On the inaccuracies of dental radiometers

Abstract

This study investigated the accuracy of sixteen models of commercial dental radiometers (DR) in measuring the output of thirty-eight LED light curing units (LCUs) compared with a 'gold standard' laboratory-grade spectrometer integrating-sphere (IS) assembly. Nineteen Type I (fiber-bundle light guide) and nineteen Type II (light source in head) LED LCUs were tested, some using different output modes and light guides, resulting in 61 test subsets per radiometer. Gold standard (GS) output measurements (n = 3) were taken using the IS and confirmed with two types of laboratory-grade power meter (PowerMax-Pro 150 HD and PM10-19C; Coherent). One DR (Bluephase Meter II, Ivoclar; BM II) allowed power (mW) as well as irradiance (mW/cm2) recordings. Irradiance readings (n = 3) for each DR/LCU were compared with the IS derived irradiance. Individual LCU irradiance values were normalized against IS data. The GS method yielded reproducible data with a 0.4% pooled coefficient of variation for the LCUs. Mean power values ranged from 0.19 W to 2.40 W. Overall power values for the laboratory-grade power meters were within 5% of GS values. Individual LCU/DR normalized irradiance values ranged from 7% to 535% of the GS; an order of magnitude greater than previous reports. BM II was the only radiometer to average within 20% of normalized pooled GS irradiance values, whereas other radiometers differed by up to 85%. Ten radiometers failed to provide any reading for 1 LCU. When tested with the PowerMax-Pro in high speed (20 kHz) mode, eight LCUs demonstrated pulsing outputs undetectable at the standard (10 Hz) data acquisition rate. Sufficient light exposure is critical for the successful curing of dental resin-based materials. Substantial discrepancies may occur between actual and estimated radiometric data using current DRs. More accurate DRs need to be developed. Manufacturers' accuracy claims for DRs should specify compatible LCUs and testing parameters.

Fig 1

Regression analyses and fitted line plots with 95% confidence and prediction intervals for mean power recordings of the Type I and Type II LED-LCUs using (a) the PM10-19C thermopile, (b) the LabMax Pro, (c) the MARC LC and, (d) the BMII (sn: 1300001150). For each analysis, the integrating sphere gold standard power (W) data was used as the predictor variable. Several high outlier results were noted in Fig 1 (d) for Type II LED-LCUs (I LED in standard and high output modes, Pencure VII and Fusion 5 (Blue Head) in Plasma mode). As the BM II is only designed to generate irradiance data for light guide tip diameters between 6 and 12 mm, the values generated for the 4 mm and 13 mm tips of the Demetron A2 LED LCU were based on programming the nearest tip diameter value (6 mm and 12 mm, respectively) to obtain the readings which were then corrected for area differences.

Fig 2. Pooled normalized mean power readings for all LED-LCUs relative to the integrating sphere gold standard data 100% values.

Note the difference between the Type I and Type II LED-LCU data distributions for the BM II units (average of the three tested units shown) compared with the two laboratory-grade power meter instruments and the MARC LC integrating sphere / spectrometer apparatus.

Fig 3

Regression analysis fitted line plots comparing irradiance data for the (a) BMII (sn:1300001150) and (b) the CQ LIT radiometer with the corresponding Integrating Sphere derived irradiance data sets as the independent variable.

Fig 4

Normalized pooled mean irradiance for all LCUs; (a) Type I and (b) Type II tested with the sixteen models of dental radiometers—the 3 examples of the BM II are denoted by the last 4 digits of their respective serial numbers …0003, …1134, …1150 and the MARC LC instrument (calculated from mean power data) relative to the gold standard Integrating Sphere method. Note the extremely wide data spread for many radiometers.

Fig 5

Radiant power of the LCUs, that delivered a) greater than 60% of their power output below 450 nm and b) greater than 65% of output above 450 nm.

Fig 6

(a) The front surface mirror attenuator from the disassembled BM II (sn:1300001134) and the large area photovoltaic cell array light sensor which lay directly beneath it; and (b) the "optics" components (diffusers, heat and bandpass filters) from a disassembled Demetron L.E.D. radiometer and the small area photodiode sensor typical of other dental radiometers for comparison. The BM II meter incorporates a light tip diameter guide, allowing the operator to measure the light guide tip diameter for programming the instrument in "irradiance reporting" mode. (c) Beam profile images of 2 single spectral peak Type I LED LCUs (S10 and S.P.E.C.3) and one LED LCU with 2 wavelength band outputs (Bluephase Style with the original non-homogenizing light guide to show the two blue and one violet LED chips seperately). From left to right the profiles were recorded by imaging: 1, directly onto the light guide exit face (left column); 2, through the mirror attenuator of the BM II (middle column) which was disassembled after the remainder of the project was completed (Fig 6A; sn: 1300001134); and, 3, through the reassembled optics (Fig 6B) of the Demetron L.E.D. radiometer (right column). Note the relative increase in output for the violet chip (~7 o’ clock position on the image) versus the two blue chips of the Bluephase Style when imaged through the BM II mirror attenuator (middle column–bottom image) compared to imaged directly onto the light guide exit face (left column).

