We have developed a computer software program that analyzes the reproducibility of corneal topography systems. Using a Macintosh computer with Microsoft QuickBasic programming we created a program that records and analyzes the corneal power of 72,000 points per topography map (200 points over each one degree meridian) by linear interpolation of the Placido-based ring data. Reproducibility studies on 7 normal and 7 abnormal (pathological and postsurgical) corneas were performed with 2 Placido ring based topography systems with EyeSys Corneal Analysis System and the Humphrey MasterView system. Five exams were performed by two technicians each. The mean, range and standard deviation of corneal power was calculated for all points on a map, and represented pictorially for each patient. Statistical analysis of standard deviation at each point was used in assessing the reproducibility between systems and between technicians. The topographic maps of mean power range and standard deviation are presented for selected patients. The overall mean standard deviation of power and the means within concentric annular zones and quadrants are presented for each system and technician for both normal and abnormal cornea. Intraobserver variability of each system were not significantly different for both normal and abnormal corneas. There was no significant difference between technicians, nor were there regional differences in variability.standard deviation of power is presented for each system and technician comparing intersystem and intraobserver variability for both normal and abnormal corneas. A precise method of analyzing multiple variation of reproducibility in corneal topography is now available through a new proprietary software system.

The human cornea is not a surface with simple geometric shape especially after refractive surgery. Thus, conventional topographical instruments of the corneal curvature can not measure the correct values on the peripheral cornea. This is because the assumptions made in current topographical instruments are based on that the human cornea is composed of spherical surfaces. The peripheral cornea is not only important for shape formation after refractive surgery, but also important for the high contrast resolution of vision. This paper presents a new algorithm, which do not depend on the assumption of the corneal shape. The mathematical equations are based on polynomial equations and geometric optics of the measurements. By adjusting the coefficients, the polynomial equations can approach the curves of complicated shapes. The equations based on polynomial modeling are nonlinear, thus the Newton method is applied to simplify the equations. To implement experiments for the new algorithm, a setup is built at the Ohio State University. Topographical images are captured from simulated corneal surfaces and the image is processed using C programming. The curvature information is acquired through numerical analysis of polynomial equations. Spherical surfaces and aspherical surfaces are tested on this setup as well as on Topographical Modeling System and EyeSys Corneal Analysis System. The results show that the polynomial modeling can measure the radius of curvature for spherical or aspherical surfaces within 0.05 mm of error. Results also show that the aspherical surfaces measured on Topographical Modeling System and EyeSys Corneal Analysis System have about 0.3 mm of error at the periphery of 3.0 mm from optical axis. However, the polynomial modeling seems to have larger standard deviation. To improve the polynomial modeling, future studies are suggested in the dissertation.

A method for calculating pulse distribution maps for scanning laser corneal surgery is presented. The accuracy, the smoothness of the corneal shape, and the duration of surgery were evaluated for corrections of myopia by using computer simulations. The accuracy and the number of pulses were computed as a function of the beam diameter, the diameter of the treatment zone, and the amount of attempted flattening. The ablation is smooth when the spot overlap is 80% or more. The accuracy does not depend on the beam diameter or on the diameter of the ablation zone when the ablation zone is larger than 5 mm. With an overlap of 80% and an ablation zone larger than 5 mm, the error is 5% of the attempted flattening, and 610 pulses are needed per Diopter of correction with a beam diameter of 1 mm. Pulse maps for the correction of astigmatism were computed and evaluated. The simulations show that with 60% overlap, a beam diameter of 1 mm, and a 5 mm treatment zone, 6 D of astigmatism can be corrected with an accuracy better than 1.8 D. This study shows that smooth and accurate ablations can be produced with a scanning spot.

Some technical factors relating to the use of the VISX model 20/20 excimer laser for photorefractive keratectomy will be discussed and the way they affect the final optical correction will be demonstrated, as follows: (1) Discontinuation of nitrogen flow: improved corneal haze but induced a mean undercorrection of 0.8 D, 18 months postoperatively. (2) Discontinuation of the use of the fixation ring did not affect centration in a statistically significant fashion. (3) Enlargement of the optical from 5.0 mm to 6.0 mm zone did not affect the refractive outcome. Visual acuity was improved in eyes which had PRK done with a 6.0 mm optical zone.

The advantages of scanning mode for corneal reshaping are discussed including smooth ablation surface, unlimited optical zone size, and flexible ablation pattern. Optimum system parameters and clinical requirements for various refractive surgical lasers are compared. Multi-zone method to reduce haze and regression is discussed.

A scanning beam of nano-second pulses at 213 nm flattens the cornea as predicted. However, there is a considerable variability in the flattening and the ablation is not safe. Ablation for 16 D flattening with an active spot overlap of 50% induced 8.9 +/- 5.3 D (n equals 7) as measured by the TMS topography system (ring 7 average) and 5.8 +/- 4.1 D (n equals 5) as measured with the SK-1 (2 mm zone) system. Ablation for 6 D flattening with an active spot overlap of 70% induced approximately 2 D flattening to 2 D steepening (n equals 3) as measured with the TMS (ring 7) and 6.6 +/- 4.33 D (n equals 7) flattening as measured by SK-1 (2 mm zone) keratometry. There was no change in IOP at 12 weeks after as compared to before ablation. There was a net increase of central and peripheral corneal thickness at 12 weeks after the ablation as compared to preoperatively. Epithelial defects remained up to 4 weeks after ablation. After four weeks, vessels had invaded the cornea in 30% of the cases and remained throughout the three months observed. It is concluded that 213 nm nano-second pulses can be used for flattening the cornea but the system should not be used for clinical trials in humans until the adverse effects can be avoided.

