Richard Farrell

Light Scattering in the Cornea

The physical basis for the transparency of the cornea to visible light is investigated theoretically in terms of the molecular structure as depicted by electron microscopy. Electron micrographs show that the major portion of the cornea contains long cylindrical fibrils arranged in a quasi-random fashion, with local order extending over distances comparable to the wavelength of light. Heretofore, the generally accepted explanation of transparency has been in terms of a supposed crystalline arrangement of the fibrils, because this was the only distribution that could ensure transparency on a simple theoretical basis. Thus, the non-crystalline structure shown by the electron microscope has been widely regarded as an artifact due to the fixation procedure. In the present work, the light scattering from the fibrils is formulated in terms of their radial distribution function, which is determined by numerical analysis of electron micrographs. Comparison of theoretical results and experimental values for transmittance through rabbit cornea shows that the quasi-regular quasi-random structure revealed by the electron microscope is not in conflict with transparency.

Blood flow in the human eye

It is widely appreciated that impaired vascular circulation in the eye plays a significant role in the pathology of the retina, the optic nerve and the choroid. Capillary closure, microaneurysm formation, shunts, neovascularization and breakdown of the blood retinal barrier are non-specific responses to ischemia, congestion and thrombosis. Examples include diabetic retinopathy, where the microaneurysms and neovascularization reflect an underlying focal hypoxia and impaired ocular blood flow (Ashton 1953,1969; Davis 1968; Kohner et al. 1969; Gamer & Ashton 1972). Unfortunately, the identification of patients with impaired ocular blood flow, and the assessment of treatments aimed at improving the ocular circulation have been precluded by the absence of a suitable routine measurement of ocular blood flow. This situation is now changing with the recent development of non-invasive procedures for the assessment of ocular blood flow in the retina and ciliary choroidal networks. The non-invasive procedure of laser doppler velocimetry was developed for the measurement of retinal blood flow (Riva & Feke 1981; Riva et al. 1981, 1985). In this procedure the retinal blood flow is derived from the velocity (V) of red cells and the cross sectional area (A) of a given artery or vein; blood flow equals VA (mm3 min-1) for an individual vessel, and the total blood flow is the sum of flows in all the retinal arteries or veins. The mean velocity of blood was measured using a bidirectional laser doppler system and the vessel diameter assessed from monochromatic fundus photographs. In 8 healthy human eyes the average total volumetric flow in the retina was 34 p1 min-1 with a range of 18-44 pl min-I. The values of the retinal blood flow in man are similar to those determined in primate eyes by the radioactive labelled microsphere entrapment procedure, namely 25 If: 9 ~l min-l (Alm & Bill 1973). The non-invasive quantitative assessment of the ciliary choroidal blood flow is based on the analysis of the intraocular pressure (IOP) pulse. The flow of blood into the eye is pulsatile and causes a rhythmic fluctuation of the steady-state IOP. It is the magnitude and the form of the IOP pulse that is used to assess the pulsatile component of ocular blood flow. The high fidelity recording of the IOP that is essential for the evaluation of the pulsatile blood flow (PBF) is obtained with a pneumatic tonometric probe (Langham 1987). In order to quantitate, automate and rapidly analyze the IOP pulsations, the electronic signal from the probe is digitalized and fed into a modified IBM PS2/30 computer. Representative recordings of the IOP in eyes of 4 normal subjects are shown in Fig.1. Each IOP measurement is completed within 10 microseconds and the measurement repeated at intervals of 30 milliseconds, giving a total of 30 IOP readings during each cycle of the IOP. The average of all the individual IOP measurements is the steady state value, Goldmann (1954) recognized the problem of

Photon detection with high gain avalanche photodiode arrays

The detection of light emitted in fast scintillating fibers and Cerenkov radiators used for fiber calorimetry and tracking applications in high energy colliders, requires fast detector arrays with high sensitivity to short wavelength photons. Photomultiplier tubes, the traditional imaging detectors for short wavelength optical radiation, have limited spatial resolution and require expensive anti-magnetic shielding. We report on short wavelength sensitivity improvement and detection efficiency performance for a novel p-n junction planar structure silicon avalanche photodiode (APD) array, operated in Geiger mode. The APD array provides a high sensitivity detector for applications requiring the detection of light spatial distributions with single photon sensitivity.

