BEAM PROFILE CHARACTERIZATION OF A DENTAL LIGHT CURING UNIT USING A SPECTROMETER-BASED METHOD system. This was to simulate in a reproducible way operator errors in light curing, which have a deleterious effect on the light energy administered to a restoration [ 33 ]. With a homogeneous beam profile, as long as a restoration is smaller than the diameter of the light exciting window of the LCU slight positioning errors may result only in minimal decrease of exciting irradiation if any at all. However, with an LCU that has a centered beam profile as the one of the Ascent OL5 used in the present study, the positioning even within the confines of the light exiting window has significant effects on the exciting irradiation, as can be seen in Fig. 4, where a marked decrease in irradiation is noticed when the LCU is moved in the X-Y direction. The first null hypothesis that the irradiation would be at the same level all over the irradiated surface was rejected, since with both methods to characterize the LCU significant differences in local irradiance were found. In this study beam inhomogeneity could be shown as other researchers have done with different methods [ 25-28 ]. The second null hypothesis could be accepted, since the slopes of the decrease( in absolute numbers) were quite similar( Table 1). It is known that the irradiance a target surface received is a function of the exposure distance [ 25,26 ]. This is also visible in Figs. 3 and 5 confirming the results of others [ 25,26 ]. Furthermore, in Figs. 3 and 5 the irradiance dropped significantly from the center to the periphery in all measured directions(“ East”-“ West” and“ North”-“ South”). Looking at the absolute numbers of the slopes, the farther away from the LCU light exciting windows the smaller the numbers are, which means that the inhomogeneity decreases. With controlled lateral movements, the inhomogeneity of the beam could be roughly reproduced with the spectrometer. The same could be shown with the beam profiler emulating a cosine corrector light collector with a diameter of 3.9 mm, which limits the precision. It is suggested to use a smaller cosine corrector diameter for future measurements. With the methods used, area specific mean irradiations could be shown. Therefore, one has to rely on the known minimal irradiance needed to cure a specific resin-based material in order to assess the performance of a given LCU / RBC combination. The differences between the spectrometer-based vs camera-based beam profile methods are basically related to the resolution and accuracy of irradiance detection. Michaud et al. [ 29 ] used a laboratory grade integrating sphere spectrometer system to measure the irradiation and emission spectra of LCUs. Combined with a beam profiler camera they recorded the localized irradiance across the face of the light tip. The irradiation calibrated beam profile was then divided into 45 squares of 1 mm 2 each, thus being able to give more detailed beam analysis than it was possible with the method in the present paper. The beam inhomogeneity was additionally confirmed by micro hardness analysis [ 35 ]. Although the camera-based method can show higher resolution in mm 2 than the spectrometerbased method, the quantitative measurement of the irradiance using the spectrometer-based method seems to be more accurate, because the spectrometer-based method captures light directly from the light output tip, which is not the case for camera-based methods. Camera-based beam profilers are rarely able to give a direct measurement of the total irradiance of an LCU light beam. First, the LCU light beam passes through a long chain of attenuation so that the camera-sensor does not see the total power of the beam directly. Since this attenuation is put in place so as to get the energy down to the level of the camera sensor, it is not practical to calibrate each element of attenuation. Thus, the irradiance values that get into the camera are relative to the total power of the LCU light beam. Secondly, cameras do not have uniform wavelength absorption [ 36 ]. Therefore, they would have a different calibration factor for every wavelength of the LCU that is used. It would be impractical to attempt to calibrate the camera as a function of wavelength. So, after correcting for the power loss, the total irradiance energy measured by the spectrometer can then be entered into the software of the beam analysis instruments. From then on, the camera can give a readout of the total power or energy across the entire two-dimensional light beam distribution. For the clinical application, the findings of the present study mean that even with a properly positioned LCU, peripheral areas would get much less irradiation. Even small positioning deviations would aggravate this fact, e. g. looking at Fig. 4 one can see that a 1.5 mm positioning deviation in the“ WEST” direction would yield approximately 50 % of the maximal possible irradiation. At a distance of 2.5 mm this effect is slightly less pronounced. As a clinical relevance of this study, it is important for the clinicians to understand that there are a lot of broad spectrum LCUs on the market, but not all of them might be really effective. And, it is important to be aware of the implications of bad quality broad spectrum LCUs on the quality of RBCs and thus on dental practice.
5. Conclusions The analyzed LCU has an inhomogeneous beam profile, being the most intensive in the center and diminishing substantially towards the periphery. The spectrometer-based method used was able to characterize an LCU which may be helpful among other parameters to make a selection for a specific LCU for clinical use.
Author contributions JFR: Idea, experimental design, wrote the manuscript. MGR: Performed standard camera-based beam profiles, contributed extensively to introduction and discussion. CS: Performed data analysis, proofread the manuscript. MMK: Performed spectrometer experiment. DCRSdeO: Contributed extensively to introduction and discussion.
Acknowledgments DO is a Post-Doctoral Researcher at São Paulo Research Foundation- FAPESP( grant # 2016 / 05823- 3 and # 2017 / 22161-7) and MR is a PhD Researcher at São Paulo Research Foundation- FAPESP( grant # 2016 / 06019-3 and # 2017 / 22195-9).
Original Article
Stomatology Edu Journal
89