Abstract:
A system and method for analyzing the characteristics of a thin film is provided whereby the in-plane birefringence of thin films is determined by measuring the interference fringes in the transmission or reflection spectra using unpolarized light and light linearly polarized along the MD and CD directions. The three spectra can be measured simultaneously or sequentially. The in-plane birefringence data can be used to characterize clear polymer films, which are principally made of biaxial oriented polymer, as the film is being continuously fabricated on a production line.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention generally relates to apparatuses and methods for the measurement of thin film properties. Specifically, the invention relates to techniques of measuring the thickness and the in-plane degree of birefringence of plastic films.  
       BACKGROUND OF THE INVENTION  
       [0002]     Generally, in the preparation of plastic films from granular or pelleted polymer resin, the polymer is first extruded to provide a stream of polymer melt, and then the extruded polymer is subjected to the film-making process. Film-making typically involves a number of discrete procedural stages, including melt film formation, quenching, and windup.  
         [0003]     An optional part of the film-making process is a procedure known as “orientation.” The “orientation” of a polymer is a reference to its molecular organization, i.e., the orientation of molecules relative to each other. Similarly, the process of “orientation” is the process by which directionality (orientation) is imposed upon the polymeric arrangements in the film. The process of orientation is employed to impart desirable properties to films, including making cast films tougher (higher tensile properties). Depending on whether the film is made by casting as a flat film or by blowing as a tubular film, the orientation process requires substantially different procedures. This is related to the different physical characteristics possessed by films made by the two conventional film-making processes: casting and blowing. Generally, blown films tend to have greater stiffness, toughness and barrier properties. By contrast, cast films usually have the advantages of greater film clarity and uniformity of thickness and flatness, generally permitting use of a wider range of polymers and producing a higher quality film.  
         [0004]     Orientation is accomplished by heating a polymer to a temperature at or above its glass-transition temperature (T g ) but below its crystalline melting point (T m ), and then stretching the film quickly. On cooling, the molecular alignment imposed by the stretching competes favorably with crystallization and the drawn polymer molecules condense into a crystalline network with crystalline domains (crystallites) aligned in the direction of the drawing force. As a general rule, the degree of orientation is proportional to the amount of stretch, and inversely related to the temperature at which the stretching is performed. For example, if a base material is stretched to twice its original length (2:1) at a higher temperature, the orientation in the resulting film will tend to be less than that in another film stretched 2:1 but at a lower temperature. Moreover, higher orientation also generally correlates with a higher modulus, i.e., measurably higher stiffness and strength.  
         [0005]     When a film has been stretched in a single direction (monoaxial orientation), the resulting film exhibits great strength and stiffness along the direction of stretch, but it is weak in the other direction, i.e., across the stretch, often splitting or tearing into fibers (fibrillating) when flexed or pulled. To overcome this limitation, two-way or biaxial orientation is employed to more evenly distribute the strength of the film in two directions, in which the crystallites are sheet like rather than fibrillar. These biaxially oriented films tend to be stiffer and stronger, and also exhibit much better resistance to flexing or folding forces, leading to their greater utility in packaging applications.  
         [0006]     From a practical perspective, it is possible, but technically and mechanically quite difficult, to biaxially orient films by simultaneously stretching the film in two directions. Apparatus for this purpose is known, but tends to be expensive to employ. As a result, most biaxial orientation processes use apparatus which stretches the film sequentially, first in one direction and then in the other. Again for practical reasons, typical orienting apparatus stretches the film first in the direction of the film travel, i.e., in the longitudinal or “machine direction” (MD), and then in the direction perpendicular to the machine direction, i.e., the “cross direction” (CD).  
         [0007]     The degree to which a film can be oriented is also dependent upon the polymer from which it is made. Polypropylene, as well as polyethylene terephthalate (PET), and NYLON, are polymers which are highly crystalline and are readily heat stabilized to form dimensionally stable films. In the plastics industry, common biaxial oriented or “biax” films include MYLAR (biaxial oriented polyester or BOPET), NYLON (biaxial oriented polyamide or BOPA), and biaxial oriented polypropylene (BOPP). Biaxial oriented polymeric films and methods of fabricating them are known in the art and are described, for example, in U.S. Pat. No. 6,379,605 to Lin, U.S. Pat. No. 6,174,655 to Shirokura, et al., U.S. Pat. No. 5,912,060 to Kishida, et al., U.S. Pat. No. 5,552,011 to Lin, and U.S. Pat. No. 5,268,135 to Sasaki et al., which are incorporated herein by reference.  