Regression of effect as well as undercorrection are well established complications of excimer photorefractive keratectomy for the correction of myopia. In thirteen eyes initially treated with the VISX Taunton excimer laser and then retreated with the Chiron Technolas laser, the minimum follow-up time was six months. The mean postoperative refraction at six months was -0.442 +/- 0.996 D (diopters), significantly different from the pretreatment mean of -1.904 +/- 1.297 D. At six months after retreatment (10/13 eyes) 76.9% had an uncorrected visual acuity greater than or equal to 20/40 and (10/13 eyes) 76.9% were within 1 diopter of emmetropia. Only one eye had a significant increase in postoperative haze. The results of this study indicate that the majority of photorefractive keratectomy regressions and undercorrections can be successfully retreated.

As an alternative to far-UV lasers for corneal refractive surgery, the Erbium:YAG laser may be used in TEM₀₀ mode. The resulting gaussian beam profile leads to a certain amount of myopic correction per laser pulse. Although animal data suggest that the clinical outcome should be comparable to the UV-lasers, no human data were available until now. We performed Erbium:YAG laser areal ablation in 5 blind human eyes. In TEM₀₀ mode, the laser parameters were: effective diameter of laser spot equals 3.4 mm, fluence equals 380 mJ/cm², pulse duration equals 250 microsecond(s) , Repetition rate equals 4 Hz, Number of applied laser pulses equals 15. Four patients with no light perception, one with intact light projection on one eye (some of them scheduled for enucleation) were treated under topical anaesthesia. Patient selection and informed consent were agreed to by the University's independent Ethics Committee. Prior to laser irradiation, corneal epithelium was removed. A postoperative silicone cast of the cornea was analyzed with a confocal laser micro-topometer for the ablation profile. The eyes were treated with antibiotic ointment until the epithelium was closed. Clinical appearance and, where possible, profilometry of the ablated area was observed. The ablation profile in cornea was gaussian shaped with a maximal depth of 30 micrometers . During laser treatment, the corneal surface becomes opaque, clearing in a matter of seconds. Epithelial healing and clinical appearance was similar to excimer laser treatment. However, during the first week, the irradiated area shows subepithelial irregularities, resembling small bubbles, disappearing thereafter.

The refractive outcome of thermokeratoplasty depends upon the location and angle of the coagulation spots, applied with a focusing handpiece onto the corneal surface. Accuracy can be enhanced using a specially designed application mask. An astigmatism correction was performed on 10 human donor eyes (Holmium 25, Technomed, FRG, 15 Hz, 20 mJ/pulse, 25 pulses) with an optical zone of 8.1 mm, 5 eyes received a free hand laser application (marked positions) and the other 5 eyes were treated using a suctioned metal mask with drills for the handpiece (optical zone 8.1 mm). To compare the results a silicone replica was taken and analyzed by a confocal laser microtopometer. The refractive change for the steepest meridian was 10 D with a standard deviation of +/- 3.7 D for the free hand application. Using the application mask the refractive outcome was 9.8 D with a standard deviation of only 0.8 D. Using the application mask the standard deviation for the induced refractive change decreases by a factor of five.

Gel Injection Adjustable Keratoplasty (GIAK) is a refractive surgery procedure which uses an ocular ring implant made of a polyethylene oxide hydrogel to cause a refractive change in the cornea. Unlike laser photo refractive keratectomy, GIAK does not interfere with the central cornea because the ring lies around the optical axis. Thus, vision can be assessed immediately after surgery. Our in vivo study was designed to quantify GIAK's effects on tissues, the biocompatibility of the polymer and in the process investigate which ocular changes in the rabbit model can be monitored with precision using current technology. Thirty-two young rabbits underwent a delamination in one eye, 22 of which were injected with a new polymeric gel. Corneal topography, keratometry, pachymetry, and tonometry were performed on both eyes for up to 105 days. All corneas flattened with growth. In GIAK animals, we found an average flattening of 6.51 +/- 1.23 diopters (p < 0.0001) relative to the fellow eye. No statistically significant regression over the 102 days was observed. Intraocular pressure dropped slightly by 0.69 +/- 1.21 mmHg (p equals 0.025), a clinically insignificant value, while no significant change was detected in corneal thickness. Keratometry can be tracked in rabbits after GIAK surgery from POD 1. Measuring unoperated fellow eyes allows for the effects of surgery to be assessed without bias from growth. Using this protocol, GIAK was shown to be stable. It was more difficult to draw conclusions from pachymetry, tonometry, and topography data.

Photorefractive keratectomy with 193 nm excimer laser has been used for the treatment of myopia. Because of its big size and hazards of toxic gas, a 2.94 micrometers Er:YAG laser had been suggested for this purpose. The aims of the present study was to investigate the thermal effects in the corneas after exposed to Er:YAG laser with different pulse widths. 14 New- Zealand white rabbits were divided into two groups. In the first group, the corneas were irradiated by a normal spiking mode Er:YAG laser with a pulse width of 150 microsecond(s) at the fluence of 585 approximately 697 mJ/cm². In the second group, the corneas were exposed to a Q-switch mode of Er:YAG laser with a pulse width of 700 ns at the fluence of 524 approximately 562 mJ/cm² and 700 approximately 850 mJ/cm². The eyes were followed up 5 months and enucleated for histopathologic inspection 30 minutes, 2 weeks, 1, 2 and 5 months postoperatively. Corneal stromal hazes in the first group were more obvious than that in the second group. The most significant haze was observed two weeks after treatments and disappeared at two months. The adjacent thermal damage in the first group were much wider than that in the second group. To minimize the thermal and shock wave effect, the Q-switch mode Er:YAG laser with pulse width around 700 ns is recommended for corneal surgery.

