Abstract:
In accordance with embodiments of the present invention, a goniospectrophotometer is provided for quickly obtaining a goniospectrum using a goniospectrophotometer. In some embodiments, a parabolic reflector is used to optically transform the angular space of a source at the parabola focus into a linear space and facilitate the use of a single diffracting element and area camera to simultaneously measure the angular spectrum of the source. Spectra corresponding to zenith angles of light reflection by the parabolic reflector can be acquired by a detector and analyzed in a computer.

Description:
RELATED APPLICATIONS 
   The present application claims priority to provisional application 60/495,132, entitled “Novel Real-Time Goniospectrophotometer” by Jeffrey L. Guttman, filed on Aug. 15, 2003, herein incorporated by reference in its entirety. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention is related to the use of goniospectrophotometric, goniospectroradiometric, goniophotometric, or gonioradiometric measurements of primary or secondary sources, including the fields of color measurement, scatterometry, multi-angle light scattering (MALS), and beam profiling. 
   2. Discussion of Related Art 
   Goniospectrophotometers are used for characterizing the spectra of light as a function of angle for light sources such as LEDs, and also for materials painted or printed with gonioapparent or pearlescent pigments in the paint and ink. Conventional commercially-available systems obtain the goniospectra either by using an optical fiber coupled spectrophotometer and moving the fiber about the source or sample, or by using a set of spectrometers positioned at a set of fixed angles. Measurement times for multiple zenith angles at a single azimuth angle about the source are typically in the range of 1-20 minutes. 
   Conventional practice uses mechanical scanning of a detection system (such as a fiber-coupled spectrograph) about the source.  FIG. 1  shows a schematic diagram of a conventional Goniospectrophotometer  100 . Detection system  103  of goniospectrophotometer  100  can, for example, be a fiber-optic cable attached to a spectrophotometer to provide spectral data regarding light source  101 . As such, detector system  103  is highly directional in its receipt of optical radiation from source  101  and therefore can be configured to receive light at substantially a single zenith angle at a single azimuth angle from light source  101 . The distance between detection system  103  and light source  101  is typically within the “near-field” region, but may also be in the “far-field” region. The “far-field” region is characterized by a distance far enough from light source  101  such that light source  101  can be viewed as a point source. A commonly used rule of thumb boundary between the two regions is the “Five-Times Rule,” where detection system  103  is placed at a distance from light source  101  that is at least five times the lateral extent of light source  101 . 
   In operation, detector system  103  is scanned around light source  101  stopping at each of a set of predetermined zenith angles to measure a spectrum with a spectrometer. Thus, a spectral scan as a function of angular position for a single azimuth angle around light source  101  is obtained. In some embodiments, detector system  103  can be rotated along a great circle on a sphere with light source  101  at its center. Detector system  103  can also be oriented so that it collects light in a small solid angle substantially originating from the center of the sphere. 
     FIG. 2  illustrates an example where source  101  is illuminated by light beam  105 . Light beam  107  is a reflection from light source  101  of illuminating beam  105 . As shown in  FIG. 2 , light source  101  may be a material deposited on a base or substrate  109 . In some testing situations, scatter of light beam  105  is measured in detector system  103 . In some testing situations, light from light source  101  is a result of luminescence after excitation by illuminating light beam  105 . 
   Performing a spectral scan at each angular position, however, takes a significant amount of time to perform. Therefore, systems such as those shown in  FIG. 1 and 2  can be very slow. As mentioned above, a spectral scan at an individual azimuth can take up to about 20 minutes. Therefore, an angular distribution of that scan can involve a few hundred to a few thousand minutes (depending on the number of angles) to execute. 
     FIG. 3  illustrates a goniospectrophotometer  300  that mitigates the scanning time somewhat. As shown in  FIG. 3 , goniospectrophotometer  300  includes a plurality of detector systems  301 - 1  through  301 -N. Detector systems  301 - 1  through  301 -N are fixed at particular angles around light source  101  so that each receives light from light source  101  at a particular zenith angle θ 1  through θ N . As described above, each of detector systems  301 - 1  through  301 -N can include an optical fiber directed to receive light at an azimuthal angle θ 1  through θ N  from light source  101 . Each of detector systems  301 - 1  through  301 -N can include a spectrometer for providing a spectrum at the corresponding one of zenith angles θ 1  through θ N . 
