1 ms; 10 – 27 ms, respectively. The dynamics of the growth of the decay time with increasing annealing time is manifested. Issues related to the nature of possible processes after pulse electronic excitation are discussed.

Luminescence and up-conversion properties in La2Ti2O7:Eu3+,Er3+ oxides under UV and NIR radiations towards a two-color sensor

La2-x-yEuxEryTi2O7 oxides have an emission color which depends on the excitation wavelength. Under the UV excitation (310–393 nm), they emit in the red spectral region and, under NIR excitation (785–980 nm), they exhibit a green emission induced by upconversion luminescence from Er3+ ions. The structural and luminescence properties of these oxides synthesized by the solid state reaction method were studied according to the x and y values. The co-substituted oxides are obtained over a composition zone between x ≤ 0.2 and y ≤ 0.05. Beyond these values, a phase mixture consisting of a monoclinic phase (layered-perovskite) and cubic phase (pyrochlore) is observed. Under the UV excitation, the reddish emission color is mainly induced by the Eu3+ ions via an exchange interaction mechanism. The quenching concentration of Eu3+ ions is dependent on the excitation wavelength: x = 0.3 and 0.6, at 310 and 377 nm, respectively. With the measurements of photoluminescence decay curves, the lifetime kinetics was linked to the Eu3+ ions concentration. The CIE 1931 color coordinates were also determined according to the x and y values. This one shows linear evolutions when x is fixed and y is variable and vice versa. The good chemical and thermal stability of these oxides also make it possible to disperse them effectively in a glass of lithium tetraborate. Only a 2% transmittance drop is observed for the insertion of 2 wt. % of the oxide. The upconversion luminescence properties were also measured under NIR excitation and shown an emission in the green spectral region mainly due to Er3+ ions. The study of emission intensity dependence on the laser power led to the conclusion of a two-photon excitation process. Temperature-dependent upconversion emission follows a Mott-Seitz model. The activation energy for 4F9/2 → 4I15/2 quenching was found to be 52.8 meV.

Photoluminescence of Eu3+-doped CaZrO3 red-emitting phosphors synthesized via microwave-assisted hydrothermal method

Eu-doped 1, 3 and 5 % CaZrO3 red-emitting phosphors were synthesized for the first time using the microwave-assisted hydrothermal method followed by heat treatment at 1200 °C. After the crystallization, the orthorhombic CaZrO3 and the non-stoichiometric cubic Ca0.15Zr0.85O 1.85 phases were identified for all samples, and the portion of the non-stoichiometric phase increases as the Eu3+ concentration increases. The excitation spectra of the phosphors display Eu3+f-f excitation bands (namely at 395 nm and 464 nm) and the Eu3+→O2- charge transfer band at 250 nm, confirming that the CaZrO3 host acts as sensitizer for the Eu3+ luminescence. Likewise, the emission spectra feature the typical Eu3+ emission within the red spectral region, displaying CIE color coordinates of (0.642; 0.332), (0.663; 0.330) and (0.668; 0.327) for the 1, 3 and 5 % doped samples, respectively, indicating high emission color purity. By using Eu3+ as spectroscopic probe, the high-resolution selective emission spectra (14 K) confirm that Eu3+ replaces Ca2+ and Zr4+ local sites in both CaZrO3 and the Ca0.15Zr0.85O1.85 phases. Finally, the high absolute emission quantum yield of the 3 %-doped phosphor found at 393 nm (0.29 ± 0.03) confirms its potential application as red-emitting phosphor, especially in rapid identification of latent fingerprints, security ink or as red-emitting coating of near-UV LEDs for WLED applications.

