Light emitting device with improved warm-white color point

A light emitting device is disclosed and includes an emission source configured to emit a primary blue light and a wavelength-converting element configured to convert the primary blue light to a secondary light having a correlated color temperature (CCT) in the range of 1600K-2500K and color rendering index (CRI) in the range of 40-60, the wavelength-converting element including a red phosphor material having a peak emission wavelength that is less than 620 nm and a green phosphor material having a peak emission wavelength that is greater than 530 nm. The device may exhibit a melanopic/photopic ratio of less than 0.25 and/or may exhibit a radiometric power fraction of light having a wavelength below 530 nm below 0.1.

BACKGROUND

Light emitting diodes (“LEDs”) are commonly used as light sources in various applications. LEDs are more energy-efficient than traditional light sources, providing higher energy conversion efficiency than incandescent lamps and fluorescent light, for example. Furthermore, LEDs radiate less heat into illuminated regions and afford a greater breadth of control over brightness, emission color and spectrum than traditional light sources. These characteristics make LEDs a viable choice for various outdoor lighting applications ranging from street lighting to traffic lights and street signage.

SUMMARY

According to aspects of the disclosure, a light emitting device is disclosed and includes an emission source configured to emit a primary blue light, and a wavelength-converting element configured to convert the primary blue light to a secondary light having a correlated color temperature (CCT) in the range of 1600K-2500K, a melanopic/photopic ratio of less than 0.25, and a color rendering index (CRI) in the range of 40-60. The wavelength-converting element includes a red phosphor material having a peak emission wavelength that is less than 620 nm and a green phosphor material having a peak emission wavelength that is greater than 530 nm.

According to aspects of the disclosure, a light emitting device is disclosed and includes an emission source configured to emit a primary blue light, and a wavelength-converting element configured to convert the primary blue light to a secondary light having a correlated color temperature (CCT) in the range of 1600K-2500K, a radiometric power fraction of light having a wavelength below 530 nm is below 0.1, and a color rendering index (CRI) in the range of 40-60. The wavelength-converting element includes a red phosphor material having a peak emission wavelength that is less than 620 nm and a green phosphor material having a peak emission wavelength that is greater than 530 nm.

DETAILED DESCRIPTION

Outdoor lighting applications may use high-pressure sodium (HPS) light sources, which provide light emissions having a correlated color temperature (CCT) in the range of 1900-2800K. By contrast, LEDs that are typically used in outdoor applications may have a CCT of about 4000K and CRI of about 70. As existing HPS installations are converted to LED, the typical 4000K/70 LED spectrum may become non-optimal due to the relatively high short-wavelength (blue) spectral content. While 3000K/70 LED spectra may offer a reasonable compromise between reduced blue light content, color visibility, and efficacy, some applications require even lower CCTs of around 2000K. For example, light sources having a CCT of around 2000K may be required for preservation of the HPS “look” in historic districts, or for minimization of blue light in areas that are particularly sensitive from an ecological standpoint.

According to aspects of the disclosure, a light emitting device is disclosed that closely matches the emission spectrum of HPS light sources. The light emitting device includes a blue light LED as its primary emission source and a wavelength-converting element that is formed by using an improved warm-white phosphor system (hereinafter “the warm-white phosphor system”). The warm-white phosphor system may be characterized by a combination of a green phosphor and a red phosphor. The green phosphor may have a peak emission wavelength above 530 nm. The red phosphor may have a peak emission wavelength below 620 nm. The warm-white phosphor system is discussed in detail further herein.

According to some implementations, the light emitting device may provide light emissions having a CCT between 1600K and 2500K, which have a reduced blue light content. For example, the emissions may have less than 10% (0.1) radiometric spectral power in wavelengths below 530 nm and/or a melanopic/photopic ratio of less than 0.25. The color fidelity of the emissions may be characterized by a CRI Ra that is greater than 40 or TM-30 Rf that is greater than 40. Accordingly, the light emitting device may have an emission spectrum that is similar and/or superior to that of HPS light sources.

These performance characteristics of the light emitting device are made possible by the improved warm-white phosphor system that is used to make the device's wavelength-converting element. Unlike traditional phosphor systems in illumination-grade warm-white LEDs, the warm-white phosphor system used by the light emitting device does not meet the traditional color rendering requirement of having a CRI that is greater than 70. However, the warm-white phosphor system enables a lower blue light content and higher efficacy that are desirable for many outdoor lighting applications, while still providing better color rendering than HPS light sources. The improved warm-white phosphor system is discussed in detail further below.

