They come in collections and instalments, but these videos bought, sold and traded online are not Hollywood blockbusters.

They are part of an illicit trade of voyeuristic videos marketed on adult forums here, which made headlines last week when a jobless woman was convicted over taking and selling videos of other women in states of undress.

Heng Li Ying, 29, had secretly filmed the victims in the changing rooms of a gym she frequented.

She sold the videos online to 22 people and earned about $1,540, charging 10 to 20 cents per second of video. She even offered potential buyers additional videos as freebies to generate more sales.

She was sentenced to 30 weeks' jail.

Experts say Heng's act is an anomaly as most offenders do it for self-gratification instead of monetary profit.

But a quick check proves that the illicit trade is bustling online.

Compromising videos and pictures of women, which include upskirt shots and those in various states of undress in toilets and changing rooms, are highly sought after on adult forums here, with many users wanting to trade, sell and buy such materials.

In one thread, there have been 30 posts since February requesting to buy and trade, or advertising the sale of such videos. Some requests continue to pop up even after news of Heng's conviction.

The forums also have many "sniper" threads, a term referring to those who take upskirt or down-blouse clips. Users would share self-taken videos and pictures with the "community" for free.

Dr Lim Yun Chin, a psychiatrist at Raffles Hospital, said the subculture of voyeurism drives this market.

Addiction specialist Thomas Lee from the Resilienz Clinic said those caught for voyeuristic acts are often found with hundreds and even thousands of clips in their mobile phones and computer hard drives.

"These are sometimes for bragging rights or like their trophies. The more they collect, the more they want. And sometimes, they build a 'tolerance' and that's when they need a 'higher dose' and either buy, trade or film the clips themselves," he added.

Under the Films Act, it is illegal to make, reproduce or distribute obscene films. The maximum penalty for making or reproducing such films is a $40,000 fine and two years' imprisonment for first-time offenders. For distributing, first- time offenders face a fine of at least $2,000 per film or a maximum of two years' jail or both.

The act of filming such videos is classified under insulting the modesty of a woman. Police statistics show there were 597 insult of modesty cases in 2015 and 540 last year.

While the numbers have dipped since the 634 cases in 2014, psychiatrists and psychologists say they are seeing more people coming to them for treatment. They add that it is rare for people to come to them voluntarily and their clients are those who have been caught or finished serving their sentences.

Heng's defence lawyer, Mr Rajan Supramaniam, said he has seen a rise in the number of clients involved in insult of modesty cases. He said: "In the past it was occasional, but now it's at least two a month. Also, many cases go undetected especially if the person isn't caught in the act."

Dr Lim, who sees about four such cases a month, noted that people who form this group come from all walks of life, including doctors, former bankers and students. The demographic also cuts across a range of age groups, with some experts saying they have seen teenagers as young as 15 getting caught.

Dr Lim Boon Leng, a psychiatrist in private practice, said offenders usually fall under two categories: paraphilic (a psychiatric disorder that drives sexual urges) and non-paraphilic (where they are controlled by impulse and curiosity).

"Some clients get sexual pleasure from it, while others don't and they do it out of stress and boredom, and sometimes they take the videos just for the thrill," he said.

Psychologist Daniel Koh said: "They seek a challenge, and getting caught or getting away with the act is part and parcel of the arousal."

Tan Tam Mei