Abstract:
Apparatus and method are provided for measuring a dimension of an airborne fiber. The apparatus includes a flow channel for providing a laminar flow to at least a portion of the fibers in air sample and a light source for projecting a light beam along a selected beam path to impinge upon a first fiber in the sample to create scattered light. A portion of the scattered light is measured by a light detector to produce an electrical output which is related to the fiber&#39;s dimension.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is related to co-pending non-provisional U.S. patent application Ser. No. 08/743,554, entitled “Device For Measuring The Concentration Of Airborne Fibers”, filed Nov. 4, 1996, which is assigned to the same assignee hereof, and is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to devices for measuring the dimension of airborne fibers and more particularly, to non-contact optical devices for measuring the diameter and length of flowing fibers. 
     BACKGROUND OF THE INVENTION 
     Existing airborne fiber dimension measuring devices typically require that the fibers in the sampled air be separated, aligned, and analyzed individually. Furthermore, some devices require multiple sensors which determine airborne fiber dimensions by analyzing the residual amount of direct light from collimated light beams that have been attenuated or otherwise disturbed by a passing fiber. The accuracy of devices requiring multiple sensors can be adversely affected by alignment errors, calibration drift, component degradation and the like. 
     Two examples of existing methods and apparatus for the measurement of entities in fiber samples include, for example, U.S. Pat. No. 5,430,301 to Shofner, et al. (1995), entitled “ Apparatus and Methods for Measurement and Classification of Generalized Neplike Entities in Fiber Samples”  (Shofner I); and U.S. Pat. No. 5,270,787 to Shofner, et al. (1993) entitled, “ Electro - Optical Methods and Apparatus for High Speed, Multivariate Measurement of Individual Entities in Fiber or Other Samples”  (Shofner II). See also  MIE Fiber Monitor Model FM -7400 User&#39;s Manual by MIE, Inc., Billerica, Mass. 
     In Shofner I, multiple sensors are provided for measuring fiber characteristics in a sample of textile material, including small clumps or entanglements of fiber known as neps. Although this apparatus and method may be suitable for determining the characteristics of neps from textile samples, it is generally unsuitable for characterizing airborne fibers, such as glass fibers, having a diameter of less than about 10 microns. 
     Similarly, the device in Shofner II employs multiple sensors to directly measure the amount of light remaining from a collimated light beam which has been at least partially extinguished by the passage of a fiber between the source of the collimated light beam and the sensor. Shofner II analyzes ribbon-shaped cotton fibers with a typical width of approximately 20 microns. In addition, Shofner II analyzes the diffraction pattern that results from a cotton fiber passing through the sensing zone. This apparatus and device is also unsuitable for certain other fibers, particularly those of narrow diameter, such as glass fibers having a diameter of less than about 5-10 microns due to its non-monotonic response to fiber diameter. 
     What is therefore needed is an airborne fiber dimension measuring device that can accurately characterize the dimensions of small-diameter fibers. 
     SUMMARY OF THE INVENTION 
     This invention provides devices and methods for determining the dimension of airborne fibers. The device includes flow means for providing a laminar flow to at least a portion of a group of fibers in a air sample. These aligned fibers are then illuminated with a light source to create scattered light. A light detector is then used for sensing a portion of the scattered light and for generating an output from which a dimension of a first of these fibers can be provided. 
     This invention takes advantage of the characteristics of scattered light to produce very accurate measurements of fiber dimensions, such as diameter and length. In preferred embodiments, scattered light is collected from a slotted opening at an angle of about 60° to about 120° relative to the direction of the light source to produce an approximate monotonic voltage amplitude range, indicative of a fiber diameter. 
     This invention also provides a method for measuring a dimension of an airborne fiber. The method includes providing a fiber-containing air sample having a laminar flow. This air sample is thereafter contacted by a light beam to produce scattered light. The scattered light is sensed and an electrical output is produced which is representative of a sensed portion of the scattered light. This electrical output is then processed to produce a perceptible indication of a dimension of at least a first fiber in said sample. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, referenced to herein and constituting a part hereof, illustrate preferred embodiments of the device of the present invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 is a front cross-sectional illustration of one embodiment of the present invention. 
