Photometry of Light-Emitting Diodes (LEDs)

Introduction

Light-emitting diodes (LEDs) are rushing into everyday life. Robustness, long lifetime and low energy consumption as compared to conventional light sources are making them the hot topic of the day. Like any other sources of visible light, also LEDs are subject to photometry which describes the optical radiation seen by a human eye. In contrast to incandescent lamps, LEDs are much more difficult devices to characterize: they have narrow spectral and spatial distributions, not to mention the intensity dependence on operating current and wavelength shift along with the junction temperature.

Ongoing Research

International Commission on Illumination (CIE) is working to create improved international standard procedures for single LED measurements. Metrology Research Institute is developing methods for LED measurements as well. Currently, we have constructed measurement setups for LED luminous flux, averaged LED intensity and spectral measurements. The switched mode operation of LEDs is also being studied, as LEDs are often operated using pulse width modulation (PWM). This allows more flexible dimming of the LEDs compared to the traditional DC-operation. A special LED current supply / temperature controller has been recently built. The device can be used for calibration and test-measurements of both low- and high-power LEDs.

The research of single LED-components has been recently extended to E27-base retrofit LED lamps, which are becoming more popular among CFL-based energy-saving lamps. The ongoing research is mainly focused on the luminous efficacy (lm/W) of the lamps. For this, both the luminous flux and the electrical power consumption need to be measured. Metrology Research Institute is a funded partner in the European Metrology Research Programme, EMRP Metrology for Solid State Lighting.

Contact person

Tuomas Poikonen
tuomas.poikonen(at)tkk.fi

Project pictures

Figure 1. Preparation for luminous flux measurement of E27-base LED lamp.

Figure 2. Ageing of various LED lamps for test measurements.

Figure 3. A blue low-power LED in a total flux measurement.
A 30-cm integrating sphere is open for photographing.

Figure 4. Modified LED-holder and baffle design of our LED integrating sphere. A white low-power LED is attached for total flux measurement.

Figure 5. A white standard LED in its holder. Temperature
is controlled by NTC-thermistor and Peltier element.

Figure 6. A temperature-stabilized photometer used for measuring
LED components. The area of the aperture is 100 mm².

Figure 7. Standard LED aligned for averaged LED intensity measurement.

Figure 8. The angular distribution of LEDs is measured using a turntable.

Related material

Back to Optical Radiation Measurements