Examples of different light emitting devices and/or wavelength-converting element implementations will be described more fully hereinafter with reference to the accompanying drawings. These examples are not mutually exclusive, and features found in one example can be combined with features found in one or more other examples to achieve additional implementations. Accordingly, it will be understood that the examples shown in the accompanying drawings are provided for illustrative purposes only, and they are not intended to limit the disclosure in any way. Like numbers refer to like elements throughout.

FIG. 1Ais a diagram of an example light emitting element (LEE)100that includes a light emitting semiconductor structure115, a wavelength converting material110, and an optional coating105on the wavelength converting material110. Contacts120and125may be coupled to the light emitting semiconductor structure115, either directly or via another structure such as a submount, for electrical connection to a circuit board or other substrate or device. In embodiments, the contacts120and125may be electrically insulated from one another by a gap127, which may be filled with a dielectric material. The light emitting semiconductor structure115may be any light emitting semiconductor structure that emits light that may be converted to light having a different color point via a wavelength converting material. For example, the light emitting semiconductor structure115may be formed from III-V semiconductors including, but not limited to, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, II-VI semiconductors including, but not limited to, ZnS, ZnSe, CdSe, CdTe, group IV semiconductors including, but not limited to Ge, Si, SiC, and mixtures or alloys thereof. These example semiconductors have indices of refraction ranging from about 2.4 to about 4.1 at the typical emission wavelengths of LEDs in which they are present. For example, III-Nitride semiconductors, such as GaN, have refractive indices of about 2.4 at 500 nm, and III-Phosphide semiconductors, such as InGaP, have refractive indices of about 3.7 at 600 nm. Contacts120and125may be formed from a solder, such as AuSn, AuGa, AuSi or SAC solders.

FIG. 1Bis a diagram of an example light emitting semiconductor structure115that may be included in the LEE100ofFIG. 1A. The illustrated example is a flip chip structure. However, one of ordinary skill in the art will understand that the embodiments described herein may be applied to other types of LED designs, such as vertical, lateral, and multi junction devices.

In the example illustrated inFIG. 1B, the light emitting semiconductor structure115includes a light emitting active region135disposed between a semiconductor layer or semiconductor region of n-type conductivity (also referred to as an n-type region)130and a semiconductor layer or region of p-type conductivity (also referred to as a p-type region)140. Contacts145and150are disposed in contact with a surface of the light emitting semiconductor structure115, such as a surface of the semiconductor layer or region of p-type conductivity140, and electrically insulated from one another by a gap155, which may be filled by a dielectric material, such as an oxide or nitride of silicon (i.e., SiO2 or Si3N4). In the illustrated embodiment, contact145(also referred to as a p-contact) is in direct contact with a surface of the p-type region140, and the contact150(also referred to as an n-contact) is in direct contact with a surface of the n-type region130. Although not shown inFIG. 1B, a dielectric material, such as disposed in the gap155, may also line side walls of the light emitting active region135and p-type region140to electrically insulate those regions from the contact150to prevent shorting of the p-n junction.

The n-type region130may be grown on a growth substrate and may include one or more layers of semiconductor material. Such layer or layers may include different compositions and dopant concentrations including, for example, preparation layers, such as buffer or nucleation layers, and/or layers designed to facilitate removal of the growth substrate. These layers may be n-type or not intentionally doped, or may even be p-type device layers. The layers may be designed for particular optical, material, or electrical properties desirable for the light emitting region to efficiently emit light. Like the n-type region130, the p-type region140may include multiple layers of different composition, thickness, and dopant concentrations, including layers that are not intentionally doped, or n-type layers. While layer130is described herein as the n-type region and layer140is described herein as the p-type region, the n-type and p-type regions could also be switched without departing from the scope of the embodiments described herein.

The light emitting active region135may be, for example, a p-n diode junction associated with the interface of p-region140and n-region135. Alternatively, the light emitting active region135may include one or more semiconductor layers that are doped n-type or p-type or are un-doped. For example, the light emitting active region135may include a single thick or thin light emitting layer. This includes a homojunction, single heterostructure, double heterostructure, or single quantum well structure. Alternatively, the light emitting active region135may be a multiple quantum well light emitting region, which may include multiple quantum well light emitting layers separated by barrier layers.

