Low Irradiance Levels Research Articles

Modeling and simulation of photovoltaic (PV) system during partial shading based on a two-diode model

This paper proposes accurate partial shading modeling of photovoltaic (PV) system. The main contribution of this work is the utilization of the two-diode model to represent the PV cell. This model requires only four parameters and known to have better accuracy at low irradiance level, allowing for more accurate prediction of PV system performance during partial shading condition. The proposed model supports a large array simulation that can be interfaced with MPPT algorithms and power electronic converters. The accurateness of the modeling technique is validated by real time simulator data and compared with the three other types of modeling, namely Neural Network, P&O and single-diode model. It is envisaged that the proposed work is very useful for PV professionals who require simple, fast and accurate PV model to design their systems.

Accurate MATLAB Simulink PV System Simulator Based on a Two-Diode Model

This paper proposes a MATLAB Simulink simulator for photovoltaic (PV) systems. The main contribution of this work is the utilization of a two-diode model to represent a PV cell. This model is known to have better accuracy at low irradiance levels which allows for a more accurate prediction of PV system performance. To reduce computational time, the input parameters are reduced to four and the values of <TEX>$R_p$</TEX> and <TEX>$R_s$</TEX> are estimated by an efficient iteration method. Furthermore, all of the inputs to the simulator are information available on a standard PV module datasheet. The simulator supports large array simulations that can be interfaced with MPPT algorithms and power electronic converters. The accuracy of the simulator is verified by applying the model to five PV modules of different types (multi-crystalline, mono-crystalline, and thin-film) from various manufacturers. It is envisaged that the proposed work can be very useful for PV professionals who require a simple, fast and accurate PV simulator to design their systems.

A framework for predicting the yield stress, Charpy toughness and one hundred-year activation level for irradiated fusion power plant alloys

Recent papers have demonstrated that the yield stress and the Charpy ductile to brittle transition temperature shift at the high irradiation levels of a fusion power plant may be predicted from measurements at lower irradiation levels using neural networks. It was demonstrated that the extrapolation inherent in such predictions could be validated provided that network complexity was appropriately low. Simultaneous predictions of these metallurgical properties at the 100 dpa irradiation level and 400 °C irradiation temperature of a possible fusion power plant have been made for a series of ferritic/martensitic steels, albeit based on mainly fission data. Together with the readily available one hundred-year activation level, benefit functions are defined which can be used to predict the most suitable alloys for a fusion power plant from within existing databases. Our model is sufficiently flexible to allow a variety of possible benefit functions to be defined. The F82H, Eurofer and LA12 alloy series all receive a favourable rating, although all results presented here must be tempered with caution until more data at relevant irradiation levels and with relevant energy spectra become available.

PHOTOACCLIMATION IN THE PHOTOTROPHIC MARINE CILIATE MESODINIUM RUBRUM (CILIOPHORA)(1).

Mesodinium rubrum (=Myrionecta rubra), a marine ciliate, acquires plastids, mitochondria, and nuclei from cryptophyte algae. Using a strain of M. rubrum isolated from McMurdo Sound, Antarctica, we investigated the photoacclimation potential of this trophically unique organism at a range of low irradiance levels. The compensation growth irradiance for M. rubrum was 0.5 μmol quanta · m(-2) · s(-1) , and growth rate saturated at ∼20 μmol quanta · m(-2) · s(-1) . The strain displayed trends in photosynthetic efficiency and pigment content characteristic of marine phototrophs. Maximum chl a-specific photosynthetic rates were an order of magnitude slower than temperate strains, while growth rates were half as large, suggesting that a thermal limit to enzyme kinetics produces a fundamental limit to cell function. M. rubrum acclimates to light- and temperature-limited polar conditions and closely regulates photosynthesis in its cryptophyte organelles. By acquiring and maintaining physiologically viable, plastic plastids, M. rubrum establishes a selective advantage over purely heterotrophic ciliates but reduces competition with other phototrophs by exploiting a very low-light niche.

