Abstract:
A light source comprising a light emitting device and quantum dot material is disclosed. According to various embodiments, the quantum dot material is positioned relative to the light emitting device such that the quantum dot material absorbs light emitted from the light emitting device and converts the wavelengths of photons emitted from the light emitting device to longer wavelengths. Judicious selection of the quantum dot material allows the emission spectra of the light source to be tailored to meet the needs of a particular illumination application, and avoids the drawbacks associated with the use of interference filters because the quantum dot material can upconvert the wavelengths emitted from the light emitting device such that the emission spectra of the light source can include wavelengths that are not emitted by the light emitting device itself.

Description:
BACKGROUND  
       [0001]     In spectroscopy or color measurement applications which characterize the transmission, absorption, emission or reflection of a target material (such as ink on paper, paint on metal, dyes on cloth, etc.), an illumination source must be present, as well as an apparatus to measure the reflected, transmitted or emitted light. One method for providing the illumination is using light emitted from light emitting diodes (LEDs). To adequately characterize the material properties of the target that would be seen by a human observer, illumination over the entire visible wavelength range from 400 nm to 700 nm is desirable. Individual white or chromatic LEDs and even multiple-LED assemblies, however, often do not provide adequate intensity at all wavelengths in this range.  
         [0002]     One known solution for tailoring the emission spectra of a LED to cover the desired illumination range is to use an interference filter with the LED to filter out the unwanted wavelengths. Such an arrangement, however, is not practical where the source (e.g., the LED) does not emit sufficient energy at the desired wavelength. Also, such arrangements can be inefficient for certain applications because much of the energy emissions from the source may be filter out and therefore wasted.  
       SUMMARY  
       [0003]     In one general aspect, the present invention is directed to a light source comprising a light emitting device and quantum dot material. The quantum dot material is positioned relative to the light emitting device such that the quantum dot material absorbs light emitted from the light emitting device and converts the wavelengths of at least a portion of the photons emitted from the light emitting device to longer wavelengths. Judicious selection of the quantum dot material allows the emission spectra of the light source to be tailored to meet the needs of a particular illumination application, and avoids the drawbacks associated with the use of interference filters because the quantum dot material can upconvert the wavelengths emitted from the light emitting device such that the emission spectra of the light source can include wavelengths that are not emitted by the light emitting device itself.  
         [0004]     According to various implementations, the quantum dot material may comprise a host material and a plurality of quantum dot material intra-layers suspended in the host material, wherein the quantum dot material intra-layers have different light absorption/emission characteristics. Also, the quantum dot material may be positioned directly on the light emitting device, or it may be a part of a quantum dot material assembly spaced apart from the light emitting device that comprises (1) an optically transparent substrate and (2) one or more quantum dot material layers. The quantum dot material layer(s) may comprise quantum dot material and the host material, and the assembly is positioned such that light from the light emitting device is absorbed by the quantum dot material layer(s) on the substrate.  
         [0005]     In addition, the light emitting device may comprise one or a number of light emitting diodes (LEDs), one or a number of lasers, one or a number of laser diodes, a lamp, or a combination of these light emitting devices.  
         [0006]     The quantum dot material may be chosen such that the emission spectra of the light source meets a desired emission spectra profile. For example, the emission spectra profile may correspond to an adopted industry illumination standard, such as an incandescent illumination standard, a daylight illumination standard or a fluorescent illumination standard. Also, the quantum dot material may be chosen such that the emission spectra of the light source may cover a narrow band of wavelengths, for example.  
         [0007]     In addition, the light source may comprise (1) a lower lens between the light emitting device and the quantum dot material for collecting and focusing light from the light emitting device onto the quantum dot material and/or (2) an upper lens, wherein the quantum dot material is between the light emitting device and the upper lens, for collecting and focusing light from the quantum dot material on a target sample material.  
         [0008]     In another general aspect, the present invention is directed to an apparatus for measuring a spectroscopic property of a target material. The apparatus may comprise, for example, the above-described light source for emitting light photons to impinge upon the target material and an optical radiation sensing device for detecting light reflected by the target material. The apparatus may, of course, comprise other components. 
     
    
     FIGURES  
       [0009]     Various embodiments of the present invention are described herein by way of example in conjunction with the following figures, wherein:  
         [0010]      FIGS. 1 and 3 - 6  are diagrams of a light source according to various embodiments of the present invention;  
         [0011]      FIG. 2  is a diagram of the quantum dot material layer according to various embodiments of the present invention; and  
         [0012]      FIG. 7  is a block diagram of a spectroscopic apparatus according to various embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]      FIG. 1  is a diagram of a light source according to various embodiments of the present invention. In the illustrated embodiment, the light source  10  includes a light emitting device  12  mounted on a header  14 . In one embodiment, the light emitting device  12  may be a light emitting diode (LED) including a lead wire  16  that allows the LED to be biased so that it will emit light. The LED may emit photons in the ultraviolet and/or visible portions of the optical spectrum. In other embodiments, the light emitting device  12  may be, for example, a laser, a laser diode, multiple LEDs, a lamp, or combinations thereof.  
