A 43-year-old rail worker was caught masturbating in a garden in Outram Road, a court heard.
Terry Anderson, of Long Lane, Addiscombe, was spotted with his trousers round his ankles looking into bedroom windows.
At Croydon Crown Court last week he pleaded not guilty to a charge of outraging public decency and claimed police had the wrong man.
But he was convicted by a jury and fined £500 and ordered to pay £2,000 costs.
Martin Kenny, prosecuting, said it was six weeks after the incident in August last year before Anderson was arrested.
He claimed police had the wrong man and that he had not been in the area.
A witness told the court he came out of his second-floor flat and saw a man with his trousers down and masturbating.
He added: "I shouted at him, what's your game, sunshine?' He ran to the bottom of the garden. I got a glimpse of him as he disappeared around the corner of the house.
"He must have jumped over a fence to the next garden. My fiancee rang the police. We got the number of his white van."
The court heard the van was traced to Anderson's wife and he was eventually arrested on September 29.
Anderson was said to have several previous convictions but none for this kind of offence.
He told the court he had not been in the garden on that night and that residents had taken the wrong number of the van they had seen.
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