With the event of topographic steep central islands following excimer laser surgery and the potential damage to the corneal endothelium, shock waves are playing an increasingly important role in laser refractive surgery. With this in mind, we performed a comparative shock wave analysis in corneal tissue using an excimer laser, picosecond laser, and femtosecond laser. We used a Lambda Physik excimer laser at 308 nm wavelength, a Nd:YLF picosecond laser at 1053 nm wavelength and a synchronously pumped linear cavity femtosecond laser at 630 nm wavelength. The pulse widths of the corresponding lasers were 8 ns, 18 ps, 150 fs, respectively. The energy density of irradiation was 2.5 to 8 times the threshold level being 2 J/cm² (excimer laser), 86 J/cm² (picosecond laser) and 10.3 J/cm² (femtosecond laser). Shock wave dynamics were analyzed using time-resolved photography on a nanosecond time scale using the picosecond laser in corneal tissue, water and air. Shock wave dynamics using the femtosecond laser were studied in water only while the excimer laser induced shock wave during corneal ablation was studied in air only. We found the dynamics of shock waves to be similar in water and corneal tissue indicating that water is a good model to investigate shock wave effects in the cornea. The magnitude of the shock wave velocity and pressure decays over time to that of a sound wave. The distance over which it decays is 3 mm in air with the excimer laser and 600 - 700 micrometers in air with the picosecond laser. In water, the picosecond laser shock wave decays over a distance of 150 micrometers compared to the femtosecond laser shock wave which decays over a distance of 30 micrometers . Overall the excimer laser shock wave propagates 5 times further than that of the picosecond laser and the picosecond laser shock wave propagates 5 times further than that of the femtosecond laser. In this preliminary comparison, the time and distance for shock wave decay appears to be directly related to the laser pulse duration. The decay distance of the excimer laser shock waves appear to be 5 times longer than that of the picosecond laser in air while the picosecond laser shock wave distance appears to be 5 times longer than that of the femtosecond laser in water. The shorter shock wave distance of the picosecond laser is advantageous for corneal surgery by minimizing potential acoustic damage to the stromal and endothelial cells. Femtosecond lasers further minimize acoustic damage and their use should be considered in future corneal laser procedures.

Picosecond intrastromal ablation is currently under investigation as a new minimally invasive way of correcting refractive error. When the laser pulses are placed in an expanding spiral pattern along a lamellar plane, the technique is called intrastromal photorefractive keratectomy (ISPRK). We performed ISPRK on six human eye bank eyes. Thirty picosecond pulses at 1000 Hz and 20 - 25 (mu) J per pulse were separated by 15 microns. A total of 3 layers were placed in the anterior stroma separated by 15 microns. The eyes were then preserved and sectioned for light, scanning and transmission electron microscopy. Light and scanning electron microscopy reveals that picosecond intrastromal ablation using an ISPRK pattern demonstrates multiple, coalescing intrastromal cavities oriented parallel to the corneal surface. These cavities possess a smooth appearing inner wall. Using transmission electron microscopy, we noticed tissue loss surrounding some cavities with collagen fibril termination and thinning of collagen lamella. Other cavities we formed by separation of lamella with little evidence of tissue loss. A pseudomembrane lines the edge of some cavities. Although underlying tissue disruption was occasionally seen along the border of a cavity in no case was there any evidence of thermal damage or tissue necrosis. Ablation and loss of tissue in ISPRK results in nonthermal microscopic corneal thinning around some cavities whereas others demonstrate only lamellar separation. Alternative patterns and energy parameters should be investigated to bring this technology to its full potential in refractive surgery.

The presence of hydroxyl radicals in the ArF excimer ablation sites in bovine corneal tissue has been investigated by laser-induced fluorescence and Raman spectroscopy. Raman spectroscopy has also been applied to examine the time-resolved temperature of the water component of the ablation site as well as the spatially selective measurement of surface desiccation. Results indicate that the hydroxyl radical is not generate in abundance during the ablation process. Also, Raman monitoring of the ablation site demonstrates no detectable heating during ArF irradiation, while elevated temperatures are observed following exposure of the corneal surface to a Er:YAG laser. Finally, comparison of C-H and O-H Raman bands shows potential utility as a spatially selective means for monitoring tissue hydration.

Laser photoablation in the IR and UV is accompanied by an acoustic signal. NCPAS may be used for a non-contact monitoring of the ablation process. NCPAS was performed in combination with the 193 nm Excimer-, 2.94 micrometers Er:YAG-, and the FEL laser (< 11 micrometers ). A microphone (Bjoer & Kjaer, 20 Hz- 200kHz) was used. Gelatine, as a target material substitute for cornea was ablated. During ablation, the dependency of NCPAS as a function of fluence, ablation diameter, and target hydration was determined. NCPAS was analyzed in the time-signal and Fourier transformation (FT). The Fourier spectrum of the NCPAS signal shows two gaussian-like curves. As a function of fluence NCPAS showed a linear increase of the time dependent amplitude and the amplitude of the frequency interval, in the observed fluence range. In the FT the beam diameter leads to a square increase of amplitude. Up to the physiological value the hydration shows an increase in amplitude of the FT. NCPAS allows the discrimination of different neighboring biological materials by a typical frequency shift in the FT. On-line detection of NCPAS can be used for feed back, leading to a `smart' laser control.

The hydrogen fluoride laser has been used to successfully weld corneal tissue in vivo. Previous experiments have demonstrate the success of producing watertight welds in both porcine and human cadaver corneas. Wound bursting strengths of up to three times normal intraocular pressure have been reported. In this study, an in vivo model was utilized, specifically the rabbit cornea. Twelve New Zealand white rabbits, received a 7 mm, full thickness, linear corneal incision in one eye, and stay sutures were placed. Six of the wounds were welded with a semiconductor infrared laser, and six eyes served as controls. At two and four weeks, both histologic and tensiometric studies were performed. There was a trend toward increasing wound strength when the two and four week specimens were compared. Corneal welding may prove to be an adjunct to current suturing techniques in humans. Procedures requiring the closure of corneal incisions such as cataract extraction or penetrating keratoplasty may benefit from this technique.

