In total, Santa Maria del Fiore was endowed by the Opera with no fewer than forty-five marvellous stained glass windows, forty-four of which survive (one of Uccello's was, alas, destroyed by lightening in the nineteenth century). Over recent decades, beginning in the 1970s, the still-active Opera has directed and overseen an ongoing cleaning and restoration campaign for all of them, with the work expertly done by the Florentine Studio Polloni & Co.. The Assumption oculus will be the thirty-fourth to be cleaned (if I count correctly!), and is projected to be re-installed in summer 2015.
In the meantime, visitors see the four cleaned and restored nave windows (the earliest, dating to 1394-96, all designbed by Agnolo Gaddi, whose frescoes in the main apse chapel of Santa Croce have very recently also been cleaned and restored, beautifully), and the cupola oculus windows, especially the three or five most visible from the nave side of the choir -- one of the grandest company of artistic masterpieces in Florence still in their original place today, by Donatello, Ghiberti, Uccello, del Castagno! In fact, the closest views of any of the cathedral's coloured glass windows is reserved for those who pray or attend mass in the southern apse tribune. After all, the cathedral was and is a church, not a museum!
[Image: Assumption of the Virgin, designed by Ghiberti, 1404/05: from BIVI Vetrate Italiane Banca Ipermediale delle Vetrate Italiane]