High-Speed Data-Logger Measurement of Luminous Flux and Luminous Color of Flash Operated Single LEDs

LEDs for pulsed light applications are operated in high power mode with short pulse widths and long duty cycle for low average power loss. A very effective heat sink structure on the LED is required to limit power loss and color changes as a result of any increase in junction temperature during the flash. This technical note describes a light meter with data-logger function manufactured by Gigahertz-Optik GmbH to measure the luminous flux and luminous color stability during the flash.

Luminous flux, measured in lumens, is the most common photometric light measurement quantity used to specify the light source effectiveness by comparison of the electrical power to the total flux. The color temperature in degrees Kelvin specifies the luminous color of the light source. LED electrical power is specified by the product of the forward voltage and the operating current. In single flash operated LEDs the measurement of these parameters must be done at a specified time within the flash. In binning applications this may be the point of peak intensity. In flash applications where constant flux and constant luminous color during the flash event are important and must be analyzed pulse shape data is required. State of the art light meters are able to measure and store the luminous intensity and luminous color data within a few milliseconds. For repetitive pulses the chain of measured data represents the time dependent luminous flux and luminous color profile defined as pulse shape. Using this data the ratio between the readings at the beginning and end of the pulse can be calculated and compared to the specified value.

The main requirements for a LED pulse shape light measurement instrument is a power supply capable of driving electrical pulses with a specified pulse length and a light meter for luminous flux and luminous color with short measurement processing time and data logger function with a fast sampling rate. To meet these requirements Gigahertz-Optik offers the LPS-20-1500-RM Power Supply with the P-9801-V3 Optometer, CT-4501-1 Light & Color Detector and UMBB-150 Integrating Sphere:

LPS-20-1500-RM Power Supply featuring:
During qualification of LEDs constant current is commonly the mode of operation. Because of the very wide dynamic range operating current is controllable with 16bit D/A converter resolution to ensure precise adjustable low and high current settings. The very fast rise time of LEDs requires a very short switch-on time from zero to the set current value. Also the switch-on characteristic must be free of overshoot to avoid damaging the device under test (DUT). The test LED’s forward voltage is measureable with a 16bit A/D converter for precise electrical power calculation. Pulse width is controllable in the range from less than 20ms for typical LED binning up to a few hundred milliseconds for LED flash light applications.





P-9801-V3 Light Meter model with CT-4501-1 Light Detector and UMBB-150 Integrating Sphere: To measure the luminous flux of LEDs with almost hemispherical light emitting characteristic the light output of the LED must be measured with an Integrating Sphere. Integrating Spheres act as a light integrator almost independent of the spatial light distribution of the light source under test. The integrating sphere with its associated light detector can be calibrated for luminous flux and luminous color measurements in absolute quantities traceable to national and international standards. The luminous flux and color light detector itself is a RGB type detector with three (red, green and blue) filtered photodiodes. The reason for employing a detector head with filter corrected photodiodes (integral light detector) instead of one like a diode array spectrometer with spectral measurement capability normally recommended for LED measurements is the short response time required for data-logging measurements with sampling rates in the millisecond range. Only filtered photodiode based light meters like the P-9801-V03 ensure fast enough response time with wide enough dynamic measurement range for low pulsed measurement signal levels as a result of the attenuation of the integrating sphere. Using ’gold standard’ reference LEDs is one way to reduce the added measurement uncertainty of the RGB type light detector. Built-in detector electronics consist of one analog signal amplifier with short rise time, current to voltage converter and ADC for each RGB detector. Each detector stores its readings as digital signals locally to inhibit any time loss for interface communication. During memory read-out the stored signal data is corrected by calibration factors for absolute readings and the color temperature is calculated. Instrument sensitivity enables the measurement of flashed white LEDs with a peak intensity of minimum 10lm.

S-P9801 Software: All device set-up parameters, measurements and documentation are implemented with the S-P9801 software supplied. The software enables the user to select and display specific measurement criteria and screen set-up tailored to his individual requirements. Data storage in ASCII and Excel format is supported. The extended S-P9801 with S-SDK-P9801 software development kit with DLL for C and C++ programming and Labview Vi is available for those customers interested in integrating the P-9801-V01 light meter into their own software.

Measurement Routine: For measurement the test LED (DUT) must be connected to the power supply and placed at the measurement port of the integrating sphere detector so that all light emitted by the LED enters the integrating sphere. LED measurement adapters are recommended for mounting and connection of the test LED (typically supplied by the end-user) onto the sphere port frame. This adapter also acts as a light-tight cover to prohibit ambient light from entering the integrating sphere. The front of the adapter oriented to the sphere port should be coated flat black. The measurement is initiated by the software with a start command via the RS232 or IEEE488 interface of the P-9801optometer. The power supply is triggered by an electronic signal from the optometer to ensure precise synchronization of the light flash with the light meter in data logger mode. Measured data stored in optometer memory are readout via its interface after the completion of the measurement.