Patent Description:
A phosphor wheel is a wavelength conversion device for converting (for example) blue light to yellow light. In a phosphor wheel configuration, the phosphor is deposited at the periphery of a rotating wheel that is made of metal or another thermally conductive material. A light source (e.g. blue laser) applies high power light (e.g. blue light in the power range of watts, tens of watts, or a hundred watts or more in some designs) to the phosphor which converts a portion or all of the incident (e.g. blue) light to wavelength-converted (e.g. yellow) light. An advantage of a phosphor wheel over a "static" phosphor is that heat is distributed over the annular perimeter of the wheel, thereby reducing heating of the phosphor.

<CIT> discloses a phosphor wheel that forms a plurality of wing-like projections therein to improve cooling efficiency.

<CIT> discloses a phosphor wheel including a substrate having a first main surface, a second main surface, opposite to the first main surface, and an open portion. A phosphor layer is provided on the first main surface and a fin is provided on the second main surface. The fin is nearer a center of the substrate than the phosphor layer is, and the open portion is located between the phosphor layer and the fin.

The invention refers to a phosphor wheel according to claim <NUM>. In one disclosed aspect, a phosphor wheel comprises: a disk; a wavelength conversion layer comprising phosphor disposed on the disk; and an impeller disposed on the disk. The impeller comprises vanes which are shaped as airfoils, with each vane oriented to drive outward airflow across the disk including across the wavelength conversion layer when the disk is rotated in the rotation direction. In some embodiments, the wavelength conversion layer is disposed at a larger radial position than the impeller on the disk, and may optionally be an annular wavelength conversion layer. An electric motor may further be provided, operatively coupled with the disk to rotate the disk in the rotation direction. A light source may further be provided, arranged to output a pump beam impinging on the wavelength conversion layer. The light source may, for example, be a laser or a light emitting diode (LED). For full color or white light applications, it may be useful for the light source to output the pump beam comprising blue or ultraviolet light having a largest spectral peak at or below <NUM>, and for the phosphor to emit wavelength converted light with one or more spectral peaks at or above <NUM> (e.g. yellow, red, and/or green peaks, by way of non- limiting illustrative example).

In another disclosed aspect, a wavelength conversion method comprises: rotating a disk in a rotation direction using an electric motor; while rotating the disk in the rotation direction, optically pumping a wavelength conversion layer comprising phosphor disposed on the disk to generate wavelength converted light; and while rotating the disk in the rotation direction, driving airflow across the wavelength conversion layer disposed on the disk using an impeller comprising airfoil-shaped vanes disposed on the disk wherein the impeller is operated by the rotating of the disk in the rotation direction.

In another disclosed aspect, a method of manufacturing a phosphor wheel is disclosed. The method comprises disposing a wavelength conversion layer comprising phosphor on a disk, and disposing or forming an impeller comprising airfoil-shaped vanes on the disk. The disposing of the wavelength conversion layer on the disk may consist of one of: (i) depositing material forming the wavelength conversion layer on the disk; or (ii) adhering the wavelength conversion layer onto the disk. The disposing or forming of the impeller on the disk comprises, in one embodiment, adhering vanes of the impeller onto the disk using an adhesive. The disposing or forming of the impeller on the disk comprises, in another embodiment, depositing vane material forming vanes of the impeller onto the disk. The disposing or forming of the impeller on the disk comprises, in another embodiment, forming vanes of the impeller by sheet metal stamping (in this embodiment the disk typically comprises sheet metal).

As earlier noted, a phosphor wheel reduces heating of the phosphor compared with a static phosphor by distributing the heat over the annular periphery along which the phosphor is disposed. However, the phosphor wheel does not completely eliminate heating, and the phosphor can reach temperatures in excess of <NUM> in some commercial wavelength conversion devices that employ a phosphor wheel. In general, such heating can be expected to increase with increasing optical input power (e.g. using a higher power laser to pump the phosphor), and/or with reduction in the wheel diameter and consequently reduced annular path length around the wheel periphery. Wheel diameter reduction may be advantageous in certain applications in which miniaturization is beneficial. Such heating of the phosphor can have detrimental effects such as reduced wavelength conversion efficiency (e.g. thermal quenching) and/or potential heat degradation of the phosphor and/or the phosphor binder over time. In another heat-induced failure mode, adhesive used to bond the phosphor to the wheel may be cracked or lose bonding strength. For example, when the temperature gets higher than <NUM>, silicone glue used in bonding some types of phosphor-containing wavelength conversion layers to the disk substrate have been observed to start to fail.

