Patent Description:
The ability of a light source to render deep red colors is measured by the metric R9. Unfiltered incandescent light sources by definition render those extremely well, typically greater than <NUM>. Replacements for incandescent light sources have tended to struggle with faithful rendition of red colors. For example high pressure sodium lamps and older fluorescent lighting tubes often had negative values for R9, and reduced almost any red color to a rather dull orange appearance. Early light emitting diodes (LEDs) were notorious for poor rendering of red colors. The situation was so significant that many programs related to LED lighting only require R9 > <NUM>. This contrasts with the requirements for the general color rendering index (CRI) which are usually CRI > <NUM>.

Generally speaking, general lighting can be made more pleasing by the increase of red-green contrast, for example by the removal of yellow light, which can wash out the appearance of many objects. This phenomenon has been known in the art for many years, dating back at least to <CIT> "Incandescent lamps with neodymium oxide vitreous coatings", in which a neodymium oxide coating filters out some of the green and lot of the yellow. Based on this work, in <NUM> GE released the Enrich® line of incandescent light bulbs, and in <NUM> renamed the line Reveal®. The incandescent Reveal® product line still exists today, along with an updated Reveal® LED product, still utilizing a neodymium based filter.

<FIG> shows normalized emission spectra of a Reveal® incandescent bulb (solid line) and of a Reveal® A19 LED bulb (dashed line), showing the effect of the neodymium oxide filters in these products. One of the largest drawbacks of this methodology is that it first creates photons and then removes a significant portion of the photons which have been created. This can be seen in the rated outputs of a 60W Reveal® incandescent, <NUM> lumens, and Reveal® LED A19, <NUM> lumens, compared with the benchmark <NUM> lumens for a 60W equivalent A19.

Generally, it has been an industry goal to produce white light emitting phosphor-converted LEDs that have emission spectra that are relatively flat, sloped, and continuous in the region between about <NUM> nanometers (nm) and about <NUM>. This general shape roughly mirrors the reference illuminant, e.g. the emission spectra of black body radiators such as a standard incandescent. As shown in <FIG>, the neodymium oxide filter used in the Reveal® products introduces a dip in the emission spectra in the yellow region. Such a dip may be characterized by the residual intensity at its minimum when compared to the maximum intensity of the emission spectrum between <NUM> and <NUM>, for example about <NUM>% for the incandescent version and about <NUM>% for the LED A19 version.

Red-green contrast does not have a clear metric in the CRI/Ra system, however it can be captured to some extent by the gamut index metric (Rg) of the IES TM-<NUM>-<NUM> method. Applicant measured an Rg of <NUM> for the Reveal® incandescent versus <NUM> for a non-filtered incandescent. The Reveal® LED bulb similarly measures at a high Rg value of <NUM>. Surprisingly, despite the good gamut indices, these bulbs measure relatively poorly on the R9 deep red metric. A drawback of this method is that the neodymium filter used subtracts a significant amount of the light generated. Photons in the wavelength region impacted by these neodymium filters are especially particularly bright, typically ranging from <NUM> to <NUM> lumens per optical Watt compared with the maximum of <NUM> lumens per optical Watt. The Reveal® LED bulb is rated to deliver <NUM> lumens using <NUM> W, while a similar Correlated Color Temperature (CCT) and CRI LED bulb from the Relax® line delivers <NUM> lumens using those same <NUM> W.

Typically, white light emitting phosphor-converted LEDs comprise a two or sometimes three phosphor blend, with a combination of a broad green or yellow phosphor having a full width at half maximum (FWHM) of about <NUM> - <NUM> and a peak wavelength of about <NUM> - <NUM> and a broad red phosphor having a FWHM of about <NUM> - <NUM> and a peak wavelength of about <NUM> - <NUM>, or more usually about <NUM> - <NUM>.

A red phosphor with peak emission at <NUM>-<NUM> provides higher efficacy, due to the better overlap of the red phosphor emission and the photopic response curve of the typical human eye, but this choice of red phosphor emission is generally to the detriment of R9. Conversely, a red phosphor with a peak emission closer to <NUM> provides better red rendering, however, but at a cost of efficacy because the longer wavelength red emission contributes little to the overall brightness of the LED. There is generally an inverse relation between the deep red rendering of a light source as measured by R9 and the spectral efficiency or luminous efficacy of radiation (LER) of the spectrum.