         [0008]     On-line measurements of the thickness, basis weight, and molecular orientation of plastic films can be employed to control the process of fabricating biaxial oriented plastics. Orientation within a film can be described by the index of refraction ellipsoid, which is defined by the indices of refraction along the three axial directions, i.e., machine direction, cross-direction and thickness. Birefringence is the difference between two of these refraction indices. In thin films, birefringence of particular interest is the in-plane birefringence, which is defined as the difference between the indices of refraction along the machine direction (MD) and the cross-direction (CD). Birefringence in polymers is a result of the anisotropy in the molecular orientation. Such anisotropy occurs in the biax fabrication process wherein stretching of the film leads to molecular orientation in the machine and cross directions.  
         [0009]     An optical technique for determining thickness measures the amounts of light absorbed by a sample in two or more wavelength bands of the infrared (IR) spectrum. In the simplest case, two bands are used, a measure band and a reference band. The measure band is selected to coincide with a strong absorption in the target material (film to be measured), and the reference band is selected to match a weakly absorbing region of the target material.  
         [0010]     The transmission measurement is based on Beer&#39;s Law, which states I=I 0 e −μw , where I 0  is the signal with no sample, I is the signal with sample, μ is the absorption coefficient, and w is the weight of the sample. Equivalently, this may be written as w=(1/μ) ln(I 0 /I). Thus for a given wavelength of IR radiation, the weight, or thickness of the film, is proportional to the logarithm of the attenuation.  
         [0011]     In practice the accuracy of such transmission techniques is limited when measuring in the thin film regime due to an interference fringing effect. Fringes in the transmission and reflection spectra of the measured film appear due to interference of the light reflected from the film surfaces with light transmitted through the film. An example is illustrated in  FIG. 1 , which shows interference fringes  31  forming when the transmission of a 16 μm polyamide film is measured at different wavelengths. As a result, the sensor calibration error for such films increases significantly making measurements inaccurate. The lower limit for the film thickness is about 15-30 microns and depends on the material of the film.  
         [0012]     To understand the fringing effect, consider a thin film with thickness d and index of refraction n 2 , deposited on another material as shown in  FIG. 2 . Both the top and bottom of the film will reflect a portion of the light. The total amount of transmitted light contains contributions from these multiple reflections. Because of the wavelike nature of light, the reflections from the two interfaces may add together constructively or destructively, depending on their phase relationship. Their phase relationship is determined by the difference in the optical path lengths of reflections from these two interfaces, which in turn is determined by the thickness of the film d and the index of refraction n. Reflections are in-phase and therefore add constructively when the light path is equal to an integral multiple of the wavelength of light. For light perpendicularly incident on a film, this occurs when 2nd=iλ, where d is the thickness of the film, i is an integer, and λ is the free space wavelength of the incident radiation. Conversely, reflections are out of phase and add destructively when the light path is half of a wavelength different from the in-phase condition, or when 2nd=(i+½)λ.  
         [0013]     Qualitatively, these multiple reflections result in a transmission amplitude with a cos(4πnd/λ) component, or a transmitted intensity given by: 
 
 I=B   0   +A   0  cos(4π nd /λ).  (1) 
 
         [0014]     The reflected intensity will have a similar periodic component.  
         [0015]     From this it is apparent that the transmittance will vary periodically with wave number 2π/λ. Furthermore, at a given wavelength (index of refraction n is wavelength dependent) the frequency of oscillations is proportional to film thickness d. The transmitted light can be detected by sensors located on the opposite side of the film. A fit of the transmission spectra to Eq. 1 will give the thickness d assuming that n(λ) is known.  
