LED Measurements

As it's possible to manufacture Light Emitting Diodes in a wide variety of colours and brightness, it is also necessary to accurately measure their optical characteristics.

There are two ways of measuring LED's: photometry and radiometry.

Photometry only relates to visible radiation, like the human eye response. Radiometry is not limited to the human eye response. In both photometry and radiometry, the LED can be characterised in emitted power or in intensity. The emitted power is all the power (flux) emitted from the LED in lumens or watts, collected and measured without regard to the direction of the flux. The intensity is the flux per unit solid angle directed toward the observer, usually along the axis of the LED and is given in candela's.

Because of their size, LED's are very difficult to mask and standards are not currently available defining how such measurements should be performed.

Careful consideration in the design and use of LED test and measurement equipment is essential to achieve valid measurement results that fit the application.

The simplest and quickest way to measure the total luminous flux from an LED is to use an integrating sphere, coupled to an Avantes spectrometer. The integrating sphere a simple device for measuring optical radiation. The interior surface of the integrating sphere is perfectly diffusing and has spatially uniform reflectance. The radiant exchange from diffuse surface to diffuse surface integrates the light, resulting in equal radiance at any point on the sphere wall. The system can be calibrated with a halogen light source AvaLight-HAL-CAL-ISP.

With AvaSoft-IRRAD spectrometer software, it is possible to calculate the parameters from the measured spectral distribution and to perform an absolute irradiance measurement. Also the intensity of the measured light can be calculated, displayed and saved as spectral irradiance in μWatt/cm²/nm. Furthermore, the following output parameters are displayed in a separate window: radiometric quantities μWatt/cm², μJoule/cm², μWatt or μJoule, photometric quantities Lux or Lumen, colour coordinates X, Y, Z, x, y, z, u, v and colour temperature. In addition, raw data in Scope mode is displayed as well as the X-Y Chromaticity diagram, including parameters especially useful for LED measurements such as Dominant Wavelength, Purity, Central Wavelength, Peak Wavelength, Centroïd, etc.

A typical setup for LED measurements is shown in the illustration above.

Typical components used in LED measurements