Study Questions Class 2 Laser Safety

[joseph.greco@kodak.com]

From: Joseph M. Greco



The following may be of interest to the laser safety community.



J. M. Greco, CHP

Eastman Kodak Company

joseph.greco@kodak.com



_______________________________



Optics.org - news ? Study questions Class 2 safety

Exclusive from Opto & Laser Europe (OLE) magazine,   E-mail info@optics.org



The eye's blink reflex cannot be regarded as protection against

overexposure to laser radiation, according to a German researcher.

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."





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