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Laser Safety - Biological Effects

The Biological Effects of Laser Radiation

Why is Laser Radiation Dangerous?

The ‘light’ from powerful lasers can be concentrated to power densities (power per area or Watts/m2) that are high enough to evaporate human tissue and even metal or ceramics. In the medical field, laser radiation is used to remove tattoos or to cut human tissue. These are examples of applications which require high power lasers and as such there is a high potential risk of accidental exposure of the laser beam to the user. Because the eyes are much more sensitive to light, they are at increased risk. In fact, it is possible to cause irreversible ocular injury with just one glance into a direct or reflected laser beam even at relatively low power levels. Laser eye protection must be taken extremely seriously.

Laser light can travel over great distances as a nearly parallel, collimated beam. This means that the beam can travel a considerable distance and still represent a significant ocular hazard. Compare an ordinary light bulb with a laser and you will note an important difference. The light bulb emits light more or less equally in all directions and in accordance with the inverse squared rule, the irradiance decreases with the square of the increase in distance travelled. The power received through a 7mm limiting aperture at a distance of 1m from the lamp would be 100,000 times lower than for a laser that emits the same optical power. The 7mm aperture is not just a random area - this is the diameter of the dilated pupil in a dark adapted eye.

In addition to the quantity of light that can hit the eye, the coherent radiation produced by a laser can be focussed to extremely small sizes. While the light bulb creates an image on the retina of approximately 100μm, the laser light is reduced to a spot of just a few micrometers (~ 10μm) in diameter. Therefore, the power from the laser that hits the eye which is already 100,000 higher than for the lamp is further concentrated into a much smaller focussed spot on the back of the eye. The power density resulting from this concentration may be sufficient to cause irreversible damage to the retina. In the worst case, a single laser pulse can cause a total loss of sight in that eye.

Different Laser Wavelengths Present Different Hazards

The risk of losing your eyesight from an accidental exposure to laser radiation is due to the special optical properties of the human eye. The type of ocular hazard depends on the wavelength of laser light you are exposed to. The eye is only transparent in the wavelength range between 370 and 1400nm.

Ocular Hazards

UV Radiation Below 350nm

Ultra violet light at wavelengths shorter than 350nm either penetrates as far as the lens or is absorbed on the surface of the eye. A consequence of exposure to high power laser light at these wavelengths is damage to the cornea by ablation,  or by the creation of a cataract.

Visible Radiation from 380-780nm

Light in the visible wavelength region (380-780nm) penetrates through the cornea and lens and onto the retina. The eye sees light in this wave band and has developed natural protective mechanisms against bright lights. When the light appears too bright, we automatically turn away and close our eyes. This aversion response is called the blink reflex. This automatic reaction is effective for radiation up to 1mW power. With higher power levels, too much energy reaches the retina before the blink reflex can respond, which can result in irreversible thermal damage and loss of vision.

IR Radiation Above 780nm

The near infrared wavelengths (780-1400nm) are a type of radiation that is especially dangerous to the human eye because there is no natural protection mechanism. The radiation in this band again penetrates to the retina, but the light is invisible and a dangerous exposure is only noticed after the damage has been done. Infrared radiation at wavelengths above 1400nm is absorbed at the surface of the eye. It leads to overheating of the tissue and burning, or ablation of the cornea.

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