LED measurements

Since it is possible to manufacture Light Emitting Diodes in a wide variety of colors 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 characterized in emitted power or in the intensity. The emitted power is all the power (flux) emitted from the LED in lumens or watts,

collected and measured without regards 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 is 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 μWcm^-2nm^-1. Further, the following output parameters are displayed in a separate window: radiometric quantities μW/cm², μJoule/cm², μWatt or μJoule, photometric quantities Lux or Lumen, color coordinates X, Y, Z, x, y, z, u, v and color temperature. In addition raw data in Scope mode is displayed as well as the X-Y Chromaticity diagram, including parameters, specially useful for LED measurements, such as: Dominant Wavelength, Purity, Central Wavelength, Peak Wavelength, Centroïd, etc.

A typical setup for LED measurements is given below: