Reflectance Transformation Imaging (RTI) is a method of creating an interactive digital image of an object with a moveable light source. While not a full 3-dimensional scan, RTI gives a better view of surface relief and texture than is usually possible from conventional photography. It can produce views of marks, inscriptions and indentations on surfaces to assist with readability, develop scientific understanding, and create a visually engaging experience of the object suitable for a wide range of audiences.
Creating an RTI involves photographing the subject while varying the lighting angles in a controlled manner. Reflective spheres are included in the photographs to record the lighting angle and enable the processing software to calculate a matrix of surface ‘normals’ that indicate the direction of reflected light from the surface.
As well as producing an interactive, natural-colour representation of the surface, the image can be enhanced mathematically to exaggerate the surface relief and provide a better view of key details (the ‘specular enhancement’ technique). The ‘normals visualisation’ technique creates a false colour view of the ‘normals’ of reflection, indicating the direction each surface faces, and giving a better idea of the overall shape of the object.
RTI photography often uses a dome with a system of internal lights programmed to 
light up one by one, enabling the lighting pattern to be carefully controlled. The process is fast, reliable, and produces a high-quality end result. The size of the subject that can be captured this way depends on the size of the dome available, but this approach is generally most suitable for medium-sized to very small objects.
However, specialist equipment is not essential. RTI photography can be carried out manually, using only a camera on a tripod and a hand-held light source. The quality of the end result is dependent on manual accuracy, but this method is flexible, portable and inexpensive, and is a very useful solution for capturing large or immovable subjects out in the field.
RTI files usually require specific software to be able to view them and access the specular enhancement and normals visualisation views. However, the interactive natural-colour image can be embedded into a webpage, enabling them to be easily viewed and shared without any additional software.
Workflows and other resources for RTI photography and image processing are available on Github.
Have a look at examples of RTI that have been created in the lab in our RTI demonstration page.
RTI is one of many digitisation methods offered by the Digital Humanities Lab. Get in touch to learn more about RTI or discuss digitisation projects.