The p-contact145may be formed on a surface of the p-type region140. The p-contact145may include multiple conductive layers, such as a reflective metal and a guard metal, which may prevent or reduce electromigration of the reflective metal. The reflective metal may be silver or any other suitable material, and the guard metal may be TiW or TiWN. The n-contact150may be formed in contact with a surface of the n-type region130in an area where portions of the active region135, the n-type region140, and the p-contact145have been removed to expose at least a portion of the surface of the n-type region130. The sidewall of an exposed mesa or via may be coated with a dielectric to prevent shorting. The contacts145and150may be, for example, metal contacts formed from metals including, but not limited to, gold, silver, nickel, aluminum, titanium, chromium, platinum, palladium, rhodium, rhenium, ruthenium, tungsten, and mixtures or alloys thereof. In other examples, one or both contacts145and150may be formed from transparent conductors, such as indium tin oxide.

The n-contact150and p-contact145are not limited to the arrangement illustrated inFIG. 1Band may be arranged in any number of different ways. In embodiments, one or more n-contact vias may be formed in the light emitting semiconductor structure115to make electrical contact between the n-contact150and the n-type layer130. Alternatively, the n-contact150and p-contact145may be redistributed to form bond pads with a dielectric/metal stack as known in the art. The p-contact145and the n-contact150may be electrically connected to the contacts120and125ofFIG. 1A, respectively, either directly or via another structure, such as a submount.

Referring toFIG. 1A, the wavelength converting material110may be any luminescent material, such as a phosphor, phosphor particles in a transparent or translucent binder or matrix, or a ceramic phosphor element, which absorbs light of one wavelength and emits light of a different wavelength. The wavelength converting material110may be a ceramic phosphor element such that the ceramic phosphor element may be, for example, a ceramic phosphor plate, such as a platelet of phosphor, for generating one color of light, or a stack of ceramic phosphor platelets for generating different colors of light. The ceramic phosphor plate may have an RI of 1.4 or greater (e.g., 1.7 or greater) at the wavelengths emitted by the light emitting semiconductor structure115.

The wavelength converting material110may be applied in a layer having a thickness that may depend on the wavelength converting material used or other factors related to enhancing the color point shift as a function of drive current as described in more detail below. For example, a layer of wavelength converting material110may be approximately 50 μm in thickness while other wavelength converting materials may be formed in layers as thin as 20 μm or as thick as 200 μm. In embodiments, the wavelength converting material110, such as a ceramic phosphor element, may be pre-formed into a wavelength converting element and attached to the light emitting semiconductor structure115using an adhesive or any other method or material known in the art.

In embodiments, the light emitting semiconductor structure115may emit blue light. In such embodiments, the wavelength converting material110may include, for example, a yellow emitting wavelength converting material or green and red emitting wavelength converting materials, which will produce white light when the light emitted by the respective phosphors combines with the blue light emitted by the light emitting semiconductor structure115. In other embodiments, the light emitting semiconductor structure115emits UV light. In such embodiments, the wavelength converting material110may include, for example, blue and yellow wavelength converting materials or blue, green and red wavelength converting materials. Wavelength converting materials emitting other colors of light may be added to tailor the spectrum of light emitted from the device100.

In embodiments, the wavelength converting material110may be composed of Y3Al5O12:Ce3+. The wavelength converting material110may be an amber to red emitting rare earth metal-activated oxonitridoalumosilicate of the general formula (Ca1-x-y-zSrxBayMgz)1-n(Al1-a+bBa)Si1-bN3-bOb:REn wherein 0≤x≤1, 0≤y≤1, 0≤z≤1, 0≤a≤1, 0≤b≤1 and 0.002≤n≤0.2, and RE may be selected from europium(II) and cerium(III). The phosphor in the ceramic phosphor plate may also be an oxido-nitrido-silicate of general formula EA2-zSi5-aBaN8-aOa:Lnz, wherein 0≤z≤1 and 0<a<5, including at least one element EA selected from the group consisting of Mg, Ca, Sr, Ba and Zn and at least one element B selected from the group consisting of Al, Ga and In, and being activated by a lanthanide (Ln) selected from the group consisting of cerium, europium, terbium, praseodymium and mixtures thereof.

