17 Jun 2002
The eye's blink reflex cannot be regarded as protection against overexposure to laser radiation, according to a German researcher.
Exclusive from Opto & Laser Europe (OLE) magazine
Hans-Dieter Reidenbach from the University of Cologne found that the eye's blink reflex does not exist for coherent light sources. He says that his findings mean that Class 2 lasers can now no longer be regarded as safe and that greater care should be taken when using low-power lasers.
The current laser-safety standards are based on the assumption that most people have a blink reflex that closes the eye before 250 ms of exposure. This figure, however, was calculated using flashlamps more than 40 years ago during atom-bomb research.
"Our original work used incoherent radiation, such as a camera flashlight, which confirmed that most people have a blink reflex. When we tried the same experiment with lasers, we found that less than 20% of our volunteers exhibited a blink reflex when illuminated by a laser with a power of between 0.8 and 1.0 mW and at wavelengths of 670, 635 and 532 nm," said Reidenbach.
In fact, at 670 nm none of the volunteers showed a blink reflex. Reidenbach admits that he finds his own results surprising and he plans to conduct more research to verify these findings. "We tested 200 volunteers at the Laser exhibition in June and we want to test a total of 1000 people," he said.
He has a hypothesis to explain the results: "I believe the difference is the picture on the retina. A laser produces a small dot of about 10 to 15 µm in diameter, whereas a flashlight gives a dot of about 1 mm in diameter, thus illuminating the entire fovea. Every cell in the fovea contributes 1 pA of current and you need to pass a certain threshold to achieve a blink reflex."
Reidenbach's results have been met with scepticism from laser-safety experts. Karl Schulmeister from the Austrian Research Centre in Seibersdorf, Austria, has been involved in the development of the new edition of the international laser-safety standard and took part in Reidenbach's study. He said: "I do not want it to be said that Class 2 lasers are unsafe. This is not true. There is a large safety margin built into the standards and Reidenbach's work does not change this."
Schulmeister also questioned Reidenbach's test method. "The volunteers were told that they would be targeted, so they were not startled when it happened. The blink reflex works much better when the subject is surprised by the laser beam."
David O'Brart, a consultant ophthalmologist at St Thomas's Hospital in the UK, agrees. "The fact that people know that the laser beam is coming could affect the result. However, Reidenbach's work does raise several issues regarding the blink reflex and laser safety, and these issues need to be addressed."