LED Binning in Single Pulse Mode with BTS256-LED Tester

LEDs are considered to be the light technology of the future due to their low energy consumption and long lifetime as compared to traditional light sources. As LED luminous intensity levels increase the range of applications for LEDs expands as well. Many LED applications require that optical properties like luminous color and intensity be clearly defined. But in mass production of single LEDs small differences in these properties are inherent in the manufacturing process. As a result after fabrication all LEDs are classified according to light and color properties. This classification is called “binning” (similar diodes going into the same bin class). The binning of white LEDs is mainly done according to luminous flux, color (CIE 1931) and color temperature. Color temperature bins are defined by xy coordinates on the CIE 1931 Chromaticity diagram. This bins are grouped as quadrants around the standard chromaticity lines for a specific color temperature. As smaller the quadrants as tighter are the tolerance limits. In single pulse mode binning this measured data is taken during a few millisecond long light pulse at an specified operating current.

This technical note serves three purposes:

• Explain the measurement set-up and use of the BTS256 LED Tester and LED Power Supply LPS20 for short duration 20ms pulse LED binning
• Investigate what effects light pulse length have on binning parameters. In addition binning data of the same LED with and without heat sink are compared
• Examine effects of integrating sphere absorption on the measurement results.

For the measurement itself almost standard measurement devices manufactured by Gigahertz-Optik has been in use. Because of that this technical note does also offer handling and operation instructions about Gigahertz-Optiks LED tester. As test sample a Diamond Dragon white LED has been taken offering a wide range in operation current.

The following instrumentation was used:

BTS256-LED Tester
The BTS256-LED Tester is a compact measurement device designed for high accuracy measurement of luminous flux, spectral and color data of single assembled and unassembled LEDs in the visible spectrum. The BTS256’s bi-technology light sensor offers a fine photometric response photodiode for accurate wide dynamic range flux detection. Beside the integral detector a compact low stray light spectrometer is installed for spectral color measurements. The photodiode of the Bi-Tec sensor offers a fast data logger mode with 1ms sampling rate which can be used for pulse form profiling measurements. To operate the BTS256-LED a laptop or PC with USB interface is needed. The BTS256-LED is powered through its USB adapter when connected to PC.

LPS-20-1500 LED Power Supply
The LPS-20 is a microprocessor-based current and voltage source especially designed for the operation of LEDs and other semiconductor light sources requiring low noise. The LPS20-1500 operates up to 1500mA with 30μA resolution and up to 24V with 0.5mV resolution. It is set-up for full remote control operation in CW or single pulse operation mode. Either current or voltage can be measured with high 16bit ADC resolution. Its I/O interface supports trigger-in and trigger-out operation in flash mode.

Experiment Setup:

   

Verification of Thermal Stability:

First measurements were taken to help gauge any thermal effects on test parameters during the 20ms pulse. For this investigation the LED tester and LPS-20 were adjusted so that a few millisecond before the flash, the LED-tester starts to measure and collects data with its photodiode over the whole pulse width. This measurement was of the Diamond Dragon operated with heat sink at currents of 200mA, 350mA, 600mA, 1000mA and 1400mA. During the pulse the photodiode and the data logger collects data every 1ms. Picture 4 shows that there are no thermal effects visible at the end of the pulse (degradation of flux).

The differences at the beginning and the end of the pulse are caused by the measurement device electronics. The LED tester operates in a different gain range with different rise times for the two lowest current pulses (200mA and 350mA curves overlap) as that at the higher currents (600mA, 1A and 1.4A curves also overlap).

For comparison, LED operation times were lengthened and results were analyzed. Logically the LED temperature will rise during longer operation times and changes in luminous flux, luminous color and temperature should develop. While the test device LED were operated with a constant current of 350mA, luminous flux and color were measured over a time interval of 60 seconds. This measurement was performed twice, with and without heat sink. Results are shown in Pictures 5 and 6. As the LED heats up luminous flux and color characteristics are affected. The effects are noticeably greater without the heat sink attached. Over sixty seconds only 0.6% of initial luminous flux is lost for the LED with heat sink. The color temperature changes by 30K. The LED without heat sink luminous flux drops to 93% of its initial value while color temperature increases 310K. To sum up, thermal effects were shown to cause a ten times difference in flux and color temperature for the test LED with and without heat sink.

       

Verification of the Test Sample Bin-classes:

According to the Diamond Dragon data sheet (LUW W5AP-MYNY-4C8E) at an operating current of 1400mA, typical color temperature should be about 6500K, be located in one of the brightness groups from MY to NY (meaning luminous flux between 210 and 390 lm). Also it should fall into one of the xy Chromaticity coordinate bin classes from 4C to 8E (Picture 8). According to the data sheet the availability of the LEDs is only with binning for brightness and chromaticity group, not for single bins.

For the binning measurements the BTS256-LED tester and LPS-20 LED power supply was set for single pulse of 20ms at an operation current of 1400mA to see if the same binning data resulted and, for this case, how precise information can be supplied.
BTS256-LED tester and the LPS20 were adjusted so that 10ms before the 20ms long flash the spectrometer starts measuring and stops 10ms after the flash. The photodiode collects data every 1ms starting and finishing 3ms before and after the flash.      

Measurement Results:

Colour temperature: 6570 K
Luminous flux: 250 lm
Chromaticity coordinate x: 0.312
Chromaticity coordinate y: 0.323

First the results show that all of the measurements values fell into same the same bin groups as stated in the data sheet. Secondly the higher resolution single bin-class can be specified. So the measured LED is part of brightness group MZ (240 lm -280 lm) and it belongs to Chromaticity coordinate group 5D (Picture 10).

Next question is what impact does verifying current on the binning results have. To find an answer testing of the LED operated at 200mA, 350mA, 600mA and 1000mA is repeated. Looking at Pictures 8 & 9, you can see that not only luminous flux changes with current, but also color temperature and Chromaticity coordinates. Subsequently the Chromaticity coordinate of the LED changes too. As shown in Picture 10, at 1400mA the
LED is located in 5D, but the same LED operated at 1000mA and 600mA
falls into 6D whereas operated at 350mA and 200mA it is 6E.

         

Spectrometer Measurement Plausibility Check:

Different to photodiodes the sensitivity of diode array sensors is controlled by their integration time. Short integration time is for low sensitivity were as long integration time is for high sensitivity. In short duration binning application the integration time and therefore sensitivity is set to a fix value. The measureable flux range is therefore limited by the dynamic range of the diode array. To avoid measurement errors by to low signal to noise ratio or saturation of the pixel signal it is recommended to check the diode array raw signal in counts of the ADC output. A graphical measurement display in the software supplied with the LED tester displays the spectral flux in Counts. In our application the raw data at 46 lm (200mA) and 250 lm (1400mA) are 475 counts and 2850 counts at the wavelength of peak signal. On the assumption that the signal to noise ration of the ADC signal should be at minimum about 100 : 1 the measurement range of the LED tester with the white LED measured is about 10 lm to 300 lm. For higher flux levels the measurement time of 20 ms needs to be shorten. For lower intensity the measurement time would need to be longer. For longer measurement time the temperature influence must be checked as described in this article.

LED-Binning with large diameter Integrating Sphere:

The LED tester can be combine with integrating spheres up to one meter in diameter. Because of that the influence of the integrating sphere to the measurement values is examined. For that the BTS256-LED tester is plug by its bayonet adapter to the ISD-21-V01 integrating sphere from Gigahertz-Optik. That sphere offers a diameter of 210mm and a measurement port with 50mm diameter which can be increased to 75mm or reduced by optional port reducers. The detector port is baffled to the measurement port to avoid direct illumination of the measurement device by the source measured. The sphere is set-up with an auxiliary lamp to measure and compensate the self absorption of the test sample.

The luminous flux and spectral flux of the test LED is measured with 200mA, 350mA, 600mA, 1000mA and 1400mA operation current.
The measured signal in counts related to the luminous flux

confirm an attenuation of the sphere by factor 53. With that additional attenuation the measurement signal of the LED tester becomes less than 100 counts for all measured intensities and therefore less than the recommended min. signal to noise ratio of 100:1 counts. Single LEDs with maximum luminous flux of 500 lm should therefore be measured with the BTS256-LED tester only. Higher power LEDs can be measured with the large size integrating sphere as well as large size high power LED matrix or LED luminaires. In case of lower flux than 500 lm longer integration time of the diode array spectrometer are recommended. For the operation with longer pulse length the influence of the LED operation temperature must be evaluated as described in this article.

The measurement itself, the measurement devices set-up and measurement documentation was done with the software S-BTS256-LED supplied with the BTS256-LED tester. The graphical user interface can individual be configured with numerical data and graphic displays available by the operator. The configuration example shows a desktop with CIE-1936 x,y-color coordinate diagram, numerical data of the measured flux and the LED spectrum. The software development kit S-SDK-BTS256-LED is optional offered. It includes DLLs for C and C++ programming as well as Labview VI.