BEAM PROFILE CHARACTERIZATION OF A DENTAL LIGHT CURING UNIT USING A SPECTROMETER-BASED METHOD
Original Article
Figure 1 . LED positions in the Ascent OL5 LCU .
Figure 2 . Measuring points as related to the position within the light exit window . Blue cross = geometrical center of the light exiting window . Orange circles = “ WEST ”, Light blue circles = “ EAST ”, Purple circles = “ North ” and green circles = “ SOUTH ”.
Inc ., Newton , NJ , USA ) placed in front of the camera lens . A bandpass filter centered first at 400 nm with a 40 nm full width at half maximum ( FB400-40 ) was used to identify the LED chips with spectrum emission peaks at 400 nm ( violet light ). A different bandpass filter , centered at 450 nm with a 40 nm full width at half maximum ( FB450-40 ) was used to identify the LED chips generating emission peaks near 450 nm ( blue light ). To produce calibrated images and data showing the irradiance patterns across the surface of the broad spectrum LED , the mean power values , obtained using the MARCR resin calibrator were entered into an open source optical analysis software ( Fiji , ImageJ , National Institute of Health , Bethesda , MD , USA ) [ 32 ] to process the camera-based method beam profile images . The total spectral power output was integrated with each bandpass filter spectral optical density to calibrate each image . The scaled numerical data associated with each image were exported into a computer graphic software ( Origin Pro , OriginLab Co ., Northampton , MA , USA ). The beam profiles were captured at 1 , 1.5 and 2.5 mm from the light exciting window of the LCU and further analysis of the local intensities was performed the identical way as done with the MARC ® resin calibrator . A virtual aperture of 3.9 mm was created using the computer graphic software and the average irradiation on this surface was measured by first placing this aperture in the geometrical center of the beam profile and then moving it into 1 mm steps in the “ EAST – WEST ” and “ NORTH – SOUTH ” direction . Bar graphs comparable to the ones created with the spectrometer-based method were generated . In a second step , to calculate the magnitude of the decrease in the light irradiance according to the distance from the center of the LCU , a linear correlation fit was made to compare the slopes produced by the spectrometer-based method with the slopes generated from the camera-based method .
3 . Results At 1 mm distance from the light exciting window , the broad-spectrum LED had a total irradiance of 1159 mW / cm 2 , however , 1088 mW / cm 2 was within the blue wavelength range ( 420 - 495 nm ) and only 71 mW / cm 2 was within the violet wavelength range ( 380 – 420 nm ). As shown in Fig . 3 , it is clear that the majority of the power emitted ( mW ) by the broad-spectrum was within the blue wavelength range and the violet emission from this LED is considered insignificant . The results of the irradiance measurements at 1 , 1.5 and 2.5 mm distance from the light exciting window are shown in Fig . 4 . As expected , there is a decrease in irradiance the farther away the measuring area is from the center . To allow better direct comparisons , the slopes of these decreases were calculated as shown in Table 1 . Higher absolute value means greater rate of reduction in irradiance with distance from the geometric center of the LCU . Note that the farther away the light source is from the sensor , the flatter the slope , which means that the inhomogeneity of the beam profile decreases . Fig . 5 shows the beam profiles obtained at 1 , 1.5 and 2.5 mm distance from the light exciting window of the LCU . Note the spread of the beam and the inhomogeneity of the irradiation . Fig . 4c , Fig . 4d and Table 1 show the data as extracted from the beam profiles . Note the similarity of the pattern .
4 . Discussion The LCU used in this study was selected because based on its design an inhomogeneous beam profile was to be expected . The objective of the study was to use a spectrometer-based method to detect inhomogeneity in the light beam profile output of a LCU . Therefore , it makes sense to use a LCU , where this characteristic is to be expected . The Ascent OL5 broad spectrum LCU had a significant higher amount of blue light being emitted than violet
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Stoma Edu J . 2018 ; 5 ( 2 ): 84-91 . http :// www . stomaeduj . com