In other embodiments, the wavelength conversion material110may also have a general formula (Sr1-a-bCabBacMgdZne)SixNyOz:Eua 2+, wherein 0.002≤a≤0.2, 0.0≤b≤0.25, 0.0≤c≤0.25, 0.0≤d≤0.25, 0.0≤e≤0.25, 1.5≤x≤2.5, 1.5≤y≤2.5 and 1.5≤z≤2.5. The wavelength conversion material may also have a general formula of MmAaBbOoNn:Zz where an element M is one or more bivalent elements, an element A is one or more trivalent elements, an element B is one or more tetravalent elements, O is oxygen that is optional and may not be in the phosphor plate, N is nitrogen, an element Z that is an activator, n=2/3m+a+4/3b−2/3o, wherein m, a, b can all be 1 and o can be 0 and n can be 3. M is one or more elements selected from Mg (magnesium), Ca (calcium), Sr (strontium), Ba (barium) and Zn (zinc), the element A is one or more elements selected from B (boron), Al (aluminum), In (indium) and Ga (gallium), the element B is Si (silicon) and/or Ge (germanium), and the element Z is one or more elements selected from rare earth or transition metals. The element Z is at least one or more elements selected from Eu (europium), Mg (manganese), Sm (samarium) and Ce (cerium). The element A can be Al (aluminum), the element B can be Si (silicon), and the element Z can be Eu (europium).

The wavelength conversion material110may also be an Eu2+ activated Sr—SiON having the formula (Sr1-a-bCabBac)SixNyOx:Eua, wherein a=0.002-0.2, b=0.0-0.25, c=0.0-0.25, x=1.5-2.5, y=1.5-2.5.

The wavelength conversion material110may also be a chemically-altered Ce: YAG (Yttrium Aluminum Garnet) phosphor that is produced by doping the Ce: YAG phosphor with the trivalent ion of praseodymium (Pr). The wavelength conversion material110may include a main fluorescent material and a supplemental fluorescent material. The main fluorescent material may be a Ce: YAG phosphor and the supplementary fluorescent material may be europium (Eu) activated strontium sulfide (SrS) phosphor (“Eu: SrS”). The main fluorescence material may also be a Ce: YAG phosphor or any other suitable yellow-emitting phosphor, and the supplementary fluorescent material may also be a mixed ternary crystalline material of calcium sulfide (CaS) and strontium sulfide (SrS) activated with europium ((CaxSr1_x)S:Eu2+). The main fluorescent material may also be a Ce:YAG phosphor or any other suitable yellow-emitting phosphor, and the supplementary fluorescent material may also be a nitrido-silicate doped with europium. The nitrido-silicate supplementary fluorescent material may have the chemical formula (Sr1-x-y-zBaxCay)2Si5N8:Euz 2+ where 0≤x, y≤0.5 and 0≤z≤0.1.

In embodiments, the wavelength conversion material110may include strontium-lithium-aluminum: europium (II) ion (SrLiAl3 N4:Eu2+) class (also referred to as SLA), including MLiAl3N4: Eu2+ (M=Sr, Ba, Ca, Mg). In a specific embodiment, the luminescent particles may be selected from the following group of luminescent material systems: MLiAl3N4:Eu (M=Sr, Ba, Ca, Mg), M2SiO4:Eu (M=Ba, Sr, Ca), MSel-xSx:Eu (M=Sr, Ca, Mg), MSr2S4:Eu (M=Sr, Ca), M2SiF6:Mn (M=Na, K, Rb), M2TiF6:Mn (M=Na, K, Rb), MSiAlN3:Eu (M=Ca, Sr), M8Mg(SiO4)4Cl2:Eu (M=Ca, Sr), M3MgSi2O8:Eu (M=Sr, Ba, Ca), MSi2O2N2:Eu (M=Ba, Sr, Ca), M2Si5-xAlxOxN8-x:Eu (M=Sr, Ca, Ba). However, other systems may also be of interest and may be protected by a coating. Also combinations of particles of two or more different luminescent materials may be applied, such as e.g. a green or a yellow luminescent material in combination with a red luminescent material.

In embodiments, the wavelength conversion material110may be a blend of any of the above-described phosphors.