An endoscope allows visualization of the anterior chamber angle in porcine eyes despite the presence of cloudy corneas. The pectinate ligaments in the anterior chamber angle are a surgical model for primary infantile glaucoma. This study investigated the histologic results, one month after treating the anterior chamber angle with a goniotomy needle, the holmium:YAG laser, or the erbium:YAG laser coupled to a small endoscope. The anterior chambers were deepened with a viscoelastic material in one-month-old anesthetized pigs. An Olympus 0.8 mm diameter flexible endoscope was externally coupled to a 23 gauge needle or a 300 micron diameter fiber. The angle was treated for 120 degrees by one of the three methods, and the probe was removed. During the acute study, all three methods cut the pectinate ligaments. The histologic findings one month after healing demonstrated minimal surrounding tissue damage following goniotomy with a needle and the most surrounding tissue damage following treatment with the holmium:YAG laser.

Action on biotissues of power laser pulses about 20 nsec was investigated by using high level mathematical model. The system of equations contains five unknown functions: temperature, pressure, axial and radial convection velocity, density. The 2D solution gives these functions depending upon the laser pulse shape and duration, power and spectral band of radiation. Calculations have been carried out for eye's fundus tissues. It has been found a kind of feedback between radiation power and rate of local tissue cooling. An increase in the irradiation power is accompanied by the growth of convective heat transfer and rate of medium cooling. Peculiarity of this phenomenon have been studied.

In this paper, we consider a laser pulse normally incident on the surface of a stratified tissue. The Green function technique in the time domain is used to solve the inverse problem of the transport equation that describes the behavior of energy transport in tissue. The novel algorithm is applied to reconstruct the optical parameters of tissue, which provides the possibility of noninvasive diagnosis and image. The structure of the fundamental solution is analyzed and the equation for the Green function is given together with the initial and boundary conditions. The initial value of the Green function is related to the optical parameters of tissue, and this condition is employed to reconstruct the scattering coefficients or absorption coefficients when the other parameters are assumed to be known. The reflection data are required as the input data in the reconstruction algorithm. It is shown that one-sided reflection data are sufficient to reconstruct one of the optical parameters. The numerical results are presented for both the clean and noisy data to check the stability of the algorithm. This method also provides an efficient way to calculate the internal light distribution field as well as to reconstruct the internal optical parameters distribution.

We designed and built an automated imaging dark adaptometer (AIDA) to increase accuracy, reliability, versatility and speed of dark adaptation testing in patients with hereditary retinal degenerations. AIDA increases test accuracy by imaging the ocular fundus for precise positioning of bleaching and stimulus lights. It improves test reliability by permitting continuous monitoring of patient fixation. Software control of stimulus presentation provides broad testing versatility without sacrificing speed. AIDA promises to facilitate the measurement of dark adaptation in studies of the pathophysiology of retinal degenerations and in future treatment trials of these diseases.

We present measurements of the Er:YAG laser drilling rate in human cataractous lens and describe a simple model for mid-infrared laser thermoablation. Lens slabs were immersed in saline and subjected to the output of a free-running Er:YAG laser (2.94 micrometers ). The laser output was directed through a fiberoptic to a 400 micrometers quartz tip. With a radiant exposure of 10 J/cm², and a repetition rate of 10 Hz, we measure an ablation rate of approximately 7 micrometers per pulse and an ablation threshold of 0.6 J/cm². The thermoablation rate varies linearly with radiant exposure. Our model divides the laser pulse into a series of sequential, identical micropulses, (2) solves for the ablation depth for each micropulse, (3) sums the contributions from all micropulses, and (4) specifies that the ablation depth be maximized as a function of the number of micropulses. Predictions of ablation depth and the threshold for tissue ablation are generated. The model yields a linear dependence of penetration depth on radiant exposure and a much larger single pulse penetration depth when compared with the more superficial ablation and logarithmic dependence predicted by Beer's law. Improved correlation of our model with measurements in lens tissue and available experimental data is demonstrated.

Transscleral laser microsurgery has gained wide acceptance recently. Many problems in this field are due to laser beam decay in an optically nonhomogeneous sciera. The sciera light transmission in defined by scattering processes on its fibers. Because of high density in the arrangement of sciera fibers, the phenomena of interferential interaction between scattered radiation and multiple scattering are essential. Our paper is aimed at the theoretical and experimental validation of light scattering decrease on collagen fibers by substituting the sciera base substance with substance exhibiting higher refraction index. As a model of sciera, system of long dielectric cylinders was chosen, which were surrounded by isotropic base substance with lower refraction index. The fibril diameters are varied from 10 to 100 nm. The indexes of refraction for the fibrils and embedding substance differ appreciably from each other and are equal2, respectively) to 1,47 and 1.345. The thickness of sciera is I mm. The statistic characteristics of spatial arrangement of fiber centers were described at an approximation of exduded volume. The scattering amplitude function for a single cylinder was calculated using Mie theory. Under calculation it was assumed that the axes of all the fibers are parallel to the sciera surface and their orientation in this plane is random. Two complex coefficients for waves, polarized in parallel and normally to the fiber axes, were found by means of method describing the propagation of coherent waves through random systems. According to this approach, scattering medium is represented as homogeneous but a having new effective refraction index. The imaginary part of the index takes into account energy extinction due to scattering.

Influence of laser irradiation (wavelength 632.8 nM) of the retina on visual evoked potentials (VEPs) in response to flashes of diffuse light have been studied. VEPs were recorded by tungsten-in-glass semimicroelectrode blocks at 700 (mu) M below cortical surface. It was revealed that VEPs were modified at all used doses of laser irradiation (power at cornea from 0.5 to 17 mW, exposure from 0.1 to 1000 s). During the initial 5 - 70 s of laser irradiation VEPs completely disappeared. After this silent period there appeared VEPs splitting into 2 - 4 distinct components and strong suppression or disappearance of VEPs first negative wave was observed. When laser irradiation was switched off VEPs negative waves were restored while the amplitude of splitting components was diminished. Restoration (frequently incomplete) of VEPs passed through a phase of increased negative wave amplitude. After the dose of laser irradiation was increased, this phase was followed by periodic changes in the amplitude of all VEPs components. Besides, the cortical zone that displayed the disturbances of the VEPs, became more extended. Long-lasting disturbances of VEPs occurred at irradiation doses close to those described in literature for ophthalmologically detected injuries. It is supposed that reversible (functional) disturbances may be identified by means of the above-mentioned phenomena. The discovered phenomena suit well the scheme which supposes disbalance and disinhibition of lateral connections between the irradiated retinal loci and the surrounding site.

