LEDs made with KSF phosphor—such as Luminus LUX COBs—have advantages in both display and general lighting applications [1, 2]. For general lighting, these advantages include: a narrow emission band, improved color rendering, a favorable excitation spectrum, good thermal quenching, improvements in concentration quenching, higher efficacy, and they are free of any rare-earth minerals. These advantages are discussed below.
Narrow Emission Band: KSF phosphors exhibit a narrow emission band, leading to more visible red light and higher attainable luminous efficacy of the radiation. This narrow emission band contributes to the overall energy efficiency of the LEDs.
Improved Color Rendering: KSF phosphors offer the ability to produce saturated red emission below 650 nm, without low visibility red “tails” in the SPD, which is desirable for achieving better color rendering in white LEDs while maintaining a high efficacy.
Luminus KSF development has focused on high-CRI products. There is no real advantage in developing a low-CRI KSF product, they typically use more yellow and green content to improve efficacy at the expense of color rendering accuracy.
The images in Figure 1 are abbreviated TM-30-18 reports (based on the internationally accepted ANSI/IES color rendition standards). Some examples of full TM-30 reports are included in Appendix B. A more detailed explanation of TM-30 and other color quality metrics can be found in the Luminus White Paper “Achieving Optimal Color Rendition with LEDs”. The TM-30 method uses 99 color samples to calculate the reflection from the test spectrum and a black body reference spectrum (100 CRI) which has the same CCT as the sample.
The circle graphic is a concise summary of the differences between these two spectra where the 99 samples are grouped into 16 color bins. The reference source is always a black circle, and the red polygon shows the fit of the product to the reference spectrum by color bin: the closer the fit, the more accurate color rendition.
Modifying the spectral composition of specific colors can have a negative impact on the naturalness and accuracy of illumination. This effect is particularly noticeable when it comes to red hues. Figure 1 provides a visual representation of this phenomenon, highlighting the significant difference in R9 values between the 80 CRI and 90 CRI parts. The R9 value for the 80 CRI part is only 6, while the 90 CRI part boasts a much higher R9 value of 55.
When we compare the spectral power distribution (SPD) and shape fit to the hue bins in the circle graphic, we observe that the 80 CRI plots lack the same level of balance between the red, green, and blue spectral contributions as the 90 CRI plots. The KSF phosphor based 90 CRI part has nearly the same Luminous Efficacy of Radiation (LER)* but significantly better color rendition.
Figure 1. Comparison of TM-30-18 for nitride-based phosphor Gen 6 80 CRI and KSF phosphor-based LUX 90 CRI 3000K COBs. These two parts have equivalent LER and LPW metrics, but the LUX part has superior color rendition.
Figure 2 compares the color quality metrics of the SPDs of two parts that have very similar color quality, but the LUX part has a significantly higher efficacy.
Figure 2. Comparison of TM-30-18 Gen 6 90 CRI and LUX 90 CRI 3000K COBs showing better efficacy for the KSF system with similar color quality.
Favorable Excitation Spectrum: Unlike some other phosphors, the TriGain KSF that Luminus uses has broad excitation bands that are separated from the emission bands. This minimizes reabsorption issues when combined with yellow or green phosphors, resulting in improved light conversion efficiency.[3] Excitation curves (Figure 3) show the wavelengths where a phosphor absorbs photons and emits longer wavelength photons, comparing KSF, a nitride, and a YAG (yttrium aluminum garnet) phosphor.[4]
There is a strong excitation peak near 450 nm for all of these phosphor types that enables a single blue pump LED to be used in this type of white LED system. The efficiency associated with each phosphor type is based on some fraction of the absorbed photons that generate heat instead of light. KSF does not absorb YAG or Nitride light emission. This increases efficacy and reduces phosphor heating for lower-CCT color temperature parts with high red nitride phosphor loading.
Figure 3. Excitation and emission spectrums for KSF, a comparable nitride and a typical YAG phosphor showing that KSF-phosphors have a favorable excitation spectrum. Overlap between an excitation curve and any of the emission curves shown indicates absorption at these wavelengths. Some of this absorption generates heat in the system. KSF phosphors have very little overlap. (Note: There are many varieties of yellow, green, and red phosphors and these are just exemplars.)
The nitride excitation curve in Figure 3 shows that YAG phosphor emission is affected by the amount of nitride phosphor in the system, which lowers efficacy and contributes to phosphor heating for warmer CCT parts. The nitride excitation curve shows that there is significant absorption of the YAG phosphor and also some from the nitride itself (self-absorbing).
Good Thermal Quenching Threshold: A key performance metric of a phosphor is its thermal quenching (TQ). TQ is a decrease in the photoluminescence intensity (luminance efficiency) of phosphors as temperature increases. During operation. KSF phosphors demonstrate thermal quenching behavior comparable to traditional red nitride phosphors.[5]
Concentration Quenching Improvements: Concentration quenching is the loss of phosphor quantum efficiency (QE) at high activator concentrations, a common issue with phosphor materials. The critical activator concentration is the point where the fluorescent intensity of the blue-pumped phosphor starts to decrease. GE reports on improvements that can be gained using phosphor synthesis process optimization.[3]
Rare-Earth-Free Composition: KSF phosphors belong to a new class of rare-earth-free red fluoride phosphors. This composition is environmentally friendly, making them a sustainable choice for white light technology.
KSF Performance Compared to Traditional Nitride-based White LEDs
The performance advantage of using white LEDs with KSF phosphor are evident when compared to traditional nitride-based white LEDs. Figure 4 shows a comparison of nitride-based LEDs to a 95 CRI KSF-based LED. The LER of the 80 CRI nitride-based LED and the 95 CRI KSF-based LED are very similar while the higher CRI nitride-based parts have lower LER. The data shown in Table 1 provides a more detailed comparison of the LER efficiency advantages of LUX LEDs.
Figure 4. Comparison of 80, 90, 95 CRI nitride-based white LEDs to a 95 CRI KSF-based LED. The 95 CRI sources have 262 LER for nitride and 316 LER for KSF-based LUX.
Across the Luminus COB product lines, the LUX KSF Phosphor LEDs offer higher efficacy while delivering high color rendition. The improvement from nitride LEDs to LUX is a paradigm shift in phosphor technology.
Figure 5 shows the distributions of LER versus red phosphor type for all of the CCTs in the LUX product portfolio. The LERs of 90 and 95 CRI KSF-based LEDs are similar to 70 and 80 CRI LEDs while the LERs of nitride-based LEDs are significantly lower at 90 and 95 CRI.
Figure 5. Boxplot of LER vs. CRI for KSF and nitride-based white LEDs. The 90 CRI KSF version is the sweet spot for efficacy parity with 80 CRI nitride-based LEDs.
It can sometimes be difficult to do an apples-to-apples product comparison of the various KSF Phosphor LEDs due to the rapid emergence of many new products on the market. However, not all KSF COB LEDs are the same. An important quality consideration to keep in mind is Phosphor quality. There are multiple phosphor-based products offered by multiple manufacturers, using red phosphor that can vary in quality. Luminus uses only high-quality Current Lighting Solutions TriGain phosphor.
NOTES:
* Luminous Efficacy of Radiation (LER) refers to the number of lumens produced by an LED per optical watt it generates (the ratio of its luminous flux to its optical radiation power). This value remains constant for each specific spectral power distribution (SPD) used as an input. When the same SPD shape is applied at different optical power levels, the LER remains unchanged. LER is quantified in units of lm/optical watt to convert between lumens and optical power output for an LED. It's important to note that LER only changes if there is an alteration in the SPD shape.
For more information about KSF Phosphors and Luminus’ LUX Series COB LEDs with KSF phosphor technology, read the white paper: LUX Series COBs: Achieving High CRI Lighting with High Efficacy Using Luminus LUX Technology.
REFERENCES:
[1] Osborne, R., Cherepy, N., et al. “New Red Phosphor Ceramic K2SiF6:Mn4+” Optical Materials, Volume 107, September 2020, 110140. DOI: 10.1016/j.optmat.2020.110140
[2] Hendy, I, Murphy, J., and Setlur, A., “Latest Advances in Narrow-Band Phosphors and their Role in Color Management.” Information Display, May/June 2023.
[3] Murphy, J., Garcia-Santamaria, F., et al. “PFS, K2SiF6:Mn4+: The Red-line Emitting LED Phosphor behind GE’s TriGain Technology™ Platform. Presented at SID Display Week 2015, San Jose, CA.
[4] Sijbom, H., Joos, J., et al. “Luminescent behavior of the K2SiF6:Mn4+ Red Phosphor at High Fluxes and at the microscopic level.” The Electrochemical Society, ESC Journal of Solid State Science and Technology. 5(1)R3040-R3048 (2016).
[5] Amachraa, M., Wang, Z., et al., “Predicting Thermal Quenching in Inorganic Phosphors.” Chemistry of Materials, 2020. DOI: 10.1021/acs.chemmater.0c02231.
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