Patent Grant
8016443
Downlights are lighting fixtures mounted in a ceiling for illumination directly below them. These ubiquitous luminaires generally comprise an incandescent spotlight mounted within a can. The can is typically closed except at the bottom, so any hot air becomes trapped within the can. Even in the rare cases when heat is transmitted through the can to a heat sink or heat exchanger on the outside of the can, the heat exchanger is typically in stagnant air within a false ceiling, and is not very effective. In most cases, not only is there no heat exchanger, the can is actually insulated to prevent heat from being delivered into the space within the false ceiling. Since incandescent bulbs operate hot anyway, they are not thermally bothered by the can being a trap for hot air. It would be highly desirable to replace the light bulbs with lamps using light-emitting diodes (LEDs), which are more efficient. A white LED system, using blue LEDs combined with yellow phosphor, would be suitable.
LEDs, however, are sensitive to excessive temperatures and thus find downlights to be a more difficult lighting application than anticipated. This is because their heat cannot safely be dissipated passively into the stagnant hot air of the typical downlight can. This typically limits the total wattage that can be handled in a solid state LED downlight to a maximum power of approximately 4 Watts. This limit can only be overcome if the can is dramatically widened to aid in cooling for the sake of heat management, a severe limitation on the situations in which the LED downlight can be used. Furthermore, the best commercially available 4 Watt LED sources have an efficacy of 60 lumens per Watt including driver losses. This limits the solid state downlight to a flux of only approximately 250 lumens. A flux output of 600 to 1000 lumens is desirable for a downlight, and it is desirable for the downlight to be able to operate in a standard size, typically 4″-6″ (10 to 15 cm) diameter ceiling can. This is achievable for an LED or comparable solid-state downlight if the heat management can handle a minimum of 10 Watts.
One embodiment of the present invention provides a luminaire comprising one or more blue LED chips, collimating apparatus operating upon the output light of said chips, a phosphor patch situated at a distance from said LED chips such that said collimator illuminates said phosphor patch, and a beam-forming reflector surrounding said phosphor patch.
The aforementioned thermal limitation of LEDs is overcome in the present application by separating the blue LED and the yellow phosphor in white LEDs. Then the heat-producing LEDs can be situated at the front (bottom) of the downlight, facing backwards (upwards, into the can), so that only the remote phosphor need be at the back (top). This allows the LEDs to have a heat sink that is located at the open (bottom end) face of the can or, if needed, just outside the can. It also allows for an active cooling device to be attached to the can instead of, or in addition to, a passive heatsink. An example of a commercially available active cooling device suitable for this purpose is the Nuventix Synjet cooler, which can easily handle 15 to 20 Watts.
An embodiment of a luminaire comprises one or more blue LED chips, a collimator operative on the emitted light of said chips to produce collimated light, a phosphor situated at a distance from the LED chips such that the collimated light illuminates the phosphor, and a beam-forming reflector surrounding the phosphor and arranged to produce an output beam of light from the phosphor past the LED chips.
Another embodiment of a luminaire comprises a housing with an open end and a closed end, a phosphor patch in the closed end of the housing, a light source spaced from the phosphor patch in the direction from the closed end of the housing to the open end of the housing, and arranged to emit light so as to illuminate the phosphor patch, wherein light from the phosphor patch is emitted through the open end of the housing past the light source.
A further embodiment of a luminaire comprises a shroud having an open end and a closed end, an opaque reflector in the closed end of the shroud, a phosphor patch in the closed end of the shroud, between the opaque reflector and the closed end of the shroud, and a light source in the open end of the shroud, operative to direct onto the phosphor patch light of a frequency effective to excite the phosphor patch, wherein light from the phosphor patch exits through the open end of the shroud past the light source.
In an embodiment, the beam-forming reflector may produce an output beam centered on an axis from a center of the phosphor or phosphor patch through a center of the LED chips or other light source. The near field beam may then be annular, because the light source creates a shadow in the middle, but by shaping the beam to include converging rays, the field can close at the center further from the luminaire.
In an embodiment, the beam-forming or primary reflector may comprise a conicoidal reflector having a narrow end encircling the phosphor patch, and may further comprise a cylindrical reflector extending from a wide end of the conicoidal reflector to the open end of the luminaire.
In an embodiment, the light source may comprise a collimator operative on the output light of the light source to illuminate the phosphor patch with light from the light source, and preferably to illuminate substantially the whole phosphor patch with substantially all the light from the light source, either directly or by reflection from the output beam-forming reflector.