<CIT> discloses a white light illumination system comprising: a phosphor package; a first radiation source for providing co-excitation radiation to the phosphor package, the source emitting in wavelengths ranging from about <NUM> to about <NUM>; and a second radiation source for providing co-excitation radiation to the phosphor package, the source emitting in wavelengths ranging from about <NUM> to about <NUM>; wherein the phosphor package is configured to emit photoluminescence in wavelengths ranging from about <NUM> to about <NUM> upon co-excitation from the first and second radiation sources, and wherein the phosphor package comprises at least one narrow band green phosphor with a photoluminescence peak with a full width at half maximum of less than <NUM>, and wherein the narrow band green phosphor is configured to emit photoluminescence in wavelengths ranging from about <NUM> to about <NUM>.

<CIT> relates to a semiconductor light-emitting device comprising a semiconductor light-emitting element that emits blue light, a green phosphor that absorbs the blue light and emits green light, and an orange phosphor that absorbs the blue light and emits orange light. <CIT> and <CIT> relate to a light emitting device combining a blue LED with a green and a red phosphors.

In one aspect of the invention, Applicants have discovered that phosphor converted white light LEDs comprising a narrow green phosphor rather than a conventional broad green phosphor may simultaneously exhibit high R9, high CRI, and high Luminance Efficacy of Radiation without use of a deep red phosphor to maintain desired red color rendering. For example, in such devices the longest wavelength phosphor peak emission may be shorter than about <NUM>.

In another aspect, Applicants have discovered that phosphor converted white light LEDs comprising a narrow green phosphor rather than a conventional broad green phosphor may provide an emission spectrum exhibiting a significant dip in the yellow region of the spectrum and thereby provide high red-green contrast without use of a filter. Because this yellow dip is in the light emission, rather than caused by a filter, no emission power is lost to filtering. Further, Applicant has discovered that with use of narrow green phosphors the yellow dip may be shallower than in prior art products, and the device may therefore be brighter, while maintaining desired CRI and R9 (red color rendering). The minimum intensity in the yellow dip is, greater than about <NUM>% of the peak intensity in the total emission spectrum of the device between about <NUM> and about <NUM>.

These and other embodiments, features and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following more detailed description of the invention in conjunction with the accompanying drawings that are first briefly described.

The following detailed description should be read with reference to the drawings, in which identical reference numbers refer to like elements throughout the different figures. The drawings, which are not necessarily to scale, depict selective embodiments and are not intended to limit the scope of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention. As used in this specification and in the claims, the term LED is intended to include light emitting diodes and semiconductor laser diodes.

Applicants have developed new families of phosphors that may be excited by a blue emitting LED and in response emit narrow band green light. These phosphors generally emit at a peak wavelength of about <NUM> to about <NUM>, with the peak having a FWHM of about <NUM> to about <NUM>. Examples of these phosphors are described later in this specification and also in <CIT> titled "Phosphors With Narrow Green Emission" referred to above.

In addition, Applicants have simulated total emission spectra from white light emitting phosphor-converted LEDs comprising a blue LED, a green phosphor excited by the blue LED, and a red phosphor excited by the blue LED. In these simulations, the blue LED has a peak emission at about <NUM> with a FWHM of about <NUM>. The green phosphor has a peak emission at about <NUM> to about <NUM> with a FWHM of about <NUM> to about <NUM> (as do various of Applicants' new narrow green phosphors); in some cases the green phosphor may be a blend of <NUM> or more slightly different phosphors. In some simulations, the red phosphor has a peak emission at about <NUM> with a FWHM of about <NUM>, generally corresponding to emission from Intematix Corporation ER6436 red phosphor or Mitsubishi Chemical BR-102C. In other simulations, the red phosphor has a peak emission at about <NUM> with a FWHM of about <NUM>, generally corresponding to emission from Mitsubishi Chemical Corporation BR-<NUM>/Q red phosphor. No other light sources (e.g., no additional LEDs or additional phosphors) contribute to the total emission from the simulated devices. However, white light-emitting phosphor converted LEDs as described in this specification may in some embodiments optionally comprise additional phosphors, for example additional green emitting phosphors and/or additional red emitting phosphors.

In these simulations, the red phosphor peak and bandwidth were held constant, the LED emission peak and bandwidth were held constant, the green phosphor emission peak and bandwidth were varied, the ratio of green phosphor emission intensity to blue LED emission intensity was varied, and the ratio of red phosphor emission intensity to blue LED emission intensity was varied. (Varying the ratios of green and red phosphor emission intensity to blue LED emission intensity is analogous to varying phosphor concentration and loading in a phosphor-converted LED).