쇠미역사촌과 미역 포자체의 배양 및 야외 개체군의 생장 비교

【The effects of temperature, light, and salinity on the growth of Costaria costata and Undaria pinnatifida juveniles were examined in laboratory cultures. In a cultivation farm, the monthly yield and density were also investigated between December and April for C. costata and between December and March in 2007 and 2008 for U. pinnatifida. The relative growth rates (RGRs) were greater at $20{\sim}60\;{\mu}mol$ photons $m^{-2}s^{-1}$ than at low ( $0{\sim}10\;{\mu}mol$ photons $m^{-2}s^{-1}$ ) and high ( $100{\sim}180\;{\mu}mol$ photons $m^{-2}s^{-1}$ ) irradiance levels. The optimal growth conditions for the two species were $17^{\circ}C$ , 35 psu, $60\;{\mu}mol$ photons $m^{-2}s^{-1}$ , and a daylength of 12 h, indicating that C. costata and U. pinnatifida have very similar growth responses to temperature, light, and salinity. However, the growth responses of the two species to various environmental factors were different; C. costata grew faster than U. pinnatifida but the latter species grew well at low salinity. The monthly yield of C. costata and U. pinnatifida increased steadily over the study period, and it was maximal in March for both species, but the yield of U. pinnatifida was greater than that of C. costata.】

Global and regional trends in ionospheric total electron content

[1] A statistically significant positive trend of 0.6 ± 0.3 total electron content unit (TECU; 1 TECU = 1016 el m−2) per decade is detected in the 15 year record of daily averaged global total electron content obtained from multiple GPS observations between 1995 and 2010. The trend is extracted using a multiple regression analysis that simultaneously accounts for the comparatively larger global ionospheric responses to solar irradiance variability, solar-modulated annual and semiannual ionospheric oscillations, and geomagnetic activity. Geographical maps of regional trends in total electron content reveal both positive and negative local secular change during the past 15 years, with an overall larger increase in the Northern Hemisphere than in the Southern Hemisphere. Largest regional changes of as much as ±3 TECU per decade occur in the vicinity of 60°W to 60°E longitude and 15°S to 30°N latitude. TEC trend magnitude depends sensitively on the specification of solar EUV irradiance variations in the multiple regression model. The +0.6 TECU per decade trend pertains to equal solar irradiance levels during the 1996 and 2008 solar activity minima. When the specified solar irradiance is 15% lower in 2008 than in 1996, the derived global ionospheric trend increases to 3 TECU per decade. We contend that such a large global trend is implausible and that the associated anomalously low level of EUV irradiance in cycle 2008 minimum, reported in earlier publications, is unlikely to be real.

IncorporatingPhaeocystisinto a Southern Ocean ecosystem model

Phaeocystis antarctica is an important phytoplankton species in the Southern Ocean. We incorporated P. antarctica into the biogeochemical elemental cycling ocean model to study Southern Ocean ecosystem dynamics and biogeochemistry. The optimum values of ecological parameters for Phaeocystis were sought through synthesizing laboratory and field observations, and the model output was evaluated with observed chlorophyll a, carbon biomass, and nutrient distributions. Several factors have been proposed to control Southern Ocean ecosystem structure, including light adaptation, iron uptake capability, and loss processes. Optimum simulation results were obtained when P. antarctica had a relatively high α (P-I curve initial slope) value and a higher half-saturation constant for iron uptake than other phytoplankton. Simulation results suggested that P. antarctica had a competitive advantage under low irradiance levels, especially in the Ross Sea and Weddell Sea. However, the distributions of P. antarctica and diatoms were also strongly influenced by iron availability. Although grazing rates had an influence on total biomass, our simulations did not show a strong influence of grazing pressure in the competition between P. antarctica and diatoms. However, limited observations and the relative simplicity of zooplankton in our model suggest further research is needed. Overall, P. antarctica contributed ∼13% of annual primary production and ∼19% of sinking carbon export in the Southern Ocean (>40°S) in our best case simulation. At higher latitudes (>60°S) P. antarctica accounts for ∼23% of annual primary production and ∼30% of sinking carbon export.