Eu2+-doped ultra-broadband VIS-NIR emitting phosphor

The discovery of ultra-broadband VIS-NIR emission phosphors is a major challenge for miniature/portable devices in special applications like bio-sensing. In different solid materials, the Eu2+ emission band usually covers the blue to red spectral region. The near infrared spectral range is extremely difficult to obtain with the activator Eu2+, even in nitride host materials. In this work, we design and report a record-breaking oxide-based phosphor, Ba3ScB3O9:Eu2+, which yields VIS-NIR light (510–1100 nm) in response to ultraviolet-blue light excitation. Under a preferred excitation at 376 nm, Ba3ScB3O9:Eu2+ shows a broadband long-wavelength emission with the main peak, full width at half maximum and Stokes shift of 735 nm, 205 nm and 12991 cm−1, respectively. The spectral position and band width can be rationalized by combining the nephelauxetic effect and crystal field. By covering the title phosphor on a blue LED chip (450 nm), a VIS-NIR pc-LED containing Eu2+ emission with NIR out power of 2.3 mW@100 mA and photoelectric efficiency of 9.3%@100 mA is obtained. According to ultra-broadband emission in visible-near infrared region, application in LEDs for real-time non-destructive examination occurs promising.

Synthesis of bio-functional nanoparticles from sono-responsive amino acids using high frequency ultrasound.

A simple, one-pot high frequency ultrasonication (490kHz) methodology to convert hydrophobic and amphipathic amino acids into nanostructures was investigated. The approach involved the oxidative coupling of aromatic amino acids (phenylalanine and tryptophan) in aqueous solutions to form high molecular weight dimers and oligomers. The role of cavitation bubble surface and ultrasonic power to trigger the out-of-equilibrium self-assembly of dimers and trimers to spherical and uniform nanostructures with controlled size has been discussed. The synthesized particles exhibited fluorescence in blue, green and red spectral regions and a strong antioxidant activity.

New conjugated polymer nanoparticles with high photoluminescence quantum yields for far-red and near infrared fluorescence bioimaging

The development of new aqueous conjugated polymer nanoparticles with high photoluminescence quantum yields (PLQYs) at the far red and near infrared (NIR) spectral regions (>650 nm) as alternative polymer probes for fluorescence imaging is reported.

Reactivity of ZrO(MFP) and ZrO(RP) Nanoparticles with LnCl3 for Solvatochromic Luminescence Modification and pH-Dependent Optical Sensing.

The luminescence of the inorganic–organic hybrid nanoparticles ZrO(MFP) (MFP=methylfluorescein phosphate) and ZrO(RP) (RP=resorufin phosphate) was modified by addition of different rare earth halides LnCl3. The resulting composite materials form dispersible nanoparticles that exhibit modified nanoparticle fluorescence depending on the rare earth ion. The resulting chromaticity of the luminescence is further variable by the employment of different solvents for ZrO(MFP)‐based composite systems. The strong solvatochromic effect of the MFP chromophore leads to different luminescence chromaticities of the composite materials between green, yellow, and blue in THF, toluene, and dichloromethane, respectively. The luminescence of ZrO(RP)‐based composite particles can be modified between the red and blue spectral regions in dependence on the applied reaction temperature. Beside a luminescence shift that is derived from nanoparticle modification by LnCl3, a strong turn‐on effect of ZrO(RP) particles results after contact with different Brønsted acids and bases in combination with a respective chromaticity shift. Both effects enable the potential employment of such particles as highly sensitive optical pH sensors.

Композитные наночастицы на основе лексанполимерной матрицы, иттербиевых комплексов порфиринов и оксида железа для БИК-люминесцентной диагностики и тераностики новообразований

Описаны методика синтеза и спектрально-люминесцентные свойства наночастиц, содержащих иттербиевый комплекс диметилового эфира протопорфирина IX в полимерной матрице на основе лексана, перспективных для использования в диагностике новообразований. Наночастицы обладают увеличенным временем жизни люминесценции в ближней инфракрасной области 700–1100 нм и практически полным отсутствием флуоресценции свободного основания порфирина в красной области спектра, что показывает перспективность использования полученных наночастиц для диагностических целей. Для целей тераностики проведен синтез многофункциональных наночастиц, содержащих ядро оксидов железа и лексан-полимерную оболочку, включающую в себя иттербиевый комплекс тетраметилового эфира гематопорфирина IX. Предполагается, что наночастицы оксидов железа ответственны за проведение процедуры локальной гипертермии.

