Patent Application
20070262714
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.
One known solution for tailoring the emission spectra of a LED to cover the desired illumination range is to use an interference filter in combination 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 where much of the energy emissions from the source may be filter out and therefore wasted.
In one general aspect, the present invention is directed to an illumination source. The illumination source may comprise a light emitting device, such as one or more LEDs, one or more lasers, one or more laser diodes, one or more lamps, or a combination of these things. The illumination source also comprises at least one photoluminescent material layer. The photoluminescent material layer may comprise quantum dot material and/or phosphors. The photoluminescent material layer may absorb light emitted from the light emitting device and convert the wavelengths of at least a portion of the photons emitted from the light emitting device to longer wavelengths. Also, the illumination source comprises at least one filter positioned between the light emitting device and the photoluminescent material layer. The filter is substantially transmissive of light emitted by the light emitting device and substantially reflective of light emitted by the photoluminescent material layer, which may be omnidirectional. That way, light emitted from the light emitting device and the photoluminescent material layer may be directed in a common direction that is generally away from the light emitting device. Also, the properties of the photoluminescent material layer may be chosen to achieve a desired emission spectra for the illumination source.
According to various embodiments, the filter may be dielectric filter, comprising layers of material with different refractive indices. Also, multiple photoluminescent material layers may be used, and each may have different light absorption/emission characteristics. Such multiple layers may further facilitate achieving a desired emission spectra for the illumination source. Also, multiple dielectric filters may be employed. In addition, the photoluminescent material layer may be located on an optically transparent substrate that is between the photoluminescent material layer and the filter. Additionally, optical elements, such as lenses, may be positioned before the filter and/or after the last photoluminescent material layer.
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 illumination source for emitting light photons to impinge upon the target material and an optical radiation sensing device for detecting light reflected by or transmitted through the target material. The apparatus may, of course, comprise other components.
Various embodiments of the present invention are described herein by way of example in conjunction with the following figures, wherein:
FIGS. 1, 3-5 and 7-8 are diagrams of an illumination source according to various embodiments of the present invention;
The illumination source 10 illustrated in
The filter 19 may be constructed such that the light emitted from the photoluminescent material layer 20, which may be generally omnidirectional due to the properties of the photoluminescent material, is reflected back in a direction generally away from the light emitting device 12. That is, the filter 19 may allow the shorter wavelengths from the light emitting device 12 to pass through to the photoluminescent material layer 20, but reflect back the longer wavelengths emitted from the photoluminescent material layer 20 in a direction generally away from the light emitting device 12. This will tend to increase the efficiency of the illumination source 10 as light emitted from the photoluminescent material layer 20 may be directed in a substantially common direction.
According to various embodiments, the photoluminescent material layer 20 may comprise quantum dot material and/or phosphors 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 or 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. Phosphors can also upconvert the light emitted from the light emitting device 12.
The substrate 22 on which the photoluminescent 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 photoluminescent material layer 20. According to various embodiments, the substrate 20 may be made from glass, such as sapphire glass.
According to other embodiments, as shown in
The filter 19 may be any optical device that is capable of allowing all or most of the photons from the light emitting device 12 to pass through to the photoluminescent material layer 20, but which reflects all or most of the longer-wavelength photons emitted from the photoluminescent material layer 20 in a direction generally away from the light emitting device 12. The light then can be collected by an optical component (See
The assembly 18 maybe spaced-apart from the light emitting device 12 as shown in
In an embodiment where the photoluminescent material layer 20 comprises quantum dot material, the photoluminescent material layer 20 may comprise a composite of different quantum dot intra-layers 21a-c suspended in the host material 23, as shown in
According to various embodiments, the illumination source 10 may comprise multiple photoluminescent material assemblies 17.
In such an arrangement, the photoluminescent material layer 20a of one of the assemblies 17a may have different absorption/emission characteristics than the photoluminescent material layer 20b of the other assembly 17b. 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 photoluminescent material layer 20a may convert a portion of the light from the light emitting device 12 to a certain, longer wavelength range, and the second photoluminescent material layer 20b may convert a portion of that light to an even longer wavelength range, and so on. According to another embodiment, the second photoluminescent material layer 20b may transmit the longer wavelengths emitted from the first photoluminescent layer 20a, 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 from the first photoluminescent layer 20a), and so on. The thicknesses of the various photoluminescent material layers 20a,b could also be selected to tune the intensity of the emitted light. In addition, one or more of the photoluminescent material layers 20a,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
In other embodiments, rather than using two (or more) substrates 22a,b as in the embodiment of
According to other embodiments, as shown in
By careful selection of various options, including the characteristics of the photoluminescent material layer(s) 20 (including the number and characteristics of the intra-layers 21, if any), the number of photoluminescent 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<400 nm), and the photoluminescent material assembly 17 may convert the pump light to longer 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 photoluminescent material assembly 17 may emit light at sufficient intensities over the 400 nm to 700 nm range.
According to other embodiments, the quantum dot material layer(s) 20 may be chosen such that the emission spectra of the illumination 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 illumination source 10 is a narrow band, the difference between the wavelengths at which emission intensity of the illumination source is half the maximum intensity is less than or equal to 50 nm.
According to other embodiments, the photoluminescent material layer(s) 20 may be chosen such that the emission spectra of the illumination 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.
One or more of the illumination 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.
In another embodiment, the wavelength discriminating device 34 and the optical radiation sensing device 36 may be positioned on the opposite side of the target material 32 from the illumination source 10. That way, the optical radiation sensing device 36 may detect light transmitted through the target material 32. Also, in yet another embodiment, the apparatus 30 may comprise one optical radiation sensing device in front of the target 32 for detecting light reflected by the target 32 and a second optical radiation sensing device behind the target for detecting light transmitted through the target 32.
One or more of the illumination 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.
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.
