City and federal authorities are investigating reports that a person or people have been shining beams from green laser pointers at aircraft over O'ahu, a potentially dangerous prank.

There have been at least four reports this year of flight crews over O'ahu being "lased," including two involving Coast Guard C-130 landings at John Rodgers Field at Kalaeloa.

No injuries have been reported, and there were no effects on flights. But the light from green laser pointers, whose sales are not regulated, can be extremely dangerous. A Mayo Clinic study published in the Archives of Ophthalmology journal found that exposure of just 60 seconds from a commercially available Class 3A green laser pointer can harm a person's retina.

Green laser pointers, the size of a pen, can project a beam 5,000 to 9,000 feet.

Honolulu Police Department spokeswoman Michelle Yu said the department's criminal intelligence unit knows of four incidents this year but that no arrests have been made.

The Coast Guard is offering a $1,000 reward for information leading to identifying a suspect in its investigation into green laser incidents in February and March at Kalaeloa. Anyone with information can call the Coast Guard Investigative Service at 541-2301.

Both Coast Guard incidents occurred while aircraft were conducting instrument approaches at John Rodgers Field. In both cases, the crews were on final approach to the runway in a "very vulnerable position," said Cmdr. Chris Moss, operations officer at Air Station Barbers Point.

"Landing is a precarious operation, the crew is completely focused and procedures need to be exact," Moss said. "To be distracted by the laser is dangerous in itself, but the eye damage from the laser can be instantaneous and permanent."

Thirteen crewmembers from the two airplanes were examined and cleared at Tripler Army Medical Center.

Under federal law, interfering with anyone engaged in the operation of an aircraft is an offense punishable by up to 20 years in prison and fines of $500,000 or more.

John Rodgers Field lies on the approach path for Honolulu International Airport, and the suspected "sky pointer" used in the Coast Guard incidents has a range of 9,000 feet and costs about $100, said Coast Guard spokeswoman Chief Petty Officer Marshalena Delaney.

"That's what our investigators believe because of the range where the air crews were when they felt the laser on their eyes," Delaney said. "They were fairly far from the possible source."

From December 2004 through January 2005, police conducted air and ground searches in response to reports of lasers being pointed at pilots at undisclosed locations. Police did not release information on the other two lasing reports received this year.

Reach Rod Ohira at rohira@honoluluadvertiser.com.