Abstract:
Radiation scattering is one of the main contributors to the uncertainty of near infrared (NIR) measurements. Enhanced absorption-measurement accuracy for NIR sensors is achieved by using a combination of NIR spectroscopy and time-of-flight techniques to select photons that are the result of a given mean free path within a moving sample target. By measuring absorption as a function of path length or by windowing signals that are attributable to excessive scattering of NIR radiation within the sample, this technique affords the calculation of more accurate and more universal calibrations. The NIR sensor employs short or ultra-short laser pulses to create NIR that is directed to the moving sample and emerging radiation is detected over time. Windowing effectively truncates non-contributing measurements.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to scanning sensors that employ near infrared radiation for detecting the presence of specific components in paper, plastic, powders and like products on a continuous basis. In particular, the sensors employ windowing or time-correlated single-photon detection techniques that reduce the adverse effects of scattering on absorption measurements. 
     BACKGROUND OF THE INVENTION 
     Various sensor systems have been developed for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. Sensors for continuous flat sheet production processes typically employ single or dual-sided packages with on-line sensors that traverse or scan traveling webs of sheet material during manufacture. Near infrared (NIR) spectroscopy is the method of choice for measuring composition or component weight and moisture content in a multitude of products. These include materials produced in sheets such as paper and plastic. The technique is fast, inexpensive, and is compatible with on-line measurement, which allows the process to be controlled in a closed-loop fashion. NIR spectroscopy is accurate if a suitable calibration model can be obtained for the product to be measured. A specific calibration model is required for two main reasons. One reason is that a number of overlapping absorption bands exists in the NIR. Typically, a number of components in the product contribute to the measured absorption bands and a model is required to separate the contributions from the individual components. The second reason is related to light scattering: when light interacts with a sample it gets absorbed and scattered and the amount of scattering depends on the chemical as well as the structural properties of the sample. Paper, in its simplest form, is a mixture of cellulose fibers surrounded by air. Due to index of refraction changes, the cellulose/air interfaces lead to significant light scattering. The scattering power of paper can change dramatically as fillers or even moisture fill the gaps between the cellulose fibers thereby displacing the air. Scattering affects the NIR absorption technique through changes in the average path length through the sample. Scattering, especially in products like paper and powder samples, can significantly reduce the accuracy of absorption-type measurements due to changes in the photon mean free path. As calibrations are not only dependent on a single component but on many components in a non-linear fashion, calibration curves cannot be simply computed. For example, the calibration curves for measuring moisture in paper are multidimensional and depend on cellulose, ash, and furnish contents and concentrations. Simpler calibrations would greatly assist end users by improving the accuracy and robustness of on-line measurements. 
     SUMMARY OF THE INVENTION 
     The present invention is based, in part on the recognition that increased absorption-measurement accuracy for near infrared (NIR) sensors can be achieved by using a combination of near infrared spectroscopy and time-of-flight techniques to select photons that are the result of a given mean free path within the target. In particular, measurements of absorption as a function of path length are conducted and by fitting a model that correlates the absorption and the scattering of NIR in the target to the data, scattering-free absorption measurements are obtained. Alternatively, an average absorption per unit path length can be calculated from the data. The average absorption normalized by unit path length is by definition free of the scattering contribution. 
     In another embodiment, the signals that are attributable to excessive or minimal scattering of NIR radiation within the sample of interest are removed through windowing. Another possible technique is referred to as time-correlated single-photon counting (TCSPC) where the processor operates by measuring the arrival time of every photon as represented by electrical detection signals and uses an algorithm to determine at least one property of the material being monitored. TCSPC is particularly useful where there are restraints to the intensity that can be employed and is more accurate for shorter pulses or for targets where the maximal amount of scattering is less. 