     FIG. 4  illustrates a goniospectrophotometer  300  utilized to characterize a light source  101  illuminated by beam  105 . Light source  101 , which can be on substrate  109 , reflects, luminesces, or otherwise emits light in response to illumination beam  105 . Illumination beam  105  is reflected from light source  101  into reflection beam  107 . 
   Although the embodiments of goniospectrophotometer  300  shown in  FIGS. 3 and 4  reduce the amount of time required to provide a spectral scan of light source  101 , they are expensive in that each of detector systems  301 - 1  through  301 -N is an independent detector system that requires an independent spectrometer. Further, goniospectrophotometer  300  suffers from the need to calibrate multiple detector systems  301 - 1  through  301 -N. Also, there may be difficulty in measuring continuous angular spectra due to mechanical obstructions from mounts utilized to hold other detector systems  301 - 1  through  301 -N in place. 
   Therefore, there is a need for economical, faster methods of obtaining the angular dependence of color spectra of light from material samples or light sources for applications requiring goniospectrophotometric analyses. 
   SUMMARY 
   In accordance with the present invention, a goniospectrophotometer includes a parabolic reflector that allows for optical transformation of the angular space of a light source at the parabola focus into a linear space. A single diffracting element and area camera can then be utilized to simultaneously measure the angular spectrum of the source. 
   A method of obtaining an angular spectrum according to some embodiments of the present invention includes parabolically reflecting light from a light source to provide a beam of light, the beam of light including light from a range of zenith angles at a particular azimuthal angle from the light source; spectrally dispersing light from the beam; and capturing a goniospectrum corresponding to the beam, the goniospectrum including spectra corresponding to the range of zenith angles. In some embodiments, the beam of light includes a set of discrete beams, each beam in the set of discrete beams corresponding to a zenith angle in the range of zenith angles. In some embodiments, the set of discrete beams may be collimated. In some embodiments, spectrally dispersing light from the light beam includes diffracting the light beam in a diffraction grating. In some embodiments, the beam may be focused before being spectrally dispersed. In some embodiments, capturing a goniospectrum includes detecting a spectrum from the diffraction grating corresponding to a plurality of zenith angles within the range of zenith angles. In some embodiments, the spectrum is a zero&#39;th order spectrum, although a higher order spectrum may be utilized. In some embodiments, the spectrum from the diffraction grating can be projected on a screen. In some embodiments, detecting the spectrum includes digitally capturing an image of the spectrum in a digital camera. The camera may be a CCD array. In some embodiments, the light source can be a sample illuminated by an illumination beam. In some embodiments, the illumination beam results from a source of light coupled to the same through an optical fiber. 
   A goniospectrophotometer according to some embodiments of the present invention can include a parabolic reflector positioned to receive light from a light source and producing a light beam; a spectral element positioned to receive the light beam from the parabolic reflector and produce a diffracted beam; and an optical detector system coupled to detect a goniospectrum from the diffracted beam. In some embodiments, the parabolic reflector can include alternating strips of reflective and non-reflective material. In some embodiments, the goniospectrophotometer can include a collimating element positioned between the parabolic reflector and the diffraction element. The collimating element can be a set of parallel plates. In some embodiments, the spectral element can be a diffraction grating. In some embodiments, the optical detector system includes a camera positioned to capture the goniospectrum. In some embodiments, the camera can be a CCD array. In some embodiments, the camera can be a digital camera. In some embodiments, the goniospectrophotometer can include focusing optics coupled between the spectral element and the detector to focus light into the camera. In some embodiments, a computer can be coupled to the optical detector system to analyze the goniospectrum detected by the optical detector system. 
   A method of inspecting a work piece with a goniospectrophotometer according to some embodiments of the invention includes providing an illumination light source; positioning the work piece such that a portion of the work piece is illuminated by the illumination light source; collecting scattered light from the portion of the work piece corresponding to a range of zenith angles in a parabolic reflector and forming a light beam reflected from the parabolic reflector; diffracting the light beam to form a diffracted beam; detecting a goniospectrum from the diffracted beam; determining whether the goniospectrum falls within a specification; and passing or rejecting the work piece based on the result of determining whether the goniospectrum falls within the specification. In some embodiments, providing an illumination light source includes coupling light from a lamp into an optical fiber, the optical fiber being positioned above the work piece. In some embodiments, diffracting the light beam to form a diffracted beam includes positioning a diffraction grating in the light beam. In some embodiments, detecting the goniospectrum includes measuring the diffracted beam with a position sensitive detector. The position sensitive detector can be a CCD array. 
   Embodiments according to the present invention provide a faster method of obtaining the color spectra versus angle of material samples or light sources for applications requiring goniospectrophotometric analyses. Methods according to the present invention apply to many other applications, such as goniometric instruments, fixed multi-angle instruments, and scatterometers with elliptical mirrors. 
   These and other embodiments of the invention are further described below with respect to the following figures. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  illustrates an embodiment of a goniospectrophotometer where a detector system is scanned around a light source. 
       FIG. 2  illustrates an embodiment of a goniospectrophotometer as illustrated in  FIG. 1  where the light source is an illuminated material. 
       FIG. 3  illustrates an embodiment of a goniospectrophotometer that includes multiple detectors/spectrometers located at fixed angles about the light source. 
       FIG. 4  illustrates the embodiment of goniospectrophotometer shown in  FIG. 3  where the light source is an illuminated material. 
       FIG. 5  illustrates a goniospectrophotometric instrument according to some embodiments of the present invention. 
       FIG. 6  illustrates an embodiment of a parabolic reflector that can be utilized in embodiments of the present invention. 
       FIG. 7  illustrates an embodiment of a collimator that can be utilized in embodiments of the present invention. 
       FIG. 8A  illustrates a cross sectional view of an embodiment of goniospectrophotometer according to the present invention. 
       FIG. 8B  illustrates a cross section view along the AA direction shown in  FIG. 8A  of an embodiment of a goniospectrophotometer according to the present invention. 
       FIG. 8C  illustrates a sample goniospectrum detected by a detector in an embodiment of a goniospectrophotometer according to the present invention. 
       FIG. 9  illustrates utilization of a goniospectrophotometer according to some embodiments of the present invention in a production process. 
   

   In the figures, elements having the same designation have the same or similar function. 
   DETAILED DESCRIPTION 
   Some embodiments of the present invention obtain the full goniometric spectra (a goniospectrum) over an angular range greater than 90° in about 30 milliseconds. Use of parabolic reflector with the source of interest positioned at the focus of the parabola allows for simultaneous capture of data at a set of discrete angles. With this configuration, the angular light emission transforms into a collimated beam propagating parallel to the axis of the parabola. Thus, the light emanating angularly is then distributed linearly and radially with respect to the axis of the parabolic reflector along the cross-section of the reflected beam, with a quadratic one-to-one correspondence of angle and radius. 
   Placement of diffraction gratings or other spectrally dispersive elements in the reflected beam coupled to CCD area sensors provide for simultaneous measurement of the entire spectra for all the angles collected by the parabolic reflector. In some embodiments, a slit along the parabolic reflector can serve as an entrance aperture of the diffraction grating spectrometer. Alternatively, the parabolic reflector itself can be a thin segment of a parabola. An optional Angular Field-of-View (FOV) element can limit the angular extent of the light incident on the grating. An optional light baffle or baffles can be positioned between the source and the grating. Such a baffle can block light emanating directly from the source. In some embodiments, a collimator can be positioned between the parabolic reflector and the spectrally dispersive element. In some embodiments, a set of parallel beams where each beam corresponds to a discrete zenith angle can be produced, either by a collimator or by the parabolic reflector. The angular intensity profile is obtained by integrating the spectra at each zenith angle. Alternatively, the intensity profile can be obtained using a linear array positioned in the collimated beam space without the spectrally dispersive element. 
     FIG. 5  illustrates an embodiment of goniospectrophotometer  500  according to the present invention. Light source  501  is positioned at a focus point of a parabolic reflector  502 . Light source  501  can be an illumination source such as a LED, light bulb, or other light-producing source. Alternatively, light source  501  can be an illuminated material that reflects (scatters) light or fluoresces in response to an illumination beam (not shown in  FIG. 5 ). Further, a slit (not shown) can be positioned between light source  501  and parabolic reflector  502  in order to serve as an entrance slit. Such an entrance slit can be positioned to allow light from light source  501  corresponding to the range of zenith angles θ while only allowing a narrow band of light around a particular azimuthal angle φ to enter. 
   Light source  501  can be oriented at an arbitrary angle, zenith and azimuthal (θ,φ), with respect to the parabola axis of parabolic reflector  502 . Parabolic reflector  502  can be any parabolic shaped surface that reflects light in the wavelength range measured by goniospectrophotometer  500 . In some embodiments, parabolic reflector  502  can be a segment of an off-axis parabolic mirror. Parabolic reflector  502 , then, captures light corresponding to a range of zenith angles θ at a particular azimuthal angle φ. 
   In some embodiments, light source  501  can be formed from an illuminated sample. In some embodiments, a lamp and a multimode optical fiber bundle can be utilized to deliver the illumination beam to the sample to produce light source  501 . In some embodiments, a 2.4 mm diameter fiber bundle with individual component fibers of nominally 40 μm diameter with numerical aperture of about 0.55 can be utilized to deliver light to a material to form light source  501 . Light from a high intensity tungsten filament lamp driven by a DC power supply or light from a high intensity pulsed Xenon flashlamp can be coupled into the optical fiber. Any other lamp may be utilized, depending on the range of optical wavelengths that are under investigation. 
   Light emanating from light source  501  between the zenith angles θ 1  and θ 2  is subtended by parabolic reflector  502 . Further, parabolic reflector  502  can be set at particular azimuthal angles. In some embodiments, parabolic reflector  502  may intersect zenith angles of |θ 2 −θ 1 | greater than 90°, but parabolic reflectors intersecting less of a spread of azimuthal angles can also be utilized. In some embodiments, parabolic reflector  502  can be set at a set of azimuthal angles as well. In some embodiments of the invention, parabolic reflector  502  may include slit apertures or otherwise narrow strips of reflective material to reflect light at particular discrete zenith angles between the angles θ 1  and θ 2 . In some embodiments, parabolic reflector  502  can be a continuous reflector, reflecting light corresponding to all angles in the range of zenith angles θ 1  to θ 2 . In some embodiments, parabolic mirror  502  can be thin so as to collect light only from a narrow range of azimuthal angles around a predetermined azimuthal angle φ. In some embodiments, parabolic reflector can be formed in a replication process from an originally machined part. 
     FIG. 6  illustrates a cross-section of a portion of parabolic reflector  502  according to some embodiments of the present invention. The embodiment of parabolic reflector  502  shown in  FIG. 6  includes reflective regions  601  and non-reflective regions  602 . Nonreflective regions can be absorbing strips on parabolic reflector  502  or may be slits formed in parabolic reflector  502 . Light beam  603  originate by reflection from light source  501 , then, can include a set of discreet light beams with each beam in the set of discreet light beams corresponding to light collected at a corresponding particular zenith angle θ n . Light from light source  501  are reflected from parabolic mirror  502  substantially in parallel rays of light with each ray of light associated with light emanating from light source  501  at a corresponding zenith angle from light source  501 . In some embodiments, about five to about eight zenith angles of data can be taken, which is consistent with the ASTM Draft Standard WK1164 for measurement of gonioapparent materials. Light beam  603  reflected from parabolic reflector  502 , then, can be in the form of pseudo-collimated thin rays of light, each ray corresponding to a different unique zenith angle between the angles of θ 1  and θ 2 . Some embodiments of parabolic reflector  302  provide for a continuous beam  603  where light emitted at a particular zenith angle θ n  corresponds to a particular position in light beam  603 . 