We describe a new, real-time, flying slit confocal microscope, that has unique features and imaging characteristics for in vivo human ocular imaging. In vivo real-time confocal microscopy is currently used to investigate the tear film, renewal of the ocular surface, the role of epithelial innervation in epithelial cell proliferation, wound healing, kinetics of drug penetration, the effects of laser refractive surgery on the keratocyte activation and distribution in the stroma, and the nature of endothelial defects. The following clinical examples will be presented and discussed: confocal microscopy of normal human basal and wing cells in the epithelium, confocal microscopy of lamellar and penetrating corneal grafts, confocal microscopy of corneal ulcer, confocal microscopy of scar formation after herpes keratitis, and confocal microscopy of corneal innervation. The use of scanning slit confocal microscopes has unique advantages over other instrumental systems based on pinhole-containing Nipkow disks (tandem-scanning confocal microscopes) for clinical in vivo confocal microscopy.

During the last years a new optical technique for measuring intraocular distances, partial coherence interferometry, was developed. It is based on the special coherence properties of multimode semiconductor laser diodes or super luminescent diodes. A special dual beam version of this method eliminates any influence of longitudinal eye motions on the results. It was shown that the axial length of the eye (the distance cornea - retina) can be measured with a precision of 20 micrometers in human eyes in vivo. This technique has further been improved: In case of well defined, sharp boundaries between two layers of different refractive indices (e.g. the internal limiting membrane), it is now possible to determine the absolute position (with the cornea as the reference surface) of individual retinal layers with a precision (standard deviation) of a 5 micrometers in human eyes in vivo. The thickness of fundus layers can be measured with the same precision, provided the reflectivity of the corresponding interfaces is high enough to separate the signals from noise. By performing these measurements to different points of the retina, the contours of retinal layers can be determined and thickness profiles of individual layers can be obtained with a precision down to 5 micrometers .

We have used the small eye of the snake in combination with confocal scanning laser ophthalmoscopy to evaluate Argon laser retinal lesions. The snake eye has high optical power and high image quality. With these physiological optical attributes and confocal ophthalmic capability, we were able to examine laser induced retinal lesions over a very large dynamic range. Clear visualization of the photoreceptor matrix suggested alterations in the photoreceptor optical properties not previously noticed with conventional ophthalmoscopy and larger mammalian eyes. Inspection of anterior retinal levels suggested long term development intraretinal fibrosis and nerve fiber layer damage, as well as alteration to the choroidal vasculature at the lesion site. No alteration was observed at the retinal vascular level.

In this paper, we describe a military laser accident case where bilateral Q-switched laser exposure resulted in bilateral macular damage with immediate visual acuity loss in one eye (OS) and delayed visual acuity loss in the other exposed eye (OD), where retinal damage appeared more parafoveal. At 6 weeks post exposure, OS had recovered to 20/17 and OD had dropped to 20/100 Snellen activity. Retinal nerve fiber damage was observed in both eyes at this time. Contrast sensitivity measurements made in OS were suppressed across all spatial frequencies, even though Snellen acuity measured in the normal range. More severe high spatial frequency loss in contrast was measured in the right eye as well as low spatial frequency loss. Both OS and OD revealed a parafoveal preferred retinal locus with scanning laser ophthalmoscopy contrast sensitivity measurements, suggesting parafoveal retinal compensatory processes.

A scanning laser ophthalmoscope (SLO) uses a novel principal to view the retina while simultaneously projecting high resolution targets onto the retina. Consequently, the SLO is potentially an excellent instrument to measure fixation stability in humans. We used a commercially available Rodenstock SLO to project a small cross onto the retina of normal subjects with the HeNe laser and simultaneously view their retinal vasculature with the SLO's infrared diode laser. The video images from the SLO were continuously recorded (at 30 frames per second) with a video-cassette recorder. The video was digitized in 16 consecutive frame increments with a 4 Mbyte frame grabber in a microcomputer. By marking the audio track of the video tape and measuring from the audio mark before digitizing, long segments (1,000's of consecutive frames) can be digitized. We developed software to automatically map the locations of venous crossings at multiple retinal locations. In early work with the system, we verified that the digitization process worked correctly by time-stamping the video and confirming that all video frames were captured without duplication or drop out. We measured the location of 2 retinal points in 1024 consecutive frames of 4 normal subjects and showed that these points could be accurately followed with the automatic system.

The pulse-synchronous pulsations of the eye fundus are measured by laser interferometry. The eye is illuminated by the beam of a single mode laser diode. The light is reflected at the front side of the cornea and the retina. The two remitted waves product interference fringes, from which the distance changes between cornea and retina can be determined. The interferometer is coupled to a fundus camera, so that fundus pulsations can be measured at preselected points on the retina with high transversal resolution. This technique was used to study the influence of phenylephrine (a peripherally vasoconstricting drug), isoproterenol (a predominantly positive inotropic drug), sodium nitroprusside (a peripherally vasocilating drug) on fundus pulsations in healthy volunteers. The effect of isoproterenol to increase pulse pressure amplitude was detectable even at low doses. Neither sodium nitroprusside nor phenylephrine had a significant influence on ocular fundus pulsations. These results show that measurement of fundus pulsations in the macula estimates the pressure pulse amplitude in choroidal vessels. Measurements of fundus pulsations at preselected points of the retina, show that fundus pulsations in the macular region are larger than in peripheral parts of the retina but smaller than in the optic disc region under baseline conditions.

