The medical literature on the ocular hazard of ultraviolet laser radiation indicates that, at the wavelength and fluence of the tetanizing weapon, there is little possibility of injury to the cornea and none to the retina or the other internal structures of the eye. This is because the cornea absorbs virtually all such shortwave radiation. Only those wavelengths between about 1200 nm in the infrared and 200 nm in the ultraviolet are able to reach the retina (Verhoeff and Bell, 1916). More precisely, Lembares et al. (1997) found that "corneal absorption is relatively weak from 266 to 248 nm, increases steeply from 248 to 213 nm, and remains strong from 213 to 193 nm." At 193 nm the corneal absorption coefficient is extremely high, Petit and Ediger (1996), having calculated a value of 39,900 cm-1.
However, if the tetanizing weapon were directed at the eyes for several minutes the target subject would experience actinic conjunctivitis and photokeratitis. Conjunctivitis is the inflammation of the conjunctiva, which is the membrane that covers the anterior surface of the eye and the eyelid. Photokeratitis is a painful irritation of the cornea that is accompanied by a sensation of foreign matter in the eye, minor swelling, lacrimation and photophobia. There is no permanent damage and the effects usually disappear within six to twenty-four hours (Pitts, 1973).
All ocular damage is a function of both wavelength and intensity. A beam of shortwave ultraviolet radiation can ionize the oxygen molecules in air enough to conduct an electrical current (Scheps, 1997) without harming the cornea (Pitts, 1973). Puliafito et al. (1965) determined that the lowest per-pulse ultraviolet fluences that will ablate human corneas are 46 mJ cm-2 at 193 nm and 58 mJ
cm-2 at 248 nm. Moreover Trokel et al. (1983) found no evidence of thermal damage during 193 nm corneal ablation even at an energy level of 1 J cm-2.
The relative ocular safety of shortwave ultraviolet radiation is in contrast to the extreme hazard of equally-ionizing visible light, as may be produced by a femtosecond-pulsed laser. For example, at 193 nm molecular oxygen has a two-photon absorption cross section, whereas through most of the visible range it has a six-photon cross section. This means that the peak power needed for the ionization of air by a visible-light beam is much too high for safety. At such wavelengths and intensity it would penetrate the cornea unhindered and deeply burn the retina.
In summary, the cornea protects the internal ocular structures from shortwave ultraviolet radiation. If the fluence is below the ablation threshold at the corneal surface it may cause irritation and annoyance, but no lasting damage.
