This paper introduces an uncertainty analysis model and its experimental implementation for chromaticity coordinates and luminous flux measurements of light-emitting diode (LED) sources. The uncertainty model applies the theory of the numerical method to estimate both the chromaticity coordinates and luminous flux uncertainties. The modeling process follows the steps described in GUM for determining the uncertainties. First, the mathematical functions for chromaticity coordinates and luminous flux are expressed according to both the sphere calibration and the LED measurement procedures. Based on the functions, the uncertainty contributors are identified as the input quantities of the model, and luminous flux and chromaticity coordinates are the output quantities. Second, the uncertainty contributors are categorized as random variables and systematic variables. Contributors such as spectrometer wavelength and spectral value repeatability are random variables; thus, their standard uncertainties are analyzed with statistical methods. The other contributors, such as spectrometer wavelength offsets and stray light, are systematic variables; thus, their standard uncertainties are estimated with non-statistical methods. In order to measure these contributors, several simple methods are developed for spectrometers and source measure units (SMU). Third, the sensitivity coefficients for the uncertainty contributors are calculated based on the numerical approach by calculating the output quantities with a change of the input quantities. Fourth, the uncertainties caused by each contributor are calculated using their standard uncertainties and sensitivity coefficients, and then combined. Finally, the expanded uncertainty is obtained with a coverage factor (k=2). The calculation for each step is conducted by a Matlab program.
Free-space optical (FSO) links for high-speed communications between buildings must consider detrimental environmental effects including interference from sunlight in the receiver's instantaneous field of view (IFOV). Sunlight can degrade receive sensitivity resulting in link disruptions, even with significant optical filtering. Thus it is important to characterize this environmental effect for designing and testing optical transceivers. Background light levels are highly dependent on the geometry and environmental conditions of a specific link making general statements difficult. However, we have characterized the likelihood and frequency of direct or reflected sunlight passing into or near a terminal's IFOV. We have also measured detector solar power levels under sunny and partly cloudy conditions, and measured detector sensitivity degradation as a function of background light levels. This paper presents a summary of our results.
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