The voltage transfer function is a rapid and visually effective method to determine the electrical response of liquid crystal (LC) systems using optical measurements. This method relies on crosspolarized intensity measurements as a function of the frequency and amplitude of the voltage applied to the device. Coupled with a mathematical model of the device it can be used to determine the device time constants and electrical properties. We validate the method using photorefractive LC cells and determine the main time constants and the voltage dropped across the layers using a simple nonlinear filter model. Typically, such devices rely on having a LC layer aligned between two substrates that can be reoriented with an applied electric field. The alignment is usually controlled through thin layers of polymers or surfactants on the substrates that, in most cases, play a passive role when considering the optical or electrical response of LC devices. However, this assumption is not always valid as even nominally passive polymers, and their interface with LCs, can influence the electric field profile extending into the LC bulk and can introduce ionic effects [2]. Therefore, one of the most important parameters that has to be established in systems with LCs integrated with other layers is the actual value of the voltage dropped across the LC bulk. This question is particularly relevant if the alignment layers themselves respond to an electrical or optical field. Such "active" layers can be used to enhance the functionality of LC devices. Suitable examples include the use of electrically commanded surfaces of ferroelectric LC polymer combined with nematic LCs [3] and those that include photoconducting polymers such as polyvinyl carbazole (PVK) [4,5].To optimize the performance of different polymer-LC systems, a reliable and universal technique is needed to determine a cross-section of the voltage profile across each of the layers and visualize their frequency response. Such a technique can provide an informative snapshot of the suitability of the aligning layer and LC materials for effective and stable light modulation [6].We have developed an optical technique that we have termed the voltage transfer function (VTF) to visualize the electrical response of an LC device and, in particular, to measure the ratio of the voltage dropped across the LC layer and the voltage applied to the whole device. This method is based on crosspolarized intensity measurements at different frequencies and amplitudes of the voltage applied to the device. As the electro-optic response of the LC is independent from the other layers, the VTF method can be applied to a wide variety of LC cells and can be used to calculate important parameters, such as birefringence and elastic constants. The VTF function can be shown in a visually instructive way and data can be extracted from the cells by comparison with suitable LC models that may include ion motion [7] and the effect of Debye layers [8]. Here we use, for illustration purposes, a simple, but intui...