         [0016]     Because the spectral position of the fringes depend on the film thickness, there have been efforts to extend current transmission sensors into the thin film regime by measuring interference fringes and extracting the film thickness from the fringe parameters.  
         [0017]     In on-line monitoring applications, birefringence is usually obtained directly by measuring the optical retardation using polarimetry techniques. Such a technique is described in U.S. Pat. No. 5,864,403 to Ajji, et al. Retardation is the product of birefringence and thickness of a material. Therefore, it decreases with decreasing birefringence and with decreasing thickness. In the limit of very thin films (below 20-30 μm), retardation is difficult to measure. This is due to the fact that it becomes small and that interference fringes can affect the measurement. The present invention is directed to the use of interference fringes for the measurement of birefringence of thin films.  
       SUMMARY OF THE INVENTION  
       [0018]     The present invention is based in part on the development of a technique for measuring the in-plane birefringence of thin films by measuring interference fringes in the transmission or reflection spectra using unpolarized and light linearly polarized along the MD and CD directions.  
         [0019]     The invention can be implemented in thin film and birefringence sensors that are employed to characterize clear polymer films, which are principally made of biaxial oriented polymer, that are continuously fabricated on production lines.  
         [0020]     In one embodiment, the invention is directed to a method of analyzing one or more characteristics of a film, which is moving in a machine direction, said method including the steps of:  
         [0021]     (a) providing a broadband source of radiation that creates a probe beam;  
         [0022]     (b) directing the probe beam along a beam path onto or through the film such that the probe beam is reflected from the film to form a first output beam or is transmitted through the film to form a second output beam;  
         [0023]     (c) providing an analyzer to determine the intensity of the first output beam or the second output beam at desired wavelengths or wavelength bands;  
         [0024]     (d) positioning a neutral density filter, a first polarizing filter, or a second polarizing filter in the beam path such that the neutral density filter, the first polarizing filter, or the second polarizing filter is located in the beam path between the broadband source of radiation and the analyzer; and  
         [0025]     (e) utilizing differences in the spectral fringes of unpolarized and linearly polarized light to calculate birefringence characteristics of the film.  
         [0026]     In another embodiment, the invention is directed to a system for analyzing one or more characteristics of a film, that is moving in a machine direction, the system including:  
         [0027]     (a) a broadband source of radiation that creates a probe beam;  
         [0028]     (b) means for directing the probe beam along a beam path onto the film such that the probe beam is reflected to form a first output beam or such that the probe beam is transmitted through the film to form a second output beam;  
         [0029]     (c) analyzer means for determining the intensity of the first output beam or the second output beam at desired wavelengths or wavelength bands;  
         [0030]     (d) a first polarizing filter;  
         [0031]     (e) a second polarizing filter wherein the first polarizing filter has a direction of linear polarization that is parallel to the machine direction and the second polarizing filter has a direction of linear polarization that is perpendicular to the machine direction, wherein the first polarizing filter and the second polarizing filter are located between the broadband source of radiation and the analyzer means; and  
         [0032]     (f) calculation means for utilizing differences in the spectral fringes of unpolarized and linearly polarized light to calculate birefringence characteristics of the film.  
         [0033]     In a preferred technique for thickness and birefringence measurements of thin films, spectra of transmitted or reflected light that is measured simultaneously with unpolarized light, linearly polarized light along the MD direction, and linearly polarized light along the CD direction is generated. Thickness and birefringence values can thus be ascertained at specific points along a moving web of plastic film.  
         [0034]     In another technique, which is especially applicable when the production process is in steady state, the thickness and birefringence profiles of the web are obtained by measurements using (i) unpolarized light, (ii) linearly polarized light along the MD direction, and (iii) linearly polarized light in the CD direction. Only one analyzer is required when the measurements are performed sequentially. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a graph of measured percentage transmission of unpolarized radiation vs. wavelength for a 16 μm thick polyamide (NYLON) film which shows interference fringes.  
         [0036]      FIG. 2  shows a basic concept diagram of a light wave passing through a thin film with thickness d.  
         [0037]      FIG. 3A  shows a schematic drawing of the system with a multichannel detector assembly and polarizing filters positioned between the source beam and the web where the (i) filters and source beam and (ii) assembly are on opposite sides of the web.  
         [0038]      FIG. 3B  shows a schematic drawing of the system with a multichannel detector assembly and polarizing filters positioned between the source beam and the web where the (i) filters and source beam and (ii) assembly are on the same side of the web.  
         [0039]      FIG. 4A  shows a schematic drawing of the system with polarizing filters between the web and the multichannel detector assembly where the source beam and assembly are on opposite sides of the web.  
         [0040]      FIG. 4B  shows a schematic drawing of the system with polarizing filters between the web and the multichannel detector assembly where the source beam and assembly are on the same side of the web.  
         [0041]      FIG. 5A  shows a schematic drawing of a system wherein three spectrometers are used for simultaneous measurements where the source beam and assembly are on opposite sides of the web.  
         [0042]      FIG. 5B  shows a schematic drawing of a system wherein three spectrometers are used for simultaneous measurements where the source beam and assembly are on the same side of the web.  
         [0043]      FIG. 6A  shows a schematic drawing of the system with a single spectrometer for sequential measurements where the source beam and assembly are on opposite sides of the web.  
         [0044]      FIG. 6B  shows a schematic drawing of the system with a single spectrometer for sequential measurements where the source beam and assembly are on the same side of the web.  
         [0045]      FIG. 7  illustrates a polarizing filter wheel including a neutral density filter, a first linear polarizing filter and a second linear polarizing filter.  
         [0046]      FIG. 8  shows a source assembly of an infrared film thickness measurement system.  
         [0047]      FIG. 9  shows a receiver assembly.  
         [0048]      FIG. 10  shows channels mounted to the center column of the receiver assembly.  
         [0049]      FIG. 11  shows the interference fringes formed from measuring 5 μm and 16 μm NYLON films.  
         [0050]      FIG. 12  is a graph of percentage of transmission vs. wavelength that shows the predicted change in interference fringes of a 5 micron thick MYLAR film using unpolarized and linearly polarized light, assuming a birefringence of 0.05.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0051]     In accordance with the invention, a novel infrared (IR) film measurement system is provided. The invention is based, in part, on the recognition that the only requisite for thickness measurements is the directional average (along MD and CD directions) index of refraction in the wavelength range of measurement. A very good estimate of this index value can be obtained from the literature. A more accurate value can be obtained during calibration on-line with an unpolarized light source or in a laboratory using commercial products.  
         [0052]     Once the thickness of the film is determined, the indices of refraction n MD  and n CD  in the MD and CD directions respectively, can be obtained by measuring the spectrum of linearly polarized light transmitted through or reflected by the film. For the MD and CD index, the light is linearly polarized in the MD and CD direction, respectively. The transmission spectra have the following forms. 
 
 I   MD   =B   1   +A   1  cos(4π n   MD   d /λ).  (3) 
 
 I   CD   =B   2   +A   2  cos(4π n   CD   d/ λ).  (4) 
 
         [0053]     Fitting measured data to Eqs. 3 and 4, yields n MD  and n CD , assuming that the thickness d is known from the measurement using unpolarized light. The in-plane birefringence is defined as Δ=n MD −n CD , or the difference of the MD and CD indices of refraction. It should be noted that n MD  and n CD  are assumed to have the same wavelength dependence as the average index. Thus, this fringe measurement technique can be employed to measure birefringence in the thin film regime.  
         [0054]     It has been assumed that as a consequence of the stretching process the axes of the refractive index ellipsoid of the film are oriented along the MD, CD and normal directions. Polarization of linear polarized light in the CD or MD direction is thus unchanged by transmission through the film. Effect of retardation or double refraction of the unpolarized beam is considered negligible due to the very thin nature of the film.  
         [0055]      FIGS. 3-7  illustrate various embodiments of the invention wherein unpolarized and linearly polarized light are employed to measure the difference in spectral fringes from thin films to determine their thickness and in-plane birefringence. The process is particularly suited for clear plastic films that are typically less than 1 μm to 50 μm and preferably less than 1 μm to 20 μm thick. When implemented sequentially, the process only requires one analyzer. When performed simultaneously, the process preferably employs three analyzers. The analyzer typically has a working spectral range of from 400 nm to 5000 nm.  
