LED Goniometry – Angular Optical Characterization of LEDs

Table of Contents

Gonimetric or Angular Characterization


Light emitting diode (LED) is a PN junction semiconductor diode. When operated in a forward biased direction the diode emits a single color (monochromatic) light. An LED consists of a die or light emitting semiconductor material, and a lead frame for placing the die. An LED also consists of the encapsulation epoxy that surrounds the die to offer protection for the die.

LEDs are used virtually everywhere. They are used as standard indicators on toys, stereo equipment, laptops, and in automotive lighting, variable message signs and traffic lights. The application of LEDs has been steadily increasing and the trend seems to continue further.

This high growth is due to the high levels of brightness that have been made possible with new materials and wafer fabrication methods, as well as the invention of blue and white LEDs for RGB (full color) applications and general illuminations. Accurate measurements of the LEDs optical properties have to be made to match the rising sophistication levels for using LEDs. This article deals with LED goniometry, particularly with the angular optical characterization of LEDs.

Gonimetric or Angular Characterization

A goniometer measures the radiation pattern or spatial distribution of an LED (Figure 1). This can be realized by tilting the LED while the detector remains stationary or moving the detector around the LED. In both cases many output measurements are taken for each angle when rotations from 0° to 180° are performed. A pattern of the radiated beam in one plane is achieved upon completion of the measurements. As many LEDs are round it is usually thought that the radiation pattern will be symmetrical. This assumption is supported by the graphical representations of viewing angle supplied by many LED manufacturers.

a graphic to show the spatial distribution or radiation pattern of an LED

Figure 1. Spatial distribution or radiation pattern of an LED

This is generally not the case. The assembly and geometric variations that occur during LED manufacture largely influence its optical properties. It becomes necessary to perform an extra scan and observe several planes of view. Additionally, as some LEDs have an oval or elliptical shape, and have two radiation patterns (e.g. 30° x 70°), a 0° scan as well as a 90° scan of the device is required. If a goniometer is unavailable, a crude radiation pattern can be obtained utilizing a photodetector, with a manual rotation of the detector or LED. The output levels are recorded and the data points are plotted, all of which can be time-consuming and tedious.

Accuracy in the measurement of light cannot match the accuracy of measurements of certain electrical parameters, such as current, voltage or resistance. Light measurement errors are induced by several factors such as; device geometry, color, temperature, LED alignment in the test fixture etc. Accuracies better than ű2.5% are realizable, although the industry acceptable level is only ű5%.

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This information has been sourced, reviewed and adapted from materials provided by Marktech Optoelectronics.

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