Perspectives on Measurement of LED Illumination Sources

Chris MacLellan, Optronic Laboratories Inc.

The development of optical measurement instrumentation for LEDs has followed the requirements of the LED component manufacturers. More recent changes include the use of high-speed spectroradiometers for online component or wafer testing, the adoption of new measurement geometries, the measurement of high-brightness devices and the use of pulsed power supplies. And with the development of UV devices, the spectral range of the LED spectroradiometer has been extended to cover from 250 to 780 nm or from 380 to 1100 nm for near-IR applications.

Following the introduction of white and high-power LEDs, many companies are developing products that aim to replace traditional illumination sources. The performance specification and the high cost of an LED illumination source restrict its application to specialized areas such as displays, traffic signals, avionics, medical, architectural lighting and machine vision markets. However, with technical advances in, for example, thermal management, the LED source is being tested in new markets.

The test requirements of the larger diode illumination sources are similar to those of an incandescent or fluorescent lamp rather than to those of an LED component. As a result, the instrumentation manufacturers use equipment originally developed for the lighting industry, which includes large total flux integrating spheres, intensity stages and goniophotometers. The use of conventional lamp test equipment does differ, however, in a number of areas. The application below describes the technical requirements for adapting a total flux photometer into an LED total flux spectroradiometer.

The number of applications for LED arrays is growing. This multi-LED source is placed on a cooled mount in a large integrating sphere.

Consider the following: The same source-size-to-sphere-diameter condition exists for LED sources, yet many of them emit into a solid angle of substantially less than 2π steradians and can therefore be placed on an entrance port on the wall of the sphere instead of at its center. This can facilitate more rapid placement of a test source and reduce the time of test cycles. However, because of the spatial differences between the calibration standard — which usually extends over a full 4π steradians — and the LED test source, the sphere must have excellent spatial uniformity. Designers provide this capability by using small baffles and a cosine-corrected detector port, while also ensuring that the LED source is directed at a uniformly reflecting wall on the sphere.

Currently, the sources can command a premium price, but then customers demand high quality. Unfortunately, LEDs suffer from significant production spread in their optical and electrical parameters, often resulting in a requirement for 100 percent testing.

Integrating spheres are easy to use and facilitate reliable and repeatable measurements. However, the insertion of the test sample at the center of the sphere or on a sphere port can delay test times. The use of plug-in holders will reduce the time of test cycles and minimize the risk of damaging the internal coating of the integrating sphere. Fully automated software controlling both the spectroradiometer and LED power supply will also reduce cycle times and improve measurement accuracy.

For measurements such as correlated color temperature, chromaticity and color rendering index, a photometer supplied with a total luminous flux system must be replaced with a high-performance spectroradiometer. Key specifications for this device include signal linearity over a large dynamic range, temperature stability, wavelength accuracy, low stray-light performance, sensitivity and measurement speed. International standards for total luminous flux measurements require the use of an auxiliary lamp in addition to the calibration lamp standard. The same techniques would apply to the measurement of LED sources; however, the spectroradiometer’s application software also should include the calibration routines to allow the system response calibrations using the standard and auxiliary lamps.

Ultimately, up to 95 percent of the electrical power to an LED appears as heat. Also, the optical and electrical parameters of an LED are dependent on its junction temperature. Thus, the design of test mounts for large high-power diodes should include sufficient heat sinking to ensure that the junction temperature remains within operating parameters or at a precisely known temperature. However, it can take several minutes for a test source to reach thermal equilibrium.

Operating the LED at a low duty cycle by pulsing it helps maintain the junction at a near-ambient or lower temperature level and reduces the need for lengthy warm-up periods and test cycle times. The forward voltage across the diode can be used to indicate the junction temperature during test. Algorithms can then be applied to adjust the low-temperature test measurement data to normal operating condition values.

With new LED applications appearing on a continuous basis, careful consideration must be applied to adapting standards and measurement instrumentation to provide accurate data in a format that is useful to the manufacturer and the end user alike.

Meet the author

Chris MacLellan is an applications engineer with Optronic Laboratories Inc. in Orlando, Fla.; e-mail: cmaclellan@olinet.com.