Alexander M. Clarke

An equilibrium thermal model for retinal injury from optical sources.

A uniform absorption thermal model is described which allows the calculation of the temperature rise in the retina due to steady state or continuous optical irradiation. Temperature rises of 9-10 degrees C are found to correspond to the production of threshold lesions. For a worst case approximation, a power of 1-2 mW entering the eye and focused onto a 10-micro diam area for 250 msec or longer can be shown as sufficient to cause irreversible damage.

OCULAR EFFECTS OF LASER RADIATION. PART I

Although this paper is confined primarily to the biological effects of laser radiation on the mammalian retina the results should be applicable to other organs and other tissues. The physiological implications for the eye of laser beams have been discussed by Solon (l), (2); the production of ocular lesions by ruby lasers has been described by Zaret (3), (4), and, more recently, by Ham (5) and Geeraets (6). For convenience, retinal lesions produced by lasers will be discussed under 3 separate types of operation, steady state operation, normal pulsed operation, and Q-switched operation. Before proceeding to a detailed discussion of laser effects, it will be helpful to summarize some of the data about retinal damage as produced by more conventional sources in order to provide a background which can be extended to high power densities and short exposure times.

The Effect of Threshold Macular Lesions and Subthreshold Macular Exposures on Visual Acuity in the Rhesus Monkey

The purpose of this research program was to evaluate retinal threshold burns and subthreshold exposures of the mammalian macula in terms of visual acuity. Rhesus monkeys {Macaca mulatto) were trained by a reward system to respond to the automated presentation of Landolt rings. After appropriate training, these animals were exposed to threshold and subthreshold levels of retinal energy density ranging from 3.2 to 10.7 J/cm2, exposure time approximately 135 ms. spectral quality approximately that of color temperature 6000° K with wavelengths above 900 nm removed,, and image sizes on the retina of about 1 mm in diameter, covering a major portion of the monkey macular area. Results, in terms of visual acuity decrement (monocular), indicated that energy densities on the retina below 5 J/cm2 were not statistically significant, whereas energy densities greater than 5 J/cm2 produced losses in visual acuity (monocular) which were significant. These results indicate that, at levels of energy density on the retina w...

THE EFFECT OF PULSED RUBY LASER ON RETINAL PIGMENT EPITHELIUM IN VITRO

Since the introduction of the laser in 1960, many attempts have been made to ascertain threshold values for damage to the sensitive retinas of various mammalian eyes employing in vivo methods. In v i t r o studies have been made by Rounds (1965) and Rounds et al. (1965) on cells and tissues in culture and by King & Geeraets (1968) on the effects of Q-switched ruby laser radiation upon explants of ten-day old chick embryo retinal pigment epithelium. The investigation presented here has been patterned after the latter study, substituting a pulsed ruby laser, thereby affording a pulse duration in the microsecond instead of the nanosecond range. Previous investigations (Geeraets et al. 1965, Ham et al. 1958, 1965, 1966) have demonstrated the dependence of retinal threshold damage by thermal energy density (irradiance) upon both duration of the exposure and the spot size of the retinal image. The power density required to produce a threshold lesion has been shown to be inversely proportional to the diameter of the spot size for relatively long exposure times (> 100 ms) as a result of the greater heat dissipation associated with the smaller lesion geometry. The threshold energy density

A reading aid for patients with macular blindness.

A new reading aid (called ‘kraspegig’) for patients with macular blindness is described. It is based on the principle that, in peripheral vision, our attention is strongly drawn to moving objects. Accordingly, it sets the reading matter moving with respect to the patient’s retina. Its essential part is a patterned mask into which is cut a window through which a few words of text can be seen. As the mask is slid along, the patient follows it with his eyes, so that the text lags behind. A magnifier is part of the device.

Phototherapy of the Cornea: Some Aspects to be Considered

To safeguard against and reduce possible harmful and undesired side effects of photon interaction with ocular structures during phototherapy of certain corneal diseases, precautionary measures should be taken. These include (1) Selection of the proper light source, strongly emitting in the effective spectral range. (2) Removal of potential harmful spectral bands which make no significant contribution to the therapeutic action. (3) Determination of the most optimal exposure energy (W/cm2). (4) Selection of the most optimal time-dose relationship. (5) Selection of the proper interval between vital staining and radiation exposure and considering possible repetition of therapeutic exposures. (6) Careful calibration of the light source to be used. (7) Adequate eye examination for visual acuity, pupillary diameter, signs of intraocular inflammation, mainly iritis with anterior chamber activity, and possible fundus pathology with special attention to maculopathy. These findings should be properly recorded before, during and after the period of phototherapy.