In an embodiment, the luminaire may comprise an inner cylinder surrounding the collimator, the exterior of the cylinder being a specular mirror or other reflector. Any spider or other structure supporting or carrying power or control lines to the light source may also be reflective.
In an embodiment, the luminaire may comprise an opaque reflector behind the phosphor patch at the closed end of the housing. The phosphor patch may then cover only part of the area of the reflector, for example, as a pattern of phosphor dots, or a pattern of phosphor with holes in it.
In an embodiment, the phosphor patch may be cooled by a heat sink situated on the opposite side of the opaque reflector from a side facing the light source.
In an embodiment, the light source may comprise at least one blue LED. The emitted light may then comprise blue light from the blue LED reflected at the phosphor patch and light produced by conversion of the blue light from the blue LED by the phosphor patch. The emitted light may then be white or whitish. The CRI and/or color temperature of the white light may be adjusted by using additional or secondary LEDs of a different color, for example, red or a longer-wavelength blue. Any additional LEDs may be included in the light engine of the primary light source, or may be mounted in the phosphor patch.
In an embodiment where the phosphor patch is not a continuous layer of phosphor, secondary LEDs may be mounted in parts of the reflector that are not coated with phosphor.
In an embodiment, the luminaire may have a tunable color temperature. Where the luminaire has LEDs or other light sources of more than one color, the tuning may be provided by separately controlling the intensities of LEDs of different colors.
In an embodiment, the LED chip or other primary light source may be cooled by a heat sink situated on a side of said at least one LED chip opposite from a side to which said LED chip emits light. In the case of a downlight, the heat sink may be arranged so that when the downlight is installed in a ceiling the heat sink will project into the room being lit, below the visible ceiling.
For a preferred embodiment, directly substituting for a typical 2 to 5 inch (50 to 125 mm) diameter downlight producing a beam of 30-40° half angle, the remote phosphor patch will be much larger (typically an inch or two, 25 to 50 mm, across) than the LED source, (typically a chip 1 mm across or a small array of such chips). Thus, the heat load of the remote phosphor is typically not a problem, because the large area of the phosphor results in a low concentration of heat energy to be dissipated. There is typically a secondary optic on the blue LED, so that all its light will shine only on the remote phosphor at the back (top) of the downlight. The most practical secondary optic is a cone-sphere combination, because a conical reflector can use high-reflectivity films manufactured flat. The conical reflector is oriented with its open smaller end downwards, with the LED light source simply placed within the small lower opening of the cone so that all the light emission from the LED is captured by the cone and reflected upwards.
In the cone-sphere embodiment, a plano-convex lens entirely covers the cone's large upper opening and sends all the LED's blue light to the remote phosphor or near enough to it that a primary reflector on the inside of the can will redirect onto the phosphor any rays that do not reach the phosphor directly. The relatively large remote phosphor that the blue LED excites will have relatively low luminance as compared to much smaller conventional white LEDs, eliminating or substantially reducing any glare factor. The heat sink for the blue LEDs can be located down low, exposed to the ambient air below the visible ceiling, enabling adequate cooling even for a 10-20 Watt blue LED package. Such power levels are too much to be easily accommodated in an installation in the top of a sealed hot can, closed to outside air, even with a fan. With the current proposal, only the phosphor heats the interior of the can, so the interior of the can becomes less hot than if the LEDs were in the top of the can. In addition, only the phosphor is in the top of the can, and the phosphor is far less vulnerable to heat damage than the LEDs themselves.
It is also desirable to have a solid state downlight with a high CRI of 92 or better, with a color temperature ranging from 2500 to 4000° K. This can be achieved using currently available phosphors in conjunction with blue LEDs. An alternative preferred embodiment uses a combination of a blue LED chip with a red LED chip, configured in a two-dimensional array at the base of the cone. In order to achieve high uniformity of both red and blue light on the phosphor, homogenizing lenslets can be added to the inner flat face of the plano-convex lens. Alternatively, a holographic or other shaping diffuser can be used after the lens. By individually tuning the currents supplied to the red and blue LEDs, a wide range of color temperatures can be achieved, all with very high CRI.
The above and other aspects, features and advantages of the present invention will be apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:
A better understanding of various features and advantages of the present invention will be obtained by reference to the following detailed description of embodiments of the invention and accompanying drawings, which set forth illustrative embodiments in which various principles of the invention are utilized.