A microwave-assisted rapid route to synthesize ZnO/ZnS core–shell nanostructures via controllable surface sulfidation of ZnO nanorods

We demonstrate a new strategy for preparation of ZnO/ZnS core–shell nanorods via microwave-assisted in situ surface sulfidation of ZnO nanorods. This is a facile and rapid process, requiring only a low level of microwave irradiation (400 W), through which a conformal ZnS nanoparticle layer is deposited onto the ZnO nanorods, while retaining the uniformity of the original ZnO nanorods in the as-prepared ZnO/ZnS hybrid structures with excellent fidelity. Furthermore, the thickness and nanoparticle size of the ZnS shell can be conveniently varied or controlled by the concentration of the organo-sulfur source thioacetamide (TAA). The as-prepared products exhibit narrowed band gap and strong orange luminescence at 621 nm, due to the interstitial oxygen ion defect present in hydrothermally grown ZnO. However, the PL intensity gradually decreases with the increase of the thickness of ZnS shell, indicating charge transfer between the two components of the ZnO/ZnS hybrids. Further investigation has revealed that the hybrid nanostructures possess significantly higher visible light photocatalytic activity which is twice that of the original ZnO nanorods. The mechanisms of the optimal ZnS shell thickness to reach the maximum photocatalytic activity in the ZnO/ZnS core-shell hybrid are proposed and discussed. It is believed that this facile, rapid microwave-assisted strategy is scalable and its application can be extended synthesize other oxide/sulfide (MOx/MSy) core-shell nanostructures for different applications.

Crassulacean acid metabolism under severe light limitation: a matter of plasticity in the shadows?

Despite the increased energetic costs of CAM compared with C(3) photosynthesis, it is hypothesized that the inherent photosynthetic plasticity of CAM allows successful acclimation to light-limiting conditions. The present work sought to determine if CAM presented any constraints to short and longer term acclimation to light limitation and to establish if and how metabolic and photosynthetic plasticity in the deployment of the four phases of CAM might facilitate acclimation to conditions of deep shade. Measurements of leaf gas exchange, organic acids, starch and soluble sugar (glucose, fructose, and sucrose) contents were made in the leaves of the constitutive CAM bromeliad Aechmea 'Maya' over a three month period under severe light limitation. A. 'Maya' was not particularly tolerant of severe light limitation in the short term. A complete absence of net CO(2) uptake and fluctuations in key metabolites (i.e. malate, starch or soluble sugars) indicated a dampened metabolism whilst cell death in the most photosynthetically active leaves was attributed to an over-acidification of the cytoplasm. However, in the longer term, plasticity in the use of the different phases of gas exchange and different storage carbohydrate pools, i.e. a switch from starch to sucrose as the major carbohydrate source, ensured a positive carbon balance for this CAM species under extremely low levels of irradiance. As such, co-ordinated plasticity in the use of C(3) and C(4) carboxylases and different carbohydrate pools together with an increase in the abundance of light-harvesting complexes, appear to underpin the adaptive radiation of the energetically costly CAM pathway within light-limiting environments such as wet cloud forests and shaded understoreys of tropical forests.

Innovative High-Surface-Area CuO Pretreated Cotton Effective in Bacterial Inactivation under Visible Light

This study presents the first report on enhanced bacterial inactivation of E. coli by RF-plasma pretreated cotton with high-surface-area CuO powders compared with nonpretreated cotton textiles. The high-surface-area CuO (65 m/g) powder was fully characterized. The E. coli inactivation proceeded in the dark and was accelerated under visible and sunlight irradiation even at very low levels of visible light irradiation. The effect the RF-plasma pretreatment of the cotton on the binding of CuO, applied light dose, the amount of CuO loading and initial E. coli concentration on the inactivation kinetics of E. coli is reported in detail.