         [0056]     As the sensor is scanned across the web, i.e. in the CD direction, the thickness and birefringence profiles of the film are obtained. The basis weight profile can also be calculated using the known density of the plastic.  
         [0057]     As illustrated in  FIG. 3A , a system for analyzing a characteristic of the film includes an analyzer that comprises a multichannel detector  70  that is positioned on one side, e.g., above, the web  88  of thin, mostly clear plastic, for example, which moves in the machine direction (MD). The cross direction (CD) is transverse to the MD. The multichannel detector  70  includes a body in which three beam splitters  74  are positioned. Each optical channel comprises a detector  72 , lens  76 , and associated an IR band selection filter. The six filters are designated as  78 A,  78 B,  78 C,  78 D,  78 E, and  78 F. These filters are typically interference filters that have a spectral transmission band surrounded by two blocking bands that allow only a portion of the spectrum to pass. This result in high transmission centered about the chosen wavelength. At the distal portion of the detector  70  is a mirror  75  and at the proximal or entrance end is a focusing lens  86 . In this diagram only six optical channels are shown for clarity; the detector can have additional optical channels and corresponding detectors. Suitable detectors  72  include photoconductive or photovoltaic detectors that have an element that is formed from PbS, PbSe, InGaAs, Si, mercury cadmium telluride (MCT), InAs, Ge, and InSb.  
         [0058]     As further shown in  FIG. 3A , the system further includes a source IR radiation  82  and a parabolic mirror  84  for focusing the light from the source to create a probe beam. A polarizing filter wheel  80  is positioned so that the probe beam passes through the polarizing filter wheel before reaching the moving web  88 . As shown in  FIG. 7 , the polarizing filter wheel  80  includes first and second linear polarizing filters  80 A,  80 B, and a neutral density filter  80 C. The polarizing filter wheel  80  includes a mechanism that rotates the wheel so that the filters  80 A,  80 B, or  80 C can be inserted between the radiation source  82  and the web  88  at predetermined intervals, i.e. after every acquisition or after a CD scan or after many CD scans. Light passing through filter  80 A is preferably linearly polarized in a direction of linear polarization that is parallel to the machine direction and light passing through filter  80 B is preferably linearly polarized in a direction of linear polarization that is in the CD, i.e., that is perpendicular to the machine direction. The neutral density filter  80 C reduces or attenuates the intensity of the radiation that passes through but leaves the radiation unpolarized. As used herein, the term “neutral density filter” is meant to encompass an open aperture as well where there is zero reduction in intensity. Using a neutral density filter that reduces the intensity is preferred as opposed to the case where the neutral density filter is an opening. The latter situation can be employed, for example, if electronic gain can be adjusted by appropriate software.  
         [0059]     When the radiation source  82  is positioned below the moving web  88  as shown in  FIG. 3A , the multichannel detector  70  measures the transmittance through the web  88 . Alternatively, as shown in  FIG. 3B , when the radiation source  82  is located on the same side as the multichannel detector  70 , radiation that is reflected from the moving web  88  is measured.  
         [0060]     In operation, broadband radiation from the IR source  82  is reflected from the parabolic mirror  84  to form a collimated probe beam that passes through polarizing filter wheel  80  before being incident on the web  88 . The radiation that emerges from the web  88 , either transmitted or reflected radiation, is focused by lens  86  into the multichannel detector  70  where the beam is separated into a plurality of parallel beams by the beam splitters  74 . Each parallel beam passes through a narrow bandpass filter, e.g.,  72 A, before reaching a detector  72 . As the polarizing filter wheel  80  rotates, the linear polarizing filters  80 A,  80 B, or the neutral density filter  80 C is inserted into the path of the probe beam at predetermined time intervals which can be the same. It should be noted that the order that the filters  80 A,  80 B, and  80 C is inserted sequentially is a matter of design choice. That is the polarizing filter wheel  80  can be rotated clockwise or counter-clockwise. The birefringence, thickness, and other characteristics of the web  88  can be calculated using the differences in the spectral fringes of unpolarized and linearly polarized light.  
