composite resin restorations have begun to constitute a significant portion of dentist 's routine practice due to the patient 's increasing demand for an esthetic appearance .
proper color match of a composite restoration with the adjacent teeth is important not only at the initial stage of the restoration , but also over a longer period of time .
however , despite the recent developments , color stability of light cured composites after long term intraoral exposure remains a concern .
extrinsic factors include intensity and duration of polymerization , exposure to environmental factors including ambient and ultraviolet radiation , heat water or food colorants .
intrinsic factors include composition of resin matrix , type of photo initiator and percentage of remaining double bonds ( c = c ) .
the latter is considered a fundamental parameter influencing a variety of mechanical , chemical and biological properties .
a lot of research has been undertaken on various light curing units and methods to cure the composite ( light intensity , depth of cure , area of cure , irradiance of the light curing gun);however , the lack of maintenance of the light curing unit and constant measurement of the logarithmic total exposure can play a deleterious effect on the esthetic light cured restorations .
the purpose of the study was to investigate the effect of intensity of the light curing unit and its relation to color stability of the polymerized composite material .
forty specimens of composite resin disc ( filtek z250 ivoclarvivadent ) with 3 mm diameter and 1.5 mm thick were prepared by using spherical polyethylene moulds , and the samples were divided into two groups of 20 samples each .
two light curing units were taken from general dentists and their intensities were measured by using a radiometer .
the first group was cured with the light curing unit which had an intensity of 380mw / cm and the second group was cured with the light curing unit with an intensity of 680mw / cm .
mylar strips were used on the surfaces and polymerization was done according to manufacturer guidelines ( 20 samples for each group ) .
after polymerization , the composite resin samples were removed from moulds , placed in an empty test tube and each specimen was placed in an incubator ( 372c ) for 24 hours .
later they were individually immersed in 1 ml of 2% methylene blue solution and placed at 37c  2c .
after 24 hours , the specimens were rinsed under running distilled water for 1 minute and stored at 37c  2c at relative humidity for 24 hours .
the specimens were immersed in new test tubes containing 1 ml of absolute alcohol for 24 hours .
the solutions were filtered and centrifuged for 3 minutes at 4,000 revolutions per minute , and the supernatant was used to determine dye absorbance in a spectrophotometer and calorimeter at 590 nm .
results were analyzed statistically for spectrophotometer and calorimetric values by using the mann - whitney test ( graph 1 ) .
spectrometric analysis showed that the composite cured with a low intensity light had higher dye absorption ( p value 0.009 ) ; and , the results for calorimetric analysis were statistically insignificant ( p value 0.171 ) .
the irradiance of the light source , the exposure time and light transmission of composite are significant variables that affect the hardness or conversion profile .
it is suggested that the depth of cure corresponds to the minimum amount of energy required to initiate polymerization . for a resin composite ,
the properties depend among other things , such as , the degree of conversion of the monomer .
a high degree of conversion gives hardness and strength to the material , as well as color stability .
thus , a reduction in remaining double bonds to the lowest possible level is normally considered a desirable feature of a polymerization system . staining susceptibility of resin composite might be attributed to their degree of water sorption and hydrophilicity of matrix resin , that is , if the resin composite can absorb water it can also absorb other fluids like methylene blue dye , tea and coffee .
the glass filler particles will not absorb water into the bulk of the material;however , can adsorb water onto the surface
. extra water sorption may decrease the life of the resin composites by expanding and plasticizing the resin component , hydrolyzing the silane and leading to microcrack formation .
therefore , the micro - cracks or the interfacial gaps at the interface between filler and matrix allow stain penetration and discoloration . as a consequence of the high water sorption and solubility of restorative resins , studies have shown that composite restorations may have decreased mechanical properties and reduced longevity .
the present study measures the dye penetration and indirectly reflects the amount of conversion of the double bonds as it is given that composites containing more than 35% of unconverted c = c bonds are susceptible to discoloration . in the present study methylene blue
was used , as it is the most commonly used dye for microleakage studies and the size of this dye is 120 or 1.2 nm which is less than that of bacteriae .
two composite light curing ( lc ) units were used for this study ; these lc units were taken from a general dentist who was using them on a regular basis . justifying our use ,
various studies have concluded that there is no difference in curing found between composite resins light - cured by light emitting diode or halogen light - curing units.to prevent the bias regarding the surface finishing and polishing , we used mylar strips on the surface of the disc so that a smooth surface is achieved . for analyzing the dye absorbance ,
the discs were placed in methylene blue and then in absolute alcohol by which there was a release of methylene blue absorbed by the composite discs into the alcohol .