The simulated spectra were characterized by calculating various parameters including for example CCT, Duv (distance in a CIE chromaticity diagram from the Planckian locus), CRI, R9, R11, LER, and the intensity at the minimum of a dip (depression) in the yellow region (e.g., about <NUM> to about <NUM>) of the spectrum measured as a percentage of the maximum intensity in the emission spectrum in the range from about <NUM> to about <NUM>.

Some exemplary results of these simulations and some related measurements are presented below.

Table 1A below characterizes three simulated spectra for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>, <NUM>, or <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>, and the CRI is greater than <NUM>.

The trend with green phosphors of <NUM> FWHM combined with a red phosphor with about <NUM> FWHM seems to be that R11 has a maximum at a phosphor wavelength of <NUM>. R11 values then decrease as peak wavelength decreases. The R9 value trend is very dependent upon not just green phosphor peak wavelength, but also CCT and duv. For this particular combination of CCT and CRI, R9 appears to show a relative maximum with the <NUM> - <NUM> phosphors, with the highest value of <NUM> being seen with the <NUM> peaked green phosphor. For peaks of <NUM> through <NUM>, R9 increases as CCT increases and to a lesser extent as duv decreases. For green phosphors with a peak of <NUM>, R9 holds fairly constant through CCT and duv changes when CCT is around <NUM>.

Table 1B below characterizes three simulated spectra for a white light emitting phosphor converted LED comprising a blue LED, a BR102/Q red phosphor, and a green phosphor having peak emission at <NUM>, <NUM>, or <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>.

The narrower (ca. <NUM> FWHM) and slightly blue shifted (<NUM>) BR102/Q has the expected result of slightly lowering the maximum achievable CRI and R9 while increasing the overall LER; additionally, the shifting of the red phosphor spectrum shifts the range of green phosphors that pair with it to give maximum values of R9, CRI and R11. The various trends outlined above are similar with the shifted red phosphor, though also shifted. For example, maximum R11 values are observed with phosphors of a <NUM> peak wavelength. R9 trends outlined above also hold true regarding the changing of R9 with CCT and duv, none of the phosphor blends examined show a relatively constant R9 as was observed with the <NUM> green and ER6436.

<FIG> shows normalized simulated spectra of <NUM> LEDs for green phosphor emission having a <NUM> FWHM peaking at <NUM> (dotted line,) <NUM> (dashed line), and <NUM> (solid line).

Table <NUM> below characterizes three simulated spectra for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>, <NUM>, or <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>, and the CRI is greater than <NUM>.

The trend with green phosphors of <NUM> FWHM seems to be that R11 has a maximum at phosphor wavelengths of <NUM>. R11 values then decrease as peak wavelength moves to <NUM>, or decreases to <NUM>. The R9 value trend is very dependent upon not just green phosphor peak wavelength, but also CCT and duv. For this particular combination of CCT and CRI, R9 appears to show a relative maximum with the <NUM> phosphor, with the highest value of <NUM>. For peaks of <NUM> through <NUM>, R9 increases as CCT increases and to a lesser extent as duv decreases.

<FIG> shows normalized simulated spectra of <NUM> LEDs for green phosphor emission having a <NUM> FWHM peaking at <NUM> (dotted line), <NUM> (dashed line), and <NUM> (solid line).

It is generally accepted that CRI for a white light emitting phosphor-converted LED increases as the color point of a particular phosphor blend moves lower in CIE color space, usually characterized by a decreasing duv. <FIG> plots duv on the vertical axis against CRI on the horizontal axis for simulated phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>. This plot shows the expected trend of CRI increasing as duv decreases over a CRI range of about <NUM> to about <NUM>.

<FIG> plots duv on the vertical axis against CRI on the horizontal axis for simulated phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>. This plot shows a relatively consistent CRI of about <NUM> to about <NUM> across the entire range that would be considered "white light.

<FIG> plots duv on the vertical axis against CRI on the horizontal axis for simulated phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>. This plot also shows a relatively consistent CRI of about <NUM> to about <NUM> across the entire range that would be considered "white light.

Similar to the CRI versus duv trends discussed above for simulated white light emitting phosphor-converted LEDs comprising green phosphors with <NUM> FWHM emission, simulated white light emitting phosphor-converted LEDs comprising green phosphors with peak emission at <NUM> to <NUM> and <NUM> FWHM show expected trends, with CRI ranges of <NUM> across the <NUM> white region. For green peak emission around <NUM>, the CRI range begins to narrow, and with phosphors of <NUM>, the blends only create CRIs within <NUM> point across the white region. Once the peak emission wavelength of the green phosphor decreases to <NUM>, the blend shows the property of increasing CRI with increasing duv, and the CRI ranges approximately <NUM> points across the white region.

