BEAM PROFILE CHARACTERIZATION OF A DENTAL LIGHT CURING UNIT USING A SPECTROMETER-BASED METHOD factors . This was observed for the first LCUs operating in the visible light range , based on Quartz-tungsten halogen bulbs which had been developed for other purposes than dentistry [ 18 ]. These light curing units had the advantage of a broad light spectrum [ 19 ]. However , since only the blue light was usable to cure RBCs , infrared-blocking blue bandpass filters ( at 510 nm ) were built in the LCUs . These filters would deteriorate over time , diminishing the light emission properties of the LCUs [ 20 ]. There were more characteristics that basically made them problematic [ 21 ], such as instability of the output , spectral distribution and unreliable timers . Some LCUs had a quite homogeneous beam profile , which means that the surface of the irradiated composite gets the same energy density on every area . This is possible because in a halogen light the hot wire creating the light is almost a point light source , which can be captured with a reflector and guided into a beam . However already in 1986 it was shown that some LCUs had an inhomogeneous light beam [ 21 ]. This was confirmed using an acrylic optical fiber with a 1 mm diameter moved at 1mm steps in x-y direction in front of the light emitting tip of the LCU . With this method the local light intensity of three QTH lights , a plasma arc light and a LED light was measured [ 22 ]. For many reasons LED LCUs were a big improvement . LEDs are not considered a point light source , since they have the shape of a flat surface emitting the light . Early LEDs had very little power ( 1.2 mW ) and thus were not usable for light curing devices [ 20 ]. But , over the years they became much more powerful ( e . g . 123 mW ), thus multiple LEDs were used as a light source which made it difficult to bundle the emitting light [ 20 ]. Modern LED LCUs use 1-8 LEDs in an array which is flat [ 23 ], which makes bundling the light more difficult and may result in an inhomogeneous beam profile , despite being a mono-wavelength unit ( peak at around 450-470 nm ). Finally , the last step in development is the broad spectrum LED LCU , which uses different types of LEDs , emitting light with different wavelengths ( violet light ( 380 – 420 nm ) and blue light ( 420 – 495 nm ) [ 8,20 ]. Therefore they are able to activate different types of photoinitiators [ 24 ]. However , these LED LCUs show more or less pronounced inhomogeneity of the beam profiles [ 25- 28 ]. This means that not every point on an irradiated surface gets exposed to the same level of irradiation from the different wavelengths , especially in depth [ 29-31 ]. Thus , if the dentist aims for a short exposure time , some areas of the irradiated RBC may not obtain the minimally required light radiant exposure for optimal monomer conversion [ 24 ]. The power of the light source determines the energy it can emit ; the design of the optical system to capture and bundle the light emitted from the light source determines the degree of spread of the light after it exits the light curing fiber bundle or lens . Turbo tips , which concentrate the light on a smaller surface to increase the irradiation usually have a larger spread than conventional tips [ 24 ]. Usually , the spread is considerable , yielding to less irradiation the farther away the target RBC is from the light exciting window [ 25,26 ].
A light beam profile is the 2D irradiance intensity plot for a camera-based beam profiling system , requires a thermopile ( power meter ), a spectrometer , a CCD or CMOS camera , a modern computer with a frame grabber card to digitize the signal , and software for controlling the frame grabber card , displaying beam profiles and making respective quantitative calculations . Also , an optomechanics apparatus , such as diffusive glasses and band pass filters , is almost always needed to attenuate and / or filter the light beam before going into the camera [ 27,28 ]. In general , it cannot be bought together , and requires knowledge to buy the complete and right equipment . Moreover , the operator needs to know how to use all these parts together . Thus , the objective of the present study was to characterize a broad spectrum LED LCU with a known inhomogeneous beam profile using a spectrometerbased method and correlate it with a standard camera-based beam profile method . The following null hypotheses were tested : 1 ) The irradiation is at the same level all over the irradiated surface . 2 ) The irradiation as captured with the spectrometerbased method is similar to the irradiation shown by a standard camera-based beam profiler .
2 . Materials and Methods For this method-validation study a broad-spectrum LED LCU ( Ascent OL5 , CAO Group South Jordan , UT , USA ) was used . Its design leads to the assumption that it will generate an inhomogeneous beam profile , since it has one LED emitting blue light mounted in the center and 4 small LEDs emitting violet light mounted at the periphery ( Fig . 1 ). The LCU was attached to an x-y-z positioning device mounted on an optical bench in order to standardize the positioning of the light beam centered above the cosine corrector light signal collector of a spectrometer ( MARC ® Resin Calibrator , BlueLight Analytics , Halifax , Canada ) with the handle towards the right side (“ EAST ”, Fig . 2 ) at an exposure distance of 1.0 , 1.5 or 2.5 mm . The diameter of the cosine corrector was 3.9 mm . Using the translation stage , the position of the geometrical center of the LCU was first aligned with that of the cosine corrector and then moved in 1-mm steps in the x-y plane (“ EAST ” – “ WEST ” and “ NORTH ” – “ SOUTH ”) ( Fig . 2 ). The irradiance was assessed at each above described condition . The irradiance loss was visualized using bar graphs for the East-West ( long axis of the LCU ) and the North – South direction . The slopes of the irradiation decrease were calculated and compared . To obtain standard camera-based beam profile images , the same LCU was attached to an x-y-z positioning device mounted on an optical bench in order to standardize the positioning of the light beam in contact with a diffusive surface of a frosted diffuser target ( DG20-1500 , Thorlabs , Inc ., Newton , NJ , USA ) while the resulting image was recorded using a camera ( NEX-F3 , Sony Corporation , Tokyo , Japan ) with a 50 mm focal length lens . To assess the irradiance distribution of the different LED emission wavelengths from the broad spectrum LED , the beam profiler was used with the addition of bandpass filters ( Thorlabs ,
Original Article
Stomatology Edu Journal
85