Over a period of time, a sales engineer in Singapore amassed 280 obscene films, many of which depicted women in various stages of undress in public bathrooms and changing rooms.

The unsuspecting women, including schoolgirls, were filmed with secret cameras, and Joel Chew Weichen, 27, had collected the films for distribution.

Based on checks by The Straits Times, a worrying trend has emerged - the sale of such cameras is on the rise.

On online shopping platform Lazada, which has more than 600,000 hidden camera products available, sales of such cameras have grown 1.5 times this year compared with last year. The cameras come disguised as clocks, pens and even smoke detectors.

A spokesman said spy pens, which can cost about $12, are the most popular. Spectacles with built-in cameras may cost about $85.

Lawyer Rajan Supramaniam said there are no restrictions in buying such cameras, "and some do so to monitor strangers who may come to their house, such as loan-shark runners", or use them to watch out for other illegal activities.

OMG Solutions, which specialises in homecare and aged care, noted the increased demand for hidden cameras and expanded its business to include nanny cameras last year.

It can sell up to 100 such products in a month, with those disguised as clocks the most popular, but founder Pieter Tjia said customers generally use such items to monitor elderly family members at home, or their domestic helpers, to ensure that no accidents occur.

He added that it is possible to identify customers with no ill intentions as they often buy other products for the elderly, such as anti-slip mats. Others who buy hidden cameras from his firm include private investigators, he said.

But there are those who buy such products with nefarious intentions.

Chew, who was sentenced to six months in jail this month, was the first of five individuals to plead guilty to having the obscene films for distribution.

He was also part of groups that share and download such videos.

The victims were secretly filmed while in bathrooms in cafes, schools, offices, changing rooms of popular fashion outlets, and bathroom showers in private homes.

According to lawyers, the act is classified as insulting the modesty of a woman.

The police investigated 597 insult of modesty cases in 2015, and 540 last year.

Though not all the cases may be related to voyeuristic pictures and videos, the increased sales of spy cameras and the number of those caught insulting the modesty of women has people like Ms Pamy Tan, 28, worried.

"I can't begin to comprehend why people want to do things like that," said Ms Tan, 28, who works as a writer. "With public toilets, I usually trust that the building or institution responsible for its maintenance would do checks on (whether there are) such spy cams."

But some are not just content with secretly filming women.

The others arrested in relation to Chew's case are accused of making and trading the videos online as well, which carries a maximum penalty of a $40,000 fine and two years' jail for first-time offenders.

As a result, some retailers say they are selective about who they sell such cameras to.

Sim Lim Square's Choicecycle staff member Jason Chen said: "Most (inquiries) come from men of all ages but, occasionally, there may be women looking to catch a cheating husband."

When potential buyers show tell-tale signs of having criminal intentions, such as asking if a hidden camera is waterproof, staff may refuse to sell them the cameras.

Despite such efforts, Mr Supramaniam noted that some people can still abuse the use of such cameras.

"They can install cameras in a bathroom, or in a room rented to tenants," he said. "Restrictions on hidden cameras could be tighter by tracking the identity of buyers, which may serve as a deterrent, or by having a licence for sellers, which will help to deter illegal peddlers."