         [0014]     The light source  10  illustrated in  FIG. 1  also includes, in the path of the emitted light from the light emitting device  12 , an assembly  18  comprising quantum dot material layer  20  placed on a substrate  22 . The quantum dot material layer  20  may comprise quantum dot material incorporated in an inert host material, such as epoxy, resin, gel, etc. Quantum dots have the characteristic that by adjusting the size and chemistry of the quantum dot particles, the optical properties of the material, such as light absorption or light emission, can be tailored to meet desired characteristics. For example, quantum dot material, which may be made from CdSe, CdS, ZnS or other materials, may have absorption in the blue and UV portion of the optical spectrum and emission wavelengths in the visible part of the optical spectrum. This allows these materials to be used for a variety of spectroscopic applications which require illumination in the visible spectral region.  
         [0015]     In the light source  10  of  FIG. 1 , the quantum dot material layer  20  may absorb all or part of the light from the light emitting device  12  that impinges on the quantum dot material layer  20 . That energy may then be re-emitted at longer wavelengths (i.e., lower energy). That is, the light emitting device  12  may optically pump the quantum dot material layer  20 , which may convert at least a portion of the short wavelength photons emitted by the light emitting device  12  into longer wavelength photons. By correctly selecting the quantum dot material, therefore, a desired illumination wavelength can be obtained.  
         [0016]     According to various embodiments, the quantum dot material layer  20  may comprise a composite of different quantum dot intra-layers  21   a - c  suspended in the host material  23 , as shown in  FIG. 2 , each intra-layer  21   a - c  having different absorption/emission characteristics. For example, the first quantum dot material intra-layer  21   a  may convert a portion of the light from the light emitting device  12  to a certain, longer wavelength range, and the second quantum dot material intra-layer  21   b  may convert a portion of that light to an even longer wavelength range, and so on. In another embodiment, the second intra-layer  21   b  may transmit the longer wavelengths emitted by the first intra-layer  21   a , and may also convert another portion of the shorter wavelengths from the light emitting device to a second, higher wavelength, and so on. In addition, the thicknesses of the various quantum dot material intra-layers  21   a - c  could also be selected to tune the intensity of the emitted light. This may allow the illumination spectra to be further tailored to have specific features, such as multiple sharp emission peaks or broad band illumination that covers a wide range of the optical spectrum. Also, one or more of the intra-layers  21   a - c  may comprise phosphors rather than quantum dot material according to various embodiments.  
         [0017]     The substrate  22  on which the quantum dot material layer  20  is placed may be optically transparent such that all or most of the light from light emitting device  12  passes through the substrate  22  and impinges on the quantum dot material layer  20 . According to various embodiments, the substrate  20  may be made from glass, such as sapphire glass. The substrate  22  may be spaced-apart from the light emitting device  12  as shown in  FIG. 1  and may be supported by a frame (not shown), for example. The quantum dot assembly  18  and the light emitting device  12  may additionally be encased in a casing (not shown).  
         [0018]     According to various embodiments, the light source  10  may comprise multiple quantum dot assemblies  18 .  FIG. 3 , for example, shows an embodiment of the light source  10  comprising two quantum dot assemblies  18   a - b . In such an arrangement, the quantum dot material layer  20   a  of one of the assemblies  18   a  may have differently tailored absorption/emission characteristics than the quantum dot material layer  20   b  of the other assembly  18   b . That way, for example, like the embodiment discussed above where multiple quantum dot material intra-layers  21  are suspended in a common host material, the first quantum dot material layer  20   a  may convert a portion of the light from the light emitting device  12  to a certain, longer wavelength range, and the second quantum dot material layer  20   b  may convert a portion of that light to an even longer wavelength range, and so on. According to another embodiment, the second quantum dot material layer  20   b  may transmit the longer wavelengths emitted from the first quantum dot material layer  20   a , and convert another portion of the shorter wavelengths emitted from the light emitting device  12  to another, longer wavelength range, which may be longer or shorter than the wavelengths emitted by the first quantum dot material layer  20   a , and so on. In this particular embodiment the light emitted from layer  20   a  will be transmitted through layer  20   b , but both layers will absorb light photons emitted from the light emitting device. The thicknesses of the various quantum dot material layers  20   a,b  could also be selected to tune the intensity of the emitted light. In addition, one or more of the quantum dot material layers  20   a,b  may comprise a composite of different quantum dot intra-layers or phosphors suspended in the host material, each which different absorption/emission characteristics, as described above in connection with  FIG. 2 .  