In embodiments disclosed herein, a phosphor wheel includes a disk, a wavelength conversion layer comprising phosphor disposed on the disk, and an impeller disposed on disk to drive airflow across the wavelength conversion layer when the disk is rotated in a rotation direction. This approach advantageously leverages the existing rotational motive force applied to the phosphor wheel by a motor to provide convective cooling of the wavelength conversion layer. The impeller typically comprises vanes which in illustrative embodiments are shaped as airfoils. In some embodiments, each vane is shaped as an airfoil having its leading edge disposed radially inward of its trailing edge on the disk. Each impeller vane is oriented to drive outward airflow across the disk including across the wavelength conversion layer when the disk is rotated in the rotation direction. In this way, direct convective cooling of the phosphor layer is obtained. The airfoil shape typically has a rounded leading edge and a sharp trailing edge, and may have a widest cross-section at a point between the rounded leading edge and the sharp trailing edge, with the more detailed shape chosen to provide specific airflow characteristics.

In operation, the disk is rotated in a rotation direction (e.g. clockwise or counterclockwise) using an electric motor. While rotating the disk in the rotation direction, the wavelength conversion layer comprising phosphor disposed on the disk is optically pumped to generate wavelength converted light. As mentioned above, by way of the rotation the heat deposited by the optical pump beam, e.g. a laser beam in the watts to hundred(s) watts range in some embodiments, is spread out over an annular path. If the annular wavelength conversion layer is placed at a large radius, e.g. at or near the periphery of the disk, the length of this annular path is maximized so as to maximize heat distribution. Additionally, while rotating the disk in the rotation direction, airflow is driven across the wavelength conversion layer disposed on the disk using the impeller disposed on the disk. The impeller is operated by the rotating of the disk in the rotation direction. Hence, the convective cooling is obtained for free due to the already-imposed rotation of the phosphor wheel, except for a certain amount of additional drag imposed by the impeller. This drag can be reduced by constructing the impeller to be of low profile, e.g. the impeller protrudes from a surface of the disk on which the impeller is disposed by <NUM> or less in some embodiments, and/or by constructing the impeller to be highly aerodynamic and to impose minimal drag, for example by shaping the vanes as airfoils as in some illustrative embodiments described herein.

With reference to <FIG>, an illustrative phosphor wheel of the above design is illustrated in front view (<FIG>), side-sectional view (<FIG> showing Section S- S indicated in <FIG>), and in perspective view further including motorization and optical pumping (<FIG>). The phosphor wheel includes a disk <NUM> on which is disposed a wavelength conversion layer <NUM> comprising a phosphor typically disposed in a binder material. The disk <NUM> may, for example, be a circular metal plate made of sheet metal or the like. Other materials are contemplated for the disk <NUM> (e.g. plastic, ceramic). The disk <NUM> preferably has high thermal conductivity to provide some conductive heat sinking of the wavelength conversion layer <NUM>.

As mentioned above, the wavelength conversion layer <NUM> comprises a phosphor typically disposed in a binder material. In an illustrative example, the phosphor is operative to convert the short wavelength light (e.g. blue or ultraviolet light having a largest spectral peak at or below <NUM>) to wavelength converted light (e.g. yellow light, or as another example a mixture of phosphors outputting green and red light components, respectively, in illustrative embodiments the wavelength converted light has one or more spectral peaks in the visible spectrum at or above <NUM>). To form the wavelength conversion layer <NUM>, the phosphor is suitably dispersed in a light-transmissive binder at a concentration effective to provide the desired amount of light conversion. By way of some non- limiting illustrative examples, some suitable yellow phosphors include cerium-substituted yttrium aluminum garnet (YAG:Ce), cerium-substituted terbium aluminum garnet (TAG:Ce), europium-substituted barium orthosilicate (BOS), or another suitable phosphor operative to convert the blue light to longer-wavelength visible light, e.g. yellow light. The binder may, for example, comprise a ceramic, a silicone, an epoxy, a plastic (e.g. styrene, a styrene-acrylic copolymer, polycarbonate, polyolefin, polymethylmethacrylate) or so forth.

As illustrated in <FIG>, during operation the disk <NUM> on which the wavelength conversion layer <NUM> is disposed is rotated in a rotation direction R (e.g., clockwise in some embodiments, or counterclockwise in other embodiments) by an electrical motor <NUM> operatively connected with the disk <NUM> by way of an illustrative drive shaft <NUM> or other suitable rotational coupling. (Note, the motor <NUM> and shaft <NUM> are drawn in dotted lines in <FIG> to distinguish from other illustrated components). In one design, the shaft <NUM> shown in <FIG> mechanically connects with a central opening <NUM> of the disk <NUM> is shown in <FIG>, but this is merely one nonlimiting illustrative operative coupling of the motor <NUM> and disk <NUM>. As another nonlimiting example of an operative coupling (not shown), the disk may have no opening and the shaft may be welded or otherwise connected to the disk.