     These techniques afford the calculation of more accurate and more universal calibrations. As scattering is one of the main contributors to the uncertainty of NIR measurements, the inventive method of extracting the effects of scattering produces more accurate absorption measurements. To remove the effects of scattering, the inventive NIR sensor employs short or ultra-short light pulses and a way of discriminating the measurements by time-of-flight. As described above, the technique can be implemented by modeling the absorption measurements to calculate the scattering, by calculating an average absorption per unit path length or by measuring absorption of the photons that are selected for their similar path lengths or time-of-flight through the sample composition. 
     Accordingly, in one aspect, the invention is directed to a sensor for measuring at least one property of a composition of a moving sample that includes: 
     a light source, which emits broadband optical pulses at a sample of the composition; 
     a receiver operable to detect reflected or transmitted radiation from the sample and to provide electrical detection signals; 
     synchronization means for receiving electrical pulses from the light source or optical pulses from the receiver and for providing electrical synchronization signals to a processor; and 
     a processor that receives the electrical detection signals and the electrical synchronization signals and that is operable to determine at least one property of the composition with substantial independence of the measurement from the effects associated with scattering in the composition. 
     In another aspect, the invention is directed to a system for continuous on-line measurement of a characteristic of a moving sample that includes: 
     a broadband light source, which emits optical pulses, operable for emitting pulsed radiation at the moving sample, wherein the ultrafast light source travels over the cross direction of the moving sample; 
     a receiver operable to detect reflected or transmitted radiation from the sample and provide electrical detection signals and wherein the receiver travels over the cross direction of the moving sample; 
     synchronization means for receiving electrical pulses from the light source or optical pulses from the receiver and for providing electrical synchronization signals to a processor; and 
     a processor that receives the electrical detection signals and the electrical synchronization signals and that is operable to determine at least one property of the composition with substantial independence of the measurement from the effects associated with scattering in the composition. 
     In yet another aspect, the invention is directed to a method of measuring at least one property of a moving sample that includes the steps of: 
     (a) directing radiation at the moving sample; 
     (b) measuring reflected or transmitted radiation from the sample and generating electrical signals therefrom; and 
     (c) determining at least one property of the sample from the electrical signals whereby electrical signals associated with scattering within the sample are processed with the knowledge or consideration that scattered photons have longer transit times. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 3  illustrate scanning NIR sensor systems employing time-correlated single-photon counting; 
         FIG. 2  is a depiction of the photon paths through a sample; 
         FIG. 4  illustrates a scanning NIR sensor system employing an acousto-optic tunable filter; 
         FIG. 5  illustrates a scanning NIR sensor system employing windowing; 
         FIG. 6  is a graph of photons vs. transit time measured at different wavelengths; and 
         FIG. 7  shows a sheet making system implementing the NIR sensor. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows the structure of a NIR sensor apparatus for monitoring at least one property of the moving sheet or web of material  50 . The NIR sensor is particularly suited for measuring properties of continuous web materials such as sheets of paper or plastic. The sensor can also be readily adapted to measure a continuous stream of discrete materials, such as powder compositions, that is conveyed past the sensor. The sensor generates NIR radiation  52  that is directed to material  50  and measures the radiation that emerges therefrom using time-correlated single-photon counting (TCSPC). The principle of TCSPC is the detection of single photons and the measurement of their arrival times with respect to a reference signal, usually a light source. TCSPC is a statistical method and a high repetitive light source is employed to accumulate a sufficient number of photon events for a required statistical data precision. For example, a light source can be employed to generate both the (i) reference light pulses that are converted to references electronic (reference) pulses and (ii) sample light pulses that are directed to a sample target. Single photons emerging from the sample are converted to single photon (signal) pulses. The TCSPC electronics can be viewed as receiving two inputs, with the electronics being initiated when it receives a reference pulse and the electronics being stopped by the signal pulse. The time interval is measured. The intensity of the measured photon is not critical aside from discriminating against double count events; it is the timing of the signals that is of importance. TCSPC uses intensity filters, for example, to reduce the photon intensity to a level where the probability of a photon being detected by each detector from each pulse is substantially less than unity. This is then timed to a ‘zero scattering’ photon. With many pulses a curve that contains much information is generated and from which analysis yields additional and more complex measurements. TCSPC is further described in “Advanced Time-Correlated Single Photon Counting Techniques,” Becker, W., Springer (2005);  Time correlated single - photon counting  ( TCSPC )  using laser excitation , Phillips, D.; Drake, R. C.; O&#39;Connor, D. V.; Christensen, R. L. Source:  Analytical Instrumentation , v 14, n 3-4, p 267-292, September-December 1985 and U.S. Pat. No. 6,342,701 to Kash, which are incorporated herein by reference. 
