For two decades his insistence on wearing sunglasses, even when indoors, has been seen simply as part of his rockstar image.

But Bono, 54, has revealed that his trademark shades are instead there to alleviate difficulties caused by a chronic eye condition.

Speaking on BBC1’s Graham Norton show, the U2 frontman explained that for the past 20 years he has had glaucoma, a condition that can make eyes more sensitive to light.

Asked by Norton whether he ever removes his shades, Bono replied: “This is a good place to explain to people that I’ve had glaucoma for the last 20 years. I have good treatments and I am going to be fine.”

The singer added: “You’re not going to get this out of your head now and you will be saying, ‘Ah, poor old blind Bono.’”

Glaucoma is caused by a buildup of pressure in the eyeball which can damage the optic nerve and lead to blindness if not treated. Early diagnosis can prevent mean further deterioration of someone’s sight– with laser treatment, eye drops and surgery used to stop it worsening. The damage to the optic nerve, however, cannot be undone.

U2 are currently promoting their newest album, Songs of Innocence, which was released commercially this week after previously being given away to half a billion iTunes customers. It was a move that caused much controversy, with many complaining they did not want the album automatically added to their iTunes, and last week Bono admitted the band had “this beautiful idea and we got carried away with ourselves”.

Speaking to Norton about the unconventional album release, Bono added: “We wanted to do something fresh but it seems some people don’t believe in Father Christmas. All those people who were uninterested in U2 are now mad at U2. As far as we are concerned, it’s an improvement.”