Abstract:
For identifying black plastics effectively and rapidly with a laser-powered identification probe, it is desirable to maintain full laser power while reducing the power density. This is achieved by providing the probe with a moving lens that disperses the 0.5 mm laser spot over a larger area typically of about 5 mm in diameter. The entire signal from the larger (5 mm) diameter is collected at the same spot in the fiber bundle within the probe that leads to a Raman or other spectral analyzer. There are no other modifications required for the rest of the system as the moving lens does not affect the collection efficiency of the characteristic signal from the sample.

Description:
This application claims benefit of Provisional application No. 60/212,174 filed Jun. 16, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to spectral analysis systems that can be used for polymer identification. More particularly, the invention particularly relates to improvements in Raman polymer identification systems to permit effective and rapid identification of darkly colored plastics. 
     2. Description of the Prior Detection Method 
     Many plastics that should be recycled, particularly in the automotive industry, are black or highly pigmented. Such darkly colored plastics have proven to be the most difficult to identify using existing plastic identification technologies. Due to the strong optical absorption of black plastics, most of the signal needed to perform a spectroscopic identification is absorbed by the sample and thus unavailable for detection. At the same time, absorption may also lead to a significant thermal change such as a rapid heating, melting and even burning of the plastic sample during the identification process. Thus, not only are the signal levels from black plastics very small, but also these weak signals, particularly Raman signals, may be further obscured by large interfering backgrounds due to the thermally induced changes in the plastics including smoking. For example, white plastics can be easily and rapidly identified in 0.1 seconds with a Raman spectrometer, such as that disclosed in International Publication WO 99/01750, using a 1 Watt diode laser power, while black plastics cannot be identified under the same conditions due to laser induced detrimental changes. The power density reaches 5 W/mm 2 when a 1 W laser at a wavelength of 800 nm is focused to a 0.5 mm diameter focal spot. 
     In order to avoid laser induced detrimental changes in the plastic, it is necessary to decrease the laser power density on the surface of the black plastic. One way to reduce laser power density is to reduce total laser power that illuminates the surface of the black plastic. But at same time, in order to accumulate enough signal for identification, the signal collection time has to be increased proportionally. Obviously, this is not acceptable for rapid identification. The other way to reduce the power density of the laser is to increase the size of the laser spot that illuminates the surface of the plastic, while still maintaining a sufficiently high laser power of 1 Watt to allow rapid identification. Experiments have shown that to avoid laser induced detrimental changes in black plastic samples, in the case of 1 Watt total laser power at wavelength 800 nm, the size of the laser spot illuminating the surface of a black plastic sample needs to be increased 40 times, to a size that is greater than 3 mm in diameter. As a consequence, the signal acceptance area of the collection fiber bundle and the acceptance area of the spectrograph (slit-height times slit-width) must also be increased 40 times. It is almost impossible to achieve this from a technical point of view. Enlarging the laser spot size without changing the optical train and components would cause the signal from the sample to overfill the collection fiber bundle and thus decrease the collected signal intensity. 
     Thus, there exists a need for a quick yet effective way to identify materials such as darkly colored plastics using spectral analysis, particularly Raman spectroscopy. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to aid in the understanding of the present invention, it can be stated in essentially summary form that it is directed to a moving objective lens in a traditional system employing spectral analysis such as a Raman polymer identification system. By moving the objective lens, the laser beam can be distributed to a focal plane, while the spectral signal from the moving laser spot can still be collected back to the same point as if the objective lens were stationary. As a result, the average power density of the moving laser spot can be reduced to a point that no light induced detrimental degradation such as undue heating, melting and burning of the plastic sample will occur, while still maintaining the same laser beam power level. At the same time the power level of the spectral signal being returned from the sample is maintained at a level sufficient to make very rapid identification of the character and composition of the sample by an analysis of the Raman or other spectral signal. 
     In a preferred system, an optical fiber bundle conducts the spectral signal from the sampling optics situated to receive the characteristic spectrum produced from the sample to a spectral analyzer. Even though the lens is moving, the spectral signal returns from the sample to the same point of the entrance end of the fiber bundle as if the lens were stationary. This means the spectral signal is not reduced by the movement of the lens. The terms moving and movement as employed in this application is to be given the broadest possible meaning and include a patterned or random movement of the lens so that the laser energy directed toward the sample is distributed over an area in the focal plane of the objective lens larger than would be achieved were the lens not subjected to movement. Examples of movement that are easily achieved to obtain the desired results include rotation of an eccentrically positioned objective lens, and one-or two-dimensional translational vibration of an objective lens in a plane roughly parallel to the sample surface. 
     The invention provides a convenient way to solve the problems faced by traditional Raman or other spectral polymer identification methodologies that prevent the rapid and effective identification of darkly colored plastics. The invention can be better understood from the following description when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows that when increasing the laser spot size so as to decrease or reduce the power density to a manageable level with a stationary objective lens the Raman signal acceptance area of the fiber bundle needs to be increased. 
     FIG. 2 shows the laser beam and Raman signal path in a traditional Raman Polymer Identification system. 
     FIG. 3 shows the laser beam and Raman signal path in a traditional Raman Polymer Identification system when the objective lens F is off center. 
     FIG. 4 shows the laser beam and Raman signal path in a modified Raman Polymer Identification system with an embodiment of the inventive moving objective lens. 
     FIG. 5 is an exploded perspective view of an identification probe incorporating a spatial filter of the present invention. 
     FIG. 6 shows an objective lens system including a moving objective lens. 
     FIG. 7 shows that tilting movements do not affect the collection efficiency of Raman signal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following portion of the specification, taken in conjunction with the drawings, sets forth the preferred embodiments of the present invention. The embodiments of the invention disclosed herein are the best mode contemplated for carrying out the invention in a commercial environment, although it should be understood that various modifications can be accomplished within the parameters of the present invention. 
     In a traditional Raman Polymer Identification system, such as that disclosed in International Publication WO 9901750, the general signal collection setup can be illustrated as in FIG.  2 . The objective lens F is placed at (0,0,0). The collimated diode laser beam is focused by the objective lens F at focal point (0,0,f). The Raman signal from the excitation focal point (0,0,f) is re-collimated by the objective lens F and passed through the Holographic Notch Filter (HNF), then collected by lens F1 to fiber bundle at focal point (0,0,L). 
     Experiments have shown that to avoid laser induced detrimental changes in black plastic samples, in the case of 1 Watt total laser power at wavelength 800 nm, the size of the laser spot illuminating the surface of a black plastic sample needs to be increased 40 times, to a size that is greater than 3 mm in diameter. FIG. 1 schematically shows such an enlargement, as compared to FIG. 2, of the illuminated surface to cover the area centered on (0,0,f) but extending from (a,0,f) to (-a,0,f). As a consequence of such an enlargement of the laser illumination area, the Raman signal is spread over an area centered on (0,0,L) extending from (A,0,L) to (-A,0,L) thus requiring an acceptance area of the collection fiber bundle, shown in FIG. 2, to be correspondingly increased. As described earlier, the enlargement of the laser spot size without changing the optical train components would cause the signal from the sample to overfill the collection fiber bundle and thus decrease the collected signal intensity to an unsatisfactory level. 
     When the center of the objective lens F is moved to (a,0,0) as in FIG. 3, the focal point of laser beam will be moved to (a,0,f). The Raman signal from the excitation focal point (a,0,f) will be re-collimated by the objective lens F, which is centered at (a,0,0) and passed through the HNF. The Raman signal is then collected by lens F1 at the same point of fiber bundle, (0,0,L). In other words, no matter how the objective lens is moved, as long as it is on the same vertical plane, the Raman signal will always be collected at same point (0,0,L). 
     When the objective lens F is vibrated on a plane that is perpendicular to the optical axis shown in FIGS. 3 &amp; 4, the laser spot that is formed on the sample surface will vibrate on the focal plane synchronously with the objective lens movement in the X and Y directions. With the increase of amplitude of the vibration on the objective lens, the scan area of the focal spot will increase. By adopting random, noisy or complex functions for the amplitude of the objective lens in the X and Y directions, the movement of the laser spot will effectively smear or spread out nearly uniformly over the spot area. As a result, the average power density of the vibrated laser spot can be reduced proportionally until it is lower than the laser induced detrimental change threshold of the sample. 