FIG. 2Ais a diagram of an example light emitting device (LED)200A. In the example illustrated inFIG. 2A, the LED200A includes the light emitting semiconductor structure115ofFIG. 1B, which is mounted to a submount205that includes the contacts120and125. The light emitting semiconductor structure115may be mounted to the submount205by an electrical coupling between the contacts145and150on the light emitting semiconductor structure115and submount electrodes on an adjacent surface of the submount205(not shown inFIG. 2A). The submount electrodes may be electrically connected by vias (not shown) to the contacts120and125on the opposite surface of the submount205. In embodiments, the LED200A may be mounted to a printed circuit board (PCB)215. In such embodiments, the submount205may be mounted via the contacts120and125to the PCB215. Metal traces on the circuit board may electrically couple the contacts120and125to a power supply, such that an operational or drive voltage and current may be applied to the LED when it is desired to turn the LED on.

The submount205may be formed from any suitable material, such as ceramic, Si, or aluminum. If the submount material is conductive, an insulating material may be disposed over the substrate material, and the metal electrode pattern may be formed over the insulating material. The submount205may act as a mechanical support, provide an electrical interface between the n and p electrodes on the LED chip and a power supply, and provide heat sinking. In embodiments, a heat sink may alternatively or additionally be provided on the PCB215, such as a metal core PCB-MCPCB heat sink220illustrated inFIG. 2A. While the heat sink220is illustrated inFIG. 2Aas being attached to the bottom of the PCB215, one of ordinary skill in the art will recognize that other arrangements are possible without departing from the scope of the embodiments described herein.

In the example LED200A, the wavelength converting material110completely surrounds the light emitting semiconductor structure115on all surfaces except the surface that electrically connects the light emitting semiconductor structure115to the submount205. The optional coating105may be disposed in direct contact with the wavelength converting material110. The coating may not be a separate layer, may be a coating on the individual phosphor particles or may be formed on the ceramic phosphor, and this coating may include pores. These pores may be filled with a binder or matrix material and may be part of the wavelength converter110. Coatings of phosphor materials are described in U.S. patent application Ser. No. 15/802,273, which was filed on Nov. 2, 2017 and is incorporated by reference herein in its entirety. Phosphor coatings of sol-gel, atomic layer deposition (ALD), evaporation, sputtering, dip and dry, or spin coating methods include SiO2, Al203, HfO2, Ta2O5, ZrO2, TiO2, Y2O3, and Nb2O5. Coatings may be thick enough to include pores that may be formed during or after deposition.

FIG. 2Bis a diagram of another example LED200B. In the example LED200B, the wavelength converting material110is deposited on the light emitting semiconductor structure115. An optional coating105may be disposed in direct contact with the wavelength converting material110. A structure210, such as a frame, is disposed adjacent side surfaces of a stack formed by the light emitting semiconductor structure115, the wavelength converting material110and the optional coating105and may surround the stack. The entire structure210, but at least inner surfaces of the structure210that are adjacent the stack, may be formed from or coated in a light reflecting material, such as an interference layer or a strongly scattering layer, to further minimize absorption of any scattered light.

FIG. 3is a schematic perspective view of a device300that is light emitting and which includes a wavelength-converting element360, which may be the same as or similar to wavelength converting material110ofFIG. 1, formed using the warm-white phosphor system discussed above.FIG. 4is a schematic perspective view of the device300with the wavelength-converting element360removed to expose a light source350, which is situated underneath it.FIG. 5is a schematic cross-sectional view of the light emitting device300.

As illustrated inFIGS. 3-5, the device300includes a base310comprising a first electrically conductive lead frame312that is coupled to a second electrically conductive lead frame314by an electrically non-conductive element316. A reflective layer320is formed over the base310, and a sidewall330is formed above the reflective layer320to define a reflector cup340. The reflective layer320may include a first portion322and a second portion324. The first portion322and the second portion324of the reflective layer320may be electrically insulated from one another by the non-conductive element316. Contacts354and356may be integrally formed on the bottom surfaces of the lead frames312and314to provide means for connecting the device300to various types of electronic circuitry. Lead frame312may be coupled to a contact pad372of the light source350and the lead frame314may be coupled to a contact pad374of the light source350. A non-conductive underfill element376may be formed between the contact pads372and374to further bolster the bond between the light source350and the base310.