   In some embodiments, parabolic reflector  502  can have a focal dimension of about 10 mm. In general, however, parabolic reflector  502  can have any focal dimension. With a 10 mm focal dimension, light source  501  can have a lateral extent of about 1 mm and maintain angular resolution to a few degrees. In operation, light source  501  is positioned at a focus of parabolic reflector  502 . 
   In some embodiments of the invention, beam  603  reflected from parabolic reflector  502  can be further collimated by collimator  504  shown in  FIG. 5 . As shown in  FIG. 7 , in some embodiments, collimator  504  can be a mechanical array of thin plates  701  separated by spacers (not shown) that further narrow the view. In some embodiments, collimator  504  can include any arrangement of alternating transparent and reflective surfaces that further collimates light beam  603 . In embodiments where beam  603  includes a set of discrete light beams, collimator  504  can include a corresponding set of parallel plates. In some embodiments, where beam  603  is a continuous beam, collimator  504  may include a set of parallel plates so that beam  603  exciting collimator  504  includes a set of discrete beams. In some embodiments, collimator  504  can include a pair of plates to collimate beam  603  as a whole. 
   Further, in some embodiments a light baffle  503  ( FIG. 5 ) can be fixed adjacent light source  501  to block light from light source  501  that is outside the range of azimuthal angles between θ 1  and θ 2  from entering collimator  504 . In embodiments where a continuous beam  603  is utilized, collimator  504  is absent. 
   With continuing reference to  FIG. 5 , beam  603 , which may or may not include discrete beams, is directed onto spectral element  506 . Spectral element  506  can be any device that spectrally disperses light in beam  603 . 
   In some embodiments, focusing optics  505  can be introduced to direct light beams  603  onto spectral element  506 . In some embodiments, focusing optics  505  can be cylindrical beam-shaping optics that reduce the linear extent of the parallel light beams prior to incidence upon spectral element  506 . In some embodiments, focusing optics  505  can include a focusing lens  511  followed by a collimating lens  512 . 
   Spectral element  506  can include any device that separates a light beam into its respective wavelengths, for example a diffraction grating or a prism. In some embodiments, a diffraction grating having a transmission grating with 1500 lines/mm optimized for maximum efficiency over a spectral range of from about 360 to about 800 nm is utilized. Typically, light beam  603  is diffracted by spectral element  506  into multiple orders of spectra. The goniospectral distribution can then be determined by measuring the light intensity over one order of diffraction of beam  603 . The diffracted beam from spectral element  506  is shown in  FIG. 5  as spectra  507 . Spectra  507  includes all of the orders of diffraction.  FIG. 8C  illustrates an embodiment of goniospectrophotometer according to the present invention that utilizes the zero&#39;th order diffraction. One skilled in the art will recognize that a goniospectrum can also be obtained from higher diffraction orders. 
   With reference to  FIG. 5 , the light from a single order of diffraction in spectra  507  can be collected by lens system  508  and directed onto a detector  509 . Lens system  508 , when present in embodiments of goniospectrophotometer  500 , can include a focusing lens  513  followed by a collimating lens  514 . 
   Detector  509  can be any spatially sensitive detective device, for example a charge-coupled detector (CCD) array. In some embodiments, such as, for example, that shown in  FIG. 8 , the spectra can be projected onto a screen  801  before being detected by detector  509 . Other spatially sensitive light detectors can also be utilized, such as, for example, a CMOS camera or Vidicon camera. The signals from detector  509 , which can be either analog or digital depending on the particular detector system, are then input to an analyzer or computer system  510 . Computer system  510  then provides the goniospectrum from the data collected for recordation, further processing, or display. 
     FIG. 8A  illustrates a cross-sectional view of goniospectrophotometer  800  according to some embodiments of the present invention. As described with  FIG. 5 , light from light source  501  corresponding to a range of zenith angles θ 1  to θ 2  is captured by parabolic reflector  502  and directed into beam  603 . As before, beam  603  can be a continuous beam or a discrete set of beams with each separate beam corresponding to a discrete zenith angle θ n . In a continuous beam, light from zenith angle θ n  can be detected by position in the beam. 