     In particular, the NIR sensor includes an ultrafast laser  2 , which produces laser pulses  14 , and that is coupled to a supercontinuum generator  8 . For example, a pulsed laser source that is coupled to a nonlinear fiber can generate supercontinuum light pulses  24  over the desired wavelength range. The required duration of supercontinuum light pulses  24  depends on the amount of scattering. For paper samples, it is expected that the detected pulses be about 200 ps in length, requiring input light pulses of about 1 ps in duration. For such requirements, a preferred laser is an ultrafast modelocked laser with supercontinuum generation. However, if the NIR sensor is to measure only one or two wavelengths, such as for a moisture measurement in paper, the required wavelengths can also be generated by non-linear wavelength mixing and other means. NIR radiation  24  is focused by objective lens  10  and directed by mirror  28  into moving sheet  50 . Instead of traveling through free space, in some applications, radiation can be launched into and transmitted through a delivery fiber optic cable or optical fiber. 
     In this embodiment, receiver  4  is configured as a three-channel NIR detector for measuring three properties in material  50  and from these properties other characteristics, such as moisture content, can be derived. Receiver  4  includes dichroic mirrors or optical filters  36 ,  38  and  40  and corresponding detectors  42 ,  44  and  46 . Each dichroic beam splitter is configured for high transmissivity for certain parts of the radiation spectrum and/or high reflectivity in certain other parts of the radiation spectrum. Each detector  42 ,  44  and  46  can comprise a photomultiplier tube (PMT) or other fast optical detector or photodetector. Optionally, separate infrared band pass filters and/or intensity filters  37 ,  39 , and  41  can be positioned before detectors  42 ,  44 , and  46 , respectively; in this fashion, each detector measures the intensity of only the portion of the NIR beam spectrum that falls within the band pass of the associated filter. When both band pass and intensity filters are employed, the intensity filter can be positioned immediately downstream of the band pass filter. Each PMT detector  42 ,  44  and  46  captures selected regions of NIR  54  that emerges from moving sheet  50 . Each detector generates output electrical detection signals corresponding to the intensity of photons measured. A spectrometer can be employed instead of the optical filters (dichroic) and associated individual detectors. 
     The gap or displacement distance “z” between sensor heads  60 ,  62  through which the sample traverses can vary particularly when the dual sensors are in motion as part of a scanner. To account for this z “wander,” the gap separation can be continuously measured. Dynamic measurements can be achieved with conventional devices, such as, for example, a displacement sensor, that employs inductive or magnetic measuring device  31 A,  31 B, which is described in U.S. Pat. No. 7,199,884 to Jasinski et al., which is incorporated herein by reference. Distance signals  56  from z measurements are communicated to processor  12  that calculates the time delay based on the z measurements and generates time delay signals to delay device  16 . 
     In this embodiment, the source of NIR radiation also provides the synchronizing signals so that the steps of directing radiation to the sample and measuring reflected or transmitted radiation from the sample are synchronized as part of the process of measuring absorption as a function of time-of-flight. Synchronization signal  18  is generated by a mode locker driver  6  of the laser  2  and is directed to electrical delay device  16  to take into account of (and corrected for) the z wander during scanning. Other devices, such as an in-built photodiode, can be employed to generate this signal. Electronic delay device  16 , which delays synchronization signal  18 , is configured to provide electrical synchronization signals  22  to processing system  64  to effectively switch on NIR receiver  4  in a synchronous detection scheme. 
