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Liquid crystal display response time estimation for medical applications
Tobias Elze and Thomas Tanner
Purpose: Accurate characterization of diagnosis instruments is crucial in medical applications such as radiology and clinical neurosciences. While classical CRT medical displays have been replaced almost exclusively with liquid crystal devices (LCDs), the assessment of their temporal properties (response times) is still largely based on heuristic methods, which have not been evaluated thoroughly yet. The authors introduce a novel approach and show that it improves the accuracy and reliability compared to the common heuristic recommended by ISO~9241-305 substantially for a wide range of settings.
Methods: The approach is based on disentangling the signal from the modulatory backlight through division (division approach). They evaluated this method in two different ways: First, they applied both methods to luminance transition measurements of different LCD monitors. Second, they simulated LCD luminance transitions by modeling the LCD optical responses according to a physical liquid crystal director orientation model. The simulated data were generated for four different response times, each with four different backlight modulation frequencies. Both the novel and the ISO convolution method were applied to the data.
Results Application of the methods to the simulated data shows a bias of up to 46% for the ISO approach while the novel division approach is biased at most 2%. In accordance with the simulations, estimates for real measurements show differences of the two approaches of more than 200% for some LCD panels.
Conclusion:The division approach is robust against periodic backlight fluctuations and can reliably estimate even very short response times or small transitions. Unlike the established method, it meets the accuracy requirements of medical applications. In contrast, the popular convolution approach for estimating response times is prone to misestimations of time by several orders of magnitude, and tend to further worsen as advances in LCD technology lead to shorter response times.