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This paper describes the analysis, simulations, and proof-of-principle experiment on transparent monitoring of relative rise time using asynchronously sampled amplitude histograms in optical transmission systems. The method described is based on the count in a selected amplitude window between the mark and space levels. The method is evaluated under various amplitude windows and Q-factor values and is compared with synchronously sampled eye-diagram-based measurements. The results demonstrate that the 40-60% amplitude window is generally suitable for system rise-time monitoring purposes without clock information.