2D surface thermal imaging using rise-time analysis from laser-induced luminescence

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1 Home Search Collections Journals About Contact us My IOPscience 2D surface thermal imaging using rise-time analysis from laser-induced luminescence phosphor thermometry This article has been downloaded from IOPscience. Please scroll down to see the full text article Meas. Sci. Technol ( View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: The article was downloaded on 12/06/2012 at 22:38 Please note that terms and conditions apply.

2 IOP PUBLISHING Meas. Sci. Technol. 20 (2009) (9pp) MEASUREMENT SCIENCE AND TECHNOLOGY doi: / /20/2/ D surface thermal imaging using rise-time analysis from laser-induced luminescence phosphor thermometry School of Mechanical, Aerospace and Civil Engineering, University of Manchester, Manchester, M60 1QD, UK and Received 17 July 2008, in final form 10 November 2008 Published 13 January 2009 Online at stacks.iop.org/mst/20/ Abstract The purpose of this paper is to demonstrate a novel technique for imaging 2D temperature distributions using rise-time analysis from luminescence exhibited from a Y 2 O 3 :Eu thermographic phosphor. In phosphor thermometry, it is usually the lifetime-decay temporal response that is used to determine temperature; the rise component is usually ignored. We claim to be the first to obtain 2D thermal imaging using the rise-time response. This was demonstrated using flame impingement experiments. A 1 Mfps state-of-the-art high-speed Shiamadzu Hypervision camera was used to capture the phosphors temporal response, and was later processed in Matlab. The resulting thermal map clearly indicated a variation in temperature and showed an uncertainty of 20% at 400 C. This is relatively high, and suggestions to improve this are proposed. A calibration of rise time versus temperature is taken between 200 and 700 C. This paper builds on previous work in the field, and the results presented in this paper confirm the extended temperature sensing capability of Y 2 O 3 :Eu using rise-time characteristics. Keywords: phosphor thermometry, rise time, 2D thermal imaging, temperature, flame impingement, laser-induced fluorescence, luminescence, phosphorescence (Some figures in this article are in colour only in the electronic version) 1. Introduction The use of thermographic phosphors for temperature measurement is not new and dates back to the late 1930s [1]. Since then a number of studies have been undertaken covering a wide application area. Khalid and Kontis [2] have recently reviewed developments in the past 15 years and provided a good theoretical background to phosphor thermometry. Another excellent review is provided by Allison and Gillies [3]. The most established modes for temperature measurement are the intensity ratio and the lifetime decay mode. However, there still exist a number of modes, illustrated in figure 1, which can also be calibrated to reveal temperature. This paper focuses on one such response mode the rise-time mode. Historically, the biggest drawback of temporal approaches was instrument limitations only feasible to provide discrete point measurements. The intensity method, despite its problems, was more attractive as 2D thermal maps could easily be obtained using CCD imaging. Temperature maps using temporal responses were traditionally built up using point measurements coupled with an XY scanning device. In recent years, there have been many advances in imaging technologies making it practical for luminescence temporal responses to be imaged to reveal temperature of 2D surfaces. Researchers at the Lund Institute of Technology have adopted this technique and have been undertaking lifetime decay imaging experiments for the past couple of years [4 6]. From the best of our knowledge, we claim to be the first to obtain 2D temperature distribution maps via imaging using analysis from the other temporal response of rise time. The purpose of /09/ $ IOP Publishing Ltd Printed in the UK

3 Emission line shift and line width analysis. Decay lifetime analysis Spectral Temporal Absorption of excitation wavelength Thermographic phosphor characteristics Rise-time analysis Intensity of emission Spectral Intensity Ratio of emission lines Figure 1. Different response modes for thermographic phosphors. Intensity Initial laser pulse Rise component Time Decay component Figure 2. Typical response to a laser pulse from a Y 2 O 3 :Eu thermographic phosphor. this paper is to demonstrate this novel methodology on flame impingement experiments. 