Spectral reflectance of the human ocular fundus.

Reflectance spectra from discrete sites in the human ocular fundus were measured with an experimental reflectometer in the visible and near-infrared parts of the spectrum. The principal study population consisted of ten subjects 22 to 38 years of age with a wide range of degree of fundus melanin pigmentation. Reflectance spectra were obtained from the nasal fundus, the fovea, and an area 2.5 degrees from the fovea. Spectra were also recorded from several older subjects and from one aphakic patient with a coloboma. The reflectance spectra were found to be influenced by the degree of individual and local melanin pigmentation of the fundus, the amount of blood in the choroid, the transmission properties of the ocular media, and the discrete reflections in the stratified fundus layers. Mathematical models of the optical properties of the stratified layers are proposed and are fitted to the experimental fundus reflectance spectra. The models account for the absorption by blood, melanin, macular pigment, and ocular media, and incorporate tissue scattering and discrete reflectors corresponding to anatomical layers.

Fundus reflectometry for photocoagulation dosimetry.

A dosimetry technique has been developed which utilizes three-wavelength fundus reflectometry, a quantitative model of fundus reflectance, and a model of thermal tissue damage to control photocoagulation lesion size. The fundus reflectance model uses Lambert-Beer's law exponential attenuators to describe the ocular media, retinal pigment epithelium (RPE), and choriocapillaris transmission characteristics while the choroid is described as a Kubelka-Munklike homogeneous scatterer/absorber. Three reflecting layers are included in the model at the retinal inner limiting membrane, Bruch's membrane, and the sclera. Measured lesion size variability contained components which resulted from variations in choroidal blood and melanin and RPE melanin concentration. Photocoagulation dosimetry was found to reduce the photocoagulation lesion size coefficient of variation for red light from 45% to 10% for the control and dosimetry cases, respectively. Similar improvement was noted for yellow photocoagulation light.

A new solid-state, frequency-doubled neodymium-YAG photocoagulation system.

We have developed a solid-state laser system that produces a continuous green monochromatic laser beam of 532 nm by doubling the frequency of a neodymium-YAG laser wavelength of 1064 nm with a potassium-titamyl-phosphate crystal. Photocoagulation burns of equal size and intensity were placed in two rabbit eyes with the solid-state laser system and the regular green argon laser system, respectively, using the same slit-lamp mode of delivery. Histologic findings of lesion sections revealed no important differences between the two systems. In theory, the longer wavelength of the solid-state laser offers the advantages of less scattering in ocular media, higher absorption by oxyhemoglobin, and less absorption by macular xanthophyll than the 514-nm wavelength of the regular green argon laser. The solid-state laser has impressive technical advantages: it contains no argon-ion gas tube that wears out and is expensive to replace; it is much more power efficient, and thus considerably smaller and compact; it is sturdier and easily movable; it does not require external cooling; it uses a 220-V monophasic alternating current; and it requires little maintenance.

Retinal illuminance using a wide-angle model of the eye.

A computer model of relative retinal illuminance, based on our optical wide-angle model of the eye, is proposed for the cases of Ganzfeld illumination and the Maxwellian view through a range of visual field angles from 0 degrees to 80 degrees. The proposed model is designed to be functionally correct and to represent closely the anatomical parameters of the eye. Unlike earlier proposed models, this model is based on our previously reported measurements of spherical aberration in 100 eyes in vivo and is designed to be correct in the peripheral field and with large pupils. Data are reported for pupil diameters of 2, 4, and 8 mm. The effect of crystalline lens extinction on retinal illumination is also estimated for average eyes of young (age 19 years) and old (age 63 years) individuals at two wavelengths (410 and 532 nm). In the Ganzfeld case, illumination of the retina decreases with increasing visual angle. In the Maxwellian view, retinal illuminance increases with increasing visual angle.

[A universal laser for intraocular treatment]. / Laser universel pour traitement intraoculaire.

The creation of a single apparatus incorporating the different lasers applicable to intraocular pathology derives from the idea of possessing the means, with one device, of objectively comparing the effects and parameters of various lasers in order to pursue a more precise line of treatment. This has led to our fabrication of the universal intraocular laser. The word universal means the ability of the machine to act on each kind of intraocular tissues with all of the various infrared or other-colored radiations, as well as with different modalities. The unit is transportable. It only requires a source of electric power (110 or 220 V) and includes an independent cooling system. The Nd: YAG laser has selectable operating parameters: pulsed or continuous excitation, Q-switched or mode-locked mode, mirror or fiber optic transmission. It is used also as the basic system of the coloured module. The colour module can provide the three clinically useful radiations: green, red, yellow. The green (532 nm) is obtained by transmission of the I.R. beam brought a birefringent crystal (KTP). Red (650 nm) and yellow (575 nm) come from two incorporated dye lasers excited by the green radiation.

A step-zoom probe for laser endophotocoagulation: I. Design.

A step-zoom probe for laser endophotocoagulation of the eye has been developed, using fiber optics and a gradient index (GRIN) rod lens. It provides the operator with five or more focused spots ranging in size from 50 to 500 microns, and the spot size on the target is independent of the media filling the vitreous cavity, i.e., fluid or air. Each spot is located at the waist of the beam and therefore can be easily placed on the target, reducing the potential for overtreatment. A prototype model is currently undergoing animal testing.

A new method to perform the cover test.

A new method is presented to perform the cover/uncover test and the alternate cover test. With the subject's head in a chin rest, he fixates an object at either near or far distance, and one eye is illuminated from above with a collimated infrared beam. A high-sensitivity infrared TV camera observes the first and fourth Purkinje images from below. The examiner covers the nonilluminated eye and, viewing a TV monitor, compares the relative positions of the two images before and after covering. By changing the incident angle of the beam, the amount of deviation can be measured. To perform the alternate cover test, the illuminated eye is covered with an infrared transmitting filter.