     The outputs from detectors  42 ,  44 , and  46  are electrical signals that initiate the TCSPC electronics. For example, a signal processing system  64  is coupled to detectors  42 ,  44 , and  46  to receive the electrical detection signals. The signal processing system  64  comprises a memory  66  for storing calibration and normalization data to permit calculation of the moisture content, caliper or basis weight in the case where material  50  is paper. Signal processing system  64  also includes processing electronics and a processor or analyzer  68 , such as a digital signal processor, that receives processed electrical signals (amplified, filtered and converted to a digital signal) from the processing electronics. The processor  68  combines the signals received to determine at least one property of the material. For example, the processor operates by measuring the arrival time of every detected photon as represented by the electrical detection signals and uses an algorithm to determine at least one property of material  50  with substantial independence of the measurement from the effects associated with scattering in the composition. 
     As shown in  FIG. 1 , when operating in the transmissive mode, the light source that includes the ultrafast laser  2  and supercontinuum generator  8  can be housed in sensor head  60  and NIR receiver  4  can be housed in sensor head  62  that is on the opposite side of material  50 . The NIR sensor can also operate in the reflective mode, in which case, both NIR source and receiver are positioned on the same side as material  50 . Typically, the remaining components of the NIR sensor, such as processor  12  and electrical delay device  16 , are housed in module  58  that can be located remotely from the sensor heads. 
       FIG. 2  depicts sample  50  that is positioned within a measurement gap that is defined by surfaces  72 ,  74  of two oppositely facing sensor heads. NIR  76  that is directed to sample  50  from a NIR source (not shown) interacts with components within the sample before exiting the sample and being detected in NIR receiver (not shown). A portion of the radiation will be absorbed and scattered. The degree of scattering in the sample depends on, among other things, the composition of the sample, temperature, and wavelength of the NIR. Highly scattered radiation  78  remains in the sample a longer period of time before exiting. Radiation  76  and  78  are arbitrarily shown to experience an identical number of scattering events. The highly scattered radiation  78  could experience a number of scattering events significantly larger than that of radiation  76 . 
     In operation of the NIR sensor, the system preferably undergoes an initial standardization procedure with respect to the material being monitored. In one standardization technique, with the sensor in the “off-sheet” mode so that no product is in the measurement gap of the sensor, a flag that consists of a thin layer of PTFE (TEFLON) or aluminum oxide (Al 2 O 3 ) is inserted into the gap between the NIR source and receiver. Thereafter, the NIR sensor is activated and the integrated photon counts at all NIR wavelengths of interest over a fixed delay time period are recorded. The ratio of the integrated photon counts at time zero over the now integrated photon counts yields a standardized correction value for each wavelength. The correction value is applied to normalize each subsequent measurement in order to correct for variations in the radiation source, gap alignment and other operating parameters. Another standardization technique is to use a flag that has the appropriate physical properties in term of density, thickness and composition so that it contains path lengths that are similar to those in the material to be measured. Thereafter, the instrumental function is measured at all NIR wavelengths of interest. The ratio of the instrument functions at time zero with the now instrumental functions yields a standardized correction value that can be applied to subsequent measurements. As all the detectors have different optical paths, the standardization signal can be used to synchronize the different detection channels. 
