Abstract:
A method of controlling the illumination angle onto a target, including, illuminating onto the target with light from at least two light sources of pre-selected wavelengths; wherein each point on the target is illuminated by light from the light sources with a respective maximal illumination angle relative to a main illumination axis extending from substantially the center of the at least two light sources to the target, selecting a dichroic filter that transmits light from the at least two light sources as a function of the angle of incidence upon the filter, positioning the dichroic filter between the at least two light sources and the target to limit the transfer of light to light of the pre-selected wavelengths; and wherein the dichroic filter is selected to limit the maximal illumination angle illuminating each point on the target.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to control of the illumination angles of an object using dichroic filters. 
     BACKGROUND OF THE INVENTION 
     Many applications require illumination of points on a target at specific illumination angles to achieve a desired contrast for examining the surface, for example to analyze the surface of electrical circuits on a printed circuit board (PCB). Typically in process of fabrication of electrical circuits, such as printed circuit boards, interconnect devices, Integrated Circuits and flat panel displays, an automated optical inspection operation is used to identify defects in the electrical circuits or on the substrate. Typically examination of a PCB is achieved by illuminating the surface of the PCB line by line with one or more line light sources and scanning the line for analysis. A line light source may be created by various methods, for example by forming a row of glass fibers that originate from a point light source (e.g. a halogen light), or by forming a row of LED sources. A lens may be placed along the line between the line light source and the PCB to focus the light in the plane perpendicular to the line to achieve an optimal angle of illumination on each point along the line. 
     Along the longitudinal axis each point of the line on the PCB is illuminated from the multiple light sources forming the line light source. The overall angle of illumination of the point is determined by the dispersion angles of the point light sources that the line light source is made up from, for example a Halogen light source tends to generate light that spreads out forming an angle of about +/−25° from the focal point of the source along the longitudinal axis. LED light sources generally generate light that spreads out forming an angle of about 70° in all directions. 
     To optimally analyze a point on the PCB it is desired to illuminate the point symmetrically, to eliminate the need to perform calculations to compensate for the asymmetry of the lighting. However controlling the illumination angles of the points that make up the line light source in all directions constitutes a geometrical challenge for lens makers. Such a solution may be impossible or very costly depending on the size and number of points forming the line light source. Therefore other methods of controlling the dispersion angles of the light are desirable. 
     SUMMARY OF THE INVENTION 
     An aspect of an embodiment of the invention relates to a system and method for controlling the illumination angles of a beam of light from two or more points toward a line or area on a target by placing a dichroic filter between the light source and the illuminated target. The dichroic filter selectively transfers light depending on the wavelength and the angle of incidence of the light source. Selection of a dichroic filter that transfers a desired wavelength at specific angles of incidence enables the control of the resulting illumination beam upon the target. 
     In an exemplary embodiment of the invention, one or more rows of light sources are used to illuminate a line or area on a target. Optionally, a lens is used to focus the light in the direction perpendicular to the row of light sources, thus controlling the illumination angles for that direction. In some embodiments of the invention, each row illuminates a segment of the full angular coverage. In the direction parallel to the row of light sources the dichroic filter transfers specific wavelengths of light at specific ranges of angles of incidence upon the filter, so that the illumination angle in the parallel direction can also be limited. In some embodiments of the invention, the illumination angles on the target are the same in all directions. Alternatively, the light in the perpendicular direction may be focused using the lens to a narrower illumination angle than in the parallel direction using the dichroic filter. Optionally, if the lens focuses the light to a beam with a wider angle than provided by the dichroic filter, the filter will limit the illumination angle also in the perpendicular direction. 
     In some embodiments of the invention, the dichroic filter is placed parallel to the row of light sources, so that all of the light sources are handled symmetrically. Alternatively, the dichroic filter is placed at an angle relative to the row of light sources, so that some of the sources will transfer more light than others and for example to allow illumination from some light beams at obtuse angles relative to the row of light sources. 
     In some embodiments of the invention, the light beams illuminated from the light sources toward the target are transferred through more than one filter to enhance control of the illumination angles. Optionally, the dichroic filter may be a multi-band pass filter that supports more than one range of wavelengths that are transmitted through the filter if arriving at the filter at the designated range of angles supported by the filter. 
     There is thus provided according to an exemplary embodiment of the invention, a method of controlling the illumination angle onto a target, comprising: 
     illuminating onto the target with light from at least two light sources of pre-selected wavelengths; wherein each point on the target is illuminated by light from the light sources with a respective maximal illumination angle relative to a main illumination axis extending from substantially the center of the at least two light sources to the target; 
     selecting a dichroic filter that transmits light from the at least two light sources as a function of the angle of incidence upon the filter; 
     positioning the dichroic filter between the at least two light sources and the target to limit the transfer of light to light of the pre-selected wavelengths; and wherein the dichroic filter is selected to limit the maximal illumination angle illuminating each point on the target. Optionally, the dichroic filter is positioned perpendicular to the main illumination axis. Alternatively, the dichroic filter does not form a right angle relative to the main illumination axis. 
     In an exemplary embodiment of the invention, the angle of the dichroic filter is user controllable. Optionally, multiple dichroic filters are positioned sequentially along the main illumination axis. In an exemplary embodiment of the invention, the multiple dichroic filters are positioned at an angle relative to each other. Optionally, the dichroic filter is a multi pass filter. In an exemplary embodiment of the invention, the method further includes placing a lens between the at least two light sources and the target to focus the light from the at least two light sources onto the target along a first direction; and wherein the light illuminating the target along a second direction perpendicular to the first direction have substantially the same illumination angles as the first direction as a result of the positioning of the dichroic filter. 
     In an exemplary embodiment of the invention, the at least two light sources are provided in multiple rows of light sources parallel to each other and wherein the beams from each row of light sources is focused with a lens onto the target along a first direction, such that the combined beam that illuminates the target along the first direction has the same illumination angles as along a second direction perpendicular to the first direction as a result of the positioning of the dichroic filter. Optionally, each of the at least two light sources selectively provides light of multiple wavelengths and the controlled illumination angles differ for each wavelength. 
     There is further provided according to an exemplary embodiment of the invention, a system for controlling the illumination angle onto a target, comprising: 
     at least two light sources to illuminate the target; 
     a dichroic filter that transmits light from the at least two light sources as a function of the angle of incidence upon the filter; wherein the dichroic filter is positioned between the at least two light sources and the target; 
     wherein the light of the at least two light sources have pre-selected wavelengths; 
     wherein each point on the target is illuminated by light from the light sources with a respective maximal illumination angle relative to a main illumination axis extending from substantially the center of the at least two light sources to the target; 
     wherein the filter is selected to limit the maximal illumination angle illuminating each point on the target. 
     In an exemplary embodiment of the invention, the system further comprises a lens positioned between the at least two light sources and the target to focus the light from the at least two light sources onto the target along a first direction; and wherein the light illuminating the target along a second direction perpendicular to the first direction have substantially the same illumination angles as the first direction as a result of positioning the dichroic filter between the target and the at least two light sources. 
     There is further provided according to an exemplary embodiment of the invention, an illumination device for illuminating a target, comprising: 
     one or more rows of light sources, wherein each light source provides light of one or more wavelengths; 
     a lens parallel to each row of light sources to focus the light from the rows of light sources so that the combined light beam that illuminates the target has a specific angle relative to a main illumination axis extending from substantially the center of the at least two light sources to the target; 
     a dichroic filter placed between the target and the lens that transmits specific light wavelengths incident on the filter at a specific range of angles, such that the resulting illumination angles for at least one wavelength, on the target along the direction perpendicular to the row of light sources is symmetrical with the illumination angles along the direction parallel to the row of light sources. 
     Optionally, the system includes multiple dichroic filters. In an exemplary embodiment of the invention, the resulting illumination angles on the target along the direction perpendicular to the row of light sources is symmetrical with the illumination angles along the direction parallel to the row of light sources for a single wavelength. Optionally, the resulting illumination angles on the target along the direction perpendicular to the row of light sources is symmetrical with the illumination angles along the direction parallel to the row of light sources for more than one wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings. Identical structures, elements or parts, which appear in more than one figure, are generally labeled with a same or similar number in all the figures in which they appear, wherein: 
         FIG. 1  is a schematic illustration of a cross sectional view of a device for optically analyzing electrical circuits, according to an exemplary embodiment of the invention; 
         FIGS. 2A and 2B  are schematic illustrations of a side view of illumination of a point on a target along the longitudinal axis with and without use of a dichroic filter respectively, according to an exemplary embodiment of the invention; 
         FIGS. 3A-3C  are schematic illustrations of a dichroic filter transmission spectrum as a function of the angle of incidence, according to an exemplary embodiment of the invention; 
         FIG. 4  is a schematic illustration of a graph of transmission versus the angle of incidence through a dichroic filter, according to an exemplary embodiment of the invention; 
         FIG. 5  is a schematic illustration of the use of two tilted dichroic filters and a mirror to transfer specific light rays from a light source and to block others thus controlling the angles by which a target is illuminated, according to an exemplary embodiment of the invention; 
         FIG. 6A  is a schematic illustration of a line light source, wherein each point on the line is made up from two light points of different wavelengths, according to an exemplary embodiment of the invention; and 
         FIG. 6B  is a schematic illustration of a graph showing transmission versus the angle of incidence using two light points of different wavelengths, according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic illustration of a cross sectional view of a device  100  for optically analyzing electrical circuits, for example as manufactured on a PCB or other surface, according to an exemplary embodiment of the invention. Optionally, device  100  includes a base  120  to mount optical elements for controlling illumination onto a target  180  and sampling reflection from target  180 . In an exemplary embodiment of the invention, three line light sources  130  are mounted on base  120  to illuminate target  180 . Optionally, target  180  is made up from a material that serves as a Lambertian surface selectively coated with a metallic conductor based on a circuit design. Typically, the Lambertian surface and the metallic conductor have different levels of reflection so that by sampling light reflected from target  180  the circuit manufacture can be verified, for example by detecting manufacturing errors. 
     In an exemplary embodiment of the invention, a light signal  125  is formed by the reflectance of light sources  130  from target  180 . Light signal  125  is reflected toward a semi-transparent mirror  150  that passes light signal  125  on through a mirror  160  and/or through any other optical path toward a sensor  170 . Sensor  170  may be for example a CCD (Charged Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor), a photomultiplier or any other adequate sensor. In an exemplary embodiment of the invention, sensor  170  samples light signal  125  and provides the sample as digitized information to a computer to analyze target  180  by comparing it to the circuit design. Alternatively or additionally other methods known in the art can be used in performing Automatic Optical Inspection (AOI) of electrical circuits. 
     In some embodiments of the invention, three light sources  130  are used to illuminate a line  185  on the target, for example the first light source  130  may be positioned to illuminate from one side of line  185 , the second light source  130  may be positioned to illuminate from the opposite side of line  185  and the third light source  130  may be positioned to illuminate from directly above line  185 . In some embodiments of the invention, a greater number of light sources may be used (e.g. 4, or 5 or more) or a smaller number of light sources may be used (e.g. 1, or 2). Optionally, a lens  135  is placed in the path of the light toward line  185  to focus the light from light sources  130  so that the combined light incident onto line  185  illuminates the line at a predetermined angle  190  (e.g. 30°, 55°, or 100°) in a plane parallel to the X coordinate as shown in  FIG. 1 . In some embodiments of the invention, each light source  130  contributes by illuminating over the entire angle  190 . Alternatively, each light source  130  contributes a segment of angle  190 . In an exemplary embodiment of the invention, a dichroic filter  145  is placed in the light path, for example after lens  135  to control the angle by which each point of line  185  will be illuminated in the Y direction as described below. In some embodiments of the invention, the position of the optical elements may vary, for example dichroic filter  145  may be placed before lens  135 . Optionally, some of the light paths may be more complex and some may be less complex, for example the center light source in  FIG. 1  is not placed along the same arc as the other two light sources, but instead is positioned remotely and transferred through a mirror  140 , a curved mirror  115  and a semi-transparent mirror  150 , to allow the reflected light to be returned in the same direction via semi-transparent mirror  150 . 
     It should be noted that the light source for implementing the invention is not limited to a point light source or a line light source as generally used in this description. Optionally, other types of light sources (e.g. a multiline source an area source, or other variations) may also be used and the details described herein are equally applicable to these cases. 