ZnO thin films synthesis by RF magnetron sputtering deposition

Investigation of ZnO thin films synthesis using RF magnetron sputtering deposition in the Ar/O2 plasma gas mixture at different O2 composition and at different growth temperatures is presented. The effect growth process on structural (morphology and orientation films, grain sizes, lattice parameters) and optical (transmittance, absorption, refractive index, photoluminescence) properties are examined. It is shown that synthesized ZnO thin films at a temperature from 200 to 300 °C are polycrystal and textured. Notice that ZnO thin films, which are synthesized using pure Ar plasma gas, are porosity. The optical transmittance and absorption spectra have an absorption edge near 380 nm. The PL spectra of synthesized ZnO films show exciton peaks at 370-400 nm and the wide emission at the yellow and red spectral regions.

Calibration of the Pulse Signal Decay Effect of Full-Waveform Hyperspectral LiDAR.

Full-waveform hyperspectral LiDAR (FWHSL) is able to obtain spectral and spatial information by utilizing a single instrument, and it has become more and more commonly used in vertical distribution studies of structural and biochemical characteristics of vegetation. However, the pulse-echo arrival times of multiple spectral channels of the FWHSL are not consistent and this causes range ambiguity in spectral channels. In this paper, the pulse signal decay effect on range measurements was studied by measuring the varying trends of pulse signal decay between spectral channels with different material properties. The experiments were repeated at different distances. All of the spectral channels were compared for different materials. The results suggest that the channels in the red edge spectral region of vegetation have good stability and accuracy for range measurements of varied distance and materials properties. Finally, based on the geometric invariability in a specific red edge channel, a practical calibration approach for the pulse signal decay effect is also presented. The validation tests showed it could improve the pulse signal decay effect of full-waveform hyperspectral LiDAR.

Fast, Infrared-Active Optical Transistors Based on Dye-Sensitized CdSe Nanocrystals.

We report an optically gated transistor composed of CdSe nanocrystals (NCs), sensitized with the dye zinc β-tetraaminophthalocyanine for operation in the first telecom window. This device shows a high ON/OFF ratio of 6 orders of magnitude in the red spectral region and an unprecedented 4.5 orders of magnitude at 847 nm. By transient absorption spectroscopy, we reveal that this unexpected infrared sensitivity is due to electron transfer from the dye to the CdSe NCs within 5 ps. We show by time-resolved photocurrent measurements that this enables fast rise times during near-infrared optical gating of 47 ± 11 ns. Electronic coupling and accelerated nonradiative recombination of charge carriers at the interface between the dye and the CdSe NCs are further corroborated by steady-state and time-resolved photoluminescence measurements. Field-effect transistor measurements indicate that the increase in photocurrent upon laser illumination is mainly due to the increase in the carrier concentration while the mobility remains unchanged. Our results illustrate that organic dyes as ligands for NCs invoke new optoelectronic functionalities, such as fast optical gating at sub-bandgap optical excitation energies.

Colloidal CdSe Quantum Wells with Graded Shell Composition for Low-Threshold Amplified Spontaneous Emission and Highly Efficient Electroluminescence.

Semiconductor nanoplatelets (NPLs) have emerged as a very promising class of colloidal nanocrystals for light-emitting devices owing to their quantum-well-like electronic and optical characteristics. However, their lower photoluminescence quantum yield (PLQY) and limited stability have hampered the realization of their outstanding luminescent properties in device applications. Here, to address these deficiencies, we present a two-step synthetic approach that enables the synthesis of core/shell NPLs with precisely controlled shell composition for engineering their excitonic properties. The proposed CdSe colloidal quantum wells possess a graded shell, which is composed of a CdS buffer layer and a CdxZn1-xS gradient layer, and exhibit bright emission (PLQY 75-89%) in the red spectral region (634-648 nm) with a narrow emission line width (21 nm). These enhanced optical properties allowed us to attain low thresholds for amplified spontaneous emission (down to ∼40 μJ/cm2) under nanosecond laser excitation. We also studied the electroluminescent performance of these NPLs by fabricating solution-processed light-emitting diodes (LEDs). In comparison to NPL-LEDs with CdSe/CdS core/shell NPLs, which exhibit an external quantum efficiency (EQE) value of only 1.80%, a significantly improved EQE value of 9.92% was obtained using graded-shell NPLs, the highest value for colloidal NPL-based-LEDs. In addition, the low efficiency roll-off characteristics of NPL-LEDs enabled a high brightness of up to ∼46 000 cd/m2 with an electroluminescence peak centered at 650 nm. These findings demonstrate the paramount role that heterostructure engineering occupies in enhancing the optoelectronic characteristics of semiconductor NPLs toward practically relevant levels.