Table 4A below characterizes four simulated spectra for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>, <NUM>, <NUM>, or <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>, and the CRI is greater than <NUM>.

Table 4B below characterizes a simulated spectrum for a white light emitting phosphor converted LED comprising a blue LED having peak emission at <NUM> with a FWHM of <NUM>, a BR102/Q red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. The CCT for this spectrum is between <NUM> and <NUM>, nominally <NUM>.

Table 4C below characterizes a measured spectrum for an example white light emitting phosphor converted LED (sample number JM388F9-28ma) comprising a blue LED having peak emission at <NUM> with a FWHM of <NUM>, a BR102/Q red phosphor, and a green phosphor (sample KB3-<NUM>-<NUM>) having peak emission at <NUM> and a FWHM of <NUM>. The CCT for this spectrum is between <NUM> and <NUM>, nominally <NUM>.

The trend with green phosphors of <NUM> FWHM seems to be that R11 has a maximum at phosphor wavelengths of <NUM>. R11 values then decrease as peak wavelength decreases. The R9 value trend is very dependent upon not just green phosphor peak wavelength, but also CCT and duv. For peaks of <NUM> through <NUM>, R9 increases as duv increases and as CCT decreases. For green phosphors with a peak of <NUM>, R9 holds fairly constant through CCT and duv changes when CCT is around <NUM>. The highest R9 values were obtained with phosphors peaked at <NUM> and some with <NUM> (R9 > <NUM>).

<FIG> shows normalized simulated spectra of <NUM> LEDs for green phosphor emission having a <NUM> FWHM peaking at <NUM> (long dashed line), <NUM> (dotted line), <NUM> (dashed line), and <NUM> (solid line).

<FIG> shows a simulated spectrum for a <NUM> LED for green phosphor emission having a <NUM> FWHM peaking at <NUM> (solid line), and a measured spectrum for the example phosphor-converted LED characterized in Table 4C (dashed line).

Table 5A below characterizes a simulated spectrum for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. A green phosphor having these spectral characteristics has been prepared as sample number YBG170620-<NUM> (<NUM>-<NUM>). The CCT for this spectrum is between <NUM> and <NUM>, nominally <NUM>, and the CRI is greater than <NUM>.

Table 5B below characterizes a simulated spectrum for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. A green phosphor having these spectral characteristics has been prepared as sample number KB3-<NUM>-<NUM> (<NUM>-<NUM>). The CCT for this spectrum is between <NUM> and <NUM>, nominally <NUM>.

Table 5C below characterizes a simulated spectrum for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. A green phosphor having these spectral characteristics has been prepared as sample number ELT-<NUM> (<NUM>-<NUM>). The CCT for this spectrum is between <NUM> and <NUM>, nominally <NUM>.

The trend with green phosphors of <NUM> FWHM seems to be that R11 has a maximum at phosphor wavelengths of <NUM>. R11 values then decrease as peak wavelength increases to <NUM> or decreases down to <NUM>. The R9 value trend is very dependent upon not just green phosphor peak wavelength, but also CCT and duv. For peaks of <NUM> through <NUM>, R9 increases as duv decreases and as CCT increases. For green phosphors with a peak of <NUM>, R9 holds fairly constant through CCT and duv changes when CCT is around <NUM>, also corresponding with some of the highest R9 values obtained (R9 > <NUM>).

<FIG> plots duv on the vertical axis against CRI on the horizontal axis for simulated phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>. Through most of the white range CRI ranges from <NUM>-<NUM>, increasing as duv decreases.

<FIG> plots duv on the vertical axis against CRI on the horizontal axis for simulated phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>. The CRI is very consistent ranging only about <NUM> point through the entire white region of <NUM>.

<FIG> plots duv on the vertical axis against CRI on the horizontal axis for simulated phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>. This plots shows a tight grouping of CRI values similar to that in <FIG>.

<FIG> plots duv on the vertical axis against CRI on the horizontal axis for simulated phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>. In this plot CRI increases as duv increases.

Table 6A below characterizes two simulated spectra for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>, or <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>, and the CRI is greater than <NUM>.

Table 6B below characterizes a simulated spectrum for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. Green phosphors having these spectral characteristics have been prepared as sample numbers ELT047C (<NUM>-<NUM>) and YBG <NUM>-4B (<NUM>-<NUM>). The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>.

The trend here seems to be that R11 is a maximum at phosphor wavelengths of <NUM> and <NUM>. R11 values then decrease as peak wavelength increases to <NUM> or decreases down to <NUM>. The R9 value trend is very dependent upon not just green phosphor peak wavelength, but also CCT and duv. For peaks of <NUM> through <NUM>, R9 increases as duv decreases and as CCT increases. For green phosphors with peaks of <NUM> and <NUM>, R9 holds fairly constant through CCT and duv changes when CCT is around <NUM>, also corresponding with some of the highest R9 values obtained (R9 > <NUM>).

<FIG> shows normalized simulated spectra of <NUM> LEDs for green phosphor emission having a <NUM> FWHM peaking at <NUM> (dotted line,) <NUM> (dashed line), and <NUM> (solid line).

Table 7A below characterizes three simulated spectra for a white light emitting phosphor converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM>, <NUM>, or <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>, and the CRI is greater than <NUM>.

Table 7B below characterizes a simulated spectrum for a white light emitting phosphor converted LED comprising a blue LED having a peak emission at <NUM> with a FWHM of <NUM>, a BR102/Q red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>.

Table 7C below characterizes a measured spectrum for an example white light emitting phosphor converted LED (sample JM388-E3-<NUM>) comprising a blue LED having peak emission at <NUM> with a FWHM of <NUM>, a BR102/Q red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>. The CCT for this spectrum is between <NUM> and <NUM>, nominally <NUM>.

As seen in Table <NUM> below, for example, the trend here seems to be that R9 increases with distance above the Planckian locus and also R9 increases with decreasing CCT. There is a clear trend of increased R11 and increased phosphor wavelength with the phosphors between <NUM> and <NUM> peak wavelength. Table <NUM> reports R9 values for selected CCT and duv for simulated spectra for a white light emitting phosphor-converted LED comprising a blue LED, an ER6436 red phosphor, and a green phosphor having peak emission at <NUM> and a FWHM of <NUM>.

<FIG> shows normalized simulated spectra of <NUM> LEDs for green phosphor emission having a <NUM> FWHM peaking at <NUM> (dotted line), <NUM> (solid line) and <NUM> (dashed line).

<FIG> shows a simulated spectrum for a <NUM> LED for green phosphor emission having a <NUM> FWHM peaking at <NUM> (solid line), and a measured spectrum for the example phosphor converted LED characterized in Table 7C (dashed line).

The trend here seems to be that R9 increases with distance above the Planckian locus and also R9 increases with decreasing CCT. For this particular combination of CCT and CRI, R9 appears to show a relative maximum with the <NUM> phosphor. There is a clear trend of increased R11 and increased phosphor wavelength with the phosphors between <NUM> and <NUM> peak wavelength. CRI versus duv follows the expected trend of increasing CRI with decreasing duv for phosphors of longer wavelength, such as <NUM>-<NUM>, with a range of <NUM> points across the region generally considered white. The CRI range compresses down to about <NUM> and shows no real correlation with duv for phosphors of peak wavelengths <NUM> and <NUM>. Shorter wavelength phosphors show a wider range of CRI, but with the inverted trend of increased CRI with increased duv.

<FIG> shows normalized simulated spectra of <NUM> LEDs for green phosphor emission having a <NUM> FWHM peaking at <NUM> (dotted line), <NUM> (solid line), and <NUM> (dashed line).

The trend here seems to be that R9 increases with distance above the Planckian locus and also R9 increases with decreasing CCT. For this particular combination of CCT and CRI, R9 appears to show a relative maximum with the <NUM> phosphor. There is a clear trend of increased R11 and increased phosphor wavelength with the phosphors between <NUM> and <NUM> peak wavelength with a slight relative maximum with phosphors with a <NUM> peak wavelength.

<FIG> shows normalized simulated spectra of <NUM> LEDs for green phosphor emission with <NUM> FWHM peaking at <NUM> (dotted line), <NUM> (solid line), and <NUM> (dashed line).

<FIG> plots CRI and R9 against green phosphor emission peak wavelength, on the horizontal axis, for simulated white light phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and a narrow green phosphor. The CCT for these simulated devices is nominally <NUM> with a duv of +<NUM>. The FWHM of the green phosphor emission varies. This plot shows that using a shorter peak wavelength green phosphor results in a higher value for R9.