The detection of alterations in the microcirculation requires both the measurement of the blood-flow and the measurement of the oxygen saturation in whole blood. The basis for the non-invasive estimation of the oxygen saturation is the difference between extinction-spectra of hemoglobin and of oxyhemoglobin. Further in whole blood the scattering at the erythrocytes has to be taken into account. In principle spectral measurements at three neighboring wavelengths are sufficient for the calculation of the oxygen saturation, the concentration- thickness-geometry product and the scattering intensity. Caused by the maximal permissible exposure, the signal/noise ratio is very low in fundus reflectometry. But if the wavelength- range from 520 nm to 620 nm is evaluated, the requirement at the signal/noise ratio is reduced. This reduction corresponds to the square root of the number of discrete wavelengths at which the ocular fundus reflectance is measured. So the oxygen saturation can be calculated with an error lower than +/- 4%. For this purpose the extinction spectrum of whole blood is approximated by a model, including besides the above mentioned unknowns the spectral dependency of the scattering. The experimental arrangement for the measurement of the oxygen saturation is an imaging ophthalmo-spectrometer which allows reflectance measurements with a good spectral (< 3 nm) and local (> 3 micrometers ) resolution simultaneously at a vessel and in its neighborhood. The extinction of blood is calculated as the logarithm of the ratio of the reflectance of the neighborhood and of the vessel. In this calculation the influences of the ocular media, of the background and of eye movements are eliminated. The sensitivity of the detector system has to be very high in order to detect the light which is reflected at a dark background and travels through the blood. The new method was tested by a comparison the oxygen saturation of the blood in an arteriole and a venule in the brain of a piglet measured by reflectometry with the oxygen saturation of the left ventricular and the venous blood measured by a laboratory hemoximeter. First results of the measurement of the oxygen saturation in human retinal vessels are demonstrated.

An analysis of laser light fluctuations may indicate a state of a bloodery. In the study spectral characteristics of a dynamic speckle field scattered by skin with different microcirculation are investigated in detail. Among them: a power of fluctuations, a ratio of spectral amplitudes for two different harmonics, an average frequency of fluctuations, a spectral width and so on. A variation of these parameters during blood flow changing are discussed. An real time algorithm is used to get a map of microcirculation.

Color indirect effects (CIE) means the physiological and psychological effects of color resulting from color vision. In this paper, we study CIE from the viewpoints of the integrated western and Chinese traditional medicine and the time quantum theory established by C. Y. Liu et al., respectively, and then put forward the color-automatic-nervous-subsystem model that could color excites parasympathetic subsystem and hot color excites sympathetic subsystem. Our theory is in agreement with modern color vision theory, and moreover, it leads to the resolution of the conflict between the color code theory and the time code theory oncolor vision. For the latitude phenomena on athlete stars number and the average lifespan, we also discuss the possibility of UV vision. The applications of our theory lead to our succeeding in explaining a number of physiological and psychological effects of color, in explaining the effects of age on color vision, and in explaining the Chinese chromophototherapy. We also discuss its application to neuroimmunology. This research provides the foundation of the clinical applications of chromophototherapy.

The HYDROVIEW^TM intraocular lens is a biconvex foldable hydrogel lens configured as a conventional one-piece design. The unfolding time is approximately constant with lens dioptric power at an average of 2.4 seconds, and increases with the length of time the lens is held in air prior to implantation. Mechanical recovery can be modeled as a non-linear critically damped harmonic oscillator, which demonstrates the controlled manner of unfolding. The Modulation Transfer Function at 30 cycles/degree reaches an average of 95% of its pre- fold value within one hour after being released from folding and recovers completely to its pre-fold values within 24 hours. Interferometrically measured root-mean-square transmitted wavefront error and Strehl Ratio both recover to pre-fold values within 24 hours after folding. Dehydration for a period of 10 minutes followed by folding, then rehydration and unfolding was found to have no significant effect on lens power (< 0.15 Diopter) or resolution. Neither temperatures in excess of 120 degree(s)C, which may be used during sterilization, nor temperatures as low as -85 degree(s)C were found to have any significant effect on lens power and resolution.

MTF data can be a dependable criterion for assessing the performance of an optical system if the testing is done under conditions that closely resemble the actual application. For testing IOL performance, this requirement can be met by inserting the lens in a suitable eyes model. A dynamic eye model in which the corneal power can be varied to simulate the range of conditions found in different human eyes is ideal, but if the vergence angle is chosen carefully, a fixed eye model can be sufficient. For the resulting MTF data to be valid, it is necessary to consider the entire spread function of the system. In the case of a multifocal IOL, the failure to accurately sample the complete spread function is a major source of measurement errors. The detector must be able to characterize in considerable detail the central maximum of the in focus segment. At the same time it must also deal with the complex and diffuse `halo' from the out of focus segment of the lens. Additionally, it must be able to measure the stray light found beyond the halo, that results from the transitions between the various zones of the multifocal lens. Test systems based on the direct measurement of MTF have a substantial advantage in these cases where the spread function is large and complex.

A compact fiber optic system, utilizing a lensless backscatter fiber optic probe, and a semiconductor laser is used as a non-invasive tool for in vivo characterization of the proteins in the eye lens of several animals. The system exploits the extremely sensitive technique of dynamic light scattering, which uses a laser beam to probe the temporal characteristics of the proteins present in eye lens fluid. The technique, with appropriate electronics and signal processing provides a rapid means of determining the size of the (alpha) -crystallin in the protein-water system. Changes in the size of the protein molecules can be tracked over the age of the eye lens; an abrupt increase in size is associated with the early cataractous formation. This paper describes the fiber optic system and discusses results obtained from measurements made on sedated rabbits, pigs and cats. A clear difference in the size of the (alpha) -crystallin of normal and cataractous lenses is observed.

The basic principles of light scattering methods which should be very useful for ophthalmic disease diagnostics and monitoring are discussed. As an example a human eye lens tissue was considered. Angular-dependent scattering spectra and scattering matrix elements M₁₂, M₃₃, M₃₄ and M₄₄ are suggested as informative parameters for eye lens aging and cataract monitoring.