alcohol uptake of the dye may be due to the hydroxide groups that are suitable for cationic dye removal like methylene blue .
since instrument measurements eliminate the subjective interpretation of visual - color comparison , spectrophotometers and colorimeters have been used to measure color changes in dental materials .
the colorimeter and spectrophotometer works on the basis of beer - lambert 's law , which states that the absorption of light transmitted through a medium is directly proportional to the concentration of the medium .
colorimeter is an instrument for psychophysical analysis , provides measurements that correlate with human eye - brain perception .
it consists of sensor and simple data processor and has a set of illuminant and observer combination .
spectrophotometer is an instrument for physical analysis , and provides wavelength - by - wavelength spectral analysis of the reflecting and/ or transmitting properties of objects without interpretation by human .
dye penetration method was used since the uncured resin matrix is more susceptible to dye absorption and also release of dye when placed in absolute alcohol .
the degree of conversion of a given resin composite is influenced by the energy density .
insufficient energy density results in less than maximal conversion , and the demonstrated differences in quantity of remaining double bonds are reflected in other polymerization characteristics . energy density ( j / cm ) is equal to the product of light intensity and the irradiation time .
the peak wavelength among light curing units varies from about 450 - 490 nm , and the irradiance ranges from 400 - 800mw / cm .
a 2 mm thick resin composite restoration requires a radiant exposure of 8 j / cm(400mw / cmx20s=8000mw / cm ) . in the present study keeping the time of exposure constant ,
it was observed that the energy density decreased for the samples cured with low intensity .
loupes et al . , reported that longer curing time is required to generate sufficient energy density to cure 2 mm increments of composite resins in lights with low output intensity .
however , in another study it was revealed that there is only a modest increase in the depth of cure ( < 25% ) , even upon doubling the exposure time .
decrease in line voltage of 6% shows a corresponding reduction in output of about 25% . a remarkable difference in intensity ( 40 - 50mw / cm )
was also observed when sterilization sleeves or jackets were worn on the tip of the light curing unit .
vivek hegde et al . , reported that only 10% led machines and 2% quartz - tungsten - halogen machines used by practitioners were in good condition .
most of the curing lights examined had low output intensity ranging from 200 - 400 mw / cm .
most of the curing unit tips had resin composite remnants on them.barghi n reported 30% of curing units had power densities <
199mw / cm and were considered inadequate for curing the composite resin . in coordination with studies
, the results suggest that there is a greater staining of the composite cured with low intensity light , which might be related to unpolymerized organic matrix .
it must be considered that esthetic restorations especially on anterior teeth look pleasing initially ; however , in due course of time in the oral environment , are deemed to stain resulting in failure of restoration , and lead to an early replacement of the restoration causing a negative psychological effect and financial loss to the patient . when discussing the clinical applications of these results
, it must be considered that the oral environment differs in several ways from in vitro conditions .
factors such as the variety of food , thermal and mechanical stresses , saliva and their interactions may intensify discoloration in vivo .
a few limitations exists in this study like the effect of saliva which was not taken into consideration , and also the effect of alcohol , which softens and effect the surface integrity of composite resin while measuring the dye uptake was not considered.before concluding , we would recommend further evaluation with a larger group of light curing units with low intensity .
the role of different intensities of light on the color stability of the composite resins was determined .
the specimens cured with low intensity light had a higher dye absorption , signifying the role of light intensity on the physical properties of composite resins like color stability .
 aim / objective : to understand the importance of intensity of light in polymerizing light cured composites and its relation to color stability.materials and methods : forty specimens of composite disc with 3 mm diameter and 1.5 mm thick were divided into two groups of 20 samples each . 
 group1 : twenty samples were cured with a light curing unit of380mw / cm2 . 
 group2 : twenty samples cured with a light curing unit of 680mw / cm2 . 
 these polymerized samples were immersed in methylene blue dye for 24hoursand later washed and immersed in absolute alcohol for 24 hours . 
 the amount of color released into absolute alcohol was assessed by spectrophotometric and colorimetric analysis.results:results were analyzed for spectrophotometric and colorimetric values by using the mann - whitney test . 
 the group cured with low intensity light stained more compared to the group cured with a normal intensity of light.conclusions:intensity of light plays a crucial role in staining of the polymerized light cured composite . 
 the intensity of the curing unit has to be maintained in acceptable limits to achieve good clinical results . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSIONS