PATENT ABSTRACT
A combination color filter having at least two dual-hue color-changing absorptive substrates. A desired hue and saturation is achieved by using light absorbing gels for a plurality of selected hues, preferably the three subtractive primary colors. The unique arrangement of transparent part, saturation parts and continuously graded saturation parts allow fast color bumping between any two primary colors without flash of white and presence of other intervening colors.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
     The present invention relates to optical filters for generating light of a variety of hues and saturations by using a white light source. More specifically, the present invention describes variable color filters used in scrolling color changers. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     Generation of hues and saturations in visible spectrum with a broad-spectral white light source has a number of applications including stage lighting. Notably, many colored spotlight projectors use one or more colored optical filters to produce a light of desired hue and saturation by projecting a white light beam through the filters. 
     Two approaches are usually used to generate colored light. One is to first generate three light beams of three primary additive colors (i.e., red, green, and blue) and then combine them to produce the desired hue and saturation. White light sources can be used to generate three white light beams. These beams subsequently pass through three color filters respectively to obtain color from the three additive primary colors. Dichroic beam combiners, light intensity filters and other optical elements are used to combine the three beams into a single output beam. Adjusting intensities of the three beams in primary colors relative to one another allows the output beam to have various desired hues, saturations, and brightness. One of the disadvantages of such a system is the precise overlapping alignment of the three beams in primary colors is subject to change due to vibrations and other factors. The optical alignment requires frequent maintenance. The optical elements required in this type of system add manufacturing cost. 
     Another often-used approach uses two or more variable color filters in three subtractive primary colors (i.e., magenta, cyan and yellow) of different saturations to sequentially filter a single white beam, resulting in output light with desired hues and saturations. This approach simplifies the optical alignment using fewer optical elements in comparision with the former approach. In particular, this approach obviates the problem of having three displaced colors in the peripheral region of the output beam present in the former approach. 
     One key component of the latter approach is implementation of variable color filters, or a color changer based on subtractive primary colors. Many such color changer systems have been developed such as those described in U.S. Pat. Nos. 4,459,014 to Theabult, 4,600,976 to Callahan, 4,745,531 to Leclercq, 4,602,321 to Bornhorst, and 3,260,152 to Aston. However, these prior-art systems have limitations either in uniformity of color filtering or in light intensity handling capacity. 
     Scrolling color changers use two or more flexible transparencies coated with color changing materials having strong absorption at different selected hues and saturations of color. Different portions of the scrolling transparencies produce different hues and saturations in the light transmitting therethrough. These transparencies work combination to change the color of the output light from a single white light source. This can be understood with the well-known color triangle  100  shown in FIG.  1 . The three additive primary colors red, green and blue are represented by the three vertices  102 ,  104  and  106 , respectively. The three subtractive primary colors magenta, yellow and cyan are represented by  122 ,  124  and  126 , respectively. The center  130  of the triangle  100  is the white color. For example, fully saturated red can be achieved by using a filtering portion for fully saturated yellow and a filter portion for fully saturated magenta. 
     The above-referenced flexible filters made of transparencies having color changing absorbing materials can be made with materials and processes disclosed in the U.S. patent application Ser. No. 08/286,969, disclosure of which is incorporated herein by reference. The flexible tranparent substrates can be polycarbonate, polyestere or polypropylene films. The color changing materials can be a mixture of dye for a particualr color and a polyester binder for holding the dye molecules. 
     The prior-art scrolling color changers can produce a uniformly filtered output beam and have a wide range of hues and saturations. However, these systems are usually slow and are usually obliged to produce undesired intervening colors in changing from one primary color to another. 
     One notable prior-art scrolling color changer is disclosed in U.S. Pat. No. 5,126,886 to Richardson et al., the disclosure of which is incorporated herein by reference. The &#39;886 patent uses two or three layers of elongated flexible scrolling substrates coated with light absorbing gels for different subtractive primary colors. In a three-scroll configuration shown in FIG. 2 of the above referenced US patent, each substrate has a surface with a graded portion having a gel of continuously graded concentration along a gradient axis in the elongated direction. The graded distributed gel ranges from a full saturation at one end of the substrate to a total transparent portion at the other end (FIGS. 4,  5 , and  6  of the referenced patent). Each substrate corresponds to one subtractive primary color. One drawback of this system is long scrolling time in changing from one primary color (e.g., red) to another (e.g., blue) since at least two of the three substrates have to be scrolled virtually all the way through their respective paths. This can take about two seconds of movement at the full speed operation of a commonly used scrolling motor. Another drawback is the flash of white in the output beam during the above operation along with presence of other intervening colors during the slow scrolling. 
     The &#39;886 patent further discloses a two-scroll color changer using two dual-hue substrates. The dual-hue substrate combination  200  having a first substrate  210  and a second substrate  220  is illustrated in FIG.  2 . The first hue substrate  210  has three sections: a yellow section  12 , a transparent portion  214  for white color, and a cyan section  216 . The yellow section  212  has a full saturation end portion  213  and the concentration of the absorbing gel for yellow color continuously decreases towards the transparent section  214  along a gradient axis  217  in the elongated direction. The cyan section  216  is similarly constructed, having a full saturation end portion  215  and a continuous decreasing concentration of the absorbing gel for cyan color towards the transparent section  214  along a gradient axis  219 . The second hue substrate is similarly constructed with a cyan section  222 , a transparent section  224  and a magenta section  226 . 
     The &#39;886 patent&#39;s dual-hue configuration still has some of the previously mentioned limitations associated with many of the prior-art scrolling color changers although the flash of white during bumping between primary colors is eliminated. This can be shown in changing the output light from blue to red. Assume that the system initially is set to generate the blue output, thus optic axis  230  being used and both cyan section  216  and magenta section  226  being at the center. An optic axis  234  has to be used to produce the red color. Therefore, the first hue substrate  210  needs to roll almost all the way from one end to the other for changing the output from blue to red. This is a significant duration (e.g., 2 seconds) although no white color is present in the output. However, during the process, the output color goes through other colors in between the red and blue such as magenta as the transparent section  214  of the first substrate scrolls to the center (i.e., optic axis  232  is along the light path). 
     The present invention describes an improved scrolling color changer system based on a new dual-hue substrate design. In particular, the preferred embodiment of the present invention uses a dual-hue substrate design with five sections and a combination of magenta and yellow in one substrate and a combination of magenta and cyan in another substrate. According to the present invention, the flash of white is eliminated in changing output color between any two primary colors. Importantly, the intervening colors between the two primary colors are eliminated from bumping between any two primary colors. Furthermore, the different color sections in each substrate are designed to minimize the scrolling amount, thus resulting in a high-speed operation that is rarely possible with the prior-art scrolling systems. 
     The advantages, sophistication, and significance of the present invention will be more apparent in the light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows the color triangle. 
     FIG. 2 illustrates a prior-art dual-hue color changer. 
     FIG. 3 is a block diagram of a scrolling color changer in accordance with the present invention. 
     FIG. 4 shows the preferred embodiment of the dual-hue substrates in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a functional block diagram of a dual-hue scrolling color changer in accordance with the present invention. A white light source  302  with a reflector  304  produces a white light beam  303 . The white light beam  303  propagates through a color changer  305 . 
     The color changer  305  preferably has two scrolling optical filters. The first optical filter has a first flexible elongated color filtering substrate  306  coated with two light absorbing gels, each corresponding to a selected hue and a range of saturation. Similarly, the second optical filter has a second flexible elongated color filtering substrate  312  coated with two light absorbing gels, each corresponding to a selected hue and a range of saturation. The selected hues are preferably the three subtractive primary colors, i.e., magenta, yellow and cyan. The first substrate  306  is disposed on a first pair of motor-driven spindles  307  and  308  and the second substrate  312  is disposed on a second pair of motor-driven spindles  309  and  310 , respectively. The motor-driven spindles operate to roll the substrates along the elongated direction to position any portion thereof to the center is the optical path of the light beam  303 . The optical path of the beam  303  is preferably perpendicular to both substrates  306  and  312 . 
     A control module  330  can vary the intensity of the white light beam  303  from the light source  302  and control the motion of both substrates. The control module  330  further operates to control the hue, saturation, and brightness of the output beam  320 . 
