From next week, visitors to France’s Chartres Cathedral will be able to plunge into the stories unfolding across dozens of medieval stained glass windows courtesy of a unique AI-driven app.

Launching on 10 November, “Lire les vitraux” (Read the Windows) will decipher the legends and narratives in 60 of the 172 windows that adorn the 13th-century gothic masterpiece.

Initially available only in French, explanations on the app will be offered in English and German from spring 2026 – when developers also hope to expand the technology to cover the cathedral’s entire 2,500 square-metre expanse of stained glass.

“You just take a picture of a window, and instantly, you get all the information to understand what’s in front of you,” said Jean-François Lagier, who coordinated the team of engineers, technicians and historians behind the app.

“So instead of just being amazed without context, your admiration is now enriched by knowledge which deepens appreciation for the stained glass itself.”

A demonstration of the new smartphone app that reveals the stories told in Chartres Cathedral’s stained glass windows, on 14 October 2025. © AFP - JEAN-FRANCOIS MONIER

A mere 15 years ago, he said, such a tool would have been impossible to imagine. “Back then, the only option was to print heavy books, which limits access because they’re expensive and cumbersome.”

It was during a meeting about those weighty tomes eight years ago that new technology was first mentioned.

“We realised our previous books were out of print,” Lagier explained. “So we had to decide: reprint them, or find something more powerful, broader and more accessible. During those discussions, someone suggested exploring artificial intelligence and new algorithms.

“We then found an engineering team willing to take on the challenge. It had never been done and still hasn’t been done elsewhere – using AI to recognise scenes in a huge building like Chartres Cathedral.”

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Technical feat

Two types of algorithms drive the app. The first gives it the ability to recognise objects in variable conditions, with different angles or lighting situations.

“That one is very useful for stained glass, since light changes constantly from sunny to cloudy and it affects what you see,” said Lagier. “Once the system has identified an object, it moves to facial recognition and those algorithms identify the exact design or figure on each stained glass panel.”

The free app uses different algorithms to identity objects and characters in the stained glass windows of Chartres Cathedral. © AFP - JEAN-FRANCOIS MONIER

The cathedral’s windows are typically made up of around 30 panels, each displaying characters, symbols and colours – the iconography of medieval stained glass.

“Beyond recognising an object, you need to interpret its forms,” added Lagier. “So we’ve combined these technologies with our own custom code, written by our developers, to create a recognition tool that works inside the cathedral.”

Hidden history

Notre-Dame de Chartres, some 80 km south-west of Paris, was constructed between 1194 and 1220 on the site of at least five earlier cathedrals that have dominated the land since the 4th century.

The present majesty was arguably saved from destruction during World War II by the actions of an American colonel, Welborn Barton Griffith Jr.

In August 1944, as Allied forces battled the Germans, who they suspected had set up positions in the cathedral, the order came to blitz it. Dubious, the officer took it upon himself to brave enemy lines with his driver to check.

After searching the cathedral and finding it empty, he raised the American flag in the bell tower and rang the bells. The order to bombard was cancelled.

Beneath those same spires, 80 years on, visitor Corentin Rouault said the cathedral had left him amazed.

[Corentin Rouault stopped off to visit Chartres Cathedral after completing a section of the Paris to Mont St-Michel bike trail. © Paul Myers/RFI ](https://www.rfi.fr/en/france/© Paul Myers/RFI)

“It’s magnificent, beautifully restored,” beamed the 31-year-old engineer, who had stopped off in Chartres after completing a section of the Paris to Mont St-Michel cycle path. “It was my first visit and it was stunning.”

On the prospect of an app to assist his next visit, he added: “That would be absolutely fantastic... I looked at the stained glass windows. They’re beautiful but it’s true that I don’t really know the stories behind them.”

Félicité Schuler does. A leading specialist in medieval iconography, she has worked for the best part of 30 years at the International Centre for Stained Glass, situated an inadvisable stone’s throw away from the cherished windows.

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For the past two years, as well as her duties as a guide and lecturer on the windows and their meanings, she has been sifting through her cornucopia of knowledge for use in the application.

“The most difficult problem has been to do a resume of a window in a specific amount of words,” she admitted. “We didn’t want to put too much text. So if somebody wants to read it very rapidly, they just take the headline. If they want to learn more, they can read the whole text.”

Even the smallest details can be revealing, she explained. A short tunic, for example, indicates that its wearer is a pagan. “But kings will never be shown with a short robe, even if they are pagans. Because they’re kings, they must be clothed correctly.”

‘Duty of memory’

The €270,000 cost of producing the app came from private sponsorship and donations in France and the United States. It will be available for free on all iOS and Android platforms.

“Having seen the stained glass windows a few years ago, I wanted to see how they had been cared for and enhanced to get them back to their former glory,” said another visitor Soraya Saidi, after her moment in the cathedral.

“I spent time in front of the windows and also at the centre of the nave, to meditate and pray in silence, as one can do here.”

The nave of Chartres Cathedral, with light streaming though its monumental stained glass windows. © AFP - JEAN-FRANCOIS MONIER

The 47-year-old careers assistant from Clermont-Ferrand, central France, added: “I found the light, the gentleness and the energy flowing through the place quite extraordinary. The colours coming from the sunlight streaming in through the stained glass was beautiful.

“There’s such richness in the windows that hasn’t been passed on. There’s a duty of history and memory. We must honour what was created by our ancestors.”