Infrared analysis can be used to detect erased or abraded inscriptions in graphite pencil, to distinguish among different formulations of writing inks (an application of great interest to forensic document examiners studying altered checks and fraudulent wills), and to detect drawings under layers of oil paint.
Infrared analysis is carried out by allowing only one narrow band of the "light" (more accurately called radiant energy because we cannot see the infrared portion of the spectrum) used to illuminate the drawing to reach the camera. The full spectrum of light is broken down into three distinct bands, of which only one narrow portion (400–700 nm) is visible to humans. The ultraviolet region (UV) has the shortest wavelengths (<400 nm) and is invisible to the human eye; the infrared region (IR) has the longest wavelengths (>700 nm) and is also invisible to the human eye.
The equipment used to photograph the Titian drawing was a Fuji IS Pro digital camera having a CCD (charge-coupled device) that can detect and convert into electrons energy ranging from the ultraviolet region (380 nm) to the near infrared region (1000nm) and a range of Peca filters (87A, 87B and 87C). The results allowed conservators to distinguish between two types of ink used in the drawing.
In the examination of oil paintings, infrared imaging is most often used to reveal underdrawings—preparatory drawings sketched directly onto the ground layer of the panel or canvas support. In order to be successfully imaged, the medium of the underdrawing must be a dark carbon-containing substance, such as black chalk or charcoal that absorbs radiation in the infrared region of the spectrum. In a traditional oil painting, the infrared radiation penetrates the paint layers to reveal the underdrawing on a white ground.
In the Titian painting, infrared imaging revealed an underdrawing of what appears to be a long javelin held in Actaeon's raised left hand. The complete underdrawing is not visible due to areas of the painting that were reworked in opaque pigments.