     An important key is to recognize that the time it takes for light from the laser to travel from the objective lens to the sample and for the excited Raman signal to travel back from sample to the objective lens to be re-collimated is less than 1 nano-second (if focal length f&lt;15 cm). If the vibration frequency is 50 Hz and the vibration amplitude is &lt;2.5 mm, the mechanical movement of the objective lens F due to the vibration is less than 1 nm during this 1 nano-second time interval. That means the Raman signal will be collected through lens F and F1 and reach the same point of fiber bundle (0,0,L), so the signal will not be weakened due to the mechanical movement of objective lens F. The result of lens vibration with 50 Hz and 2.5 mm amplitude makes the lens spot from 0.5 mm to equivalent laser spot of 5 mm in diameter, which leads to a 100-fold reduction in power density. 
     This key point of invention is that random, noisy, complex or other movement of the objective lens in the X-Y plane has the same effect as increasing the size of laser spot, yet there is no need to modify the optical system that collects the Raman signal and still maintain the same signal collection efficiency. By applying this invention, the average laser spot size can be easily changed by modifying the amplitude limits on the movement of the objective lens. The present invention has the advantage of averaging the Raman signal that is collected from the test sample over the spot area, which is important when the sample has non-uniformly distributed chemical components. Generally the movement of the objective lens should have a linear velocity component of between about 0.1 to 100 cm/sec. 
     In a commercial embodiment, the probe  12  is configured to illuminate and collect light scattered from samples, not shown, that are situated in front of optical window  26  at a front end of nose cone  24  as shown in FIG.  5 . Probe  12  includes a housing  14  in the form of a generally cylindrical member  22  and includes a nose cone  24  containing an optical window  26 . The optical window  26  can comprise a simple opening through which light can pass, but in a preferred embodiment the optical window  26  comprises a sapphire window mounted within the nose cone  24  to protect the optics within probe  12  from airborne dust and assorted particles. The probe  12  can be easily positioned relative to a sample by means of handle  28  that can constitute a coupling structure for robotic manipulation. A trigger  30  is situated on the handle  28  for easy operation by an operator&#39;s index finger. Alternatively, the trigger  30  can be computer controlled. A longitudinal rail  32  is fixed to handle  28  or equivalent robotic coupling structure to provide a foundation for the optical components within the probe  12 . The generally cylindrical housing member  22  includes a longitudinal slot  16 , the edges  18  of which contact opposing edges of the longitudinal rail  32 . The housing  22  is completed by back wall  34  having an outer perimeter  64 . In the preferred embodiment, the generally cylindrical housing member  22  has an internal diameter of about 6.0 cm. It is understood, however, that the internal diameter and other dimensions of housing member  22  can vary in accordance with the constraints imposed on the system by its intended use as well as the components to be housed therein. In the preferred embodiment, the housing member  22 , nose cone  24 , longitudinal rail  32 , and back wall  34  are construction of aluminum that has been black anodized. However, a wide variety of metals, copolymers, and composites can be used to construct probe  12  in accordance with the present invention. 
     The longitudinal rail  32  includes a lower surface  31 , an upper surface  33 , a rearward end  35 , and a forward end  37  as shown in FIG. 5. A plurality of lateral slots  39   a  through  39   g  are milled into the upper surface  33  of the longitudinal rail  32  generally perpendicular to the length dimension of the longitudinal rail  32 , except slot  39   c  which is inclined at an angle of about 10°. Pivot pins  38  are fixed in the center of each of the lateral slots  39   a  and  39   c  to permit small adjustments in the alignment of the supports fastened therein. Probe  12  employs sampling optics  42  to collect the scattered Raman radiation, discriminating with an extinction ratio of about 10 6  (1 ppm) or better for the Raman-shifted component. Support  46  is fastened in slot  39   e  to hold lens  36  adjacent the exit end  41  of optical fiber  66  carrying light from a laser source  67 . Support  49  is fastened in slot  39   b  to hold a band pass filter  48 , which controls the wavelength and deviation of the source light directed toward the sample through optical window  26 . Support  51  is fastened in slot  39   a  to hold an objective lens system  54  and mirror  50 . Supports  46  and  49  also support the ends of baffling tube  47  creating a specific segregated region  44  within the housing  22  between the lens  36  and band pass filter  48 . 
