Udo Krüger

Comparison of the Richardson–Lucy method and a classical approach for spectrometer bandpass correction

Bandpass correction in spectrometer measurements using monochromators is often necessary in order to obtain accurate measurement results. The classical approach of spectrometer bandpass correction is based on local polynomial approximations and the use of finite differences. Here we compare this approach with an extension of the Richardson–Lucy method, which is well known in image processing, but has not been applied to spectrum bandpass correction yet. Using an extensive simulation study and a practical example, we demonstrate the potential of the Richardson–Lucy method. In contrast to the classical approach, it is robust with respect to wavelength step size and measurement noise. In almost all cases the Richardson–Lucy method turns out to be superior to the classical approach both in terms of spectrum estimate and its associated uncertainties.

Virtual experiment for near-field goniophotometric measurements.

Near-field goniometric measurements are employed to determine the photometric characteristics of light sources, i.e., the spatial and angular distribution of the emitted light. To this end, a complex measurement system consisting of a goniometer and a CCD-based imaging photometer is employed. In order to gain insight into the measurement system and to enable characterization of the whole measurement setup, we propose to apply a computer model to conduct virtual experiments. Within the computer model, the current state of all parts of the virtual experiment can be easily controlled. The reliability of the computer model is demonstrated by a comparison to actual measurement results. As an example for the application of the virtual experiment, we present an analysis of the impact of axial malpositions of the goniometer and camera.

The impact of cooling on CCD-based camera systems in the field of image luminance measuring devices

From general statements made about the cooling of the CCD sensor within image luminance measuring devices (ILMD), one tends to conclude that cooling has an important effect on the luminance measurement results. However, this general statement is not valid. Based on a detailed model for the properties and the operation of ILMDs, we present results of the measurements for parameters of cooled and non-cooled systems. The influence of the two different parameter sets on the results of luminance measurements is illustrated for variations in the signal level and the temperature conditions. In summary, it can be said that only for very special applications does the cooling of the CCD have a significant influence on the measurement uncertainty associated with the luminance measurement results. Normally, one can compensate for the little drawbacks of non-cooled systems by selecting the most appropriate measuring algorithms.

Determination of tailored filter sets to create rayfiles including spatial and angular resolved spectral information

To simulate and optimize optical designs regarding perceived color and homogeneity in commercial ray tracing software, realistic light source models are needed. Spectral rayfiles provide angular and spatial varying spectral information. We propose a spectral reconstruction method with a minimum of time consuming goniophotometric near field measurements with optical filters for the purpose of creating spectral rayfiles. Our discussion focuses on the selection of the ideal optical filter combination for any arbitrary spectrum out of a given filter set by considering measurement uncertainties with Monte Carlo simulations. We minimize the simulation time by a preselection of all filter combinations, which bases on factorial design.

Hyperspectral LED models for optical simulations

Precise spectral and colorimetric simulations in commercial ray tracing software require realistic light source models, which provide spectral information as a function of angle and spatial dimension. We describe and validate a general workflow to create hyperspectral LED models as a linear combination of spectral models. The workflow only requires user defined precisions and rayfiles obtained with different optical filters. The rayfiles are transformed into histogram based models, whose precision is evaluated by normalized cross-correlation values of their intensity distributions in the near-, mid- and far field. Additionally, the concept is evaluated with a spatial and spectral well defined test device.

Camera-based investigation of high-temperature fixed points for radiometric application

A radiance camera was used to investigate the radiation field emitted by high-temperature blackbodies and fixed-point cavities up to temperatures of 3000 K. In a first step the applicability of the camera to measure the spectral radiance at such high temperatures was investigated. In a second step the radiation characteristics of several sources were studied using the camera system, in particular, large area fixed-point blackbodies of Cu, Pt–C and ZrC–C. Especially for the ZrC–C fixed points the radiation field at the cavity opening was found to be non-uniform. This observation explains systematic differences in the plateau shape of large aperture eutectic fixed-point cells for radiometers with different fields of view. Using the two-dimensional radiance distribution measured with the camera system the deviation of the temperatures measured with interference filter radiometers and radiation thermometers was studied.