The light source350may be disposed in the reflector cup340, as shown. The light source350may be the primary emission source of the device300. In some implementations, the light source350may be an LED configured to emit blue light. In some implementations, the wavelength of the blue light emitted by the light source350may be in the range of 400-550 nm (or 400-530 nm). However, it will be understood that the present disclosure is not limited to any particular blue light spectrum, as blue LEDs produced by different manufacturers may have differing blue light emission spectra.

The wavelength-converting element360may be formed by suspending a phosphor combination belonging to the warm-white phosphor system (e.g., a combination of a green phosphor material and a red phosphor material) into a silicone slurry (or another matrix material) to form an encapsulating compound, which is injected into the reflector cup340to seal the light source350inside. According to an implementation, suspending a phosphor combination, belonging to the warm-white phosphor system, in a matrix material, may form the wavelength-converting element360. According to an alternative implementation, using one or more alternative techniques may form the wavelength-converting element360. For example, the phosphor combination may be sintered to produce a tile that is then disposed on or above the light emitting surface of the light source350. As another example, the phosphor combination may be applied on a substrate (e.g., a glass tile), which is then disposed on or above the light emitting surface of the light source350. As yet another example, the phosphor combination may be used to form a coating which is then applied on the sides of the reflector cup340and/or one or more surfaces of an overlying optical element. As yet another example, the phosphor combination may be used to form a film which is subsequently laminated onto a surface of the light source350and/or a surface of the device300. As yet another example, the green phosphor and the red phosphor that make the phosphor combination may be applied in separate layers above the light source350. Stated succinctly, the present disclosure is not limited to any specific technique for using the warm-white phosphor system to convert primary (blue) light emitted by the light source350to secondary (warm-white) light.

As disclosed herein, a warm-white phosphor system may be arranged to produce warm-white light having CCT between 1600K and 2500K and may include a green phosphor and a red phosphor. The green phosphor may have a peak emission wavelength that is greater than 530 nm. And the red phosphor may have a peak emission wavelength that is less than 620 nm. In some implementations, the warm-white phosphor system may have an excitation range between 380 nm and 530 nm. Additionally or alternatively, in some implementations, the warm-white phosphor system may have a peak emission wavelength between 580 nm and 620 nm (e.g., around 600 nm).

The green phosphor may be any suitable type of green phosphor. Additionally or alternatively, in some implementations, the green phosphor may be any suitable type of YAG phosphor. Additionally or alternatively, in some implementations, the green phosphor may include any suitable type of phosphor having a peak emission wavelength between 535 nm and 560 nm. Additionally or alternatively, the green phosphor may include any suitable type of green phosphor having an excitation range between 400 nm and 480 nm. For example, in some implementations, the green phosphor may be a NYAG4355 phosphor, which is marketed and sold by Intematix Corporation of Fremont, Calif., USA. In such instances, the green phosphor may be a Ce doped YAG phosphor having density of 4.8 g/cm and particle size of 13.5 μm. Furthermore, in such instances, the peak emission wavelength of the green phosphor may be 551 nm and its excitation range may be between 430 nm and 490 nm.

The red phosphor may be any suitable type of red phosphor. Additionally or alternatively, in some implementations, the red phosphor may be any suitable type of BSSNE phosphor (e.g., a (BS)2S8N5:E phosphor). Additionally or alternatively, the red phosphor may include any suitable type of phosphor having peak emission wavelength between 580 nm and 620 nm. Additionally or alternatively, the red phosphor may include any suitable type of red phosphor having an excitation range between 350 nm and 580 nm. In some implementations, the red phosphor may be a (Ba,Sr)AlSiN3:Eu phosphor with a Ba:Sr ratio of 1:1 and an Eu concentration of 1%. Further information about red BSSNE phosphors that can be used in the warm-white phosphor system can be found in U.S. patent application Ser. No. 13/988,852, which is herein incorporated by reference.

In some implementations, the green-to-red phosphor weight ratio of the warm-white phosphor system may be between 1.5 and 0.64. As used throughout the present disclosure, the term “green-to-red phosphor weight ratio” refers to the ratio of the weight of green phosphor to the weight of red phosphor in a particular phosphor combination belonging to the warm-white phosphor system. When phosphors combinations belonging the warm-white phosphor system are suspended in a silicone slurry, the phosphor-to-silicone weight ratio of the resulting compound may be between 0.6 and 0.7. As used throughout the present disclosure, the term “phosphor-to-silicone weight ratio,” in the context of mixing a phosphor combination with a silicon slurry, refers to the ratio of the weight of the phosphor combination (e.g., a combination of green and red phosphors) and the weight of the silicone.