2. Rise-time analysis background The luminescence phenomenon, where excitation causes electrons to jump to higher energy levels, is well known. A typical temporal response of a phosphor to a pulse light source is indicated in figure 2. It is typically the decay characteristics that are used to determine temperature. An investigation by Rhys-Williams and Fuller [7] noted that there were rise times associated with the response of certain phosphors. In their investigation, it was shown that the rise times were related to activator concentration. Ranson later analysed rise-time characteristics with temperature and realized that they could be used for detecting temperature [8, 9]. The rise-time analysis can be performed in a variety of ways. Rise time could be the time after excitation to reach the maximum intensity. In electrical engineering terms, the term usually refers to the time it takes to rise from 10% to 90% of the maximum value. Rise-time analysis could involve rising rates, e.g. gradient at mid-intensity, exponential rise rate constants or Figure 3. Energy levels of Y 2 O 3 :Eu at symmetry sites C 2 and C 3i. other complex relations. Ranson et al [10] describe a relation that takes into account both the rise and decay elements of the entire curve. The derivation is shown in the next section. 3. Mathematical model This section describes the physical process that is used to explain the rise-time behaviour in Y 2 O 3 :Eu phosphors, and deduces a mathematical model linking the energy levels within the phosphors structure. This model and theory has been determined previously and has been reported on by Ranson et al [8 10]. This section summarizes the relevant findings. Ranson et al [10] note that the crystal structure of Y 2 O 3 :Eu has two sites of symmetry producing energy levels shown in figure 3. They note the previous work of Heber et al [11]who give evidence for three potential energy transfers (a, b and c) to the level 5 D 0. The energy transitions of paths a and b have been observed to be very fast compared to that of c [12]. It is this transition that gives this phosphor the rise-time characteristics. The emission of 611 nm (path d) follows the lifetime decay relation according to the following equation: N(t) = N 0 e t/τd (1) where N 0 in this case is the total number of electrons at 5 D 0. This is not fixed and depends on the transition paths a, b and c. The fast transitions a and b can be modelled as being instantaneous, but the transition of c is dependent on the decay of electrons from C 3i to 5 D 0 which decay at N c3i (t) = N c e t/τd. (2) Thus, the number of electrons accumulated from path c asa function of time is N c (t) = N c N c e t/τr = N c (1 e t/τr ). (3) The total number of electrons at D 0 is then N 0 (t) = N ab + N c (1 e t/τr ). (4) Combining the equations yields the full characterization of the decay. This is the equation that will be referred to as Ranson s relation throughout the rest of this paper: N(t) = [ ( )] N ab + N c 1 e t/τ r e t/τ d (5) where τ d is the lifetime decay, τ r is the rise time, N ab and N c are the numbers of electrons by transitions a, b and c respectively. 2

4 Nd YAG Laser Trigger Q- Switch Control Picotech USB Oscilloscope POST PROCESSING MATLAB 532nm 532nm 266nm frequency doubling crystals Transimpedance Amplifier Filter Lens Phosphor target PMT Harmonic Prism Separator & Beam Dump 266nm thermocouple Picotech Thermocouple Module Furnace Figure 4. Schematic of the calibration procedure. 4. Calibration procedure A calibration of rise time versus temperature was necessary in order for any quantitative data to be extracted from the flame impingement experiments. This was completed in advance. A Nd:YAG laser operated at 266 nm was used to excite the phosphor. A PMT was used due to its excellent responsitivity at high speeds (typical rise and fall times of 1 ns) to capture the subsequent emissions. The signal was passed through a transimpedance amplifier to a USB Picotech data acquisition system with a termination of 50. This calibration enabled the understanding of response characteristics with timescales that would be essential for optimizing the highspeed camera settings, for mathematical model verification and for post-processing. A Lindberg box furnace with a Eurotherm controller was used to heat the thermographic phosphorcoated sample. The temperature was allowed to stabilize for 15 min before readings were taken. The laser was set to 15 Hz, and a total of 150 pulses were averaged to reduce random noise errors. The proportion of the laser spike that passed through the narrowband filter was ignored, and measurements were recorded 0.5 μs after the pulse. A schematic of the calibration is shown in figure 4. Three temperature calibrations were performed. One was based on the rise constants determined from Ranson s full characteristic relation (equation (5)) [8]. This relation was expanded into a form that Matlab s curve fitting routines could interpret (equation (6)) and was used to fit the data using the nonlinear least-squares method. The equation fit very well showing R 2 values over 0.9. The analysis also computes additional terms for decay characteristics (τ d ), and to obtain a good fit, the decay segment of the curve was also captured: N(t) = N ab (e t/τd ) + N c (e t/τd ) N c (e t/τd )(e t/τr ). (6) After mathematically expressing transformations, highlighted in figure 5, a simpler calibration curve was also fitted, using only the rise segment of the curve, according to the equation N(t) = a b(e t/τr ). (7) This