Optical measurements

For optical characterisation and evaluation, we offer the following services:

  • Determination of absolute photoluminescence quantum efficiency
  • Measurement of photoluminescence excitation and emission spectra, (temperature and/or angular resolved)
  • Reflection, transmission and scattering measurements (UV-Vis-IR)
  • X-ray diffraction: crystal structure, crystallinity, crystal orientation and strain
  • Calorimetric methods for determination of specific heat capacity as well as glass transition and crystallization temperatures
  • Evaluation of phosphor stability at high temperatures and high excitation powers

Optical materials

Optical systems can be very complex. The properties of each individual component in interaction are of decisive importance. The optical material properties for LEDs determine not only the luminous efficacy but also the heat in the system. In addition to optical functions, coatings such as highly reflective or anti-reflective layers can also be designed for electrical conductivity (TCO layers) or as a heat protection layer. Furthermore, the surface properties are of great significance for reducing glare and mirror effects or for targeted light distribution.

Optical measurements and simulation

Samples are investigated for their transmittance and reflectance properties with a double-beam spectrophotometer. An integrating sphere allows to measure and distinguish between the total transmittance / reflectance so that the scattering properties of the samples can be investigated in detail.

On the basis of transmittance and reflectance measurement data, optical simulation can be used to determine the optical constants and also the thickness of the semitransparent layers. With the known optical constants, the optical spectra for a multilayer system can be determined.

© Fraunhofer AWZ Soest

Light microscopy image of a structured glass and diagram of the total and diffuse transmittance of structured glasses.

© Fraunhofer AWZ Soest

Scheme of a multilayer system for the simulation as well as diagram of the calculated transmittance and reflectance of a Bragg mirror.

Phosphors

Fluorescent materials are not only a vital component of various lighting solutions such as fluorescent lamps and LEDs, but can also be found in many other day-to-day applications. As a security feature in banknotes, they have become indispensable, and in art they enable new aesthetic approaches. Recently, they have also been used as fluorescent labels in biology or tumour therapy.

The knowledge of the optical properties is essential for the application of phosphors. The interaction of the right colours in lighting is essential for people's well-being and performance. For various technological applications, the quantum efficiency of a phosphor is crucial in addition to the corresponding colour impression.

Photoluminescence and quantum efficiency

In photoluminescence spectroscopy, the phosphor is excited at a certain wavelength and the emitted light is detected. Thus, the excitation and emission spectra of photoluminescence can be recorded. Since the emission properties of a phosphor depend on its temperature and the emission is not always isotropic, measurements of luminescence are possible with both temperature and angle resolution. In addition, the radiant lifetime of luminescence processes can be determined in time-resolved measurements.

The quantum efficiency results from the quotient of the number of emitted photons to the number of absorbed photons. Quantum efficiency is measured in an integrating sphere and can be determined for solids, powders and liquids.

© Fraunhofer AWZ Soest

Various phosphors under ultraviolet excitation.

© Fraunhofer AWZ Soest

2D plot of emission intensity versus excitation wavelength for a double phosphor doped glass.

© Fraunhofer AWZ Soest

Spectrally-resolved quantum efficiency of different commercial phosphors: yellow (a), orange (b), and white (c) phosphor powder.