       FIG. 3  shows another structure of a NIR sensor apparatus using time-correlated single-photon counting wherein the synchronization signals are generated by a fast photodetector within the receiver. Supercontinuum light  124  is generated by a pulsed laser source  102 , which produces laser pulses  114 , and that is coupled to a supercontinuum generator (nonlinear fiber)  108 . NIR radiation  124  is focused by objective lens  110  and radiation  152  is directed by mirror  128  into moving sheet  150 . Receiver  104  is configured as a two-channel NIR detector and includes dichroic mirrors or optical filters  136 ,  138  and  140  and corresponding detectors  142 ,  144  and  146 . Each detector can comprise a photomultiplier tube or other fast photodetector. Separate infrared band pass and/or intensity filters  137 ,  139 , and  141  are optionally positioned before detectors  142 ,  144 , and  146 , respectively. Each detector  144  and  146  captures selected regions of NIR  154  that emerges from moving sheet  150 . Each detector generates output electrical detection signals corresponding to the intensity and timing of photons measured. Ultrafast laser  102  and supercontinuum generator  108  can be housed in sensor head  160  and receiver  104  can be housed in sensor head  162 . 
     Fast optical detector or photodetector  142 , which is responsive to the earliest transmitted photons, generates synchronizing signals  176  to take into account the head movement so no displacement sensor is required to measure the gap distance “z” between sensor heads  160 ,  162 . Synchronization signal  176  is directed to processing system  164  to synchronize detectors  144  and  146 , which collectively measure two properties of material  150  using memory  166  and processor  168 . 
       FIG. 4  shows the structure of a NIR sensor that employs an acousto-optic tunable filter (AOTF) light source and time-correlated single-photon counting. Instead of using an AOTF, a micro-mirror array and appropriate optics can be used as the tunable grating. Moreover, other technologies such as liquid crystal tunable filters can also be used. As an alternative to the configurations of the NIR sensors as shown in  FIGS. 1 and 3  in which broadband light is directed through a material, light can be dispersed so that only a discrete wavelength is allowed through by use of an AOTF. In this case, light will scatter in the material being measured and the time of arrival at the receiver will be a strong function of the amount of scattering. This technique, which executes measurements in series or sequentially, may be slower than those depicted in  FIGS. 1 and 3  which execute the measurements in parallel. 
     As illustrated, broadband light  224  is generated by a pulsed laser source  202 , which produces laser pulses  214 , and that is coupled to a supercontinuum generator (nonlinear fiber)  208 . NIR radiation  224  is focused by objective lens  210  into AOTF  240 . Each pulse  226 , which is generated by the broadband source  208 , is filtered by AOTF  240  so that only one narrow wavelength band  226  is generated at a time and directed by minor  228  into moving sheet  250 . As AOTF  240  receives and filters pulse  226 , AOTF  240  generates corresponding wavelength information  232  to processing system  264 . Receiver  204  includes mirror  238  that directs radiation  254  emerging from material  250  through filter  239  and into PMT or fast photodetector  244 . PMT  244  generates output electrical detection signals corresponding to the intensity and timing of photons measured. Synchronization signals  282  which are illustrated as being derived from source  280  can be generated by the laser and then delayed by an electronic delay box that is controlled by a z sensor as illustrated in  FIG. 1  or it can be generated by a fast photodetector as illustrated in  FIG. 3 . In this regard, z-direction sensor  231 A,  231 B is employed in the former synchronization scenario. A signal processing system  264 , which is coupled to detector  244 , receives the electrical detection signals and comprises a memory  266  a processor or analyzer  268 . 
     While AOTF  240  is illustrated in  FIG. 4  as being configured to direct filtered radiation  252  into material  250 , the AOTF can also be positioned downstream of material  250 . In this case, the AOTF filters the broadband radiation that emerges from the material and directs a narrow wavelength band into the photodetector. Ultrafast laser  202  and supercontinuum generator  208  can be housed in sensor head  260  and receiver  204  can be housed in sensor head  262 . 