         [0019]     In other embodiments, rather than using two (or more) substrates  22   a,b  as in the embodiment of  FIG. 2 , the two (or more) quantum dot material layers  20   a,b  may be applied sequentially to a common substrate  22 , as shown in  FIG. 4 .  
         [0020]     According to other embodiments, as shown in  FIG. 5 , the light source  10  may include one or more lenses, such as a lens  24  positioned between the light emitting device  12  and the quantum dot material assembly  18  and/or a lens  26  after the quantum dot material assembly  18 . The lens  24  may collect and focus light from the light emitting device  12  onto the quantum dot material assembly  18 , which may provide more efficient use of the light energy from the light emitting device  12 . The lens  26  may collimate the light exiting the quantum dot material assembly  18 . Also, the lens  26  may collect and focus light emitted from the quantum dot material on a target sample to be illuminated by the light source  10 . This may further enhance the efficiency of the light source  10 .  
         [0021]     In other embodiments, as shown in  FIG. 6 , the quantum dot material layer  20  may be applied onto the light emitting device  12 , rather than placing it on a substrate as per the embodiments of  FIGS. 1-5 .  
         [0022]     By careful selection of various options, including the characteristics of the quantum dot material layer(s)  20  (including the number and characteristics of the intra-layers  21 , if any), the number of quantum dot material layers  20 , and the light emission spectral characteristics of the light emitting device  12 , a desired emission spectra profile may be produced (or at least approximated). For example, in one embodiment, the light emitting device  12  may emit photons in the ultraviolet portion of the optical spectrum (wavelengths &lt;400 μm), and the quantum dot material assembly  18  may convert the pump light to greater wavelengths at sufficient intensities over a broad spectrum, such as wavelengths of 400 nm to 700 nm. According to another embodiment, the light emitting device  12  may emit photons in the blue portion of the optical spectrum (wavelengths between 400 nm and 425 nm), and the quantum dot material assembly  18  may emit light at sufficient intensities over the 400 nm to 700 nm range.  
         [0023]     According to other embodiments, the quantum dot material layer(s)  20  may be chosen such that the emission spectra of the light source  10  is limited to a narrow band of wavelengths. As used herein, “narrow band” means less than or equal to 50 nm full width at half maximum (FWHM). That is, when the emission spectra of the light source  10  is a narrow band, the difference between the wavelengths at which emission intensity of the light source is half the maximum intensity is less than or equal to 50 nm.  
         [0024]     According to other embodiments, the quantum dot material layer(s)  20  may be chosen such that the emission spectra of the light source corresponds to a known spectral emission standard such as, for example, incandescent standards (e.g., CIE standard illuminant A), daylight standards (e.g., CIE standard illuminant D65 or D50), fluorescent standards (e.g., CIE standard illuminant F2 or F11), or other defined standards.  
         [0025]     One or more of the light sources  10  described above may be employed, for example, in a color measurement or spectroscopic apparatus to measure the transmission, absorption, emission and/or reflection properties of a material.  FIG. 7  is a simplified block diagram of a color measurement or spectroscopic apparatus  30  according to various embodiments of the present invention that comprises one light source  10  for illuminating a target material  32 , a wavelength discriminating device  34 , and an optical radiation sensing device  36 . Reflected light from the target material  32  can be filtered by the wavelength discriminating device  34 , which may be, for example, a prism, diffraction grating, holographic grating, or assembly of optical filters. The optical radiation sensing device  36 , which may comprise, for example, one or a number of photodiodes, may sense the light from the material  32  passing through the wavelength discriminating device  34 . A processor  38  in communication with the optical radiation sensing device  36  may determine the transmission, absorption, emission or reflection of the material  32 . Also, the system  30  may include other optical components (not shown), such as refractive or diffractive lenses or mirrors, for either directing light from the light source  10  onto the material  32  and/or directing light from the material  32  to the wavelength discriminating device  34 .  
         [0026]     One or more of the light sources  10  could be used in other equipment, including, for example, a printing press, an ink jet printer, or other color-based process monitoring equipment.  
         [0027]     While several embodiments of the invention have been described, it should be apparent, that various modifications, alterations and adaptations to those embodiments may occur to persons skilled in the art with the attainment of some or all of the advantages of the invention. For example, the materials and the emission spectra profiles described herein are illustrative only. All such modifications, alterations and adaptations are intended to be covered as defined by the appended claims without departing from the scope and spirit of the present invention.