Roving invigilators armed with signal scanners to detect mobile data and call traffic inside examination halls caught five JEE candidates using a smartphone or a smartwatch to cheat on the first day of the test.

The crackdown also netted 13 proxy candidates across Bengal. All of them were arrested.

This is the first time that radio-frequency microwave signal detectors are being used in the state to beat exam cheats at their own game. Metro had reported in March that scanners would be used at JEE centres this year.

"We had deployed people armed with radio-frequency microwave signal detectors to randomly check exam halls," Bhaskar Gupta, chairman of the JEE board, said.

"The five students caught cheating using smart tools have been barred from writing the rest of the entrance exam. The 13 people caught for impersonating candidates are in police custody."

Since radio-frequency microwave signal detectors run on the same frequency as mobile phones and smartwatches, they can help detect the presence of any such communication tool in their proximity.

"The strength of the device determines the distance from where it can detect the presence of a smartphone in an examinee's pocket or a smartwatch on someone's wrist," said an expert in cyber technology.

Smartphones have long become commonplace but smartwatches - timepieces that fuse fashion with computer and mobile phone technology - are relatively new. Smartwatches allow users to make hands-free calls and send texts and emails. They can be used in conjunction with a smartphone or independently.

All five candidates caught using technology to cheat were writing the test in north Bengal.

"The signals from devices illegally brought into the exam halls were in the form of vibrations. Once our scanners caught these signals, the invigilators were alerted so that the suspects weren't able to step out and hide their devices," a source in the JEE board said.

A signal scanner cannot catch the vibrations if a smartphone switches to flight mode, though.

"If the user puts the phone in flight mode on seeing someone with the detector in the hall, the signal can't be detected. We had to be extra cautious to ensure that nobody got the opportunity to change the smartphone mode," an official said.

Cheating examinees are known to take photographs of question papers on their smartphones and send these images as email, Messenger or WhatsApp attachments to relatives and friends, who reply with the answers.

Board chairman Gupta said carrying smartphones or any other device that could be used as a tool for cheating inside an exam hall was a serious offence punishable under law.

Until last year, the only way to catch a candidate who had smuggled a smartphone or some other tech tool into an exam hall was frisking. "Given the extent of the problem, we have had to bring in technology to supplement manual checking and the result is there for everyone to see," he added.

The 13 proxy candidates caught on Tuesday are all from other states, mostly Delhi and Rajasthan.

Two of them were impersonating candidates registered for the entrance test in Calcutta - one at Mitra Institution, Bhowanipore, and the other at RBT Vidyapith in Sinthi.

Invigilators realised they were proxy candidates when they compared their faces and handwriting with the photographs and signatures on the admit cards. "One of them had even worn a wig to look like the candidate he was impersonating but it obviously didn't fool anyone," a board official said.

Tuesday's test was on Biological Sciences.