     Support  52  is fastened in inclined slot  39   c  to hold optical filter  76 , which can be an interference or holographic filter and preferably is a long pass filter designed to reflect light having a wavelength equal to or less than the wavelength of the laser source and transmit light having a wavelength longer than the laser source. Support  56  is fastened in slot  39   d  to hold a lens  74  having a focal length selected to direct the Raman or other characteristic spectral signal passing through the optical filter  76  on to the entrance end  53  of spatial filter  55 . Support  58  is fastened in slot  39   e  to hold the entrance end  53  of spatial filter  55 . The entrance end  53  of the spatial filter  55  includes an aperture  65  that is generally round and preferably has an area of about 1 mm 2  or less. Support  40  is fastened in slot  39   g  to hold the exit end  57  of spatial filter  55  that also holds the entrance end of optical fiber bundle  62  that carries the characteristic Raman or other spectral signal produced from a sample through the fiber-optic bundle  62  to appropriate instruments capable for evaluating the spectral signal. The specific structure of the preferred embodiment of the spatial filter  55  is disclosed in U.S. patent application Ser. No. 09/447,878 filed Nov. 23, 1999, now U.S. Pat. No. 6,310,686, which is hereby incorporated by reference. 
     A convenient method of achieving the desired movement of the objective lens  60  is to mount the lens in a lens holder  59  as shown in FIG. 6 so that the lens center C is displaced from the optical axis Z. The objective lens  60  is thus eccentrically mounted with respect to the optical axis Z of the probe  12  and then simply rotated by motor  61 . This rotation causes the focal point of the lens  60  to rotate in the plane of the lens holder  59 , which is generally parallel to the surface of the sample, about the optical axis Z of the probe  12 , describing a circle that has radius equal to the eccentricity of the lens mount  59 . In the preferred embodiment, the rotating objective lens  60  is mounted adjacent to or in the nose  24  of the probe  12 . The lens mount  59  is rotated by the electrical motor  61  upon depression of the trigger  30 , which can also initiate the emission from the laser source through electrical cables  69  and  70 . The rotation of the lens holder  59  has the effect of causing the laser spot to scribe a circle on the test sample. 
     The amount of lens displacement from the optical axis and the speed of rotation can be selected so that the power density of the excitation laser beam is reduced until it is lower than the laser induces detrimental change threshold of the sample. For example, a lens can be mounted in a lens holder so that the lens center is displaced from the optical axis by about 1.5 mm, which causes the focal point of the excitation laser beam to scribe a circle having a diameter of about 3.0 mm as shown in FIG.  7 . When this lens is rotated, the power density of the excitation laser beam is distributed over a circle described by the rotation of the focal spot. In the limit of a rotation rate that is faster than thermal diffusion, the steady-state power density falls by a factor of A/A′, where A is the area of the static focal spot and A′ is the area of the ring illuminated by the rotating spot. This limiting ratio is described by the formula r/4R, where r is the radius of the focal spot and R is the radius of the ring described by the rotation of the focal spot. The eccentricity of the lens mounting can be from about 0.05 to 1.0 cm while the movement of the lens holder varies between about 0.1 and 100 rev/sec. Correspondingly, when the lens holder is vibrated at a frequency of from about 0.1 to 100 Hz, the amplitude of the vibration can be varied between about 1.0 and 0.01 cm. 
     It should be noticed that any tilt movement during the objective lens vibration will not affect the collecting efficiency of Raman or other spectral signal illustrated In FIG.  7 . In other words, there is no strict requirement for the accuracy of the objective lens movement, although significant translational movements of the objective lens in the Z direction could affect the collimation of the returning spectral signal by the objective lens  60 . 
     Although the invention has been described in detail with reference to a preferred embodiment, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.