A method and device for converting a video signal from a lower dynamic range source to produce a signal usable by target devices of a higher dynamic range comprises converting with a convertor that implements a process in which a received signal is received as a luminance component and separate colour components, A modified luminance component is then produced by applying an expansion function to the luminance component for luminance values above a threshold, applying a gamma function and then applying a scaling function. The modified luminance and separate colour components are then provided as an output signal for display on a target device of a higher dynamic range. The expansion of luminance ensures that the luminance range appropriately matches the range for a target display. By applying the expansion luminance values above a threshold, values below a threshold can be maintained with appropriate values for display.