Prediction of yield stress and Charpy transition temperature in highly neutron irradiated ferritic steels

Recent predictions have been made of metallurgical properties of low-activation ferritic/martensitic steels alloys at the high irradiation levels (displacements per atom or dpa) needed for a fusion power plant as based on measurements at low irradiation levels where more data are available. These predictions have been published for the yield stress and for the Charpy ductile to brittle transition temperature shift. The neural network model predictions use training data up to a certain dpa level to predict metallurgical properties above this level. This ‘extrapolation’ mode of neural networks is explored in some detail. Our studies revealed an increasing accuracy of predictions as the test dpa level is increased for both yield stress and Charpy shift predictions. This result suggests that a model exists for these metallurgical properties as a function of dpa level which becomes more accurate as the available irradiation range in the training data is increased. The explanation suggested is that the metallurgical annealing, which occurs as the irradiation level is increased, simplifies the microstructure and makes prediction more reliable.

The effect of solute on ultrasonic grain refinement of magnesium alloys

An experimental study has been conducted to assess the structural refinement of magnesium and its alloys by ultrasonic irradiation during solidification. It is shown that (i) ultrasonic irradiation leads to significant refinement only in the presence of adequate solute, which is alloy dependent; (ii) the attendant grain density increases linearly with increase in solute content at a given irradiation level; (iii) increasing the solute content at a low irradiation level above the cavitation threshold is more effective than substantially increasing the irradiation intensity; and (iv) the difference in the grain size between two ultrasonicated magnesium alloys is mainly determined by the solute content rather than the irradiation intensity. In view of these, the effect of ultrasonic irradiation on solute redistribution in a solidifying magnesium alloy seems rather limited even at a substantial intensity level such as 1700 W cm −2. The implications of these findings are discussed and a mechanism is proposed to account for the experimental observations.

Effect of Radiation on Human Intestinal Microvascular Endothelial Cells (HIMEC): Role of Akt/mTOR and NFκB

Radiation therapy is an essential modality in the treatment of colorectal cancers. Radiation exerts an anti‐angiogenic effect on tumors, inhibiting endothelial proliferation and survival in the tumor microvasculature. However, damage from low levels of irradiation can induce a paradoxical effect, stimulating survival in endothelial cells. We used HIMEC to define effects of low level radiation on these gut specific endothelial cells. 2 Gy of irradiation activates NFκB and the PI3K/Akt pathway, which is involved in cell cycle re‐entry and cell survival in HIMEC. A downstream target of PI3K/Akt is mammalian target of rapamycin (mTOR), which contributes to cell proliferation and angiogenesis. The aim of this study was to investigate the signaling molecules involved in the radio‐sensitizing effects of curcumin on HIMEC subjected to low level of irradiation. We have demonstrated that exposure of HIMEC to low level of irradiation induced mTOR and Akt phosphorylation, which was attenuated by curcumin, rapamycin and LY294002. Activation of NFκB by low level of irradiation was inhibited by curcumin and SN‐50. Curcumin also induced apoptosis by induction of caspase‐3 cleavage in irradiated HIMEC. In conclusion, curcumin significantly inhibited NFκB and attenuated the effect of irradiation‐induced pro‐survival signaling through the PI3k/Akt/mTOR and NFκB pathways in these gut specific endothelial cells. Curcumin may be a potential radio‐sensitizing agent for enhanced anti‐angiogenic effect in colorectal cancer radiation therapy.