     FIG. 2 is a top cross-sectional illustration of the embodiment presented in FIG.  1 . 
     FIG. 3 is a graphical illustration of a light sensor amplitude vs. fiber diameter generated in response to detected forward scattered light. 
     FIG. 4 is a graphical illustration of a light sensor amplitude vs. fiber diameter generated in response to light scattered laterally from the beam path at about 60° to 120°. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a cross-sectional view of one embodiment of device  1 , sectioned in a plane generally perpendicular to the airflow. FIG. 2 illustrates a top view of the embodiment of FIG. 1, along the plane indicated by line  2 — 2 . Referring to FIGS. 1 and 2, device  1  can include sensor  3 ; alone, or with air sampler  2 , sensor  3 , vacuum pump  4 , and flow tubes  5   a,    5   b  in combination. Air sampler  2  can be used to prefilter, or condition, the fiber-laden air  16 , or may be merely a sampling conduit. Sensor  3  is preferred to be an electro-optical sensor which provides a collimated light beam  6  using light source  7 . Light source  7  is preferred to be a laser diode. A suitable laser diode can be, for example, a model LPM03(670-5) laser diode from Power Technology, Inc., Little Rock, Ark. 
     When collimated light beam  6  strikes airborne fiber  9 , for example a cylindrical glass fiber, scattered light  8  is produced. It may be desirable to provide a light beam  6  with a preselected cross-section along the path of beam  6 , for example, an narrow elliptical cross-section. A portion of scattered light  8  is detected by light sensor  10 , which can be a photodetector. A suitable photodetector is, for example, Devar Model 509-10, Bridgeport, Conn. 
     Unlike prior art devices, which directly measure the amount of light remaining in a beam after impinging upon a fiber particle, this invention employs the characteristics of scattered light  8  as sensed by photodetector  10  and analyzed by dimension measuring circuit  11 . 
     In operation, vacuum pump  4  is attached to one end of flow tube  5   b,  and draws fiber-laden air  16  through tubes  5   a,    5   b.  The flow rate of air  16  is chosen such that the flow in tubes  5   a,    5   b  is laminar in nature. Also, the lengths of tubes  5   a,    5   b  are chosen such that there is a sufficient distance for the laminarly-flowing, fiber-laden air  16  to align the longitudinal axis of fiber  9  with the direction of the airflow. With reference also to FIG. 2, it is preferred that a small gap  15  be formed between tubes  5   a,    5   b  to permit collimated beam  6  to pass therethrough. Gap  15  can be used as a beam-steering device to preferentially direct scattered light  8  having the preselected orientation to sensor  10 . 
     In general, when fiber  9  enters the path of laser beam  6 , light is scattered. If fiber  9  is aligned with the flow of air  16 , then its longitudinal axis will be substantially perpendicular to laser beam  6  thus scattering light into a plane normal to the axis of tubes  5   a  and  5   b  (best seen in FIG.  1 ). The portion of scattered light  8  having this preselected orientation can be collected by lens assembly  13  and focused onto photodetector  10  producing a measuring signal  17 , the characteristics of which are indicative of the dimensions of fiber  9 . 
     Signal  17  can be processed by dimension measuring circuit  11 , which can produce a perceptible representation of the dimensions of fiber  9 . Responsive to scattered light  8 , photodetector  10  generates a voltage, the duration of which is essentially a function of the length and velocity of fiber  9 , and the thickness of beam  6 . If the thickness of beam  6  and the velocity of fiber  9  are substantially fixed, the length of the fiber  9  can be determined by measuring, for example, the duration of signal  17 . 
     The amplitude of signal  17  typically depends upon: (1) the wavelength of beam  6  and its intensity at the location of fiber  9 ; (2) the diameter of fiber  9 ; and (3) the angles over which scattered light  8  is collected. It is preferred that the wavelength of the light source and the light collection angles be fixed by the design of the system. It also is desirable to keep the intensity of beam  6  substantially constant in the region in which fibers  9  might be detected. Thus, the voltage amplitude of signal  17  can be made to depend primarily on the diameter of fiber  9 . 