Evaluation of the Recent Atomic Data for Fe i Lines Based on Solar and Stellar Spectra

We performed an extensive comparative analysis of the recent experimental data on Fe i transition probabilities (TP) based on the observed solar and stellar spectra. This work is part of the Vienna Atomic Line database (VALD) activities. Our motivation is to keep the VALD line list as complete as possible and provide VALD users with substantiated recommendations for the most accurate atomic data. For assessment of data quality we choose the “normal” (as opposed to peculiar and/or magnetic) Main Sequence sharp-line stars of different effective temperatures with accurately known atmospheric parameters: Sun, Procyon, HD 32115, and 21 Peg. Theoretical spectra of these stars were synthesized for 1D plane-parallel model atmospheres accounting for non-local thermodynamic equilibrium (NLTE) effects, and then they were compared with the observations. For all of the stars we derived atmospheric abundances based on the new and previous experimental TP. When using the new TPs the best agreement for iron is achieved with the TP of the Hannover group (Bard, Kock, and Kock, Astron. Astrophys.248, 315, 1991; Bard and Kock, Astron. Astrophys.282, 1014, 1994). The Hannover set is recommended as the primary source of transition probabilities in stellar abundance analysis. The new TP data significantly increase the number of spectral lines of Fe i in the optical and red spectral regions available for accurate abundance analysis of stars in a wide range of temperatures and metallicities. Comparison with the observed stellar spectra invalidates some new experimental data despite the small given uncertainty of the laboratory measurements. Finally, we note the importance of accurate line-broadening data.

Optical Properties and Electron Paramagnetic Resonance Spectra of Zirconium Fluoride- and Hafnium Fluoride-Based Glasses Activated with MnO2 and EuF2

We have studied luminescence and electron paramagnetic resonance (EPR) spectra of manganese- and europium-activated fluorozirconate glasses in the ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) system and fluorohafnate glasses in the HfF4–BaF2–LaF3–AlF3–NaF (HBLAN) system in order to assess the oxidation state and spatial distribution of the activator ions. The manganese luminescence band has been shown to shift from the green (545 nm) to the red (610 nm) spectral region, and additional lines in the EPR spectrum of manganese have been observed to emerge in the EPR spectrum of manganese as a result of BaCl2 substitution for BaF2 in the ZBLAN glass. The ratio of the concentration of free activator ions to that of clustered ions has been estimated quantitatively. It has been shown that, at high doping levels, the manganese and europium ions in the glasses are predominantly clustered, and only a small part of them are present as isolated ions.

Crystal-field and Racah parameters of Mn4+ ion in red and deep red-emitting phosphors: Fluoride versus oxide phosphor

A variety of rare earth-free phosphors enabling light emission in the red spectral region have drawn much interest for use in creating a warm white-LED device with a high color rendering index and a high luminous efficacy. The Mn4+-activated phosphor properties are discussed in detail from an aspect of spectroscopic point of view. First, we classify the various host materials used in red or deep red-emitting phosphors from a different kind of host materials and from a view point of the photoluminescence (PL) and PL excitation (PLE) properties. The ever-reported Mn4+-activated phosphors can be divided into five groups, namely, fluorides, oxides, oxyfluorides, and two fluorine-based compounds. They are then classified into totally eleven subgroups from an aspect of the spectroscopic properties, e.g., whether the zero-phonon emission peak can be clearly observed or not. Next, the PL and PLE properties of the Mn4+-activated “fluoride” and “oxide” phosphors as representatives of those phosphor groups are discussed in more detail. Finally, a method is proposed for determining reliable crystal-field (Dq) and Racah parameters (B and C) with drawing an attention to difficulty in the exact estimation of such parameters using a ligand field theory. The intra-d-shell electronic states, such as 2Eg, 4T2g, and 4T1g, of Mn4+ ions activated in the fluoride and oxide hosts are obtained and plotted against Dq in the Tanabe−Sugano diagram.