         [0061]     Due to large thickness variability in the MD direction of the web, it is preferred to employ fast detectors with high signal to noise ratio such as InGaAs detectors and to use fast chopping frequency or modulated Super Luminescent Diodes (SLDs) for the IR source.  
         [0062]      FIGS. 4A and 4B  illustrate another configuration of a multichannel detector  90  wherein a polarizing filter wheel  80  is positioned between the moving web  88  and the detector  90  along the probe beam path from the radiation source  82 . The system is essentially the same as that shown in  FIGS. 3A and 3B , in that it includes a multichannel detector  90  that is positioned one side of the moving web  88  with a source of radiation  82  that can be positioned on either of the moving web  88 . In this embodiment, the polarizing filter wheel  80  is positioned in front of lens  86  along the path of the probe beam so that either transmitted ( FIG. 4A ) or reflected ( FIG. 4B ) radiation that emerges from the moving web  88  will pass through the polarizing filter wheel  80  before entering the multichannel detector  90 . Operation of the system is essentially the same as well.  
         [0063]      FIGS. 5A, 5B ,  6 A, and  6 B illustrate embodiments of the invention in which the analyzer includes one or more spectrometers. Diffraction grating type spectrometers are preferred. Using spectrometers obviate the need for individual bandpass filters and detectors. A spectrometer measures the full spectrum in a given wavelength range. The typical number of individual wavelength bands in a spectrometer is, for example, 256, 512, or higher.  
         [0064]     In the system shown in  FIGS. 5A and 5B , three spectrometers are used so that fringe patterns are recorded simultaneously for unpolarized light and linearly polarized light in the MD direction and linearly polarized light in the CD direction. In this fashion, all measurements are performed on the same spot on the moving web  124 . Specifically, as shown, the system includes a detector assembly  100  that has two beam splitters  108  and  109  which are positioned along the middle channel and three optical channels that house spectrometers  102 ,  104  and  106 . Mirrors  114  and  116  reflect light back into the middle channel. Positioned in front of spectrometers  102 ,  104 , and  106  are a linear polarizing filter  112 , a linear polarizing filter  110 , and a neutral density filter or aperture  111 , respectively. The system  100  further includes a source of IR radiation  130  and a parabolic mirror  122 . The source of radiation  130  is positioned on the same ( FIG. 5B ) or on the opposite ( FIG. 5A ) side of the moving film  124 . In either case, radiation that is transmitted through or reflected from the web is focused by lens  118  into the spectrometers. Beam splitter  108  directs a portion of the radiation into spectrometer  106  whereas a portion of light passes to beam splitter  109  which in turn directs light into spectrometers  102  and  104 . As is apparent, spectrometer  106  analyzes unpolarized light whereas spectrometers  102  and  104  analyzes light that has passed through linear polarizing filter  112  and  110 , respectively. In this system, all three measurements are conducted simultaneously.  
         [0065]     In a further embodiment of the invention, source and receiver are on the same side of the web. The reflected intensity, not the transmitted intensity, is measured. Technically the measurement can be done in reflection geometry. This has the advantage of higher fringe visibility. However, it may suffer from sensitivity to sheet flutter.  
         [0066]      FIGS. 6A and 6B  illustrate an embodiment of a system  130  that includes a spectrometer  132  and a polarizing filter wheel  80  that filters radiation entering the spectrometer  132 . The system  130  also includes a source of IR radiation  140  and an associated parabolic mirror  138 . As illustrated, the source of radiation  140  can be positioned on the same side ( FIG. 6B ) as that of the spectrometer  132 , relative to the positioned of the moving web  150 , or it can be positioned on the opposite side ( FIG. 6A ). In either case, radiation that is transmitted through the moving web  150  or which is reflected from the moving web  150  is focused by lens  136  into the polarizing filter wheel  80 .  
         [0067]     In operation, radiation that emerges from the moving web  150  is collimated by lens  136  and directed toward the rotating polarizing filter wheel  80  so that the linear polarizing filters  80 A,  80 B, or the neutral density filter  80 C is sequentially inserted into the path of the beam of radiation at predetermined time intervals. The birefringence, thickness, and other characteristics of the web  150  can be calculated using the differences in the spectral fringes of unpolarized and linearly polarized light.  