To investigate wound healing in rabbits after corneal photoablation with a 213 nm UV scanning solid state laser. We used a frequency quintupled Nd:YAG laser to photoablate the cornea of 32 rabbits (5 mm ablation zone, 6 D myopic correction). The contralateral eyes and the eyes of 3 untreated animals served as controls. Light and electron microscopy analyses were performed on postoperative days 0, 7, 28, 90. Abnormal, light basal epithelial cells were observed during the first postoperative month, but normal epithelial maturation was evident at 3 months. Basement membrane duplication was noted. Keratocyte proliferation occurred in the anterior stroma and in a few cases, in the posterior stroma, where keratocyte activation was present. Degenerative endothelial changes were observed immediately after ablation with intracellular disorganization and junction alteration. Surface profiles with variable regularity led to a variable wound healing. Apart from differences in epithelial basal cell appearance, photoablation with a 213 nm solid state laser induced wound healing processes similar to those observed previously with prototype ArF excimer lasers. It is hoped that laser refinements will improve the surface regularity and lead to more consistent wound healing responses. However, several studies are required to assess mutagenicity, penetration depth and ablation rate of the 213 nm wavelength on ocular tissues, as well as the effects of hydration on the clinical outcome.

Ocular tissues are mainly made up from conjunctive fibers on the basis of collagen. The way they are arranged as well as the proportion of water contained in the tissues determines their transparency. As example the fibers in the sclera have diameters between 30 and 300 nm and are arranged in ribbon-like fiber bundles (15 microns thick and 140 to 150 microns wide) that cross each other in all directions [1]. Both different diameters and interlaced structure lead to a tissue displaying scattering properties. By opposition the cornea scatters visible light in a much lower amount, mainly because it is up from fibers having diameters varying only between 19 and 34 microns and that are organized in a parallel fashion within layers. Such a structure is of primary importance when objects have to be imaged through such tissues because scattering results in a blur of the image obtained. Therefore, any irregular change in the structure due to anarchical growth or in diameter as it may be the case with ageing of the crystalline lens results in a loss of visual acuity. The use of biocompatible materials has tremendously increased in ophthalmology during the last years. PMMA still represents the majority of the intraocular lenses replacing the refractive power of the crystalline lens. Viscoelastic materials are currently utilized in the anterior chamber and perfluorocarbons or other polymers like polydimethylsiloxanes (PDMS) are used as vitreous substitutes. The physical properties of such materials should be similar to those of the tissue they replace. Consequently, whenever possible flexible materials that mimic ocular tissues are developed in order to minimize local stress and tissue deformation. Among them, gels that consist of a network crosslinked in a balanced salt solution (BSS) have also been tested as intracorneal implants for GIAK (gel injection adjustable keratoplasty), a technique designed to correct myopia [2]. Because the number of intermolecular bonds per linear chain governs the rigidity of the material, various collagenous tissues can be mimicked by controlling both concentration of the network and the degree of crosslinking. However, when such materials are implanted within the optical path of the eye, their optical properties should be scrutinized. A high transparency is necessary in the visible part of the spectrum (450-700nm), associated with a high absorption in the UV and the deep blue part (400-450nm) for protection of the retina and the crystalline lens. In addition, scattering should also be minimized to avoid loss of visual acuity. Therefore, apart from an accurate shape and refractive index, the surface quality and the homogeneity of the implants have also to be as high as possible. The purpose of this study was to compare the scattering of such materials to that of ocular structures.

To avoid the post-operative hypotony that often occurs with ab interno and ab externo laser sclerestomies, trabeculectomies, setons and glaucoma implants, we designed the MESH, an implant that mimics the physiological meshwork. It is a sub miniature `T' shaped pouch made of thin porous poly(tetrafluoroethylene) (PTFE). An instrument allows its intrascleral insertion via 2 mm wide conjunctival incision with the MESH's head protruding in the anterior chamber's angle. Flow was assessed in five PTFE membranes of 5 to 80 micrometers pore size. Prototype implants were tested for integrity by scanning electron microscopy and in vivo studies were undertaken with MESH implants made of 20 and 50 micrometers (8 rabbits, 3 months follow-up) and 10 micrometers (11 rabbits, ongoing) PTFE membranes. Flow rates were 150 to 650 times the normal 2 (mu) l/mn physiological aqueous outflow. Surgery took about 7 minutes. The implants became translucent after 7 days and were found biocompatible over the 3 months follow-up period. The 50 micrometers implant was biocolonized in 21 days but only a few cells were observed at 3 months in the 20 micrometers implant which produced a longer lasting bleb and IOP lowering effect. No colonization is expected in the 10 micrometers material which has a higher flow rate and geometric integrity. The MESH appears safe and efficient in prolonging filtration in rabbits in the short term. To prevent cellular penetration and an adequate regulation of the aqueous flow, an expanded PTFE material with 10 - 20 micrometers pores and a 100 micrometers thickness seems best suited for the fabrication of artificial meshwork implants for use in glaucoma patients.

Despite advances in mechanical vitrectomy, cutting dense membranes and fibrovascular strands remains a technical challenge. Widespread acceptance of carbon dioxide and Er:YAG lasers, which cutting potential and coagulative properties were highly evaluated by experimentators, now is limited because of insufficiently convenient fiberoptic delivery systems. We investigated the use of Ho:YAG laser emitting at 2.09 micrometers to cut experimental vitreous membranes in rabbits. Radiation at 2.09 micrometers is enough strongly absorbed by water (absorption length 385 micrometers ) and can be transmitted through silica fiberoptic delivery system practically without attenuation. At first, the dependence of retinal damage thresholds and wet field thermal effects from distance `endolaser tip--target', energy, pulse rates and number of pulses were determined. With the help of multiple regression analysis models of these processes were constructed. Coagulative and cutting properties of holmium endolaser were assessed on the created transvitreal membranes. Optimal parameters for cutting and coagulation were determined. Based on own experimental studies we also used Ho:YAG endolaser in vitrectomies for diabetes and traumas. The advantages and disadvantages of the 2.09 micrometers infrared wavelength of holmium laser photoincision and photocoagulation in vitreous are discussed.

