Patent Publication Number: US-2013242533-A1

Title: Method and apparatus for a color filter

Description:
This application claims priority under 35 USC §119(e) from U.S. Provisional Patent Application No. 61/533,395, filed Sep. 12, 2011, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is in the technical field of color filters. 
     2. Description of the Related Art 
     Color filters are widely used in projection systems, stage lighting, photography and other optical applications. One type of color filter is an absorptive filter which absorbs unwanted wavelength of light and transmits others. The absorption of unwanted light will heat up the filter. Therefore absorptive filters have short lifetimes and cannot tolerate high intensity light. Another type of color filter is a dichroic filter which can reflect some wavelength of the light and transmit the remainder. As most of the light is reflected or transmitted rather than absorbed, dichroic filters do not become heated as the absorptive filters. Therefore dichroic filters have much longer lifetime and can withstand high intensity light. Dichroic filters are usually made of multilayer coatings built up on a glass substrate. According to the principle of thin-film interference, dichroic filters&#39; transmittance spectrum depends on the incident angle of the input light. To illustrate this point,  FIG. 1  shows an example of the transmittance spectrum of a particular red pass dichroic filter when the incident angle is 0, 30 and 60 degrees, respectively. The transmitted light will have very different spectra when the incident angle changes. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for a color filter that can reject unwanted wavelength of light and accept wanted wavelength. The color filter utilizes one or more wavelength down conversion materials and therefore can provide intensity gain for the wanted wavelength. 
     Additional features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a light source system which includes: a light source for generating an input light containing light in a first wavelength range and light in a second wavelength range, the first wavelength range being shorter than the second wavelength range; and a color filter disposed to receive the input light and to generate an output light in the second wavelength range, the color filter including a substrate carrying a wavelength conversion material, the wavelength conversion material absorbing the input light in the first wavelength range and converting it to light in the second wavelength range, the wavelength conversion material further transmitting the input light in the second wavelength range. 
     In another aspect, the present invention provides a method for generating a light, which includes: generating an input light containing light in a first wavelength range and light in a second wavelength range, the first wavelength range being shorter than the second wavelength range; and illuminating the input light on a color filter, the color filter including a substrate carrying a wavelength conversion material which absorbs the input light in the first wavelength range and converts it to light in the second wavelength range and transmits the input light in the second wavelength range, to generate an output light in the second wavelength range. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the transmittance spectrum of a red pass dichroic filter when the incident angle of the input light is 0, 30 and 60 degrees, respectively. 
         FIG. 2  shows excitation and emission spectra of a red phosphor. 
         FIG. 3  shows an example of transmittance spectrum of a red phosphor used as a color filter according to an embodiment of the present invention. 
         FIG. 4  shows a color filter according to one embodiment of the present invention in which the color filter is composed of a transparent plate with a phosphor film coated on its surface. 
         FIG. 5  shows a color filter according to one embodiment of the present invention in which the phosphor is coated on a transparent rotary wheel. 
         FIG. 6  shows a color filter according to another embodiment of the present invention in which the color filter is composed of a reflective plate with a phosphor film coated on its surface and other collection optics. 
         FIG. 7  shows a color filter according to another embodiment of the present invention. 
         FIG. 8  shows a color filter according to a variation of the embodiment of the present invention shown in  FIG. 7 . 
         FIG. 9  shows a color filter according to another embodiment of the present invention combining the phosphor wheel and color filter shown in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention provide a method and apparatus for a color filter that utilizes wavelength down conversion materials such as phosphors to convert light in unwanted wavelength range to light in wanted wavelength range. 
     Phosphors will be used in the following description; however, the present invention covers all down conversion materials including phosphors and quantum dots, etc. 
     Phosphors are generally used to generate high brightness color light by absorbing an excitation light and emitting a converted light at a wavelength longer than that of the excitation light. Embodiments of the present invention exploit this characteristic of phosphors to filter light using a principle different from that of dichroic filters and conventional absorptive filters. 
     A red phosphor, i.e., a phosphor that emits a converted light in the red wavelength region, is used in the description below, but other phosphors may also be used. 
       FIG. 2  shows examples of excitation and emission spectra of a red phosphor. This phosphor can absorb excitation light shown in  FIG. 2  and generate light at longer wavelengths, e.g. from about 600 nm to 700 nm. In the example of  FIG. 2 , the excitation spectrum has a dominant wavelength at about 450 nm and the emission spectrum has a dominant wavelength at about 640. With this wavelength-selective absorption property, the red phosphor has a transmittance spectrum shown in  FIG. 3 . When an input light, for example, with wavelengths ranging from 380 to 780 nm, illuminates on this phosphor, the light at wavelengths shorter than 600 nm will be substantially absorbed, while the remaining light with wavelengths between 600 nm and 780 nm will be substantially transmitted. What&#39;s more, the red phosphor will re-emit red light with wavelengths between 600 nm and 780 nm from the absorbed light. Therefore the total red light output can be even stronger than the red portion of the original input light. Thus the red phosphor acts as a red filter that has a gain for red light energy, which makes it more efficient than the passive dichroic and conventional absorptive filters. 
     Compared with dichroic filters, an additional advantage of the color filter according to embodiments of the present invention is that the passband is independent of the incident angle of input light, since neither the absorption nor the emission of phosphors is sensitive to the incident angle. Compared with conventional absorptive color filters, heat generation is substantially reduced, and the output intensity in the desired wavelength range may be increased. 
