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We present "screen codes" - a space- and time-efficient, aesthetically compelling method for transferring data from a display (e.g. a VDU or projected public display) to a camera equipped mobile device. Screen codes encode data as a grid of luminosity fluctuations within an arbitrary image, displayed on the video screen. These fluctuations, manifested as a "twinkling" within the image, are observed by the mobile device over time and decoded to reconstruct the data. Observation is passive; there is no back-channel from the camera to the display. Novel spatial and temporal coding strategies are employed, tailored to channel noise conditions. The display may be observed from any angle or orientation.