The Dean's Eye window at Lincoln Cathedral is considered to contain one of the most important examples of medieval stained glass in the world and later this year the restored glass will sit in a new stone frame.

Carving of the French Anstrude Roche Claire limestone for the £4million restoration of the window started in February last year and was carried out by the cathedral's own stonemasons - it has 30 of them as part of the second largest cathedral works department in the country. The final piece of stone was laid on the tracing floor for public viewing in January this year and the stone should be fixed in place in July.

The carving is traditional, but part of the tradition of stonemasonry includes a certain freedom for the carvers to make their own contributions and the new window includes a depiction of the current Dean of Lincoln, the Very Revd Alec Knight, and one of the mason\'s ferrets.

The window is 7.6m in diameter, although it is not quite round. It sits in the north transept, which has settled and distorted since it was built in 1220, twisting the wall 4deg out of vertical.

To make sure the new window fitted the distorted opening, both old and new technologies were used. Photogrametric surveying and computer-aided design enabled exact zinc templates to be cut of each stone. Then a tracing floor - the sort of thing that would have been familiar to the Romans - was laid on the floor of the cathedral with the pattern of the window traced on it. It was on this that the finished stones were laid out for viewing this year.

There has been concern in some quarters (disputed in others) that Anstrude Roche Claire cannot survive well in the English climate, especially as it is high on the transept and facing north. However, the new tracery will include stainless steel reinforcing between the stones and is a good 50mm thicker than the original stonework of the window.

One of the reasons for the extra thickness is that the window will be double glazed. An extra rebate cut into the stone on the outside of the window accommodates isothermal glass of the same size and shape and even the same leading as the stained glass on the inside, so it should be invisible. It is there to protect the medieval glass and will be put in place before the stained glass is fitted, work which will begin in April next year. The whole project is due to be completed in March 2006.