       FIG. 4  shows a schematic view of a color filter according to one embodiment of the present invention. The color filter includes a transparent plate  401  with a phosphor film coated on its surface. As schematically illustrated, the light incident on the plate includes shorter wavelength light  402  and longer wavelength light  403 . The phosphor absorbs the shorter wavelength light  402  and converts it to a longer wavelength light; it also passes the longer wavelength light  403 . Thus only longer wavelength light is present at the downstream side of the plate, and its energy is increased. The input light may have various incident angles while the corresponding spectra of output light will remain substantially constant. 
     In another embodiment of the present invention, the phosphor substrate is a movable substrate, such as a rotating disc, a rotating drum or linear moving plate to improve heat dissipation.  FIG. 5  shows a schematic view of a color filter according to one embodiment of the present invention in which the phosphor is coated on a transparent rotary wheel  501  which rotates around an axis  502 . The moveable substrate moves relative to the input light, and allows different areas of the phosphor material to be illuminated by the input light at different times, thereby increasing heat dissipation capability of the color filter. 
       FIG. 6  shows a schematic view of a color filter according to another embodiment of the present invention. A phosphor film is coated on a substrate, such as a rotary wheel  601 , which has a reflective surface. The reflective surface is farther away from the input light source than the phosphor material. In one example, a reflective coating is formed on the substrate surface facing away from the input light and the phosphor is formed on the substrate surface facing the input light. In another example, the reflective coating is formed on the substrate surface facing the input light and the phosphor is formed on the reflective coating. In yet another example, the reflective coating is formed on the substrate surface facing away from the input light and the phosphor is mixed in the substrate. The input light  603  illuminates on the wheel  601 . Part of the input light, at shorter wavelengths, is absorbed by the phosphor, and part of the input light, at longer wavelengths, is not absorbed and is reflected by the reflective surface of the wheel  601 . The absorbed shorter wavelength light will undergo a down conversion and some amount of light with longer wavelength will be re-emitted. The unabsorbed and reflected light  604  and the phosphor emission light  606  both have longer wavelengths, and are collected by a spherical or elliptical reflector  602  to collection optics  605 . The forward traveling part of the phosphor emitted light is reflected by the reflective surface of the wheel  601  before being collected by the reflector  602 . 
       FIG. 7  shows a schematic view of a color filter system according to another embodiment of the present invention. This system employs two phosphors. A blue light  703  excites a yellow phosphor on a substrate  702  to generate a yellow light. The blue light  703  is not fully absorbed by the yellow phosphor; the unabsorbed blue light and the phosphor-generated yellow light combines to become a white light  704  since the yellow light covers both the green and red spectral ranges. The substrate  701  coated with a red phosphor is used as a color filter. Such a color filter allows red light to pass and absorbs light at wavelength shorter than red light. The white light  704  is directed by relay optics  705  to the substrate  701 . The transmitted red light  706  on the downstream side of the color filter  701  includes both red light contained in the input white light  704  and red light generated by the red phosphor on the substrate  701 . Consequently, the method and device can generate red light more efficiently than conventional absorptive color filters or dichroic filters. In this embodiment, the phosphor substrate  701  and  702  can also be movable. 
       FIG. 8  shows a variation of the embodiment of the present invention shown in  FIG. 7 . The difference is that the two phosphor substrates  702  and  701  are disposed in parallel at a sufficiently close proximity so that the relay optics  705  between them (see  FIG. 7 ) is not needed. Alternatively, a red phosphor film and a yellow phosphor film can be formed on a same substrate, which can be either transmissive or reflective. 
       FIG. 9  shows a schematic view of a color filter according to another embodiment of the present invention. The phosphor plate  801  includes a mixture of different phosphors, for example, a yellow phosphor  803  and red phosphor  804 . An excitation light  802  such as a blue light is used to excite the phosphor plate  801 . The yellow phosphor  803  converts a part of the excitation light  802  to a yellow light, which covers both the green and red spectral range. The red phosphor  804  acts as a color filter, absorbing blue light and green light and emitting red light. The final output light  805  is a red light. The phosphor plate  801  can be movable. Similarly, phosphor plate  801  can be reflective type as well, in which case the yellow phosphor  803  and red phosphor  804  are formed a substrate that has a reflective surface. 
     The embodiments of  FIGS. 7 ,  8  and  9 , which use two phosphors, provide more flexibility for the type of lights that can be used as input to the color filter system. 
     In embodiments of the present invention, the phosphor material(s) may be either coated on the substrate or mixed in the substrate. 
     Although a red phosphor is used in the above description as an example, other wavelength conversion materials may be used. More generally, the wavelength conversion material absorbs an excitation light in a first (shorter) wavelength range and converts it to light in a second (longer) wavelength range, and substantially transmits light outside of the first wavelength range. In embodiments where two wavelength conversion materials are used, the second wavelength conversion material absorbs an excitation light in a third (shorter than the first) wavelength range and converts it to a light in the first (shorter) and second (longer) wavelength range, and (optionally) passes some of the light in the third wavelength range; the first wavelength conversion material absorbs the light in the third (if any) and first wavelength ranges and converts it to light in the second wavelength range, and passes light in the second wavelength range. 
     While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention as claimed.