relation is another form of equation (4), where a = N ab + N c and b = N c. At any given temperature, the rise constant determined by this was lower, with R 2 values that were poorer than that determined by Ranson s full relation. The reason for this may be that this relation only relates to the increase in electrons sited at the energy state 5 D 0 shown in figure 3, instead of the full characterization of the emission that is also subject to an exponential decay. The last calibration was based on the time to reach the maximum intensity. This form of calibration has not been previously reported, and although it showed the greatest sensitivity to temperature, it is yet subject to further investigation. Figure 6 illustrates all these calibration lines decreased in value with increasing temperature. 5. Experimental procedure The test chosen to demonstrate the rise-time imaging technique was that of flame impingement onto the back surface of a Initial Laser Pulse Negative exponential rise with negative rise constant (T r) Transformations Exponential decay with negative decay constant (T d) a b b e t r Subtract a Multiply -1 Time to Max Intensity Transformations mathematically expressed as: N ( t) a b e t r Figure 5. Transformation to show that the rise segment can be transformed to a simple exponential decay. 3

5 Nd YAG Laser 532nm 532nm 266nm frequency doubling crystals Harmonic Prism Separator and Beam Q-Switch Control Trigger 266nm Shimadzu Control, Image Capture and Storage Shimadzu High Speed Camera DOE Commercial Camera Lens Figure 6. Temperature calibrations based on various relations. metallic plate. A schematic is shown in figure 7. A Q- switched Nd:YAG laser in the fourth harmonic was used to excite a Y 2 O 3 :Eu (3 4%) phosphor that was chemically bound onto a nickel N75 alloy surface of approximately mm 3. A diffractive optical element (DOE) was used not only to expand the beam but also to remove any intensity peaks and change the Gaussian distribution of the beam to a more uniform, top hat distribution. The laser beam on the target was approximated to be 12 ns 5 mj over a diameter of 25 mm (490 mm 2 ), which translates to a peak power of 416 kw and an energy density and power density of 1.02 mj cm 2 and 85 kw cm 2 over the surface. A commercial butane/propane-mix torch impinged onto the back surface of the plate located approximately 120 mm away. A high-speed Shimadzu Hyper Vision HPV1 imaging camera was used to capture the rise and decay of the phosphor. A Nikon commercial camera lens was used to focus and optically magnify the area of interest, and a narrowband interference filter peaking at 611 nm with a FWHM of 8 nm was used to isolate the phosphors response from reflections/scatter from the laser, other luminescence emission wavelengths that may be rising and decaying at different rates, and to limit blackbody radiation from other wavelengths. The camera was configured to enable the fastest acquisition rate of 1 Mfps (1 million frames per second), giving a sampling window of 1 μs. Acquiring at these speeds requires immense light energy since the capture must occur within this fraction. This was problematic given that the laser beam was expanded to reduce its energy/area. Reducing the beam size would consequently reduce the detection area. A compromise was made and a beam diameter of 25 mm was chosen. The temperature of the CCD was stabilized at 10 C using a builtin electric Peltier cooler. The cameras sensitivity had to be optimized to enable detection without reaching saturation. The exposure time was set to 1/2 sample time (0.5 μs), which was POST PROCESSING - MATLAB Pre-Calibrated Data Narrow Bandpass Filter Sample with thermographic phosphor Butane/Propane Mix Flame Torch Figure 7. Schematic of the rise-time imaging experiment. the maximum for that chosen frame rate. Since the camera did not have an intensifier unit, it was configured to the maximum gain. The exposure time could have been increased to detect more light at the expense of temporal resolution. It was found that this was the optimum setup for the light levels encountered and the temperature range under investigation. The camera could only capture 102 images, and based on the sampling rate, this calculates to a maximum of 102 μs. A trigger linking the laser Q-switch to the camera was set up. A pre-trigger was configured so that five frames could be captured before the trigger to eliminate any delays in the triggering systems. To obtain a good variation of temperature to demonstrate thermal rise-time imaging, the flame was later focused at one of the corners of the detection area as shown in figure 8. This was done for two main reasons: (a) a flame focused at the centre did not provide a good distribution of temperature over the detection area; (b) although the DOE tried to homogenize the Gaussian laser beam distribution, there may have still been an optical distribution peaking at the centre. Focusing the flame away from the centre ensured that this would not be mistaken for the distribution of temperature, which may exhibit similar variations. 6. Choice of camera/methodology Apart from XY scanning from point source detectors, there exist a number of methods for rise-time imaging. The author proposes three possible methodologies using CCDs. The 4