In some implementations, the green-to-red phosphor weight ratio of the warm-white phosphor system may be approximately 1.6. Additionally or alternatively, in some implementations, the green-to-red phosphor weight ratio of the warm-white phosphor system may be approximately 1.01. Additionally or alternatively, in some implementations, the green-to-red phosphor weight ratio of the warm-white phosphor system may be approximately 0.67.

FIG. 6is a spectral plot illustrating how varying the green-to-red phosphor ratio may affect the performance of the warm-white phosphor system. More specifically,FIG. 6includes a plot showing the emission spectra of three different phosphor combinations that belong to the warm-white phosphor system. These phosphor combinations are herein referred to as first phosphor combination, second phosphor combination, and third phosphor combination. All three combinations are formed by mixing a red phosphor and a green phosphor in a silicon slurry. More particularly, the green-to-red phosphor weight ratio of the first phosphor combination may be 1.6 and its phosphor-to-silicon weight ratio may be 0.623. The green-to-red phosphor weight ratio of the second phosphor combination may be 1.01 and its phosphor-to-silicon weight ratio may be 0.638. The green-to-red phosphor weight ratio of the third phosphor combination may be 0.64 and its phosphor-to-silicon weight ratio may be 0.677.

As illustrated inFIG. 6, curve610represents the power spectral density (W/nm) of the first phosphor combination. Curve620represents the power spectral density (W/nm) of the second phosphor combination. Curve630represents the power spectral density (W/nm) of the third phosphor combination. In the example ofFIG. 6, the first, second, and third combinations may be formed using a green phosphor having a peak emission wavelength of 541 nm and a red phosphor having a peak emission wavelength of 604 nm. As can be readily appreciated, the first, second, and third phosphor combinations are provided as an example only. Accordingly, the present disclosure is not limited in any way to the examples discussed with respect toFIG. 6.

FIG. 7is a plot illustrating the color of different emissions that can be produced by the warm-white phosphor system. More particularly,FIG. 7is a CIE1931 chromaticity diagram illustrating different color points that can be achieved by respective phosphor combinations belonging to the warm-white phosphor system. Each of the points710indicates the color (in the CIE1931 color space) of light emissions produced by a particular phosphor combination belonging to the warm-white phosphor system, as disclosed herein. Points720indicate the color points of monochromatic light on the spectral locus for the listed wavelength in nm. With the color point of the light source and a reference white color point (e.g., CIE-D65) these wavelengths on the outer boundary can define the dominant wavelength of a spectral distribution.

FIGS. 8A and 8Bare plots illustrating the blue light content in emissions produced by the warm-white phosphor system disclosed herein. More particularly,FIG. 8Ais a plot having an x-axis and a y-axis where the y-axis of the plot represents melanopic/photopic ratio ranging from 0.00 to 0.30. and the x-axis represents the x-coordinates in the CIE1931 color space, and it can range from 0.30 to 0.60. The points612represent the respective melanopic/photopic ratio for different colors of light that can be produced by respective phosphor combinations belonging to the warm-white phosphor system. As illustrated, the melanopic/photopic ratio of all light emissions is below 0.25

FIG. 8Bis a plot illustrating the radiometric fraction of light of wavelength below 530 nm (e.g., blue light) for different light emissions that can be produced by the warm-white phosphor system disclosed herein. The y-axis of the plot represents radiometric power fraction values ranging from 0.00 to 0.12. The x-axis represents the x-coordinates in the CIE1931 color space, and it can range from 0.30 to 0.60. The points614represent the radiometric power fraction of light having a wavelength below 530 nm in different colors of light emissions that can be produced by respective phosphor combinations belonging to the warm-white phosphor system. As illustrated, the radiometric power fraction of light having a wavelength below 530 nm in each of the emissions is below 0.1.

FIG. 8Cillustrates the conversion efficiency of the warm-white phosphor system disclosed herein. The plot includes an x-axis and a y-axis. The y-axis of the plot represents conversion efficiency ranging from 150 lm/W to 220 lm/W. The x-axis represents the x-coordinates in the CIE1931 color space, and can range from 0.30 to 0.60. The points616represent the conversion efficiency for different colors of light that can be produced by respective phosphor combinations belonging to the warm-white phosphor system. As illustrated, the conversion efficiency of the light emissions that can be produced by the warm-white phosphor system is between 170 lm/W and 220 lm/W.

FIG. 8Dillustrates the CRI of different emissions that can be produced by the warm-white phosphor system disclosed herein. The plot includes an x-axis and a y-axis. The y-axis represents color rendering index (CRI) Ra. The x-axis represents the x-coordinates in the CIE1931 color space, and it can range from 0.30 to 0.60. The points618represent the CRI Ra values for different colors of light that can be produced by respective phosphor combinations belonging to the warm-white phosphor system. As illustrated, the CRI Ra value for the light emissions varies between 40 and 60.

As can be seen fromFIGS. 8A-D, the blue light content of emissions produced by the warm-white phosphor system disclosed herein is below the pre-defined limits of 0.25 malanopic/photopic ratio and 0.1 radiometric power fraction for light having a wavelength below 530 nm. The CRI Ra of emissions produced by the warm-white phosphor system disclosed herein is between 40 and 60. The modeled conversion efficiency (CE) of the warm-white phosphor system is around 200 lm/W.

As noted above, the wavelength-converting element360of the device300may be formed of a phosphor combination belonging to the warm-white phosphor system. With state-of-the-art wall-plug efficiency (WPE) of the device300at about 65%, the overall efficacy of the device300in this spectrum is estimated to be in the 130-135 lm/W range.

Table 1 below compares the power efficiency of one implementation of the device300to that of an example 100 W HPS light source:

Table 1 illustrates that the device300can provide a significant performance improvement over traditional HPS light sources. Specifically, Table 1 shows that the luminaire-level efficiency improvement presented by the device300may be approximately 75% in a typical roadway light. Furthermore, the light use efficiency of LED systems is higher than HSP light sources due to the smaller source size affording better optical control. Therefore, the gain in “delivered” lumens per Watt can be even higher in typical applications.

Table 2 below compares the color rendering performance of one implementation of the device300to that of an example 100 W HPS light source. More particularly, Table 2 compares the CCT, CRI, color fidelity (TM-30 Rf), and color gamut (TM-30 Rg) of the example HPS light source and the device300. The color gamut and color fidelity are measured in accordance with the TM-30 system. TM-30 is a system of measures and graphics that can be used to evaluate and communicate light sources' color rendering property.

Table 2 illustrates that the device300can provide a significant performance improvement over comparable HPS light sources. More particularly, the device300may have a considerably better color fidelity (TM-30 Rf) and color gamut (TM-30 Rg) than traditional HPS light sources due to the improved rendering of greens and reds. This, in turn, may increase color visibility in outdoor applications, such as vehicle and traffic sign recognition, and improve overall user perception.

The data provided in Table 2 is expressed graphically inFIGS. 9A-B.FIG. 9Ais a TM-30 color vector plot showing the color rendering performance of the HPS light source. AndFIG. 9Bis a color vector plot showing the color rendering performance of the device300. As illustrated inFIGS. 9A-Band Table 2, both the HPS light source and the device300produce light emissions of similar respective color temperatures (1917K and 1944K, respectively). However, the device300may have a higher CRI, color fidelity (TM-30 Rf), and color gamut (TM-30 Rg).

Although the present disclosure is provided in the context of outdoor lighting applications, it will be understood that the above-described warm-white phosphor system can be utilized in any lighting context, including, but not limited to indoor lighting, horticulture lighting, decorative lighting, etc. Although in the present example, the device300is a mid-power LED, the present disclosure is not limited to any particular type of LED. Furthermore, the present disclosure is not limited to any specific type of LED package. For example, the above-described warm-white phosphor system may be used in chip-scale packages and/or any other suitable type of LED package.

FIGS. 1-9Bare provided as an example only. At least some of the elements discussed with respect to these figures can be arranged in different order, combined, and/or altogether omitted. It will be understood the phrase “phosphor combination belonging to the warm-white phosphor system” refers to a particular species of the genus represented by the above-described warm-white phosphor system. It will be understood that the provision of the examples described herein, as well as clauses phrased as “such as,” “e.g.”, “including”, “in some aspects,” “in some implementations,” and the like should not be interpreted as limiting the disclosed subject matter to the specific examples.

Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concepts described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.