   Beam  603  is incident on spectral element  506 , which in  FIG. 8A  is a diffraction grating with rulings parallel to the plane of the cross section shown in  FIG. 8A . The diffracted beam  507  is then displayed on screen  801  and detected by detector  803 . In  FIG. 8A , spectral element  506  is illustrated so as to show the direction of the rulings on a diffraction grating. Spectral element  506  would, in this plane of reference, be oriented such that light beam  603  is incident directly on the surface of spectral element  506 . 
     FIG. 8B  illustrates a perpendicular cross section of goniospectrophotometer  800  along the direction AA shown in  FIG. 8A . A slice of beam  603  corresponding to zenith angle θ n  is shown being diffracted from parabolic reflector  502 . In this plane, the diffraction grating of spectral element  506  is set at an angle with respect to beam  603 . The rulings of the diffraction grating are perpendicular to the cross sectional plane of goniospectrophotometer  800  shown in  FIG. 8B . The zero&#39;th order spectra of the diffracted beam  507  corresponding to zenith angle θ n  is displayed on screen  801 . A detector  803 , which can be a digital camera such as a CCD camera, then detects the spectra corresponding to zenith angle θ n . 
     FIG. 8C  illustrates a full set of spectra, a goniospectrum, projected on screen  801 . The intensity of light is illustrated, for an example goniospectrum, by the shading. Detector  803 , then, can simultaneously measure the spectra as a function of zenith angle for all zenith angles in the range θ 1  to θ 2  in a single exposure. Each slice in the wavelength direction corresponds to a single zenith angle θ n  (or more accurately a small set of angles Δθ around θ n ). Therefore, the spectra for any particular angle can be determined, as is illustrated by the slice indicating the spectrum at zenith angle θ n  shown in  FIG. 8C . 
   In some embodiments, camera  803  can be a 12-bit digital CCD camera with a 1037×1376 array of 6.45 μm square pixels. The CCD image can be acquired in a computer system  510  operating a digital frame grabber software such as the Beam Profiler Software produced by Photon, Inc. In some embodiments, the image acquisition time is about 90 ms, based on the maximum rate of the digital frame grabber. 
   In some embodiments, the image captured by the digital frame grabber software can be exported to analysis software, for example MatLab, for analysis. Data analysis can involve, for example, determination of material reflectance versus angle, or transformation of angular spectral data to CIE colorimetric spaces. In some embodiments, computer  510  may only check the goniospectrum to insure that it falls within a particular manufacturing specification. In which case, computer  510  may alert an operator if the goniospectrum falls outside of the predetermined specification. 
     FIG. 9  illustrates utilization of goniospectrophotometer  901  according to some embodiments of the present invention in a manufacturing environment  900 . As shown in  FIG. 9 , a source of light  905  is coupled into an optical fiber  907  so that a work piece  903  can be illuminated. Goniospectrophotometer  901  is positioned such that a goniospectrum of scattered light from work piece  903  is obtained. In some embodiments, several goniospectra at differing positions on work piece  903  can be obtained. Goniospectrophotometer  901 , then, determines whether the goniospectrum obtained falls within a predetermined specification and then passes or rejects work piece  903  based on that determination. 
   Work piece  903  can be any part where a goniospectrum is an important parameter, for example the coloration, paint coverage, or other characteristic. Further, work piece  903  may be on an assembly line and therefore is followed by another work piece  909  once work piece  903  has been inspected. 
   Embodiments of goniospectrophotometer, as described herein, provide for faster and cheaper acquisition of goniometric spectra. Spectral data for each of a set of zenith angles around light source  501  are taken in a digital camera, which also captures each spectrum from each of the set of zenith angles simultaneously. Therefore, embodiments of the goniospectrophotometer according to the present invention can utilize only a single digital camera or CCD detector and can take simultaneous spectra in the time it takes for data acquisition by the single camera. 
   The embodiments described in this disclosure are examples only and are not intended to be limiting. As such, the invention is limited only by the following claims.