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 candelas.

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 a Melles Griot 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 13 HLS 105 calibrated halogen light source to use with 13 SIP 102 integrating sphere. With the irradiance application add-on software (13 FSS 103) 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. Further, the following output parameters are displayed in a separate window: radiometric quantities µWatt/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, especially useful for LED measurements, such as: Dominant Wavelength, Purity, Central Wavelength, Peak Wavelength, Centroid, etc.

A typical setup for LED measurements is given below:

Components used in the LED measurement setup:

Irradiance Application Add-on Software (13 FSS 103):

Radiated optical energy, as measured by Melles Griot spectrometers, can be quantified as a radiant flux, a measure in energy per second (Watts) radiated from a source. The radiated optical energy can be correlated with human vision (photometry) as defined in the CIE to obtain a spectral luminous efficiency function to characterize the vision of an average human observer.

Both radiometric and photometric quantities can be measured with an irradiance calibrated Melles Griot spectrometer system. Radiometric quantities are radiant energy (in Joule), Radiant power or flux (in Watt) or irradiance (Watt per cm²). Related photometric quantities are luminous flux (lumen) or illuminance (lux or lumen per m²).

With the Irradiance Application add-on software (13 FSS 103) it is possible to calculate the above parameters from the measured spectral distribution. A calibrated light source 13 HLS 105 or 13 HLS 205 with known energy output (in µWatt/cm²/nm) is used as a reference. This calibration can be performed, saved and loaded by the end user.

Another option is to have your spectrometer system calibrated in our irradiance calibration lab, so there is no need to have an additional calibrated light source. The calibration can be loaded into irradiance add-on software 13 FSS 103.

Color of light parameters can be expressed by the chromaticity coordinated x, y and z. These chromaticity coordinates are obtained by taking the ratios of the tristimulus values (X, Y and Z) to their sum. The tristimulus values Time Measurement mode, up to 8 functions can be displayed simultaneously against time. For each function, a different radiometric, photometric, photon or color coordinate output parameter and/or wavelength range may be selected, as well as a different spectrometer channels X, Y and Z and the spectral irradiance are computed in a wavelength range from 380 nm to 780 nm, using a 1 nm interval. These parameters, as well as u and v coordinates, and the color temperature of an external light source can be calculated and displayed in real-time.

The same experimental set up (spectrometer with fiber optics and cosine corrector or integrated sphere) is used to calculate the intensity of the light to be measured.

The calculated output can be displayed and saved in two ways:

1. In the Irradiance Chart the data can be displayed as spectral irradiance in µWatt/nm versus wavelength, like in the screen dump above. Further, the following output parameters can be displayed: radiometric quantities µWatt/cm², µJoule/cm², µWatt or µJoule, photometric quantities Lux or Lumen, color coordinates X, Y, Z, x, y, z, u, v and color temperature, and number of photons µMol/s/m², µMol/m², µMol/s and µMol. 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, Centroid, etc.
2. In Time Measurement mode, up to 8 functions can be displayed simultaneously against time. For each function, a different radiometric, photometric, photon or color coordinate output parameter and/or wavelength range may be selected, as well as a different spectrometer channel.