A downlight is a ceiling-mounted luminaire shining downwards with a restricted output angle. A downlight is generally recessed in a can from 4-6″ (100-150 mm) in diameter and 6″ (150 mm) deep. In the case of downlights intended for use in high ceilings, the output angle of the downlight is usually defined directly in degrees, but for ordinary ceilings there is frequently a zone of desired illumination, such as a tabletop. A good reflector for defined angles is the compound parabolic concentrator (CPC), while for defined target zones the compound elliptical concentrator (CEC) may be preferred. Since either one can be used as a primary reflector in the present invention, the inclusive term ‘ideal reflector’ will be used hereinafter to include both CECs and CPCs. Their shapes are so visually similar as to make their differences indiscernible in the Figures herein.
Reflector 105 is a CPC with a 45° output angle when acting as a collimator for the emitted beam. Reflector 105 acting as a concentrator will accept any blue light from LED 101 as long as the light rays are within that angle of the central axis, and convey such light to remote phosphor 104. Functionally, the combination of cone 102 and lens 103 could be replaced by any other collimator that efficiently collects the light output of LED 101 and collimates that light to produce a beam no wider in angle than ±45°.
Likewise, the dimensions of the LED optics, which are identical in
The thickness and composition of remote phosphor 104 of
Remote phosphor 104 of
As shown by the arrow marked “Down” in
It can be seen from these two preferred embodiments 100 and 300 that the general design of the present luminaire is highly adaptable to a wide variety of beam patterns and a wide range of sizes and power outputs. Thus, the presently proposed luminaires are suitable for installation within the challenging thermal environment of commercial downlights. Not shown in the somewhat schematic
The heat sink 104H, 304H for the remote phosphor 104, 304, is typically in stagnant air at the top of the downlight can, but the heat load from the phosphor is only about a third of the LED's optical output power, which in turn is only about a third of the electrical input. A phosphor's heat load will only be about one-seventh the heat load of the LED itself. This heat is from the blue light that is absorbed but does not cause fluorescence (sub-unity quantum efficiency, 80-90%) and from the lower energy of the photons of stimulated yellow light. The yellow-to-blue energy ratio, called the Stokes factor, is simply the ratio of the blue wavelength to the mean phosphor wavelength, typically about 80%. At 90% quantum efficiency, 10% of the blue light becomes heat, as well as 20% of the energy in the converted blue light, for a total heat load of 28% of the blue flux. The best LEDs currently commercially available convert about a third of their electrical power into light, so that the phosphor's heat load is about 10% of the electrical power, while the LED's heat load is ⅔ of the electrical power, giving a phosphor heat load of only one sixth that of the LED, and spread over far more area.
The heat sink 104h, 304h may be omitted if it is not needed. In many cases the removal of heat by radiation and conduction from the front of the phosphor 104, 304 to the air within the luminaire 100, 300 will be sufficient when combined with convection driven by the concentrated heat of the LED light engine 101, 301. In other cases, a thermal bridge from the phosphor 104, 304 to the primary reflector 105, 106, 305, which typically will be a metal shell acting as a heat sink, will be sufficient. The bridge may be provided by an aluminum or other metal substrate behind the phosphor 104, 304 that is continuous with the metal substrate of the primary reflector. In still other cases, a thermal bridge may be provided from the back of the phosphor 104, 304 to the can (not shown) within which the luminaire is installed.
Spider 502 has internal features on one or more of its three vanes (two shown) to enclose the wiring 503W. The arms of spider 502 are preferably sharp-edged on the edge towards the remote phosphor 104, 304 and coated with high-reflectivity material. Light falling on the spider arms is then almost all merely deflected slightly, and not lost. The spider 502 can be thermally connected to the shroud 501, the heat sink 503, the base holding the LED array (not shown) and cylindrical reflector 504. One or more vanes of the spider 502 may include a heat pipe. All the surface area of these components can help with the thermal management of the LEDs. In addition, thermal management features can be added to cylindrical shroud 501 at its base (not shown).
As an alternative to spider 502 the LED light engine may be mounted on a transparent structure, for example, a glass disk. The disk would prevent hot air from heat sink 503 from entering the can, but would prevent the formation of the convection loop that in the embodiments previously described cools phosphor 104, 304 and cylindrical reflector 504 by carrying hot air from inside shroud 501 down into the room. A spider 502 designed to occlude only a small part of the exit aperture is therefore preferred in most cases.
The optical design of the present luminaires leads to the remote phosphor being far larger than the LED chips, which incidentally results in a lower phosphor-luminance level, more gentle to the eye. This larger area and lower heat flux result in a much easier cooling task. While the placement of the remote phosphor at the top of a closed can will indeed result in an elevated operating temperature for the phosphor, that temperature can still be far below what the phosphor in a conventional white LED typically experiences.
