Light-emitting Diodes (LEDs) – the Dominating Lighting Technology for the Future

The light-emitting diode (LED) is a semiconductor device, which converts electric current into visible radiation, i.e. visible light. LEDs were discovered by accident in 1907, but no practical use was made of the discovery for several decades. Now they can be encountered virtually everywhere. LEDs are used in applications from standard indicators on stereo equipment and laptops to traffic lights and automotive lighting (Image: Wikipedia). The production and usage of LEDs has enjoyed an explosive growth over the last several years with no end in sight. In comparison with compact fluorescent and incandescent bulbs, LEDs are non-toxic, have higher damage resistance, lower energy consumption, longer lifetime and smaller size.  It is speculated that by the year 2025, LEDs will become the most widespread light source in apartment houses and offices.

As LEDs have become more sophisticated, the need for accurate measurement of their optical properties has increased. Until recently, LEDs were primarily used as indicator lights and their main characteristic was luminous intensity (expressed in millicandela). However, due to the increasing demand for LEDs as a replacement to incandescent in the general illumination market, the Lumen is now often used as a unit of measurement for light output. The measurement of the total luminous flux of light sources is generally done by one of two methods: goniophotometer or integrating sphere. The measurement by a goniophotometer may be more accurate but more time-consuming in operation than the integrating sphere method. As a result, the integrating sphere method is preferred in many industrial measurements due to its ease of operation and quick response.

It is clear that the measurement of light intensity is usually significantly less accurate compared to electrical characteristics such as voltage, current or resistance. There are many factors such as color, device geometry, alignment of the LED into a test fixture, temperature etc. that cause uncertainty in the measurement results. Systematic errors are introduced when using traditional methods (developed for the incandescent bulbs) for measuring the total luminous flux of LEDs since an LED is quite different from traditional light sources in terms of physical size, flux level, radiation spectrum and spatial distribution.

Research is currently ongoing to develop and standardize reliable measurement methods of the optical properties of LEDs.