A method of determining a dose-to-clear of a photoresist on a wafer includes providing an image of the wafer after the photoresist was exposed to a dose of energy and was developed, transforming the image of the wafer into frequency spectrum data, calculating an average frequency spectrum component of the frequency spectrum data, calculating a difference between the average frequency spectrum component and a noise average frequency spectrum component of a noise average frequency spectrum, and determining a dose-to-clear of the photoresist based on the difference between the average frequency spectrum component and the noise average frequency spectrum component.