RTI: Reflectance Transformation Imaging

Hembo Pagi

OÜ Archaeovision

RTI (Reflectance Transformation Imaging) is a method of photographing series of images which makes use of reflecting light, set at an angle, to reveal surface deformations of a photographed object. The method is useful for detailed examination and display of various objects, such as coins, sculptures, knits or ancient stone tablets.

The RTI method is based on a mathematical algorithm which calculates per pixel reflectance on a chosen set of images. The method was designed in 2001 by Tom Malzbender from the Hewlett Packard Labs. By now, in addition to Malzbender’s Polynomial Texture Mapping (PTM), other algorithms have been adopted for use, such as, for example Hemispherical Harmonics.

Photographs are taken with a regular camera and an external light source (flash). The only requirement for the camera and the flash is that they have to enable manual control, i.e. all automatic settings, including focus, aperture and shutter speed have to be set manually. A piece of string and a snooker ball are important and useful tools—while the former ensures a fixed distance of the light source from the photographed object, the latter captures the direction of light as a reflection on its spherical surface.

The 30 to 50 captured images are processed with a software to generate an interactive photograph which allows viewing the object from different light angles. Both the processing and imaging programs are freeware.

Additional information on RTI and software downloads is available at Cultural Heritage Imaging website (http://culturalheritageimaging.org/Technologies/RTI/).