Novel LED-based radiation source and its application in diffuse reflectometry and polarization measurements

Abstract

An LED sphere radiator (LED-SR) was constructed to improve the accuracy in spectral radiance factor measurements performed with the robot-based gonioreflectometer at PTB. Its properties with respect to the spectral range and coverage, the temporal stability, and the homogeneity of the radiation field are presented. Two types of matte ceramic reflection standards were used for spectral radiance factor validation measurements comparing the standardly used halogen sphere radiator (Halogen-SR) and the LED-SR. Due to its designed spectral range at the border between the visible and the UV-A spectral range, the LED-SR is well suited for many applications in diffuse reflectometry. Its use for absolute radiance factor measurements and investigations of the fluorescence properties of diffuse reflecting samples is shown. Reliable polarization-resolved measurements at wavelengths below 430 nm could be carried out with PTB’s gonioreflectometer for the first time due to the beneficial signal-to-noise ratio of the LED-SR.

Uncertainty budget for PTB’s gonioreflectometers and ways to improve it in the short VIS spectral range

“…This LED-sphere radiator (LED-SR) in its present lay-out covers wavelengths from about 360 nm to 430 nm. It generates a much higher irradiance at short wavelengths compared with the halogen source and considerably improves the signal quality and stability [10][11][12]. Both radiation sources have their advantages and can be used for the determination of the SRF as described below.…”

“…This is shown by the red curve in figure 11. The newly designed LED-SR produces a significantly higher irradiance below 430 nm, resulting in improved signal quality and stability in its design spectral range [31,32], as indicated by the measured photocurrent.…”

“…Whereas statistical contributions are greatly reduced by the application of the LED-SR (figure 13) due to its higher available radiance and very good temporal stability, a photocurrent change of only 1.7 ×10 −5 nA h −1 [12,31,32] is observed, the wavelength dependent emission of this source measured with the finite bandwidth of the monochromator has to be taken into account in the uncertainty consideration. Some bandwidth-related wavelength uncertainty is introduced by the LED-SR due to its structured spectrum with rising and falling edges, stemming from the individual form of the three LED-peaks (figure 11).…”