<FIG> plots CRI against duv for simulated white light phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and narrow green phosphors with peak wavelength of <NUM> and varying FWHM. The CCT for these simulated devices is nominally <NUM>. This plot shows that at this wavelength of green phosphors, the CRI generally increases as duv decreases. This trend follows the generally accepted trend. Additionally, the plot shows that the range of obtained CRI values decreases with decreasing FWHM.

<FIG> plots CRI against duv for simulated white light phosphor-converted LEDs comprising a blue LED, an ER6436 red phosphor, and narrow green phosphors with peak wavelength of <NUM> and varying FWHM. The CCT for these simulated devices is nominally <NUM>. This plot shows that at this wavelength of green phosphors, the CRI generally increases as duv increases. This trend is opposite of the generally accepted trend where CRI generally decreases as duv increases. Additionally, the plot shows that the range of obtained CRI values decreases with decreasing FWHM.

Table <NUM> below characterizes several simulated spectra for a white light emitting phosphor converted LED comprising a blue LED having a peak emission wavelength between <NUM> and <NUM>, a red phosphor with a nominal <NUM> peak wavelength and a FWHM of <NUM>, and a green phosphor having peak emission between <NUM> and <NUM>, and a FWHM of between <NUM> and <NUM>. The CCT for these spectra is between <NUM> and <NUM>, nominally <NUM>.

Table <NUM> below shows the attributes of commercially available LEDs purchased and tested by the applicant. These LEDs utilize green-yellow phosphors significantly broader than those disclosed herein, and as such do not display a valley in the emission spectra.

KB3-<NUM>-<NUM>, <NUM> peak, <NUM> FWHM. <NUM> Eu, <NUM> CaS, <NUM> Al<NUM>S<NUM>, <NUM> Ga<NUM>S<NUM>, <NUM> S and <NUM> AlCl<NUM> were ground and then divided into <NUM> quartz tubes sealed under vacuum. The tubes were heated together to <NUM> for one hour and then the temperature was increased to <NUM> and held for <NUM> hours. The furnace was cooled at <NUM> per hour. The tubes were opened under inert atmosphere and ground together to combine them.

KB3-<NUM>-<NUM>, <NUM> peak, <NUM> FWHM. <NUM> Eu, <NUM> Al<NUM>S<NUM>, <NUM> Ga<NUM>S<NUM>, <NUM> S and <NUM> AlCl<NUM> were ground and then divided into <NUM> quartz tubes sealed under vacuum. The tubes were heated together to <NUM> for one hour and then the temperature was increased to <NUM> and held for <NUM> hours. The furnace was cooled at <NUM> per hour. The tubes were opened under inert atmosphere and their contents ground together to combine them.

KB3-<NUM>-475a, <NUM> peak, <NUM> FWHM. <NUM> Eu, <NUM> Al<NUM>S<NUM>, <NUM> Ga<NUM>S<NUM>, <NUM> S and <NUM> AlCl<NUM> were ground and then divided into <NUM> quartz tubes sealed under vacuum. One of the tubes was heated to <NUM> for one hour and then the temperature was increased to <NUM> and held for <NUM> hours. The furnace was cooled at <NUM> per hour.

KB3-<NUM>-476a, <NUM> peak, <NUM> FWHM. <NUM> Eu, <NUM> Al<NUM>S<NUM>, <NUM> Ga<NUM>S<NUM>, <NUM> S and <NUM> AlCl<NUM> were ground and then divided into <NUM> quartz tubes sealed under vacuum. One of the tubes was heated to <NUM> for one hour and then the temperature was increased to <NUM> and held for <NUM> hours. The furnace was cooled at <NUM> per hour.

KB3-<NUM>-<NUM>, <NUM> peak, <NUM> FWHM. <NUM> Mg, <NUM> SrS, <NUM> Eu, <NUM> Al, <NUM> Ga<NUM>S<NUM>, and <NUM> S were ground and put into a quartz tube and sealed under vacuum. The sample was heated together to <NUM> for <NUM> hours and then the temperature was increased to <NUM> and held for <NUM> hours. The furnace was cooled in <NUM> hours. The sample was opened under inert atmosphere, ground and sealed in a new quartz tube. It was heated to <NUM> for <NUM> hours and cooled to room temperature over <NUM> hours.

KB3-<NUM>-<NUM>, <NUM> peak, <NUM> FWHM. <NUM> Eu, <NUM> Al<NUM>S<NUM>, <NUM> Ga<NUM>S<NUM>, <NUM> S and <NUM> AlCl<NUM> were ground and then sealed in a quartz tube under vacuum. The sample was heated to <NUM> for one hour and then the temperature was increased to <NUM> and held for <NUM> hours. The furnace was cooled at <NUM> per hour.