         [0068]      FIG. 7  illustrates an embodiment of the polarizing filter wheel which includes a neutral density filter  80 A, a first linear polarizing filter  80 B, and a second linear polarizing filter  80 C. The polarizing filter wheel  80  includes a motor which rotates the wheel a desired speed. In this fashion, a probe beam passes through each of these filters a predetermined timed interval.  
         [0069]      FIG. 8  illustrates a suitable IR source assembly  40 . An IR source transmits pulses of wideband IR to the sample between the upper and lower heads. It consists of an incandescent light  41 , a mirror  42 , and supporting hardware for modulating the IR energy. A quartz tungsten halogen lamp is used because of its compact size, and the quartz envelope is transparent to the IR energy in the wavebands of interest in the application. The small filament of the lamp makes it possible to focus most of the energy onto the window. Radiation from the quartz tungsten halogen lamp is focused at a light pipe by the mirror  42 . The only adjustment required is the focusing of the lamp by sliding it in the holder to maximize the signal strength at the analyzer or receiver.  
         [0070]     The IR energy is modulated with chopper  43 , a lightweight rotating stainless steel disc with preferably eight evenly spaced holes. It is driven by a brushless DC motor  44  that modulates the radiation at 620±25 Hz. Modulating the IR energy will prevent the signal received by the detectors in the receiver from being obscured by ambient light or by low frequency noise generated in the detector.  
         [0071]     The sample cell, in which the IR energy interacts with the sample to be measured, is located in the space between the IR source and receiver windows. The sample film to be measured is placed here to interact with the IR energy. It is very important that the IR energy transmitted be determined only by the properties of the sample and not by extraneous effects, such as dirt and head misalignment or separation.  
         [0072]     A suitable receiver assembly  50  is shown in  FIG. 9 . The function of the receiver is to simultaneously read the transmitted energy in all of the selected bands. This is done by using beam splitters ( 66  in  FIG. 10 ) to separate the energy into a series of parallel beams. Each beam is then passed through a filter designed to pass a predetermined waveband, and the total energy in that band is detected by a photoconductive or photovoltaic detector. Each detector has its own conventional electronics  57  that amplify the received signal, convert it to DC, and transmit it to the receiver.  
         [0073]     The receiver assembly  50  has capacity to support up to preferably twelve channels, and can load additional channels as needed. A central aluminum  51  column has sockets  55  that can support up to twelve channels. The central column  51  is mounted to a water-cooled plate in the ceiling (not shown) for cooling the sensors.  
         [0074]      FIG. 10  shows channels mounted to the center column  65 . In the diagram, only 6 channels are shown mounted for clarity. Additional channels can be mounted to the central column  51  at sockets  55  as required. Each channel comprises a detector  63  an IR band selection filter  67 , lens  68  and support electronics. In accordance with an embodiment of the invention, up to preferably twelve channels are utilized at once. Heat from the electronics and peltier cooling of detectors is conducted from the detector assemblies through the column  55  to the water-cooled plate.  
         [0075]     A thin film sample  69  is loaded and secured in the sample cell, and the IR source  40  operated to transmit pulses of wideband IR through the sample. The receiver reads the transmitted energy in all of the selected bands simultaneously, and the output of each of the detectors is transmitted to the signal processing circuitry  45  ( FIGS. 8, 9 ) to process the signal. The wavelength and transmittance percentage can then be electronically plotted on a graph.  
         [0076]      FIG. 11  shows the interference fringes formed from measuring 5 and 16 μm NYLON films with a Fourier Transform Infrared (FTIR) spectrometer. In order to detect the characteristic signature of interference fringes, appropriately chosen optical filters are utilized. The dashed areas under the fringes represent proposed filter wavelengths for a twelve channel IR sensor.  
         [0077]      FIG. 12  shows the predicted change in the interference fringes of a 5 μm thick MYLAR film when employing unpolarized light and linearly polarized light in accordance of the present invention.  
         [0078]     The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims and their equivalence.