A photosensor device includes a plurality of first well structures, a light shielding layer, and a plurality of second well structures. The first well structures are disposed in a substrate. The light shielding layer disposed is on the substrate; it covers a portion of the first well structures and exposes the rest portion of the first well structures. The covered first well structures are adjacent to the exposed first well structures exposed. The exposed first well structures generate a first photocurrent according to incident light. The second well structures generate a second photocurrent according to incident light. A total surface area of the second well structures is substantially equal to a total surface area of the exposed first well structures. A method for determining the incident light is also provided.