Gerritsen uses X-ray photography as a technique to reveal the underlying layers of artworks. His work, which belongs to modern methods for determining the composition of paintings, is still performed using analog techniques. Geritsen's X-ray images reveal not only the surface layers of a painting but also the internal structures, such as the wood and nails. This adds a new dimension to the artwork. Geritsen combines traditional photographic processes with advanced technologies, resulting in unique visual and scientific approaches. His photographic works thus bridge the gap between investigative photography and artistic interpretation.