Test Materials in LEDs

If ever there was doubt that the LED is the dominant technology in lighting, just look around to realize otherwise. They are virtually everywhere. From standard indicators on stereo equipment, laptops and toys, we can see more and more often at traffic lights, variable message signs and lighting for cars. LEDs have enjoyed explosive growth in recent years with no end in sight this increase.

Much of this success is due to the brightness levels increasing achieved with new materials and manufacturing processes of panels, and the arrival of the blue and white LEDs for RGB (full color) and general lighting applications . As the sophistication level rises to the use of LED, so does the need for accurate measurements of the optical properties of LEDs.

The details of optical measurement are divided into four distinct but related issues together. These are: Photometric quantities, Radiometric quantities, quantities of wavelength or chromaticity and, finally, the goniometric or angular quantities.

The photometric measurement of LEDs can be more art than exact science. There are several issues of geometry, electrical and assembly that can affect the optical properties of LEDs. Because no two LEDs are exactly alike, it is essential to increase the measurement accuracy. . These include but are not limited to:

- Know the LED optical emission from the center compared to the mechanical center. By placing a fixture LED typical tensile test, usually placed in such a way that collects the light emanating from the center of the mechanics of the device. The optical center often deviates five degrees or more from the center of the mechanics of the LED. While this may not be a major problem in measuring devices with a wide viewing angle, the above forty degrees or more, for narrow angle devices, differences in the readings can be considerable. (Note that the CIE (Commission Internationale de l'Eclairage) recommendation is to use the mechanical axis of the LED in place of the reference optical axis of measurement)

- Performance measurement in a given time interval or when stabilized finally. When the LED receives the first current, the junction temperature increases due to the electrical power consumed. (The junction temperature of LEDs is determined by Tj = Ta + (Vf * If) * Rth (ja)), and may take several seconds to several minutes before the light output has reached thermal equilibrium and a stable value. A decrease in the production of 5-20% or more is not uncommon. It is often not practical to wait long periods of time between the completion of the tests in LED, so establishing the standard of an interval of five seconds as an acceptable limit in traction stabilization.

- Ensure that the ambient temperature is constant during the test. LEDs typically change in brightness and color temperature. As temperature increases, decreases production and color changes towards the upper end of the spectrum.

- Always use a constant current source voltage (Vf).

- Use an easily reproducible test setup.