In professional industries, such as the automotive or electronics industries, there are no detailed recommendations or instructions on how to measure the parameters of high power LEDs. Therefore, the recommendations of general lighting standards are applied and industry-specific measurement methods are developed.
Maintaining the right temperature
Technical datasheets for light emitting diodes made available by manufacturers contain photometric data of the flux [lm] or luminous intensity [cd] and/or colorimetric coordinates x, y. They are determined for a LED junction temperature equal to the ambient temperature, usually 25°C. In order to verify the diodes’ parameters against the specifications after installation, it is necessary to measure the entire lighting system while maintaining the junction provided by the manufacturer in the specification. This is done to ensure adequate cooling, e.g. with a TEC mount unit (Thermo Electrical Cooling e.g. Peltier cell). This helps to improve the repeatability of the measurements and allows for the measurement of the performance of the LED module at a temperature close to that of the source. We need to remember though that when different types of diodes on the module are used it is impossible to obtain the same temperature for all junctions on one PCB due to different thermal resistances.
Single pulse or “train” methods
An alternative method is a measurement made with a short current pulse of the LED with a value and time of 20-30 ms, at which the parameters of the diode are specified in the technical specification (for LED production). The aim here is to measure the optical parameters even before the junction heats up. LED manufacturers also use impulse measurement during the “binning”, which ensures efficiency.
Another version is the so-called Train method, i.e. continuous supply of current pulses at a frequency of 100 Hz and pulse width below 0.5 ms. Such a solution makes it possible to record a dozen or so cycle periods at larger measurement times (helpful in the measurement of LED sources with high flux values) and to convert the obtained value by a coefficient corresponding to the curve. With properly selected parameters of pulsation the result is identical with the value obtained by measuring a single 30 ms long pulse.
Managing challenging measurement tasks
Since modern LED and OLED measurement includes optical measurements along with temperature measurement and stabilized, monitored power parameters, the test and measurements becomes a difficult task. In order to support engineers the GL Spectrosoft Automation can be used . It is a purpose built software to control these complex testing procedures, and communicate with different instruments at the same time providing the operator with unified interface and report interface.
Owing to the software test and quality teams can write specific procedure scripts and connect all devices to building a custom measurement setup and selecting the logical sequence from the list of available commands to the Automation editor window. These scripts may include setting initial parameters of power supply to your lamp, measurement parameters of your spectrometer and readings from a separated power meter for instance. For performance tests and measurements, you can include different current levels and check optical output of your lighting products under different conditions. The software will help you to build the measurement sequence and store all data in a user-friendly customized reports.