A paedophile has been jailed for a string of sex offences against young girls.
Jeremy Bowen, 49, of Hayes, west London, flashed at girls in parks and at school gates, the court heard.
At Harrow Crown Court he previously admitted 20 counts of indecency towards children and 14 counts of possession and distribution of indecent images.
Bowen was given an indeterminate sentence and must serve at least three years and nine months.
The court heard Bowen flashed at girls aged between eight and 14. He would touch himself and on one occasion paid a nine-year-old £5 to touch him.
He began targeting girls in the west London boroughs of Harrow, Ealing and Hillingdon in 2003, sometimes taking secret photographs of them.
Judge Alan Greenwood, said: "You are a prolific paedophile who has often engaged young girls.
"I have no doubt at all that on the basis of what you did and what you wrote you pose a very serious risk to young girls.
"I have no doubt that the sexual activities in which you engage and intended to engage in the future, to young girls creates a significant risk, that the young girls would suffer serious psychological harm."
During a raid at his home earlier this year police found more than 150,600 indecent photographs and 1,600 movie files.
But prosecutor Neena Crinnion said many more images had existed.
She said: "They had been deleted using military grade software so that it was impossible for the (police) laboratory to recover anything.
"The level of stealth this defendant deployed in secreting this material and others could be described as paranoia to avoid detection."
A search of his computer's hard drive revealed details of his visits to secure paedophile bulletin boards and forums plus personal chats with other paedophiles.
It showed how he gave advice on how to groom and seduce pre-teenage girls and to take secret photographs of them, the court heard.
During sentencing on Monday a further 22 counts of exposure were asked to be taken in to consideration.
Bowen was also disqualified from working with children and must go on the sex offenders' register for life.
He was also banned from going near schools and near certain parks and children's play areas.