YBG170620-<NUM>, <NUM> peak, <NUM> FWHM. Stoichiometric amount of Eu, Al, Ga<NUM>S<NUM> and <NUM> wt% excess S to form Eu(Al<NUM>Ga<NUM>)S<NUM> were thoroughly ground in a mortar with a pestle in the glove box and sealed in a quartz tube under vacuum. The mixtures were placed in dried silica tubes, which were evacuated and sealed at a length of about <NUM> in. Reactions were carried out in box furnaces. The temperature was raised to <NUM> and held for <NUM> hours and raised again to <NUM> and held for <NUM> - <NUM> hours then cooled to room temperature for <NUM> hours.

ELTEAGS-<NUM>-B-<NUM>, <NUM> peak, <NUM> FWHM. The reagents CaS, Eu, Al and S were combined in stoichiometric amounts to obtain the nominal composition CaAl<NUM>S<NUM>:<NUM>%Eu and loaded into an alumina crucible in a horizontal tube furnace. Following a <NUM>-min purge with Ar, the mixture was heated to <NUM>, at which point H<NUM>S gas flow was started. After <NUM> at <NUM>, the furnace was heated to <NUM> for <NUM>. Upon cooling, the H<NUM>S gas was turned off and the product was cooled to room temperature under flowing Ar.

ELTAlS-<NUM>-B, <NUM> peak, <NUM> FWHM. Eu(Al<NUM>-xGax)<NUM>S<NUM>+y was prepared by combining Eu, Al<NUM>S<NUM>, Ga<NUM>S<NUM> and S in stoichiometric ratios. <NUM> wt% AlCl<NUM> and <NUM> excess S were added prior to firing. The mixture was sealed in an evacuated silica tube and heated to <NUM> for <NUM>, then heated to <NUM> for <NUM>. The sample was cooled to room temperature at a rate of <NUM>/h.

ELTAlS-<NUM>, <NUM> peak, <NUM> FWHM. CaS, Eu, Al and S were combined in stoichiometric amounts to obtain the nominal composition CaAl<NUM>S<NUM>:<NUM>%Eu. The mixture was homogenized in a mortar and pestle under Ar, then loaded into a carbon-coated silica tube which was subsequently evacuated and sealed under vacuum. Synthesis was carried out by a stepwise heating approach: <NUM> (<NUM>), <NUM> (<NUM>), <NUM> (<NUM>) and slow-cooled over <NUM>. The product was recovered and manually reground before returning to a new carbon-coated silica tube and heated to <NUM> (<NUM>) and <NUM> (<NUM>).

ELTEAGS-<NUM>-A-<NUM>, <NUM> peak, <NUM> FWHM. CaS, EuF<NUM>, Al, Ga<NUM>S<NUM> and S were combined stoichiometrically to obtain the target composition CaAl<NUM>Ga<NUM>S<NUM>:%<NUM>. 5Eu (<NUM>% Ga). The sample was homogenized under Ar then loaded into an alumina crucible and placed in a horizontal tube furnace. Following a <NUM>-min purge with flowing Ar, the mixture was heated to <NUM>, at which point H<NUM>S gas flow was started. After holding at <NUM> for <NUM>, the sample was heated to <NUM> for <NUM>. The H<NUM>S gas was switched off at <NUM> during the cool down to room temperature over <NUM>.

ELTEAGS-<NUM>-A-<NUM>, <NUM> peak, <NUM> FWHM. CaAl<NUM>Ga<NUM>S<NUM>:<NUM>%Eu was prepared under flowing H<NUM>S/Ar from stoichiometric amounts of CaS, EuF<NUM>, Al, Ga<NUM>S<NUM> and S. The sample was homogenized in an Ar atmosphere then loaded into an alumina boat and placed in a horizontal tube furnace. Following a <NUM> purge with flowing Ar, the mixture was heated to <NUM> for <NUM>, at which point H<NUM>S gas flow was started. The sample was then heated to <NUM> for <NUM> and cooled to room temperature. The H<NUM>S gas was switched off at <NUM> during the cool down.

ELTAlS-<NUM>-G, <NUM> peak, <NUM> FWHM. EuAlGaS<NUM> was synthesized by combining Eu, Al<NUM>S<NUM>, Ga<NUM>S<NUM> and S in stoichiometric amounts under Ar. The mixture was sealed in an evacuated quartz tube and heated to <NUM> (<NUM>) then to <NUM> (<NUM>). After grinding the product and adding <NUM> excess S, a second heat treatment was followed by heating to <NUM> (<NUM>) then to <NUM> (<NUM>).

ELTAlS-<NUM>, <NUM> peak, <NUM> FWHM. Reagents Eu, Al<NUM>S<NUM>, Ga<NUM>S<NUM> and S were combined stoichiometrically to prepare EuAl<NUM>Ga<NUM>S<NUM>. Homogenization of the mixture was done in an Ar atmosphere in a mortar and pestle. 3wt% AlCl<NUM> was used as a flux and the sample was sealed in an evacuated quartz tube. The reaction was done by heating the quartz ampoule to <NUM> (<NUM>) then to <NUM> (<NUM>). The product was recovered and manually ground with a mortar and pestle.

ELTAlS-<NUM>-F, <NUM> peak, <NUM> FWHM. EuAl<NUM>Ga<NUM>S<NUM> was synthesized from stoichiometric amounts of Eu, Al<NUM>S<NUM>, Ga metal, and S. The reactants were mixed under Ar then sealed in an evacuated quartz tube. Two heat treatments were followed to obtain the final product. Heat <NUM>: <NUM> (<NUM>), <NUM> (<NUM>). Heat <NUM>: <NUM> (<NUM>), <NUM> (<NUM>). The sample was reground with <NUM> excess S and sealed in an evacuated quartz tube during the intermediate step.

ELTAlS-<NUM>-E, <NUM> peak, <NUM> FWHM. EuAl<NUM>Ga<NUM>S<NUM> was synthesized from stoichiometric amounts of Eu, Al<NUM>S<NUM>, Ga metal, and S. The reactants were mixed under Ar then sealed in an evacuated quartz tube. Two heat treatments were followed to obtain the final product. Heat <NUM>: <NUM> (<NUM>), <NUM> (<NUM>). Heat <NUM>: <NUM> (<NUM>), <NUM> (<NUM>). The sample was reground with <NUM> excess S and sealed in an evacuated quartz tube during the intermediate step.

ELTAlS-042E & F, <NUM> peak, <NUM> FWHM. Eu(Al<NUM>Ga<NUM>)<NUM>S<NUM> was synthesized from a pre-fired mixture of stoichiometric amounts of Eu, Al<NUM>S<NUM>, Ga<NUM>S<NUM> and S. The product was combined with <NUM> I<NUM> (<NUM> wt%) and <NUM> S (<NUM> wt%) before separating into two portions and sealed into two evacuated quartz tubes. Both samples were heated to <NUM> (<NUM>) then quenched in either air or water.

ELTAlS-<NUM>-B, <NUM> peak, <NUM> FWHM. CaAl<NUM>Ga<NUM>S<NUM>:<NUM>%Eu was synthesized by combining CaS, Eu, Al<NUM>S<NUM>, Ga<NUM>S<NUM> and S in stoichiometric amounts. The mixture was homogenized using a mortar and pestle under Ar, then loaded into a carbon-coated silica tube which was subsequently evacuated and sealed under vacuum. Synthesis was carried out by a stepwise heating approach: <NUM> (<NUM>), <NUM> (<NUM>), <NUM> (<NUM>) and cooled to room temperature over <NUM>. The product was recovered and manually reground with <NUM> S before adding to another carbon-coated silica tube and heated to <NUM> (<NUM>) and <NUM> (<NUM>).

Claim 1:
A light emitting device comprising:
a semiconductor light source emitting blue light, wherein the blue light emitted by the semiconductor light source has a peak at from <NUM> to <NUM> and a full width at half maximum of <NUM> to <NUM>;
a first phosphor arranged to be excited by the blue light emitted by the semiconductor light source and in response emit green light having a peak emission at from <NUM> to <NUM> with a full width at half maximum of <NUM> to <NUM>; and
a second phosphor arranged to be excited by the blue light emitted by the semiconductor light source and in response emit red light having a peak emission at a wavelength from <NUM> and less than or equal to <NUM>, and a full width at half maximum of <NUM> to <NUM>;
wherein an overall emission spectrum from the light emitting device has a depression between <NUM> and <NUM>, and the minimum intensity in the depression is greater than or equal to <NUM>% and less than or equal to <NUM>% of the maximum intensity in the overall emission spectrum in the range from <NUM> to <NUM>.