Methods are provided for creating a fluorescent watermark within an image on a substrate, such as paper. The method involves creating a halftone image using two different halftone strategies. The halftone method is combined with a binary watermark mask to form two color patterns (e.g., one in a background region of the image and one in a watermark region of the image) and two successive-filling halftone algorithms, such that the ink droplets deposited by one color pattern cover more of the substrate than the ink droplets deposited by the other color pattern, with the two color patterns having approximately the same reflectance under normal light. However, under UV illumination, a visible difference (e.g., the watermark) is seen in the two patterns.