The cabby who filmed an upskirt video of a sleeping woman passenger did so after getting away with the same crime against two other victims in the two previous months, noted a judge in explaining why he imposed a four-month jail term on the offender.

Making clear the timing of a deterrent message was all the more urgent with the "proliferation" of fee-based "taxi-like services" and their fast-paced demand, District Judge Ronald Gwee stressed that the courts would deal with such offences with a stern hand.

Cabby Tan Swee Pheng gave his female passenger "a ride she would likely rather forget", the judge wrote in judgment grounds released last week.

Tan, 60, admitted filming the woman using his mobile phone on Sept 28 last year.

The woman, who boarded the taxi in the east, fell asleep as the cab headed for Raffles Place.

But she awoke 15 minutes later to find Tan had stretched out with one hand to film "underneath her skirt" with his phone while his other hand was on the driving wheel.

Tan, who is appealing against his sentence and is out on $20,000 bail, had pleaded for the court to fine instead of jail him.

But Deputy Public Prosecutor Crystal Tan pointed to the numerous aggravating factors that justified a deterrent jail term which the judge found "compelling".

"The aggravating factors were clear to see," said Judge Gwee , adding that "there was a large public-interest factor to be borne in mind".

" Female taxi passengers were entitled to feel safe... and should never need to fear having their modesty insulted."

The judge noted that such offences are difficult to detect and Tan would have got away with his crime had the victim not woken up.

Tan admitted to two similar offences in the two previous months which were taken into consideration. The judge added: "Tan had filmed (the victim) after having probably thought he had 'gotten away' with earlier having filmed (the other two victims)."

The court made clear that the updated technology in cellphones used in such offences was an aggravating factor as the videos had "longer capture and exposure of the victims' privacy than still photographs".

Rejecting a fine as "simply inappropriate and inadequate", Judge Gwee said "any like-minded taxi driver and other like-minded driver conveying passengers for a fee had to be deterred".