6 Nickel alloy plate Area with thermographic phosphor Detection area Area covered by the laser beam Region where the butane/propane torch was focused - (Flame impingement centre) Figure 8. The positioning of the flame centre in relation to the laser beam centre. pump and probe technique relies on a camera that can be precisely triggered to take a single image with exposure time (de) after a controllable delay time (dt). A single laser pulse would not be sufficient, and the technique relies on repetitive signals with a time delay that can be incrementally increased to build up a temporal picture. Figure 9 illustrates this concept and compares it with other competing techniques. This technique would not be suited to measuring the thermal properties of transient/unsteady events. For example, if the time delay were incremented at 1 μs/pulse for a total of 300 μs, with a 15 Hz laser, it would take 300/15 = 20 s to acquire the data. The application of the technique is usually restricted by the inability of the camera to acquire at high frame rates. The system relies on repetitive laser pulses and assumes that the measurement property does not change during the camera acquisition phase, and would be suited only for steady state events. Also, the technique assumes negligible pulse-topulse variation in laser power. In some instances, the authors have tried to normalize the intensity using readings from a laser power meter. The other two methods do not have these problems, and acquisition can be made using a single laser pulse. Method 2 relies on multiple CCD cameras that are precisely triggered in sequence. Each camera captures its portion and a time sequence is built up. A problem with this technique is that each CCD may capture a different image depending on its alignment. A rotating turbine mirror is sometimes used to deflect light from one CCD to another at high speed. A prism may also be used instead to split the light equally onto each CCD. Researchers at the Lund Institute of Technology have applied this technique to obtain lifetime decay imaging using a prism-based eight-ccd camera system [5]. There may still remain errors due to CCD differences caused by manufacturing defects, making the cameras sensitivities slightly different from each other. The third option relies on newer systems that allow fast framing at the required rates. The advantage of this approach is that it does not rely on repetitive signals and acquisition can be made from a single laser pulse using a single camera. However, with the current state of technology, image spatial and spectral resolutions are lower than those of cameras used in the previous techniques. The current Shimadzu system is an 8-bit system (0 255 variations) of pixel images with a maximum sampling rate of 1 Mfps and a minimum exposure time of 250 ns. Using similar exposure times, a finer sampling rate can be achieved using the other described approaches, and it would also be possible to overlap temporal images to obtain even finer time resolutions. 7. Post-processing The post-processing that is required to reveal a temperature map was undertaken in Matlab. A more detailed account of the procedure is highlighted in figure 10. The output TIF images from the camera were loaded and stored into a matrix array (i, j time). A cropping routine was used to crop all images to include only the area of interest. This was done to save the processing time, since the curve fitting routine took approximately 0.5 s of computation per pixel. A pixel image ( pixels) results in computation times over 5 h. Rise constants per pixel were determined using a curve fitting routine based on the expanded form of Ranson s relation (6). The goodness of fit-r 2 value was also extracted for later de dt dt dt dt Frames 1 to N Option 1: Pump & Probe Option 2: Multiple cameras Option 3: High speed camera Frame Frame 1 Frame 1 Pulse 2 Frame 2 Camera 2 Frame 2 Frame 2 Pulse 3 Frame 3 Camera 3 Frame 3 Frame 3 Pulse 4 Frame 4 Camera 4 Frame 4 Frame 4 Figure 9. Methodology for pump and probe, multiple camera and high-speed camera techniques. 5