As was previously mentioned, it is possible to achieve high CRI using blue LEDs with a “warm” phosphor. However, there may be an advantage to using a cooler phosphor and combining this with the output of red LEDs, for example, around 625 nm peak emissivity. One advantage is that, because the Stokes loss in the phosphor is proportional to the ratio of the absorbed and emitted frequencies, the red phosphor output has the lowest efficiency, with about ⅓ of the blue light being dissipated as heat in the phosphor conversion. The red LEDs may be mounted before the phosphor patch is deposited, so that their light is spread out somewhat.
Alternatively, or in addition, relatively long-wave blue LEDs may be used directly to boost the amount of visible blue light emitted. For example, primary blue LEDs with a peak emissivity in the 410-460 nm range, such as 440 nm, may be used to excite the phosphor 104, 304. However, the blue light from the primary LEDs is too short in wavelength to have much visible luminance, and auxiliary blue LEDs with a peak emissivity around 490 nm may be used directly for additional visible blue light.
The use of red LEDs and/or auxiliary blue LEDs makes possible a downlight that has a white output of tunable color temperature, if the different colors of LED are separately driven by independently variable drivers. Tuning may then be adjustable by the user, adjustable by a technician when the luminaire is installed or subsequently, or preset by the manufacturer.
There are at least two possible ways auxiliary red and/or blue LEDs can be provided. The first is to put one or more red or other auxiliary LEDs in the same plane as the primary blue LEDs. In order to produce an output beam of uniform color without additional mixing, this typically requires that the collimating optics be able to homogenize the two colors such that the beam patterns on the phosphor are very similar. That can be accomplished by lenslets on the flat surface of the plano-convex lens 103 or 303. Many alternative collimator homogenizers are known to those skilled in the art of nonimaging optics.
A second way is to embed the auxiliary LEDs in the remote phosphor. If the phosphor color is only slightly too cool, the amount of red light needed to make white is relatively low, so this approach would not add a significant load on the rear heat sink loads. In the case of a discontinuous phosphor, the red LEDs can be placed in the gaps in the phosphor.
As an example of possible performance, the values shown in Table 1 are estimates for a system as described above, showing the electrical power apportioned to the auxiliary blue (490 nm) and red (625 nm) LEDs as a fraction of each electrical Watt, with the balance to the primary blue (440 nm) LEDs. The driver power supply is assumed to have 92% efficiency in converting incoming electrical power to DC to supply the LEDs. The primary reflector has a reflectivity of 92%, other surfaces have a reflectivity of 98%. The primary blue LEDs have a radiant efficiency of 40% at 250 mA per 1 mm2 chip, and the phosphor has a quantum efficiency of 85%. The characteristics assumed for the phosphor are based on the UBV_Y02 high efficacy yellow phosphor from PhosphorTech Corporation, of Lithia Springs, Ga.
The blue reflectivity values represent 10% Fresnel reflection at the front surface of the phosphor, with the balance from blue light that passes through the phosphor unconverted. The total blue reflectance thus depends on the thickness of the phosphor, and four different thicknesses are shown. For each combination of lighting, the x and y chromaticity coordinates, correlated color temperature (CCT) in Kelvin, color rendering index (CRI), lumens per Watt (LPW) including all losses, and heat generated at the phosphor as a fraction of total system power consumption are shown.
The values shown in Table 1 are believed to be achievable with luminaires as described above, using materials and components already commercially available.
Line 6, for a 25 W downlight with 24 primary blue LEDs, 3 auxiliary blue LEDs, and 3 red LEDs each running nominally at 0.75 W, allowing 2.5 W upward margin for tuning of the CCT and/or CRI, and line 10, with 24 primary blue LEDs, 2 auxiliary blue LEDs, and 4 red LEDs, are believed to be of practical interest.
If it is desired that the optical system be reduced in size, the LED collimator can be designed as described in commonly-assigned U.S. Patent Application publication No. 2008-0291682 by Falicoff et al. for “LED Luminance-Augmentation via Specular Retroreflection, Including Collimators that Escape the Etendue Limit” filed May 21, 2008, which is incorporated herein by reference in its entirety. That application reveals how collimators can be designed with a reduced diameter to escape the traditional etendue limit. That enables the LED collimator to be located closer to the phosphor, significantly reducing the overall size of the luminaire.
The preceding description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The full scope of the invention should be determined with reference to the Claims.
Although various embodiments have been described, the skilled reader will understand how features of different embodiments may be combined in a single luminaire.
This application claims benefit of U.S. Provisional Patent Applications No. 61/126,366, filed May 2, 2008, and No. 61,134,481, filed Jul. 10, 2008, which are incorporated herein by reference in their entirety.
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