Francois C. Delori

Confocal scanning laser ophthalmoscope

A confocal scanning imager moves an illumination spot over the object and a (virtual) detector synchronously over the image. In the confocal scanning laser ophthalmoscope this is accomplished by reusing the source optics for detection. The common optical elements are all mirrors-either flat or spherical-and the scanners are positioned to compensate astigmatism due to mirror tilt. The source beam aperture at the horizontal scanner is small. Light returning from the eye is processed by the same elements, but now the polygons facet is overfilled. A solid-state detector may be at either a pupillary or retinal conjugate plane in the descanned beam and still have proper throughput matching. Our 1-mm avalanche photodiode at a pupillary plane is preceded by interchangeable stops at an image (retinal) plane. Not only can we reject scattered light to a degree unusual for viewing the retina, but we choose selectively among direct and scattered components of the light returning from the eye. One (of many) consequences is that this ophthalmoscope gives crisp and complete retinal images in He-Ne light without dilation of the pupil.

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.

Maximum permissible exposures for ocular safety (ANSI 2000), with emphasis on ophthalmic devices.

After discussing the rationale and assumptions of the ANSI Z136.1-2000 Standard for protection of the human eye from laser exposure, we present the concise formulation of the exposure limits expressed as maximum permissible radiant exposure (in J/cm(2)) for light overfilling the pupil. We then translate the Standard to a form that is more practical for typical ophthalmic devices or in vision research situations, implementing the special qualifications of the Standard. The safety limits are then expressed as radiant power (watts) entering the pupil of the eye. Exposure by repetitive pulses is also addressed, as this is frequently employed in ophthalmic applications. Examples are given that will familiarize potential users with this format.

Infrared imaging of sub-retinal structures in the human ocular fundus

The interaction of infrared light with the human ocular fundus, particularly sub-retinal structures, was studied in vivo. Visible and infra-red wavelengths and a scanning laser ophthalmoscope were used to acquire digital images of the human fundus. The contrast and reflectance of selected retinal and sub-retinal features were computed for a series of wavelengths or modes of imaging. Near infrared light provides better visibility than visible light for sub-retinal features. Sub-retinal deposits appear light and thickened; the optic nerve head, retinal vessels, and choroidal vessels appear dark. Contrast and visibility of features increases with increasing wavelength from 795 to 895 nm. Optimizing the mode of imaging improves the visibility of some structures. This new quantitative basis for near infrared imaging techniques can be applied to a wide range of imaging modalities for the study of pathophysiology and treatment in diseases affecting the retinal pigment epithelium and Bruchs membrane, such as age-related macular degeneration.

Noninvasive technique for oximetry of blood in retinal vessels.

A noninvasive spectrophotometric technique for the measurement of oxygen saturation of the blood in discrete retinal vessels is described. The instrument, the retinal vessel oximeter, uses scanning fundus reflectometry to determine the optical density of a retinal vessel at three wavelengths (558, 569, and 686 nm). Oxygen saturation is determined after compensation for the effects of light scattering by the red blood cells by relating the measured densities with the corresponding extinction coefficients nf oxyhemoglobin and deoxy-genated hemoglobin. The vessel diameter is also measured continuously. All data acquisition and analysis are performed on-line by means of a microcomputer, and a vessel tracking system is used to compensate for the effects of eye movements. Oxygen saturation measurements for blood flowing through glass capillaries are presented as well as representative results of oxygen saturation measurements on normal human subjects.

Relationship of Senile Macular Degeneration to Ocular Pigmentation

We prospectively evaluated 650 consecutive white patients with senile macular degeneration and compared them to a control group of 363 patients. Ocular pigmentation (iris color and fundus pigmentation) was recorded for each patient, as was hair color (as a child and young adult) and age at evaluation. Patients were from the New England states and Florida. Our most significant finding was that 494 patients with senile macular degeneration (76%) had light-colored irides compared with 145 of the controls (40%). Fundus pigmentation closely corresponded to iris pigmentation (P less than 0.01). Hair color was blond or light brown in 370 of the patients with senile macular degeneration (57%) and in 105 of the controls (29%). Further, there was a tendency for individuals with lightly pigmented irides to have senile macular degeneration at an earlier age than those with dark irides (P less than .01). Thus, increased ocular pigmentation tends to decrease the risk of developing senile macular degeneration.

Potential Retinal Hazards: Instrument and Environmental Light Sources

Light can cause retinal damage by mechanical, thermal, or photochemical mechanisms. Mechanical and thermal injury require a very intense light exposure, but photochemical injury is caused by a prolonged exposure to light levels that probably would be well tolerated if experienced only transiently. The existence of photochemical retinal damage has prompted concern about its possible role in macular degeneration, and reexamination of the safety of clinical light sources. An analysis of the potential hazardousness of these light sources is presented, in addition to a review of retinal damage mechanisms, and suggestions for pertinent patient counseling.

Central sparing in annular macular degeneration

Using fluorescein angiography and monochromatic photography, we measured the size of the central sparing in 45 patients with annular maculopathy (mean +/- S.D., 0.34 +/- 0.15 disk diameter; range, 0.10 to 0.65 disk diameter) and compared it with the size of macular yellow pigment in 40 subjects (mean +/- S.D., 0.31 +/- 0.12 disk diameter; range, 0.1 to 0.5 disk diameter). The close approximation of these values suggested that macular yellow pigment contributed to the annular pattern through a photoprotective mechanism.

Laser Doppler Technique for Absolute Measurement of Blood Speed in Retinal Vessels

We describe an electrooptical laser Doppler system and technique of data analysis that provides absolute measurements of the speed of red blood cells flowing at discrete, selectable sites in the retinal vasculature. We present in vitro test measurements of the instrumentation as well as an example of an in vivo measurement from a patient with retinal vascular disease. We also present experimental data leading to the derivation of the relationship between the blood speeds measured in retinal arteries during the minimum diastolic and maximum systolic phases of the cardiac cycle and the time-averaged blood speed. Mean blood flow rate is calculated using the time-averaged speed and the cross-sectional area of the vessel at the measurement site. We discuss the criteria for selection of the measurement sites and assess the reproducibility of the measurements. We conclude that measurements on retinal arteries are less susceptible to experimental artifacts and provide more information than do measurements on retinal veins. The system is currently being used clinically in studies of retinal circulatory alterations in patients with diabetic retinopathy, arterial occlusive disease, retinal detachment, and carotid artery disease.

Spectrophotometer for noninvasive measurement of intrinsic fluorescence and reflectance of the ocular fundus

A spectrophotometer for noninvasively measuring the intrinsic fluorescence and the reflectance of the ocular fundus is described. The instrument uses multichannel spectral analysis for recording fluorescence emission spectra (500-800 nm) with seven excitation wavelengths between 430 and 550 nm and for the determination of fundus reflectance spectra (400-800 nm). Measurements are performed from a discrete fundus area, with a spatial resolution of a 1-2° visual angle. Calibration procedures are detailed. Representative fluorescence and reflectance spectra obtained from five normal subjects indicate that the fluorescence originates from within the fundus layers. Although the absolute fundus fluorescence measurement is affected by lens absorption and ocular refraction, it is minimally influenced by the strong fluorescence of the crystalline lens.