     For ease of analysis, it is desired that the dependency of the voltage amplitude of signal  17  upon fiber dimensions be both linear and monotonic. However, where linearity is difficult or impossible to achieve, dependency can nevertheless be determined by an approximately monotonic signal. This signal can be provided by collecting scattered light  8  over a preselected range of collection angles. 
     As an example, for a light wavelength of about 670 nm, it is preferred to collect light from about 600 to about 120° relative to the direction of laser beam  6 , thus producing an approximately monotonic voltage amplitude range, which is indicative of the diameter of a small fiber  9  of less than about 10 microns or so. Furthermore, it is preferred that beam  6  from light source  7  be very thin to simplify the measurement of the length of fiber  9 , although, even where the length of fiber  9  is generally less than the thickness of beam  6 , fiber lengths can still be measured. 
     It is preferred that a laser diode be used as light source  7  because it typically produces an inherently thin, oval-shaped beam  6 . It is preferred that light source  7  be oriented such that the wide dimension of beam  6  is generally perpendicular to the flow of air  16  and that fiber  9  passes through the thin dimension of beam  6 . To further minimize the thickness of beam  6 , a focusing lens  12 , for example, a cylindrical lens, can be used. One advantage of cylindrical lens  12  is that the width of beam  6  is not operatively reduced thereby. 
     In general, the beam intensity across the width of beam  6  is approximately Gaussian. Therefore, it is preferred to place beam block  18 , having aperture  14  therein, in the path of beam  6  to substantially block low-intensity edges of beam  6 . Typically, Fresnel diffraction can occur from the edges of aperture  14 . Although this diffraction can cause some ripple in the intensity across the width of the remaining beam  6 , the “bright edge” associated with this diffraction helps to raise the intensity where the Gaussian intensity curve otherwise would be falling. Thus, the intensity across the width of beam  6  is nearly constant with some ripple. 
     As stated previously, existing prior art devices typically analyze the amount of light directly received from the light source, as affected by the passage of an airborne fiber through the light beam. The present invention preferably does not analyze direct light signals, but rather, scattered light signals having a preselected orientation after striking the fiber. 
     The advantages of this approach can be better appreciated by examining the response of a photodetector to directly impinging light as a function of fiber diameter and the light beam being attenuated by fibers, as seen in FIG.  3 . Response curve  30  arises from the direct impingement of a collimated light beam upon a photodetector as a function of fiber diameter. Response curve  30  is neither linear nor monotonic and may not reliably produce a signal that is representative of fiber diameter. 
     However, when scattered light  8  having a preselected orientation is used to determine fiber diameter, the photodetector response can be made approximately monotonic over a predetermined range, as seen with response curve  40  in FIG.  4 . The approximate monotonicity of response curve  40  is associated with fiber sizes below about 8-10 microns, and especially below about 9 microns, using a light wavelength of about 670 nm. A skilled artisan would recognize that light at other wavelengths may be desirable for fibers of other diameters. In general, the shorter the light wavelength, the narrower the dimension of the fibers that can be accurately determined. 
     Two linear approximations can be applied over the monotonic range of curve  40  to better estimate the response. For example, one linear approximation can be employed for fiber diameters of up to about 2 microns and a second linear approximation may be used for fiber diameters between about 2 microns and about 8 microns. 
     In preferred embodiments of the present invention, the scattered light  8  sensed by light sensor  10  and its lens  13  are preferred to be at a preselected orientation of between about 60° and about 120° relative to the beam path. 
     All publications mentioned in this specification are indicative of the level of skill of the skilled in the art to which this invention pertains. All publications are herein incorporated by reference to the same extent as if each individual publication was specifically but individually indicated to be incorporated by reference. 
     While specific embodiments of practicing the invention have been described in detail, it will be appreciated by those skilled in that art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Indeed, a skilled artisan would recognize that, although the invention has been described in terms of determining the dimensions of airborne fibers, the apparatus and method illustrated in detail herein also can be used to detect, characterize, and visualize other types of particles having specific optical properties. Accordingly, the particular arrangements of the methods and apparatus disclosed are meant to be illustrative only and not limiting to the scope of the invention, which is to be given the full breadth of the following claims, and any and all embodiments thereof.