       FIG. 5  shows the structure of a NIR sensor apparatus wherein NIR radiation  353  is directed to a material and the radiation  354  that emerges therefrom is analyzed with windowing techniques. Supercontinuum light  324  is generated by a pulsed laser source  302 , which produces laser pulses  314 , and that is coupled to a supercontinuum generator (nonlinear fiber)  308 , which can be housed in sensor head  360 . Supercontinuum light can also be generated by other well-known means. NIR radiation  324  is focused by objective lens  310  and directed by mirror  328  into moving sheet  350 . Receiver  304 , which can be housed in sensor head  362 , includes dichroic mirrors or optical filters  336 ,  338  and  340  and corresponding PMT or fast photodetectors  342 ,  344  and  346 . Separate infrared band pass filters  337 ,  339 , and  341  are optionally positioned before detectors  342 ,  344 , and  346 , respectively. Each detector generates output electrical detection signals corresponding to the intensity and timing of photons measured. 
     Synchronization signals  382  which is illustrated as being derived from source  380  can be generated by the laser and then delayed by an electronic delay box that is controlled by a z displacement sensor as illustrated in  FIG. 1  or it can be generated by a fast photodetector as illustrated in  FIG. 3 . In this regard, z-direction sensor  331 A,  331 B is employed in the former synchronization scenario. A signal processing system  364 , which is coupled to detectors  342 ,  344  and  346 , receives the electrical detection signals and comprises a memory  366  a processor or analyzer  368 . To enhance the absorption-measurement accuracy of the NIR sensor, a windowing component  370  is implemented to window the photon-transit times to account for NIR scattering. The processor  368  combines the signals received to determine at least one property of the material through windowing whereby the processor gates the electrical detection signals to eliminate signals outside a fixed time window relative to the synchronization pulses and uses an algorithm to determine at least one property of material  350  with substantial independence of the measurement from the effects associated with scattering in the composition. 
       FIG. 6  illustrates time-of-flight measurements at three different NIR regions as measured by the NIR sensor. In particular, the intensity (or number) of photons emerging from a sample over time at three NIR regions, as represented by curves  80 ,  82 , and  84 , was measured. NIR that is first detected corresponds to that which passes through the sample with minimal interaction and therefore with the shortest path lengths in the sample whereas highly scattered NIR is detected later. It is expected that the degree of scattering will be a function of NIR wavelength. The windowing component selects points that correspond to a range of path lengths in the sample so as to exclude points with excessive scattering as well as points that correspond to insufficient interaction. With the components in the material. As shown, this technique effectively truncates the two-outer portions of the curves for measurements. The selection of the points of demarcation will depend on the shape of the curves with the goal of enhancing absorption-measurement accuracy. In other words, processor  368  initiates and stops measurements so that only the photons arriving between times  1  and  2  in the graph are integrated. 
       FIG. 7  illustrates one particular implementation of the NIR sensor whereby the sensor is incorporated into a dual head scanner  88  of scanner system  90  that is employed to measure properties of paper or polymer in films in a continuous production process. Upper scanner head  96 , which houses the NIR source, and lower scanner head  94 , which houses the NIR receiver, move repeatedly back and forth in the cross direction across the width of the moving sheet  86 , which moves in the machine direction (MD), so that the characteristics of the entire sheet may be measured. Scanner  88  is supported by two transverse beams  92 ,  98 , on which are mounted upper and lower scanning heads  96 ,  94 . The operative faces of the lower and upper scanner heads  94 ,  96  define measurement gap that accommodates sheet  86 . The lower scanner head  94  may include a sheet stabilization system such as an air-bearing stabilizer (not shown) to maintain the sheet on a consistent plane as it passes through the measurement gap. The movement of the dual scanner heads  94 ,  96  is synchronized with respect to speed and direction so that they are aligned with each other. 
     A technique of measuring powdered materials is to use a conveyer to continuously present the materials to a sensor of the presenting invention that is operating in the reflective mode. With a conveyer belt of limited width, sampling across the belt would not be necessary and a single stationary, point measurement may suffice. Alternatively, stationary, multiple point measurements can be implemented. 
     The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims