Fourth Case

In a British university, a student was aligning two pulsed lasers with different wavelengths for his experiment. The first one was a dye laser emitting at 720 nm, delivering 10 ns pulses carrying 10 mJ with a repetition rate of 10Hz. The second one was a pulsed Nd:YAG laser emitting at 266 nm, delivering pulses of same duration and at the same repetition rate but carrying 50 mJ. In this experiment, the beam emitted by the first laser was passing through a dichroic mirror, fully reflecting at 266 nm and highly transparent at any other wavelength. Unfortunately, the back side of the mirror was reflecting about 5% of the 720-nm-beam towards the ceiling. As he couldn't see the beam at 266 nm, the student briefly worked without safety goggles – and then the accident occurred. As he had totally forgotten the presence of the parasitic beam at 720 nm, he bent over the mirror and received a laser pulse from this reflection. He immediately noticed a blind spot in the central sight of his eye while looking at some object in the laboratory. Still, as any operator aware of the laser hazard, he was most of the time wearing safety goggles at 266 nm...

He was immediately sent to the hospital in order to make opthalmologic exams. The doctor then noticed a small burn on the fovea. The student was aware of the presence of the beam and had placed some obstacles along its trajectory during the previous experiments. However, every obstacle had been removed for this new experiment.

One must emphasize the following characteristics of this accident :

The safety goggles must never be removed. In the present case, although there were two different wavelengths, the operator should have been wearing his safety goggles : even if they were specified for one precise wavelength, they would nevertheless have protected the eye against other wavelengths – with a lower efficiency, but still. For any new experiment, the operator must evaluate the new hazard sources he will be exposed to.

Consequently to a laser accident, the eye injuries are often irreversible, except in some precise situations : it is thus very important to be subjected to medical exams as soon as possible, within the next 24 hours. One can also use a class 1 laser during the adjustment, harmless to the eye.