Modulatory effect of curcumin on survival of irradiated human intestinal microvascular endothelial cells: role of Akt/mTOR and NF-κB

Radiation therapy is an essential modality in the treatment of colorectal cancers. Radiation exerts an antiangiogenic effect on tumors, inhibiting endothelial proliferation and survival in the tumor microvasculature. However, damage from low levels of irradiation can induce a paradoxical effect, stimulating survival in endothelial cells. We used human intestinal microvascular endothelial cells (HIMEC) to define effects of radiation on these gut-specific endothelial cells. Low-level irradiation (1-5 Gy) activates NF-kappaB and the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which is involved in cell cycle reentry and cell survival in HIMEC. A downstream target of PI3K/Akt is mammalian target of rapamycin (mTOR), which contributes to endothelial proliferation and angiogenesis. The aim of this study was to investigate the signaling molecules involved in the radiosensitizing effects of curcumin on HIMEC subjected to low levels of irradiation. We have demonstrated that exposure of HIMEC to low levels of irradiation induced Akt and mTOR phosphorylation, which was attenuated by curcumin, rapamycin, LY294002, and mTOR small interference RNA (siRNA). Activation of NF-kappaB by low levels of irradiation was inhibited by curcumin, SN-50, and mTOR siRNA. Curcumin also induced apoptosis by induction of caspase-3 cleavage in irradiated HIMEC. In conclusion, curcumin significantly inhibited NF-kappaB and attenuated the effect of irradiation-induced prosurvival signaling through the PI3K/Akt/mTOR and NF-kappaB pathways in these gut-specific endothelial cells. Curcumin may be a potential radiosensitizing agent for enhanced antiangiogenic effect in colorectal cancer radiation therapy.

Modulating the light environment with the peach 'asymmetric orchard': effects on gas exchange performances, photoprotection, and photoinhibition.

The productivity of fruit trees is a linear function of the light intercepted, although the relationship is less tight when greater than 50% of available light is intercepted. This paper investigates the management of light energy in peach using the measurement of whole-tree light interception and gas exchange, along with the absorbed energy partitioning at the leaf level by concurrent measurements of gas exchange and chlorophyll fluorescence. These measurements were performed on trees of a custom-built ‘asymmetric’ orchard. Whole-tree gas exchange for north–south, vertical canopies (C) was similar to that for canopies intercepting the highest irradiance in the morning hours (W), but trees receiving the highest irradiance in the afternoon (E) had the highest net photosynthesis and transpiration while maintaining a water use efficiency (WUE) comparable to the other treatments. In the W trees, 29% and 8% more photosystems were damaged than in C and E trees, respectively. The quenching partitioning revealed that the non-photochemical quenching (NPQ) played the most important role in excess energy dissipation, but it was not fully active at low irradiance, possibly due to a sub-optimal trans-thylakoid ΔpH. The non-net carboxylative mechanisms (NC) appeared to be the main photoprotective mechanisms at low irradiance levels and, probably, they could facilitate the establishment of a trans-thylakoid ΔpH more appropriate for NPQ. These findings support the conclusion that irradiance impinging on leaves may be excessive and can cause photodamage, whose repair requires energy in the form of carbohydrates that are thereby diverted from tree growth and productivity.

PULSED DYE LASER TREATMENT FOR RECALCITRANT VIRAL WARTS: AUTO-SIMULTANEOUS LASER TREATMENT

Verruca vulgaris is a benign proliferative lesion of the skin caused by the human papillomavirus (HPV). It is commonly seen in dermatologic and/or plastic surgery clinics. The diagnosis is straightforward, but the treatment of the lesion may be difficult and lengthy. Several treatment methods have been reported but no specific therapy of choice has been developed. It has been suggested that pulsed dye laser treatment is highly effective in the treatment of viral warts which have not responded to other treatments. Pulsed dye laser treatment is a safe method with low side effects and is mostly well-tolerated by patients. Several authors have recently reported the efficiency of pulsed dye laser treatment for warts, but no report has paid attention to the influence of low levels of laser irradiation about the periphery of the treated lesion which may induce immune activation and pain alleviation. We believe these effects are mainly due to the athermal and atraumatic simultaneous Low reactive Level Laser Therapy effect on photobiomodulated cells at the periphery of the treated lesion. We call this coincident duality of thermal damage and athermal stimulation ‘auto-simultaneous laser treatment’. We report herein on pulse dye laser treatment for a recalcitrant huge viral wart and consider the vital response.

Upper critical field and Raman spectra of MgB2 thin films irradiated with low energy oxygen ion

The structure, upper critical field, and Raman spectrum of epitaxial MgB2 thin films irradiated by 300 keV O2+ ions have been investigated. Lattice parameter c expands after irradiation. There is a significant increase in upper critical field in the moderately irradiated films, while the critical temperature is reduced slightly. The values of critical field at zero temperature exhibit a maximum for samples with a moderate irradiation level for the applied magnetic field both perpendicular and parallel to the film surface. The temperature dependence of the anisotropy parameter, which is defined as the ratio of the upper critical field with the field parallel to the film surface and perpendicular to the film surface, reveals that oxygen ion irradiation mainly affects the σ band at a low irradiation level. With increasing irradiation level, π band scattering is strongly enhanced, and finally both bands are in the dirty limit. A broad peak centered around 570 cm−1 is observed in the Raman spectrum of the unirradiated films, and the peak position has a visible redshift in the irradiated samples. In particular, high-frequency spectral structures appear and become dominant, while the E2g broad band diminishes gradually with increasing irradiation fluence. The results are discussed by considering the disorder-induced change in carrier scattering within and between the σ and π bands and a violation of the Raman selection rules due to oxygen ion irradiation.

Effects of elevated CO2 on the red seaweed Gracilaria lemaneiformis (Gigartinales, Rhodophyta) grown at different irradiance levels

Zou D. and Gao K. 2009. Effects of elevated CO2 on the red seaweed Gracilaria lemaneiformis (Gigartinales, Rhodophyta) grown at different irradiance levels. Phycologia 48: 510–517. DOI: 10.2216/08-99.1.The red seaweed Gracilaria lemaneiformis (Bory) Weber-van Bosse (Gigartinales, Rhodophyta) from Nanao Island, Shantou, China, was cultured at 370 and 700 µl l−1 CO2 in aeration and at intermediate (160 µmol photons m−2 s−1) and low (30 µmol photons m−2 s−1) irradiance levels in order to examine the influences of the elevated atmospheric CO2 concentrations on growth, photosynthetic performance and some biochemical components in this commercially important species. Relative growth rate (RGR) was significantly higher in G. lemaneiformis thalli grown using CO2-enriched air with respect to nonenriched air when the algae were subjected to intermediate irradiance. However, RGR was similar between these two CO2 treatments when the algae were grown under the low-irradiance condition. Extra CO2 in the culture decreased phycobiliprotein (PB, including phycoerythrin, PE, and phycocyanin, PC) contents of G. lemaneiformis thalli at the higher growth irradiance. However, chlorophyll a (Chl a) and soluble protein contents were unchanged by the CO2 levels in culture. Both PB and Chl a contents were higher in G. lemaneiformis thalli grown at the lower irradiance than at the higher irradiance, regardless of the CO2 levels in culture. The parameters for photosynthetic responses to irradiance and inorganic carbon were mostly not altered with the increase of CO2 concentrations in culture. However, light-saturated photosynthetic rates (Pmax) and apparent carboxylating efficiencies (ACE), expressed per unit Chl a, were significantly higher in algae grown at the intermediate irradiance compared to the low irradiance. Photosynthetic rate was reduced by an increase in pH of seawater from 8.2 to 9.1, and it was also strongly inhibited by the external carbonic anhydrase inhibitor acetazolamide (AZ) in G. lemaneiformis thalli grown at each CO2 and irradiance condition. Moreover, pH compensation points were not affected by the growth conditions. These results suggested that G. lemaneiformis under both growth conditions had a similar capacity of the photosynthetic utilization of external pool in seawater. However, ACE decreased in G. lemaneiformis thalli grown at the low irradiance with respect to the higher irradiance implied that the transport of Ci towards Rubisco within the cell was weakened. Taken together, the data showed that an increase of CO2 was less effective on G. lemaneiformis than the irradiance levels. We concluded that CO2 affected photosynthesis and growth performance when light was not the limiting factor.

