A jury at Leeds Crown Court heard that John Gill pleasured himself at the premises of Clear Vision in The Arcade, Knottingley, while in a back room with a 14-year-old girl who had gone to pick up her contact lenses.

Gill, 36, of Hunters Way, Selby, denies engaging in sexual activity in front of a child but Michael Smith, prosecuting, said the alleged victim saw the defendant with his hand down his trousers while she was having a field test.

Mr Smith told the court the teenager, who is now 16, received a call from Gill on August 8 last year, telling her her contact lenses were ready for collection.

When the youngster arrived at the opticians, where Gill worked with his wife, she found him alone.

Mr Smith said Gill told her to try the lenses on and said she needed further eye tests to check they were suitable.

"He took her into a back room and did a field test on her," Mr Smith said.

It was while she had her head inserted in the device during this exam, which consists of a dots test to check vision, that she became aware of something unusual about Gill's behaviour, who was standing off to the side of her.

Mr Smith said: "She turned her head to see his hand in his trousers zipper.

"Gill told her to concentrate on the test and then she heard clothes rustling and a slapping sound."

He said the youngster then became aware of heavy breathing and carried on with the test regardless, only to find when she had finished that Gill was red-faced and throwing a tissue in the bin.

A video was played to the court of a police interview carried out with the teenager in October 2008.

In it, she told a police officer that after she had finished her test on that day: "When I looked at him he didn't have a happy smile, it was a smile like I know something you don't."

She added that Gill told her he needed to check her other eye but she had become scared by this point and made her excuses and went home to tell her mother.

Her family later told Gill they would not be coming back to the optician.

She also said that she disliked having field tests as during previous exams carried out by Gill she had felt something "soft and fleshy" on the back of her hand.

When asked what she thought this was, she replied "probably his privates."

She said she had told anyone because she was not sure what had happened and describing her overall feeling towards the optician, the teenager said: "There's friendly and then there's John."

Gill also denies a further charge of committing an act outraging public decency on a date between March 1 2007 and October 31 2007 when he is alleged to pleasured himself in the window of Clear Vision.

Two women are alleged to have seen Gill pleasuring himself as they were walking towards a Morrisons supermarket with their children.

The trial continues.