     Additionally, it should be noted that the invention is not limited to a specific type of electronic circuit or a specific type of surface serving as the target on which an electronic circuit is manufactured, for example the electronic circuit may be implemented as printed circuit boards, interconnected devices, Integrated Circuits, flat panel displays and any other form of electronic circuits or surfaces. Likewise, the invention is not limited to electronic circuits and is equally applicable to other types of targets. 
       FIG. 2A  is a schematic illustration of a side view  200  of illumination of a point  225  on a target along the longitudinal axis (Y) without use of a dichroic filter, and  FIG. 2B  is a schematic illustration of a side view  250  of illumination of a point  225  on a target along the longitudinal axis (Y) using a dichroic filter  145 , according to an exemplary embodiment of the invention. In an exemplary embodiment of the invention, in the X direction all the light is collected together toward a line parallel to Y axis  185  by lens  135  and directed to form a narrow intense illumination line limited by the desired illumination angle. Optionally, in the Y direction each light point  205  contributes to the illumination of point  225  if the dispersion of the light point covers the point. In an exemplary embodiment of the invention, light source  130  is made up from a line of light points  205   a - 205   t  (e.g. LEDs, fibers). Optionally, each point produces a light beam which spreads out by an angle  215  (e.g. 70°) relative to the normal to light source  130 , the angle depending on the properties of the light source. In an exemplary embodiment of the invention, the light passes through lens  135  towards point  225 . Optionally, the sides of base  120  are coated to form a highly reflective mirror  210  (e.g. 96% reflectance) so that the light source line can be viewed as almost infinite by points  225  and each point  225  is illuminated by many light points  205  of the line light source  130 . In  FIG. 2A , light points  205   a - 205   t  illuminate the specific point  225 , each of those points illuminate point  225  with a different angle  235  (e.g.  235 ′ and  235 ″), directly or indirectly via mirror  210 , up to the maximum angle  245  that is equal to angle  215 .  FIG. 2A  shows specifically illumination lines from some of lights points  205   a - 205   t , however it should be understood that all the points contribute and only some are shown to enhance clarity. In  FIG. 2B  a dichroic filter  145  is placed after lens  135  to limit the effective illumination angle along the longitudinal (Y) axis, so that the resulting illumination angles on point  225  will be optimal for the application desired, for example limiting illumination of point  225  up to angle  255  instead of angle  245  (angle  255 ≦angle  245 ) without filter  145 . In an exemplary embodiment of the invention, the decreasing of intensity up to angle  255  is not uniform and optionally has a slow slope so point  225  is illuminated at a higher intensity at angle  235  than at angle  255 . 
     A light ray originating from a light point  205  is incident on dichroic filter  145  at an angle  235  relative to the normal to the filter.  FIGS. 3A-3C  are schematic illustrations of the transmitted spectrum of dichroic filter  145  for different angles of incidence  235 , according to an exemplary embodiment of the invention. As shown by  FIGS. 3A-3C , for a specific wavelength (λ in nanometers (nm)), at certain angles the dichroic filter transmits most of the light (e.g. more than 60% or 70%) and at certain angles the light is reflected and only a small percent is transmitted (e.g. less than 20% or 30%). The transmission of the specific wavelength at an angle θ is equal to the transmission of a different wavelength at normal incidence. The ratio between the wavelengths (in air) is given by: 
                 λ   θ       λ   0       =       1   -         sin   2     ⁢   θ         (     N   *     )     2                 
Wherein
         θ=Angle of incidence.   λ θ =wavelength at angle of incidence.   λ 0 =wavelength at normal incidence with same transmission.   N*=Effective refractive index of the filter.       

     In an exemplary embodiment of the invention, light point  205  produces light with a specific wavelength or a specific range of wavelengths. Optionally, dichroic filter  145  is selected to match the light waves illuminated by light point  205  such that the resulting angles illuminating point  225  will be reduced relative to the original angle forming the beam produced by light point  205 . As shown in  FIGS. 3A-3C  when light point  205  produces a reddish light beam with a wavelength of about 630 nm, the rays that are incident upon dichroic filter  145  at an angle of 0° (e.g.  FIG. 3A ) will be almost completely (e.g. 96%) transmitted through the filter. For light rays that are incident upon dichroic filter  145  at an angle of 30°, only about 80% (e.g.  FIG. 3B ) will be transmitted. Light rays that are incident upon dichroic filter  145  at an angle of 60° (e.g.  FIG. 3C ) will be essentially reflected and not transmitted. Thus by selecting or producing a dichroic filter to match the wavelengths of the light produced by light points  205 , the angle of incidence on point  225  can be controlled to achieve symmetrical illumination from all sides of point  225 . 
