The present invention provides a method and a device to convert a time varying optical pattern emitted by a display into a digital data signal. More specifically the invention allows a handheld security token to convert a time-varying light intensity pattern emitted by a source such as a computer screen into a digital signal including a sequence of coded data symbols. The invention is based on the insight that the intensity of light emitted by regions of said source can be easily sampled by a simple low-cost processor if appropriate A/D conversion hardware converts the incident light into an electrical signal which is time varying, whereby the base frequency of this electrical signal is a function of the light intensity. Intensity levels used for channel coding and symbol clock can be recovered from the signal by the receiver. The invention comprises measuring this electrical signal, transforming sets of measurements into intensity samples for a plurality of sampling windows, adaptively calculating discrete intensity levels from these intensity samples, assigning intensity levels to the sampling windows, detecting symbol period boundaries, and decoding the symbols.