While rotating the disk <NUM> in the rotation direction R, the wavelength conversion layer <NUM> is optically pumped by a light source <NUM> to generate wavelength converted light. The light source <NUM> may, for example, be a laser or a light emitting diode (LED). The pump beam <NUM> impinges on the wavelength conversion layer <NUM> to generate wavelength converted light <NUM>. The phosphor wheel of <FIG> is a reflective phosphor wheel in which the disk <NUM> is reflective for the wavelength converted light (and typically though not necessarily also for the pump light <NUM>); in other embodiments (not shown) the phosphor wheel may be a transmission phosphor wheel in which the disk is optically transmissive for the wavelength converted light (and optionally also for the pump light). For a complete light source, suitable optics may be provided to collect and direct the wavelength converted light <NUM> - in the illustrative example of <FIG> such suitable optics comprise a wavelength-selective mirror <NUM> which passes (i.e. is transmissive for) the pump light <NUM> but reflects the wavelength converted light <NUM>. In a variant approach, if the output light is to include a portion of reflected pump light <NUM> then the wavelength-selective mirror <NUM> can be partially reflective for the pump light <NUM> as well. The illustrative wavelength-selective mirror <NUM> is merely an example, and any type of light collecting/directing optics conventionally used in conjunction with a phosphor wheel may be employed as appropriate for an intended commercial application.

With continuing reference to <FIG> and with further reference to <FIG>, the phosphor wheel further includes an impeller <NUM> disposed on the disk <NUM> to drive outward airflow F across the disk <NUM>, including across the wavelength conversion layer <NUM> which is disposed on the disk <NUM>, when the disk <NUM> is rotated in the rotation direction R by the motor <NUM>. By using the impeller <NUM>, the airflow F is directed outward, that is, having a principle flow component in the outward radial direction away from the center-of-rotation of the disk <NUM>. By this approach, direct convective cooling of the wavelength conversion layer <NUM> is achieved. The illustrative impeller <NUM> includes vanes <NUM> to drive the outward airflow F. In the illustrative embodiment the vanes <NUM> are shaped as airfoils. <FIG> shows an illustrative airfoil-shaped vane <NUM> that is suitable for some designs. The airfoil shape has a rounded leading edge <NUM> and a sharp trailing edge <NUM>, and as best seen in <FIG> each airfoil has its leading edge disposed radially inward of its trailing edge on the disk <NUM>. The illustrative airfoils also have a widest cross-section <NUM> of maximum width (indicated as maximum width Wmax in <FIG>) which is located between the rounded leading edge <NUM> and the sharp trailing edge <NUM>. Each vane <NUM> is oriented to drive outward airflow across the disk <NUM> including across the wavelength conversion layer <NUM> when the disk <NUM> is rotated in the rotation direction R. In diagrammatic <FIG>, the airflow F is represented by a few discrete lines, but will be appreciated to extend over most or all of the plane of the disk <NUM>. Moreover, while the illustrative lines indicating the airflow F in <FIG> are straight, it will be appreciated that the airflow may have some curvature, that is, the airflow imparted by the impeller <NUM> may not be directed precisely radially away from the center of the disk <NUM> but rather may curve due to its possessing some tangential flow component as well. The precise airflow pattern imparted by the impeller <NUM> depends on the detailed shapes and positioning of the vanes <NUM> of the impeller <NUM>, as well as on the rotational speed of the rotation of the disk <NUM> in the rotation direction R and possibly may secondarily depend on other factors such as humidity, air pressure, or so forth. The design of the detailed shape and placement of the vanes <NUM> is suitably performed using known impeller design techniques including, by way of illustrative example, computer simulation of the airflow for various designs to identify a satisfactory design.

The use of airfoil-shaped vanes <NUM> in the illustrative impeller <NUM> as best seen in <FIG> has certain advantages. The rounded leading edge <NUM> of the airfoil reduces drag. The airfoil is aerodynamic, air flow speed on the outskirt is maximized, the noise is low, and due to the low drag the counter- torque exerted on the motor <NUM> is also low. However, the vanes can have shapes other than airfoil shapes, such as being shaped as curved rib-shape vanes having curvature designed to impart the desired airflow F.

It will be further appreciated that the impeller <NUM> is typically designed to provide the desired airflow F over the disk <NUM> including over the wavelength conversion layer <NUM> when the disk is rotated in the (correct) rotation direction R. If the rotation is in the opposite direction then the impeller <NUM> may still provide some beneficial airflow over the disk <NUM> including over the wavelength conversion layer <NUM>, but the efficiency in creating the airflow will be reduced, and additionally the effect will be to draw heat toward the center of the disk <NUM>.

In the illustrative phosphor wheel, the wavelength conversion layer <NUM> is disposed on a single side of the disk <NUM>, and the impeller <NUM> comprises vanes <NUM> with all vanes <NUM> of the impeller <NUM> disposed on the same side of the disk <NUM> as the wavelength conversion layer <NUM>. This ensures that full airflow F generated by the impeller <NUM> passes over the wavelength conversion layer <NUM>.

With particular reference to <FIG>, in a variant embodiment the impeller includes a first impeller <NUM> comprising a first set of vanes <NUM> disposed on a first side of the disk (on which side the wavelength conversion layer <NUM> is also disposed), and a (second) impeller <NUM>B on a "backside" of the disk <NUM>. The second impeller <NUM>B comprises second set of vanes <NUM>B disposed on a second side of the disk <NUM> opposite from the first side of the disk. The impeller <NUM>B is shown by dotted lines in <FIG> to emphasize that including the second impeller <NUM>B is optional and forms a variant embodiment.

In the illustrative embodiment of <FIG>, the wavelength conversion layer <NUM> is disposed at a larger radial position than the impeller <NUM> on the disk <NUM>. This is advantageous because the airflow F can be designed (e.g. using airfoil-shaped vanes <NUM>) to have fastest airspeed at the outermost radius of the impeller <NUM>. The illustrative arrangement also allows for the wavelength conversion layer <NUM> to be formed as a continuous annular ring with no breaks, which can thereby provide a continuous output for the wavelength converted light <NUM>. However, in other contemplated embodiments the phosphor may be disposed in part or in whole between vanes of the impeller. This might be appropriate if, for example, the phosphor is arranged as red, green, and blue patches (with appropriate phosphor and/or reflector for each color) to provide a time sequence of red, green, and blue light for time-domain multiplexed (TDM) full color display.

In the illustrative embodiment, the airflow F is drawn into the impeller <NUM> from the side opposite from the motor <NUM>, i.e. from the same side on which the impeller <NUM> is disposed (assuming here that the second impeller <NUM>B is omitted). In a variant embodiment, an airflow path <NUM> (see <FIG>) is optionally provided via which air can be drawn into the impeller <NUM> from the motor-side, through the disk <NUM>.

With reference now to <FIG>, an illustrative approach for manufacturing and using a phosphor wheel such as that of illustrative <FIG> is described. The disk <NUM> is stamped from a disk of sheet metal <NUM>, or alternatively is cut from suitable ceramic or plastic sheet stock. In an operation <NUM>, the light conversion layer <NUM> is deposited onto the disk <NUM> (for example, using sputtering or another deposition technique), or is adhered onto the disk <NUM> using an adhesive such as glue. To form the impeller <NUM>, the vanes <NUM> are formed by a suitable approach. In illustrative <FIG>, three non- limiting examples of suitable vane formation approaches are indicated. In a first approach (i.e. operation) <NUM>, the impeller vanes <NUM> are adhered to the disk <NUM> using glue or another suitable adhesive. The vanes <NUM> in this approach are manufactured as separate components that are adhered to the disk <NUM>. As a non- limiting example, the vanes <NUM> may be machined from stock metal such as aluminum stock, and glued to the disk <NUM>, which may for example be an aluminum disk in this specific embodiment.

In an alternative second approach (i.e. operation <NUM>), the disk <NUM> is made of sheet metal which is protruding from the disk and formed by sheet metal stamping. Any suitable sheet metal stamping approach can be used to generate the appropriate deformation of the sheet metal to define the vanes <NUM>, such as sheet metal punching, sheet metal pressing, or so forth. The sheet metal stamping may merely deform the sheet metal to form the vanes <NUM>, or may partially break the sheet metal to form partially detached tabs or the like defining the vanes <NUM>.

In an alternative third approach (i.e. operation) <NUM>, the vanes <NUM> are formed by depositing vane material directly onto the surface of the disk <NUM>. As non- limiting examples of this approach, the vanes <NUM> can be made by dispensing or printing glue to form the airfoil or other desired vane shape.

It will be appreciated that typically only one of the alternative approaches <NUM>, <NUM>, <NUM> (or some other approach) will be employed to form the vanes <NUM>. Moreover, the order of the operation <NUM> creating the wavelength conversion layer <NUM> and the operation <NUM>, <NUM>, or <NUM> can be reversed, i.e. the wavelength conversion layer <NUM> can be formed before the vanes <NUM> or, vice versa, the vanes <NUM> can be formed before the wavelength conversion layer <NUM>.