Forest floor carbon exchange of a boreal black spruce forest in eastern North America

Abstract. This study reports continuous automated measurements of forest floor carbon (C) exchange over feathermoss, lichen, and sphagnum micro-sites in a black spruce forest in eastern North America during snow-free periods over three years. The response of soil respiration (Rs-auto) and forest floor photosynthesis (Pff) to environmental factors was determined. The seasonal contributions of scaled up Rs-auto adjusted for spatial representativeness (Rs-adj) and Pff (Pff-eco) relative to that of total ecosystem respiration (Re) and photosynthesis (Peco), respectively, were also quantified. Shallow (5 cm) soil temperature explained 67–86% of the variation in Rs-auto for all ground cover types, while deeper (50 and 100 cm) soil temperatures were related to Rs-auto only for the feathermoss micro-sites. Base respiration was consistently lower under feathermoss, intermediate under sphagnum, and higher under lichen during all three years. The Rs-adj/Re ratio increased from spring through autumn and ranged from 0.85 to 0.87 annually for the snow-free period. The Rs-adj/Re ratio was negatively correlated with the difference between air and shallow soil temperature and this correlation was more pronounced in autumn than summer and spring. Maximum photosynthetic capacity of the forest floor (Pff-max) saturated at low irradiance levels (~200 μmol m−2 s−1) and decreased with increasing air temperature and vapor pressure deficit for all three ground cover types, suggesting that Pff was more limited by desiccation than by light availability. Pff-max was lowest for sphagnum, intermediate for feathermoss, and highest for lichen for two of the three years. Pff normalized for light peaked at air temperatures of 5–8°C, suggesting that this is the optimal temperature range for Pff. The Pff-eco/Peco ratio varied from 13 to 24% over the snow-free period and reached a minimum in mid-summer when both air temperature and Peco were at their maximum. On an annual basis, Pff-eco accounted for 17–18% of Peco depending on the year and the snow-free season totals of Pff-eco were 23–24% that of Rs-adj.

Influence of Photoactivation Protocol and Light Guide Distance on Conversion and Microleakage of Composite Restorations

To evaluate the effect of light guide distance and the different photoactivation methods on the degree of conversion (DC) and microleakage of a composite. Three photoactivation protocols (600 mW/cm2 x 40 seconds; 400 mW/cm2 x 60 seconds or 200 mW/cm2 x 20 seconds, followed by 500 mW/cm2 x 40 seconds) and three distances from the light source (0, 3 or 7 mm) were tested. Cylindrical specimens (5 mm diameter; 2 mm tall; n=3) were prepared for the DC test (FT-Raman). Class V cavities were made in 90 bovine incisors to conduct the microleakage test. The specimens were conditioned for 15 seconds with phosphoric acid (37%), followed by application of the adhesive system Prime & Bond NT (Dentsply/Caulk). The preparations were restored in bulk. The specimens were stored for 24 hours in distilled water (37 degrees C) before being submitted to the silver-nitrate microleakage protocol. The restorations were sectioned and analyzed under 25x magnification. Statistical analyses (two-way ANOVAs and Tukey test, alpha=0.05) found significance only for the factor distance (p=0.015) at the top of the composite for the DC test. Conversion was statistically lower for the 7 mm groups compared to the 0 and 3 mm groups, which were equivalent to each other. At the bottom of the specimens, none of the factors or interactions was significant (p<0.05). The Kruskal-Wallis test showed that, in general, the soft-start method led to lower microleakage scores when compared to the continuous modes, mainly when associated with a distancing of 7 mm (p<0.01). With the exception of specimens irradiated with 400 mW/cm2 that did not demonstrate variations on scores for the distances tested, higher microleakage was observed for shorter distances from the light source. Soft-start methods may reduce microleakage when the light guide distancing provides a low level of irradiance, which also causes a discrete reduction in the DC.