Type of hazards

Why do we have to be protected from laser radiation? The "light" from powerful lasers (Class IV) can be concentrated to power densities (= power per area) that are high enough to evaporate metal or ceramics.

Consequently, it is not surprising that laser radiation also can cut or destroy human tissue. There is another potential for danger - our eyes are in particular danger as they are much more sensitive to light, and it is possible to irreversibly lose one's eyesight with just one glance into a laser beam even with lower power.

What makes a laser so dangerous compared to usual light sources? The main danger coming from a laser is due to the so called "spatial coherence”. That means that the wave trains of the laser beam have a fixed relation to space and time.

In other words, laser light is well ordered and regular regarding the direction in which it travels as well as regarding the wavelength. That means that laser light can travel over far distances almost parallel (collimated radiation) and thus the power that can meet an area e.g. the eye, is independent from the distance to the source of radiation.

Just think of a laser pointer with which you will have a beam spot that is always about the same size despite different distances.

If you compare a thermal source of radiation, e.g. an electric bulb, with a laser, you will recognise several differences. The electric bulb radiates light of a very broad spectrum of wavelengths and there is no preferred direction of dispersion. A physicist would say: The bulb supplies incoherent light.

When comparing an electric bulb with a laser, both with 1 W optical power, the power of the bulb that may reach the eye decreases with distance because the bulb is radiating in all directions. If we assume a distance of 1 metre between our eyes and the light source, then the quantity of light coming from the laser would be bigger by a factor of 100,000 compared to the light quantity from the bulb (with a pupil diameter of 7 mm - i.e. eyes adapted to darkness).

The quantity of light that may hit the eye is not the only danger. While the bulb would create a picture on the retina of approx. 100 µm, the laser light, which can be much more easily focussed, is reduced to a focus of just a few micrometers (~ 10 µm).

Thus, the light quantity that hits the eye is concentrated on a much smaller spot. The power density (power per area) resulting from this concentration is so high that any tissue in the focus will be heated up and destroyed very quickly.

Since the sensitive spot with which we can see - the yellow spot (fovea) on the retina - also has a size of just a few micrometres, it is possible to lose one's eyesight by one single laser pulse.