With continuing reference to <FIG>, the manufacturing process continues by an operation <NUM> in which the disk <NUM> (now with the wavelength conversion layer <NUM> and the vanes <NUM> disposed thereon) is operatively secured to the motor <NUM>, e.g. via the drive shaft <NUM> as shown in <FIG>. The phosphor wheel may then be employed to perform wavelength conversion as indicated by operation <NUM>, e.g. by operating the motor <NUM> to rotate the disk <NUM> in the rotation direction R and simultaneously impinging the pump beam <NUM> emitted by the light source <NUM> onto light conversion layer <NUM>. As this occurs, the rotation automatically operates the impeller <NUM> to provide cooling.

The impeller <NUM> is preferably a low profile component, e.g. protruding <NUM> or less from the surface of the disk <NUM> on which it is disposed, and in some embodiments protruding (i.e. having a thickness) of <NUM> to <NUM>. However, a larger or smaller thickness (i.e. protrusion) of the impeller <NUM> is also contemplated. In general, a larger protrusion (thickness) provides more engagement with the air and hence more efficient convective cooling, at the cost of increased drag on the motor <NUM> and a bulkier design; whereas, a smaller protrusion (thickness) provides less engagement with the air and less efficient convective cooling, but provides the benefits of reduced drag on the motor <NUM> and a lower-profile design. With sufficiently low profile, the impeller <NUM> could also be retrofitted to an existing phosphor wheel without modifying the surrounding system.

The disclosed design has been reduced to practice. In the actually constructed phosphor wheel, the diameter of the disk <NUM> was <NUM>. The wavelength conversion layer <NUM> had a width (laterally, i.e. across the plane of the disk <NUM>) of between <NUM> and <NUM>. The impeller vanes were shaped as shown in <FIG>. The number of vanes <NUM> was <NUM>, as illustrated in <FIG> and <FIG>. The vanes <NUM> were made of machined aluminum and had a total weight for all vanes of <NUM> grams. A rough measurement of air flow speed was done near the outer diameter of the disk <NUM>, both with and without attachment of the vanes <NUM>. Without the vanes <NUM> being attached, the air speed (i.e. wind speed) was <NUM>/s at a rotation speed of <NUM> RPM. With the vanes <NUM> attached, a thickness of <NUM> was added (due to the thickness of the added impeller <NUM>), and the air speed was increased to <NUM>/s at <NUM> RPM, and <NUM>/s at <NUM> RPM. The noised increased by <NUM> dBA with the vanes <NUM> added. For a contemplated projector system application, 100W pump laser power is to be applied to the wavelength conversion layer <NUM>. After the vanes <NUM> were added, the output optical power was not changed; however, the temperature of the conversion material was reduced from <NUM> to <NUM>, indicating effective cooling was being provided by the impeller <NUM>.

With reference back to <FIG> and with further reference to <FIG>, another example of an impeller <NUM> is shown. In this embodiment, the vanes <NUM> are adhered or welded to a central ring <NUM>. For example, the vanes <NUM> and the central ring <NUM> may be made of an aluminum alloy or the like. This approach facilitates modular manufacture, as the vanes <NUM> are suitably first attached to the central ring <NUM> by adhesion, welding, or the like, and then the assembly <NUM>, <NUM> is adhered, welded or otherwise attached to the disk <NUM>. Additionally, the vanes <NUM> in this embodiment are spaced apart from the disk <NUM> by the thickness of the central ring <NUM> which can advantageously improve the outward airflow across the disk <NUM> and across the wavelength conversion layer <NUM> when the disk is rotated in the rotation direction R.

Claim 1:
A phosphor wheel comprising:
a disk (<NUM>);
a wavelength conversion layer (<NUM>) comprising phosphor disposed on a first side of the disk (<NUM>); and
an impeller (<NUM>) disposed on the first side of the disk (<NUM>), characterized in that the impeller (<NUM>), comprises vanes (<NUM>) which are shaped as airfoils with each vane (<NUM>) having a leading edge disposed radially inward of a trailing edge on the disk (<NUM>), wherein the leading edge of each vane is a rounded leading edge (<NUM>) and the trailing edge of each vane is a sharp trailing edge (<NUM>), and wherein each vane (<NUM>) is oriented to draw airflow into the impeller (<NUM>) from the first side of the disk (<NUM>) and to drive airflow (F) radially outward away from a center of the disk (<NUM>) and across the wavelength conversion layer (<NUM>) when the disk (<NUM>) is rotated in the rotation direction (R).