     In some embodiments of the invention, a dual-band or multi-band dichroic filter (e.g. SEMROCK FF01-468/624-25) is used so that more than one wavelength can be provided by light point  205  and have its illumination angle controlled by the use of dichroic filter  145 . Optionally, each light point  205  may comprise multiple LEDs illuminating light of different wavelengths or each light point  205  may provide light of a different wavelength (e.g. using a Lambertian LED, or fibers). In some embodiments of the invention, target  180  may be illuminated by light of one wavelength or of multiple wavelengths. Optionally, dichroic filter  145  may be selected such that each wavelength illuminates point  225  on line  185  with different maximal angles. 
       FIG. 4  is a schematic illustration of a graph  400  of transmission versus the angle of incidence through a dichroic filter, according to an exemplary embodiment of the invention. Line  410  illustrates a transmitted light beam produced for example by a Lambertian LED. Optionally, the beam is directed toward a dual band pass filter (e.g. SEMROCK FF01-468/624-25). The normalized transmission of the filter is illustrated by lines  420  and  430 . Line  420  represents a light beam with a central wavelength of 460 nm (blue light) and line  430  represents a light beam with a central wavelength of 630 nm (red light) as the wavelengths that are transmitted by the filter. Between 0° to about 20° most of the two wavelengths are transmitted. However as the angle of incidence relative to the normal increases the transmission diminishes, and at angles above 40°-50° almost no light is transmitted. 
     In an exemplary embodiment of the invention, by selecting a single dichroic filter and the light wavelength used the angle of illuminating point  225  is determined. Alternatively or additionally, two or more dichroic filters  145  may be placed in series or in parallel at various angles relative to each other to further control the illumination angle and the strength of the light beam illuminating point  225 , for example to transfer light from large angles of incidence or to asymmetrically illuminate a point.  FIG. 5  is a schematic illustration  500  of the use of two tilted dichroic filters  510  and a mirror  520  to transfer specific light rays from a light source  505  and to block others thus controlling the angles by which a target  530  is illuminated. In an exemplary embodiment of the invention, the illumination angle may be shifted, further limited or expanded, for example to provide maximum illumination on a target at angles between 20°-50° instead of 0° to 30°. Optionally, dichroic filters  510  may be movable, for example controlled by a motor to allow user control of the light being illuminated upon a target. 
       FIG. 6A  is a schematic illustration of a line light source  600  wherein each point of the line is made up from two light points  610 ,  620  of different wavelengths, according to an exemplary embodiment of the invention. Optionally, light from light source  600  is transmitted through a multi band dichroic filter  625  that responds differently for each wavelength, such that each range of angles provides light of a different wavelength. Optionally, by controlling the intensity of each light source, the illumination angle toward a target  630  can be controlled. 
       FIG. 6B  is a schematic illustration of a graph  650  showing transmission versus the angle of incidence using two light points ( 610 ,  620 ) of different wavelengths, according to an exemplary embodiment of the invention. Optionally, line  660  shows the transmission of a Lambertian light source. Line  670  shows the transmission of a LED with a wavelength of 590 nm, and line  680  shows the transmission of a LED with a wavelength of 620 nm. According to the case depicted by graph  650  most of the intensity of the light illuminating target  630  up to an angle of about 25° from the normal will be contributed by the 620 nm LED. Most of the intensity at angles above 25° until about 60° is contributed by the 590 nm LED. In an exemplary embodiment of the invention, the use of multiple wavelengths may enhance analysis of surfaces that respond differently to different wavelengths. 
     It should be appreciated that the above described methods and apparatus may be varied in many ways, including omitting or adding steps, changing the order of steps and the type of devices used. It should be appreciated that different features may be combined in different ways. In particular, not all the features shown above in a particular embodiment are necessary in every embodiment of the invention. Further combinations of the above features are also considered to be within the scope of some embodiments of the invention. 
     Section headings are provided for assistance in navigation and should not be considered as necessarily limiting the contents of the section. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims, which follow.