Fig 7

Beam profiles of the (a) Elipar Deep Cure and (b) Elipar S10 units. The beam widths in both the horizontal and vertical directions (x and y, respectively) are given by D4σ which is 4 times σ, where σ is the standard deviation of the horizontal or vertical marginal distribution respectively. Note the more uniform color-coded irradiance distribution across the exit window face of the Deep Cure light guide compared to the central "high spot" peak irradiance region for the S10. The dotted lines represent the beam area that faced the 4 mm cosine corrector of the MARC LC lower sensor. The images explain why the irradiance ranking for these two LCUs reverses when the irradiance is measured over the central 4 mm diameter region of the beam in comparison with irradiance calculated from the total power output related to the optically active area of the light guide (9 mm diameter). Fig (c) and (d) show the local irradiance distributions for the Satelec Supercharged and Spark SK-L036A units, respectively. Readers should note that these images use different irradiance scales.

Fig 8. Fitted line plot of irradiance recorded with the BM I linear sensor radiometer versus integrating sphere derived irradiance for Type I and Type II LED-LCUs.

High outliers (circled from left to right of the graph) were noted for the Spark SK-L036A, Satelec Supercharged, and Blast Mini LED-LCUs. A similar pattern was noted with the Ledex CM 4000 sensor. These two radiometers were the only ones with relatively narrow linear sensors. When beam profiles for these three units were inspected, it was noteworthy that all three units exhibited centrally located "hot spots" with low areas of light output on the outer regions of the light guide exit faces. Fig 7C and 7D show representative beam profiles for two of these three LCUs.

Fig 9

(a) Irradiance values calculated for the four tested guide diameters (D: 4, 8, 11, 13 mm) of the Demetron A2 LED LCU based on power (mW) readings recorded (1) using the integrating sphere fiber-coupled spectro-radiometer gold standard test method (2) the MARC LC instrument and irradiance values recorded using the Demetron L.E.D. radiometer and the BM II sn:1300001150. The BMII instructions for use state that irradiance values may be recorded for light guides between 6 mm and 12 mm diameter and power (mW) values for light guides between 5 mm and 13 mm diameter. A guide with mm increments between 6 mm and 12 mm is provided on the instrument base to allow guide tip diameter measurements for programming irradiance values from power (mW) measurements. The graph displays the raw data (**) irradiance values for this instrument, as recorded for the four light guides used with the Demetron A2 and the recalculated values for the 4 mm and 13 mm tips. The latter were based on the increase in area (from 4 mm to 6 mm diameter) or decrease in the area (from 13 mm to 12 mm) between the actual guide diameter tested and the programmed instrument software input based on using the nearest available guide diameter. Note how, when this is done, the BM II data approaches the GS results and how the MARC LC data is in close accord to the GS data set. The results confirm the value of using a meter light sensor area larger than any tested light source exit window area. (b) The same data as Fig 7A normalized relative to the mean integrating sphere (100%) gold standard data.

Fig 10

(a) Stable power output over time for the Elipar Deep Cure-S LED-LCU compared with other LCUs (BA Ultimate 1400, Bluephase 16i HIP mode, Fusion 5 Plasma power), recorded using the MARC LC spectrometer-based instrument acquiring data at 30 frames per second. A relatively slower rise in power output versus time is seen for the Bluephase 16i in "High Power" output mode while the output from the Fusion 5.0 declines slightly over the 3 s Plasma output mode run time. Note pulsing nature of light output for the BA Ultimate 1400 seen at the 30 Hz rate, (black) At ~5 s, the BA Ultimate 1400 LCU switches off briefly and immediately restarts. The pulsing output was found to relate to changes in the violet component of the light output over time. (b) Four of the eight LCUs from the 38 tested exhibiting pulsing output patterns in high speed (20 kHz) data acquisition mode with the PowerMax-Pro fast response power meter (S.P.E.C.3, Omega, I LED and Xlite 2 not shown). Fig 10Bi shows Power output (W) for the S.P.E.C.3 in standard and 3K modes. Ten pulses over 0.005s equalling a rate of 20 kHz for this LCU. Note similar peak power output in both modes with lower average power output for the standard mode due to longer off duty times between pulses. The manufacturers claimed irradiance values for these modes (1600 and 3000–3500 mW/cm²) corresponded closely with GS irradiance data recorded using the I.S./spectrometer test method (~1900 and ~3400 mW/cm²). Fig 10ii to iv show the outputs for the BA Ultimate 1400, Radii Xpert, in their standard operating modes. The Cybird XD LCU in High and Plasma power output modes and the Sirius Max in Xtra power mode. The standard output mode for the Sirius Max showed steady output with no pulsing behaviour. Fig 10ii to 10iii are the same apart from the different x-axis time scales. (c) (i) shows power output for the S.P.E.C.3 in standard and 3k modes. Ten pulses over 0.005s equalling a rate of 20 kHz for this LCU. Note similar peak power output in both modes with lower average power output for the standard mode due to longer off ‘duty’ times between pulses. Depending on tip diameter (8 or 11 mm), averaged power values recorded with GS method were ~1.45–1.55 W for 3K mode and 0.85–0.9 W for standard mode and ~0.1 W less as recorded with the LabMax-Pro as displayed here. The manufacturers claimed irradiance values for these modes (1600 and 3000–3500 mW/cm²) corresponded closely with GS irradiance data recorded using the I.S./spectrometer test method (~1900 and ~3400 mW/cm²). (ii) shows the standard output mode for the S.P.E.C.3 recorded this time with the LabMax-Pro operating in Snapshot mode at 625 kHz. Note changes in pulse shape between (i) and (ii) due to ~30-fold increase in acquisition rate.