     FIG. 4 shows the preferred embodiment of the dual-hue substrates  306  and  312  in accordance with the present invention. The substrate  306  of the first color filter has five sections preferably arranged relative to one another as follows in spatial order: a yellow saturation section  402 , a graded magenta section  404 , a transparent section  406 , a graded yellow section  408 , and a magenta saturation section  410 . The width of the substrate  306  is at least larger than the beam size of the beam  303 . The yellow saturation section  402  preferably corresponds to the full saturation yellow point  124  in the color triangle as in FIG. 1, resulting in complete absorption of light of all colors except the selected yellow color. The area of the section  402  should be at least as large as the input beam size of the white beam  303 . The graded magenta section  404  has a gel for subtractive magenta of continuously decreasing concentration along a gradient axis in the elongated direction towards the transparent section  406 , tracing the magenta pastels on a straight line from point  122  to point  130  in FIG.  1 . The section  404  starts with a full saturation area  403  located next to the yellow saturation section  402 . The section  404  is generally many time larger than the beam size of beam  303  in the elongated direction dependent on the required resolution in saturation. The transparent section  406  is represented by the white color point  130  in the color triangle of FIG.  1  and is substantially similarly to the section  402  in size. The graded yellow section  408  is substantially similar to the graded magenta section  404  (e.g., the saturation distribution is at the same predetermined gradient) except that the gel is different. The section  408  starts with a yellow saturation area  407  corresponding to the point  124  of FIG.  1  and the concentration of the gel gradually decreases along the gradient axis towards the section  410  corresponding to a color change along the line from point  124  to the white point  130  in FIG.  1 . The magenta saturation section  410  indicates the color point  122  in FIG.  1 . 
     The substrate  312  of the second color filter is similarly constructed but has a different color gel arrangement. With respect to the spatial sequence of the substrate  306 , the substrate  312  has the following preferable arrangement: a magenta saturation section  422  indicative of the color point  122  of FIG. 1, a graded cyan section  424  indicative of the color line from point  126  to point  130  of FIG. 1, a transparent section  426  for white color, a graded magenta section  428  indicative of the color line from point  122  to point  130  of FIG. 1, and a cyan saturation section  430  represented by point  126  in FIG.  1 . 
     In operation, the scrolling color changer in accordance with the present invention presents significant advantages over the cited prior-art systems. One of the advantages is that the color of the output light beam  320  can be changed between any two primary colors without presence of intervening colors at a high-speed. Another advantage is the complete elimination of the unwanted flash of white in change of hues and saturations. Yet another advantage is faster speed in color switching than the prior art systems. These and other advantages will become more apparent by using the substrate arrangement of FIG. 4 to achieve any desired hues and saturations with reference to the color triangle in FIG.  1 . 
     For example, optic axis  450  in FIG. 4 represents a red saturation color at point  132  in FIG. 1, optic axis  452  represents a blue saturation color at point  134  in FIG. 1, and optic axis  454  represents a green saturation color at point  136  in FIG. 1, respectively. Assume that the system is initially set to produce the red in the output light  320 . To change to the blue, both substrates  306  and  312  will be scrolled simultaneously in the same direction for a short path between the optic axis  450  and the optic axis  452  (i.e., scrolling to the left in FIG.  4 ). No other colors will be present in the output and the scrolling distance is short. If the red output is changed to green instead, the substrate  312  scrolls to move the magenta section  422  out of the light path and move the adjacent cyan saturation area  423  of the graded cyan section  424  in the light path while the position of the substrate  306  relative to the light path remains unchanged, thus shifting optic axis  450  to optic axis  454 . Again, no other colors will be present in the operation and the scrolling path is small. If the above green output needs to change to blue output, the substrate  306  simply scrolls to replace yellow saturation section  402  with its adjacent magenta saturation area  403  in the light path, resulting a change from optic axis  454  to optic axis  452 . 
     If a white output is desired subsequent to the last operation thereabove, both substrates  306  and  312  scroll simultaneously in the same direction over the length of the section  404 , switching from optic axis  452  to optic axis  456 . If a red pastel at point  140  of FIG. 1 is needed subsequent to the a green output at optic axis  454 , both substrates  306  and  312  scroll simultaneously in the same direction with different scrolling speeds, switching from optic axis  454  to optic axis  460  that represents the respective red saturation. No flash of white happens if the scrolling speeds for both substrates are can be controlled to avoid the overlapping the transparent sections  406  and  426 . 
     Although the present invention has been described in detail with reference to a particular embodiment, one ordinarily skilled in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the following claims. For example, while the preferred embodiment does not show collimating optics or projection optics, such could be used with the present invention.