Pilot studies for laser scleral buckling made it clear that quantification of scleral shrinkage was required for precision and reproducibility of the treatment. For the quantification either the Ho:YAG (2.10 micrometers ) or the Tm:YAG (2.01 micrometers ) lasers were applied to the equatorial sclera of human cadaver eyes. Two slightly overlapping spots (2.8 mm (phi) ) were applied. Shrinkage rate was expressed as: [(Scleral length before treatment--Schleral length after treatment)/Schleral length before treatment] X 100(%). Shrinkage rate was measured changing several parameters. Total fluence, energy/pulse, scleral thickness, tissue temperature, age, and intraocular pressure. Shrinkage rate was found to be mainly function of total fluence attaining a maximum of 26 - 30% in adult and 46% in infant eyes at a 3 - 4 mm Hg intraocular pressure. Rising tissue temperature from room temperature to physiologic levels reduced the laser energy requirements but not the maximum shrinkage level. From the same shrinkage effect in the practical range of total fluence, less energy (56 - 60%) was required with the Tm:YAG laser. The data acquired in this study will help us construct an algorithm to predict the outcome of laser scleral buckling in patients.

Development of choroidal neovascularization (CNV) beneath the center of the retina (macula) occurs in a variety of conditions and is a major cause of vision loss. New surgical instruments with sizes smaller than 0.4 mm have been developed to allow direct access to the subretinal space in an effort to treat or remove these subretinal membranes. These instruments include forceps, scissors, aspiration tubes, etc. Laser photocoagulation may add another valuable tool in the treatment of these membranes. Traditional photocoagulation of membranes is achieved by applying transretinal laser, but this damages the retina itself. A direct method of endophotocoagulation via a retinotomy may allow treatment while minimizing damage to the overlying retina. In this paper we describe the development and use of a .23 mm (32 ga) laser delivery probe for endophotocoagulation of subretinal choroidal neovascular tissue. The design consists of a 100 micrometer silica core fiber in a stainless steel needle, shaped for insertion through the retinotomy and into the subretinal space. The fiber tip is designed for beam delivery 27 degrees from the fiber axis, thus allowing direct photocoagulation of the treatment area. Clinical use of the delivery system operating with argon ion (488, 514 nm) laser will be discussed.

Perimetry, Electro-Oculography, Electroretinography and visual evoked potentials practiced in Ophthalmology, are put under: contingencies, bound to the patient and the equipment itself. In Perimetry difficulties are proportional with the respect between patient and material. In visual Electro-Physiology, the identical problems are seen often amplified by the difficulties due to the setting of the data compilation. It is fitting to opt for a protocol giving in a minimum of time, a maximum of reproducible data of the state of visual functions. A good knowledge of Electro-Physiologie bases will enable the user to avoid lots of traps. The choice of material must come into considerations for the type price reliability and easy utilization. The ERG must be realized taking into account as best as possible, the problem bound to the patients' age. The recording of EOG enables, coupled with the ERG, a functional assessment and a prognosis in the Retinal disease. This examination is sensitive to mechanical, physical and physic factors. Whatever the choice VEP flash or pattern, the patients' age, child or adult, will be determinant in the choice of the method and analysis results. The VEP answer is lost in a wave of interfered results. The VEP should be amplified and taken out from this background sound, while keeping its maximal structure.

The purpose of this study was to evaluate the raster photogrammetry based Corneal Topography System by determining: inter-operator variability, reproducibility of images, effects of defocused and decentered images, and the precision of data at different optical zones. 4 human cadaver eyes were used to study the inter-operator variability. To study the reproducibility of images, 3 human cadaver eyes and a test surface doped with flourescine (provided by PAR Vision Systems Corporation) were focused and their images taken four successive times. Defocused and decentered images were taken of 4 human cadaver eyes and four times of the test surface mentioned above. The precision of defocused/decentered cadaver eyes was evaluated at the following optical zones: 3 mm, 4 mm, 5 mm, and 6 mm. All human cadaver eyes used in the above experiments had their epithelial layer removed before imaging. Average inter-operator variability was 0.06 D. In reproducibility attempts, there was an average deviation of 0.28 D for the human cadaver eyes and 0.04 D for the test surface. The defocused and decentered test surface gave an average deviation of 0.09 D. Defocused and decentered cadaver eyes resulted in an average deviation of 0.52 D, 0.37 D, 0.24 D, and 0.22 D at optical zones of 3 mm, 4 mm, 5 mm, and 6 mm, respectively. The imaging method employed by PAR Vision Systems Corporation virtually eliminates inter-operator variability. The PAR Corneal Topography System's clinical usefulness, however, could be improved by improving the reproducibility of images, decreasing the sensitivity to spatial alignment, and increasing accuracy over smaller optical zones.

Retinal nerve fiber layer (NFL) damage can be induced by retinal laser photocoagulation. This type of thermal injury involves degeneration in both descending and ascending directions from the photic injury site. We have repeated early studies in evaluation of the acute phases of the injury process. Our findings indicate that the ascending or Wallerian portion of the NFL degeneration requires less time than the descending portion; an early neural debris channel occurs in close proximity to retinal vessels and appears to enter the optic disc in close proximity to the retinal vasculature. Angiography of the ascending debris sheath suggests possible capillary pattern modulation associated with this neural debris sheath. Retinal traction evident in with other acute injuries appears at 2 weeks and disappears after 8 weeks suggesting secondary control factors other than retinal hemorrhage in the development of retinal traction bands.

A ray-tracing method has been used to investigate the refractive properties of interfaces between different ocular media and vitreous substitutes, in relation to transpupillary laser beam delivery during photocoagulative procedures. The study outlines the role of these interfaces in focusing or defocusing the laser beam along the light path within the eye.