SPONSORED CONTENT: LED TESTING
Accurate LED light measurement
T
he measurement of LED (light emitting diode) presents unique application
requirements. They can be measured in a wide variety of colours and brightnesses. Accurate measurement of the LEDs, therefore, is essential. This can be done in two ways: photometry and radiometry.
Photometry relates to visible
radiation alone, just as the response of the human eye. Radiometry goes beyond these limitations. In both photometry and radiometry, the LED can be characterised in emitted power or in intensity. 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 ratio of the flux, leaving the source and propagating in the element of solid angle containing the given direction, expressed in candelas. To guarantee the right
measurement of LED, it is therefore important to make use of intensity calibrated systems. The systems used for this are often based on a spectrometer, in combination with a fibre and integrating sphere as a measurement head. It is important to have the knowledge of how to interpret the intensity information provided by the spectrometer.
Spectroscopy – Raw data versus Intensity calibrated data The system described analyses light received by the measurement
head. As light travels through the system, all optical properties of the components inside (fibre, grating, detector, mirrors and so on) affect the raw data signal coming out of a spectrometer. So when looking at the light coming from a tungsten halogen lamp, the output of a spectrometer is not the spectrum one finds in literature, but a distorted signal such as depicted in figure 1. So, comparing the intensity (height) of the signals at 620 and 1,000nm one would, based on the raw data, state that the signal is lower at 1,000nm. However, in
“It is important to have the knowledge of how to interpret the intensity information provided by the spectrometer”
reality the signal is about twice as high. To compensate for this, the spectrometer system needs to be calibrated for intensity. With an absolute intensity,
the needed values are not only determined by converting the spectrum comparing it only to the shape of a lamp, but one also needs to determine how much power is coming from this lamp. This is done by calibrating the system against a NIST calibrated light source, as seen in figure 2. The intensity calibration contains the data transfer function for each pixel. The data transfer function is used to convert the raw
data (A/D counts) into irradiance data (in µWatt/cm2). To be able to calculate the transfer function, a calibrated light source, with known output (in µWatt/cm2/nm) is needed. This calibration is for the complete measurement setup; the spectrometer, including the fibre and sphere/cosine corrector. From the irradiance spectrum (in µWatt/cm2), a lot of light output
parameters can be calculated. For a better understanding
of this and differences between radiometry and photometry, as well as a more detailed explanation on several often-used parameters, download the white paper Intensity calibrations in spectroscopy and radiometry available at www.
electrooptics.com/white-papers.EO
www.avantes.com
Figure 2: Calibrated system with NIST calibrated light source
Figure 1: On the left a typical Halogen spectrum, on the right the Raw data signal from the spectrometer
www.electrooptics.com | @electrooptics March 2021 Electro Optics 15
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