7 TIF IMAGES FROM THE HIGH SPEED CAMERA LOAD IMAGE FILES Store in Matix Array [ i.j. x frame] CROP Crop all images to only include area of interest FOR LOOP For pixels 1:i FOR LOOP For pixels 1:j Store in Matrix Array i,j,rc,gof Extract rise constant Extract Goodness of Fit Apply curve fitting algorithm Obtain Intensity vs. time data for single pixels n Construct Image (pixel by pixel) using rise constant coefficients Apply Calibration Curve Rise-constant vs. Temp CALIBRATION DATA TEMPERATURE MAP/IMAGE based on rise-time analysis Construct BINARY Image for Goodness of Fit Construct Image (pixel by pixel) based on Goodness of Fit. IF value > 0.5 Then pixel = 1 ELSE pixel = 0 Multiply Images (pixel by pixel) TEMPERATURE MAP/IMAGE Based on rise-time analysis with bad fits eliminated Figure 10. Flow diagram details of the post-processing procedure. analysis. R 2 is a fraction between 0 and 1 that quantifies goodness of fit, with 1 being a perfect fit with no scatter. It is computed from the sum of the squares of the distances of the points from the best-fit curve. Although it should not be the only criterion used for determining whether a fit is reasonable, it provided a quick way to filter out bad-fits. These coefficients were stored in a matrix array, and a binary image was created eliminating all values that had R 2 less than 0.5. Calibration data were applied to the data to obtain a temperature map based on rise-time analysis. The images were multiplied together to obtain a temperature map that excluded data that did not fit the equation properly. 8. Results and discussions The results shown in figure 11 display a temperature map based on the rise constant derived from Ranson s relation (5). The results demonstrate successful determination of temperature and thermal imaging using rise-time analysis. The final thermal map, figure 11(C), was derived by multiplying the initial thermal map (B) with the binary image of the curvefit goodness (A), which eliminated all fits that had R 2 less than Basic uncertainty analysis The curve fitting failed at areas which had not been excited by the laser, as expected. In the final thermal map, the only area that remained was the area that had been excited by the laser beam. The curve fitting at the flame impingement centre was also poor. A profile analysis from top-left to bottomright, shown in figure 12, illustrates a temperature distribution varying from 300 to 700 C. A profile analysis of goodness of fit shows an inversely proportional relation to temperature, with R 2 being in the region of 0.8 at 300 C, varying to 0.5 at temperatures above 600 C. This seems reasonable since increasing temperature decreased luminescent intensity, whilst making both the rise and decay occur at a faster rate. This makes it more demanding for the camera to detect both a faster and less intense source. Assuming horizontal and vertical symmetry at the axis of the flame impingement centre, figure 13 demonstrates the radial distribution of temperature from the centre of impingement. Amongst various uncertainty contributions, one significant involvement is the goodness of fit. Assuming that the uncertainty in rise-time constants is directly related to computed R 2 values, upper and lower limits of the rise constant can be deduced (table 1), and be used to determine the temperature uncertainty from the temperature calibration curve. This is shown in figure 14. The uncertainty in temperature measurement, taking into consideration only the variation in rise constant values, is almost uniform at approximately ±80 C. This is concordant with temperature point variations shown in figure 12. At 6

8 (A) BINARY Goodness of Fit Image (C) Final Thermal Map X (B) Initial Thermal Map Figure 11. Images (A) goodness of fit binary image, (B) initial thermal map based on rise-time imaging, (C) final thermal map, derived by multiplying both (A)and(B). Temperature o C R 2 (expressed as a percentage) Goodness of fit % position from top-left to bottom-right % position from top-left to bottom-right Figure 12. Profile analysis of temperature and goodness of fit from the top-left to bottom-right of the detected area. Table 1. Determining uncertainty in rise constant values. Average Average Rise-time Position % temperature goodness constant distance ( C) of fit (%) variation (%) (80%) (80%) (70%) (60%) (55%) (50%) (40%) 60 Figure 13. Extended temperature map assuming horizontal and vertical symmetry. an average temperature of 400 C, a temperature uncertainty of approximately ±20% is determined using this analysis. This is relatively lower than other competing non-contact techniques, such as infrared thermal imaging and pyrometry [13], and other phosphor responses such as lifetime decay and intensity ratio [14]. Focusing on phosphor temporal responses, it has previously been shown that Y 2 O 3 :Eu only exhibits temperature dependence on the lifetime decay mode at temperatures over 600 C[15] to 1100 C[8]. The results presented in this paper confirm that the rise-time analysis does allow the temperature measurement between 200 and 700 C, 7

9 ± 50% rise constant variation at 600 degrees C Temperature ( o C) ± 20% temperature uncertainty Calibration Curve ± 30% rise constant variation at 400 degrees C ± 20% rise constant variation at 300'C 100 upper and lower limits Time (μs) Figure 14. Determination of temperature uncertainty, based on the variation in rise constant values. extending the phosphor s temperature measuring range. In other sources, the rise-time analysis is shown to reveal temperatures between C[16] and C[10]. By having a combination of both temporal responses (rise and decay), a temperature range between 25 and 1100 C could be determined [8]. Since the rise response is sensitive to activator concentrations [7], it may provide additional insight into other environmental parameters; further research is required to explore this possibility. Apart from phosphor thermometry, fluorescence lifetime decay is extensively studied in many disciplines not only to reveal temperature but also to reveal other properties such as pressure and oxygen levels, using phosphors and pressuresensitive paints [17 24]. There are many uses for lifetime imaging, such as lifetime fluorescence microscopy that is widely used in the life sciences to map a whole range of parameters such as ph, ion concentrations or oxygen saturation and cancer cells [25, 26]. On the whole, the rise-time response is largely ignored. The rise-time analysis may provide additional insight into other parameters that may not be possible to be detected using solely the decay response, and may prove to be a valuable analysis tool in such fields. 9. Future work Although the experiment presented in this paper demonstrated rise-time imaging capability to determine 2D temperature maps, the uncertainty in measurement (approximately 20% at 400 C) was relatively high. A number of suggestions for future work that may improve the uncertainty have been proposed. The absorption spectrum of Y 2 O 3 :Eu at room temperature is noted to be much greater at 266 nm than at other absorption wavelengths, approximately 20 greater than 337 nm and 355 nm [8]. Although this is the case at room temperature, the absorption needs to be considered as a function of temperature. Using 266 nm, it was found that the luminescence intensity decreased with increasing temperature. A study by Allison et al [16] noted an increase in intensity with increasing temperatures, using a 337 nm nitrogen laser. The explanation given in the study was due to increase in the absorption level that increased with temperature. The increase in luminescence intensity at higher temperatures may help the camera overcome its intrinsic noise and yield better signal-to-noise-ratios (SNR). Other methods that may help to boost SNR include the use of an intensifier unit, typically used in ICCDs. This is common in lifetime decay imaging experiments. The intensifier will pre-amplify the low light to higher values. Work by Ranson [8] performed investigations at various dopant levels to fine-tune the sensitivity of the phosphor. Rhys-Williams and Fuller [7] showed that rise times ranged from 60 μs at around 5% mole concentration to 320 μs at 0.27% mole concentration. Recent work by Allison et al [16] underwent investigations at 0.5% Eu. From these references, it is shown that the use of lower activator concentrations significantly lengthens the rise time. This will enable the camera to be operated at slower speeds, allowing for increased exposure times and hence more light being captured, which may help to improve SNR. However, this may need to be evaluated against the quantum efficiency of the phosphor at these new activator concentration levels. To reduce random error noise, images could be averaged or added together from a number of pulses. However, this would change the image capturing methodology to pump and probe, adding time for acquisition, making it unsuitable for capturing transient or unsteady thermal events. 8

10 10. Conclusions The temporal response of a Y 2 O 3 :Eu phosphor includes a rising and a decaying element. In phosphor thermometry, the rise time is often ignored. Y 2 O 3 :Eu only exhibits temperature dependence on the lifetime decay mode at temperatures over 600 C to temperatures around 1100 C[8], whereas the rise-time response can reveal temperatures from 25 to 800 C[10]. By having a combination of both the temporal responses a temperature range between 25 and 1100 C could be mapped, increasing the detection range of the phosphor [8]. In this study, the rise-time response of the Y 2 O 3 :Eu phosphor was calibrated against a temperature range of C. A flame impingement experiment was undertaken to demonstrate 2D rise-time thermal imaging for the first time. A single pulse of 266 nm laser was used to excite the phosphor, and a 1 Mfps high-speed camera was used to capture the response. Although the experiments demonstrated rise-time thermal imaging successfully, the uncertainty in the measurement was relatively high. A number of methods that could improve this were suggested. The temporal analysis of fluorescence is extensively used in a number of disciplines, with the lifetime decay being a very well established indicator of a number of properties. The rise time may provide additional insight into other parameters that may not be possible to detect using solely the decay response. Acknowledgments The authors would like to thank the Engineering and Physical Sciences Research Council (EPSRC) for financial support and the loan of the camera; Rolls-Royce plc for the loan of the laser, financial support and technical advice. They would also like to thank Hosein Zare-Behtash and Dominic Jones for useful discussions and assistance during experimental setup and post-processing. 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