1. A method of determining one or more characteristics of a target object, comprising: determining a first phase map for at least a region of a target object based on radiation directed toward the target object; determining one or more further phase maps with a higher resolution than the first phase map for a sub-region of the region of the target object; determining a number of phase wraps for the sub-region based on the one or more further phase maps for the sub-region; and determining a characteristic of the region of the target object based on the number of phase wraps for the sub-region and the first phase map.