How to Test KSF Phosphor LED Products

Testing KSF products requires a different approach from typical (non-KSF Phosphor) LED tests to get reliable results. Different settings and high-quality equipment are needed. Primarily considerations are:

• Soak time. Slowing down the test so that the KSF phosphor has stabilized before capturing the spectrum. Soak time is the amount of time the power supply is on before spectrometer integration starts; Integration time is the length of time the spectrometer captures data from the LED.
• Measurement accuracy. Measurement accuracy can be challenging; thus, customers should take care to use higher resolution test equipment with pulse-width for true measurement accuracy of Luminus phosphor devices compared to standard phosphor devices.
• Measurement parameters. In some cases, phosphor parts are measured at lower drive currents, which enables test results to be reported that show higher efficacy. Similarly, manufacturers might quote 25°C performance data, showing performance at best case conditions. Data that is shown at 85°C conditions typically provide a more realistic performance indication. Some parts also exhibit a larger CCT-temperature shift. It’s critical to compare test parameters in combination with reported results to ensure an apples-to-apples comparison can be made and to ensure purchased parts do not deliver lower performance than expected.

Soak Time

KSF is a slow reacting phosphor and takes longer than most standard phosphors to reach a stabilized (saturated) state: >40 ms according to the manufacturer. When characterizing the performance of LEDs that contain KSF phosphors, the soak time before spectrometer data acquisition (integration time) needs to be adjusted. For traditional phosphor systems, a soak time based on electronics settling times is common (see Figure 1).

For KSF systems, the soak time needs to be increased to better reflect values that will be obtained in LM-79 luminaire tests.[1] KSF phosphors take longer than traditional LEDs to reach full intensity. Thus, if the soak time is too short, the LED is measured before the KSF phosphor is fully saturated. Figure 2 illustrates this effect. The red line shows too short of a soak time where the KSF phosphor has not developed full intensity (saturation). The blue line shows the saturated KSF measurement. The inset box lists the effects of undersaturated KSF on the spectrometer calculations.

Figure 1. During an LED integrating sphere test, the timing of the drive electronics and the spectrometer acquisition time is coordinated. Soak time is 5 ms in this example.

Figure 2 – Saturated KSF (blue with sufficient soak time) compared to unsaturated KSF (red with insufficient soak time) test results. If the soak time is too short, the errors that occur are as listed in the inset. Insufficient soak times to allow the KSF intensity to fully develop leads to undermeasuring the steady state red spectral content.

Spectrometer Resolution

To achieve the accuracy required for KSF LED testing, Luminus recommends using a spectrometer with a minimum 1-nm resolution to resolve the narrow spectral peaks in KSF phosphor spectra. A lower spectrometer resolution has limited ability to pick out some of the spiky peaks and valleys shown around the main red spectral peaks. This will affect the accuracy of the intensity reading.

Figure 3 simulates the spectrum of 3000K, CRI90 Lux COB when the spectral resolutions are 1 nm and 5 nm respectively. The left curve represents the original data from the integrating sphere with 1-nm resolution. The right curve shows the spectrum sampled in every 5-nm step. One can see that the 5-nm step sampling data does not distinguish the fine peak structure. Figure 4 shows the TM-30-18 differences that occur when the spectrometer resolution is too low.

Figure 3. Comparison of 1-nm resolution (left) and 5-nm spectrometer resolution (right) of the peaks in KSF phosphors.

Figure 4. Comparison of TM-30-18 metrics for the same LED using 1-nm and 20-nm spectrometer resolution. The differences in this extreme case are significant. If this comparison was done with smooth phosphors, the differences would be less and mainly due to inaccuracy in the fairly narrow blue-chip spectra.

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]         Ghaffarzadeh, K., “Phosphors or QDs for color conversion in LCD and microLED?” Wevolver, August 16, 2022.

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