Emergency War Surgery NATO Handbook: Part IV: Regional Wounds and Injuries: Chapter XXV: Laser Injury of the Eye

Laser Principles

United States Department of Defense
Peer Review Status: Internally Peer Reviewed

1. Basics. A laser produces a beam of coherent light which travels at 186,000 miles per second, the speed of light. This beam can vary in wave length throughout the electromagnetic spectrum and can be visible or invisible. The common wavelengths of laser rays correspond approximately to the wavelengths of colors in the spectrum, specifically, the ultraviolet (below 400 nm), the visible (400-700 nm) , and the infrared spectra (above 700 nm). These various wavelengths of energy are absorbed by different layers within the eye.
   1. Ultraviolet. Lasers utilizing the ultraviolet spectrum (below 400 nm) are absorbed in the anterior segments of the eye, primarily by the cornea, as well as by the lens.
   2. Visible. Laser radiations in the visible spectrum (400-700 nm) are absorbed primarily within the retina by the pigment epithelium and the choroid. Penetration depth is greater for the longer wavelengths (red) than with shorter wavelengths (blue).
   3. Infrared. Absorbtion of lasers in the infrared spectrum (above 700 nm) occurs in two areas of the eye Lasers at the lower end of the infrared spectrum (1000 nm) damage the retina and the choroid, whereas the cornea is damaged by lasers at the top end of the infrared spectrum (3000 nm).
   Table 14 - Typical lasers and their wavelengths
   

   Krypton	350 nm	ultraviolet
   Argon	514 nm	visible
   Ruby	694 nm	visible
   CO2	10,600 nm	infrared

2. Continuous versus Pulsed Waves. Continuous wave lasers, as the name implies, are constantly emitted. These continuous wave lasers vary in energy output from fractions of a watt up to the kilowatt range. In contrast, pulsed lasers deliver lower energy levels (10-50 microwatts), but nevertheless exhibit a higher potential for eye injury. The greater destructive power of the pulsed laser lies in the very short time interval (billionth of a second, ns) over which the energy is delivered. On a comparative basis, a 20 mj pulse delivered over a 20 ns time period is comparable in power to one million watts of continuous laser emission.
3. Collimation. To collimate is to make parallel. The beams emitted from a laser, although not perfectly collimated, are very close to being parallel. The converse is true of the beams of light emitted from an ordinary incandescent light bulb, which diverge in all directions. As a result of this small divergence of laser beams, the entire silhouette of a soldier or the entire optical system of a battle tank can be covered by a single laser source six kilometers away.
4. Irradiance. Irradiance refers to the concentration of energy applied per unit area. Irradiance is expressed in watts per square centimeter. The energy output of a particular laser is a constant feature of that laser, whether it be the continuous or pulsed variety. Laser beams can be focused onto a small target or defocused by beam divergence to cover a larger area, the energy per unit area correspondingly increasing or decreasing according to the square of the target size. For example, because of divergence, the area covered by a beam at six kilometers is greater than the area covered by the same beam at one kilometer; however, the energy impacted per unit area (irradiance) is greater at one kilometer. Therefore, the "energy dose" received by the human eye at six km is less than at one km. On the other hand, optical devices such as binoculars, periscopes, and weapons-sighting devices all gather light and laser waves and magnify by converging the rays onto a smaller surface area within the eye, thereby increasing the potential for damage.
5. Tissue Effects. The biological effects produced by lasers are different for continuous and pulsed lasers. Continuous wave lasers produce primarily a thermal effect, photocoagulation. Eye examination may reveal superficial and deep burns of the cornea with opacification and tissue loss, or areas of retinal burns and necrosis. Pulsed lasers, on the other hand, produce injury faster than thermal conductivity principles would predict. Pulsed lasers produce mechanical effects, acoustic shock waves, ultrasonic waves, and high energy fields. The end result is tissue disruption (manifested as retinal tears) hemorrhage of the retina and the vitreous, and subsequent necrosis of the retina and the vitreous.

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