The underdrawings of the ‘Altarpiece of the City Patron Saints’ were made visible using infrared reflectography. This is an imaging technique used in the investigation of paintings in which infrared radiation penetrates the paint layer and is absorbed by the black lines of any underlying drawing. Special camera systems transform the thermal image into a visible rendering, known as an infrared reflectogram, of the lines of the underdrawing.
Osiris-A1 camera (Opus Instruments Ltd) with indium-gallium-arsenide array (0.9–1.7 μm), max. image size 4096 x 4096 pixel, light source: 2 photographic halogen lamps (Hedler) 350 or 600 W.
The 17-hour infrared reflektography session in time-lapse