If something is photosensitive, light does something to it. A film, a sensor, or a special paper may change colour or make an electric current when light hits it — that is how older cameras “caught” a picture, and how some detectors work. Chemists also make materials photosensitive on purpose. Though actually, doctors use the word for people too: some medicines make your skin photosensitive, so the sun burns you more easily, and a few people have photosensitive epilepsy, where flashing lights can bring on a seizure. In everyday talk you can just say “sensitive to light”; photosensitive sounds more like a lab, a factory, or a clinic.