Method and system to remove background noise with a differential approach in optical imaging is disclosed. The differential approach moves the sample position laterally over a small distance, and a differential image is generated from the images recorded before and after the lateral translation. This approach can significantly improve the image quality of objects, including single DNA molecules, for label-free optical imaging techniques, such as surface plasmon resonance imaging. Disclosed imaging technique provides high-resolution genome-wide restriction maps of single DNA molecules.