Black Light Pen

The Black Light Pen, officially known as the Stylo de Lumière Noire, is a specialized felt-tipped marker originally designed for use by museums, restorers, and forgery police to place invisible marks on items. It writes in a noncorrosive, alcohol-based fluorescent ink that is visible only under black light. Museum maintenance staffs carry these markers on their daily rounds to place invisible "tick marks" on the frames of paintings that need restoration. In the narrative, this seemingly mundane tool becomes a pivotal piece of forensic evidence and a key to unlocking the dying message of Jacques Saunière.

Discovery and Function at the Crime Scene

When Robert Langdon first examines Saunière's body, he notices the curator is clutching a large, felt-tipped marker. Captain Bezu Fache points out that Saunière was holding it when they found him, and that they have touched nothing. Langdon kneels down to see the pen's label: STYLO DE LUMIERE NOIRE. He recognizes it immediately as a black-light pen or watermark stylus. Fache then turns off the main spotlight, plunging the gallery into darkness, and uses a portable black-light source to reveal the curator's final words, which glow in luminescent handwriting on the parquet floor beside the corpse. The message reads: "13-3-2-21-1-1-8-5 / O, Draconian devil! / Oh, lame saint!" This hidden text, invisible under normal light, is the first indication that Saunière left a complex coded message, not just the pentacle drawn on his own abdomen.

Saunière's Use of the Pen for His Final Message

Saunière used the black-light pen to write his final communication, employing his own blood as ink for the pentacle on his stomach but the pen for the floor message. The pen's ink is soluble, and Fache had wiped clean a fourth line of the message—"P.S. Find Robert Langdon"—before Langdon arrived, keeping it hidden to manipulate the investigation. The pen thus becomes a tool of both revelation and concealment, as Saunière intended his message to be seen only by those with the proper forensic equipment, ensuring that the police would involve cryptographers and that his granddaughter, Sophie Neveu, would be drawn into the case. The pen's alcohol-based ink also leaves a faint smell, which Langdon later detects on the gold key, indicating that Saunière had recently cleaned or written on it.

Narrative and Symbolic Significance

The Black Light Pen functions as a bridge between the visible and hidden layers of the story. It embodies the novel's central theme of concealed truth, where ordinary objects and texts contain secret meanings accessible only to those who know how to look. The pen's use by Saunière—a master of codes and puzzles—underscores his deliberate, layered approach to communication. It also highlights the forensic methods of the police, as Fache uses black-light illumination to search for blood and other evidence, but Saunière subverts this by using the same technology to leave a message that only the police's own tools can reveal. The pen thus becomes a symbol of the interplay between secrecy and discovery, a tool that both hides and reveals, and a key piece of evidence that sets the entire plot in motion.