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  • Laser Safety Training Course
  • Laser Eye Hazards

    Eye Structure

    The potential for injury to the different structures of the eye (figure 7) depend upon which structure absorbs the energy and any repair processes that may exist for the structure.

    Figure 7 - Structures of the Eye

    Cornea

    The cornea does the initial focusing of light as well as protecting the various internal structures from the environment. 

    The cornea, because it is subjected to the environment is constantly replaced every 48 hours. 

    Damage to this layer is usually repairable.

    Lens

    The lens is the main refractive tissue in the eye which focuses images on to the retina.  Damage to the proteins that comprise the lens will result in cloudiness often referred to as a cataract. 

    There is no repair mechanism for the lens, once it has been damaged it must be replace surgically.

    Retina

    The retina is the receptor organ for light focused on it by the lens.  It consists of non-reparable specialized nerve tissue.

    Damage to parts of this tissue will result in permanent blindness or blind spots

     

    Ocular Image

    Wavelengths between 400 nm and 1400 nm are focused by the curved cornea and lens on to the retina, the optical gain is about 100,000-200,000 times. 

    Viewing a laser beam or Point Source(Fig.8) will focus all the light on a very small area of the retina, resulting in a greatly increased power density and an increased chance of damage.

    A large source of light such as a diffuse reflection of a laser beam produces light that enters the eye at a large angle is called an extended source. An extended source produces a relatively large image on the retina ( figure 9) and energy is not concentrated on a small area the retina as in a point source.

     

    Figure 8 - Point Source Viewing

    Point Source

     

     

    Figure 9 - Extended Source Viewing

    extended source

     

     

    Details of Irradiation Effects on Eyes

    Corneal Effects

    Ultraviolet-B+C (100 - 315 nm)
     
     

    The surface of the cornea absorbs all UV of these wavelengths (Figure 10) which produce a photokeratitis (weld flash) by a photochemical process which cause a denaturation of proteins in the cornea . This is a temporary condition because the corneal tissues regenerate very quickly.
    Infrared-B and Infrared-C (1400 nm to 1.0mm)


    Corneal tissue will absorb light with a wavelength longer than 1400 nm (Figure 10). Damage to the cornea results from the absorption of energy by tears and tissue water causing a temperature rise and subsequent denaturation of protein in the corneal surface.

    Figure 10 - UV-B+C & IR-B+C




    Lens Effects


    Ultraviolet -A( 315 - 400 nm)

    The lens absorbs Ultraviolet radiation of these wavelengths (Figure 12)

    Photochemical processes denature proteins in the lens resulting in the formation of cataracts.

     

    Figure 12 - UV-A

    Retinal Effects

    Visible light and Infrared-A (400 - 1400 nm)

    The cornea, lens and vitreous fluid are transparent to light of these wavelengths but it is absorbed by retinal tissue(Figure 11).

    Damage to the retinal tissue occurs by absorption of light and its conversion to heat by the melanin granules in the pigmented epithelium or by photochemical action to the photoreceptor. The focusing effects of the cornea and lens will increase the irradiance on the retina by up to 100,000 times. For visible light 400 to 700 nm the aversion reflex which takes 0.25 seconds may reduce exposure causing the subject to turn away from a bright light source. However this will not occur if the intensity of the laser is great enough to produce damage in less than 0.25 sec. or when light of 700 - 1400 nm (near infrared) is used as the human eye is insensitive to these wavelengths.
     

    Figure 11 - Visible & IR-A