Enhancing near-infrared avalanche photodiode performance by femtosecond laser microstructuring

A processing technique using femtosecond laser pulses to microstructure the surface of a silicon avalanche photodiode (APD) has been used to enhance its near-infrared (near-IR) response. Experiments were performed on a series of APDs and APD arrays using various structuring parameters and poststructuring annealing sequences. Following thermal annealing, we were able to fabricate APD arrays with quantum efficiencies as high as 58% at 1064 nm without degradation of their noise or gain performance. Experimental results provided evidence to suggest that the improvement in charge collection is a result of increased absorption in the near-IR.

On corneal transparency and its loss with swelling

The cornea is the clear front covering of the eye through which we see and is composed of collagen fibrils embedded in an optically homogeneous ground substance. It has long been recognized that these fibrils scatter light and that transparency results from interference effects due to an ordering in the spatial arrangement of the fibrils about one another. The nature of this ordering and of its disruption in abnormal corneas is of great current interest. The present study reviews experimental light scattering and electron microscopic evidence relevant to this problem. This evidence suggests that theoretical attempts to model and understand corneal transparency and its loss during swelling, in terms of ultrastructure, must account for a short-ranged ordering of fibrils in normal healthy corneas and for the formation of regions void of fibrils in swollen corneas.

Light Scattering from Cornea and Corneal Transparency

Understanding the properties of the cornea that are essential to vision—its structural stability and transparency—is a long-standing endeavor that continues to intrigue a variety of researchers ranging from ophthalmologists to physicists.1–12 The transparency of a normal cornea results directly from the fact that the cornea does not absorb visible light, and the light that it scatters is minimal. The small amount of scattered light, however, carries information about the internal structural elements from which the light is scattered. Therefore measurements of this scattered light can be used to probe structures in fresh (unfixed) corneal tissue.

Epithelial damage in rabbit corneas exposed to CO2 laser radiation

Corneal injury thresholds are determined for conditions not previously explored for CO2 laser radiation, including multiple-pulse exposures and a systematic investigation of the effect of beam diameter on single-pulse damage thresholds. Multiple-pulse exposures from pulse trains up to 999 pulses, having pulse repetition frequencies between 1 and 100 Hz and individual pulse durations between 10(-3) and 0.5 s, were explored. Damage thresholds are discussed in terms of an approximate critical temperature model, the damage integral model and other empirical correlations. Single-pulse exposures are accurately correlated by an empirical critical temperature model in which the critical temperatures have a weak dependence on exposure duration. However, certain aspects of the single-pulse damage data led us to propose a new thermal damage model that incorporates an endothermic phase transition as the damage mechanism. This physical model accurately correlates single-pulse damage for exposures between 10(-3) and approximately 10 s.

Gain and noise in very high gain avalanche photodiodes: Theory and experiment

Large area silicon avalanche photodiodes have been fabricated with maximum avalanche gains exceeding 10,000 and excellent signal to noise ratios. A model of device performance has been developed in which previously developed general expressions are numerically integrated using actual fabrication parameters. The gain, statistical fluctuations in the gain, electronic noise, and total peak broadening have been computed using this model. The results are in good agreement with measurements. The parameter keff was found to be 7.2 X 10-4, allowing a high signal to noise ratio at gains of several thousand.

Corneal small-angle light-scattering theory: wavy fibril models

Small-angle light-scattering (SALS) measurements of the cornea together with electron micrographs of the corneal stroma suggest that the waviness in the stromal collagen fibrils of corneas fixed at zero pressure is the structural feature responsible for the cross-polarized SALS patterns. This paper derives and discusses a Born approximation to the parallel- and cross-polarized SALS patterns expected from lamellae of long, thin, optically anisotropic wavy fibrils whose axes are parallel to each other and are spatially distributed about one another in a quasi-ordered fashion. The predicted scattered intensity depends on three factors: (1) the fibrils within a given lamella wave in unison, which produces scattering that is characteristic of a wavy sheet (as opposed to that characteristic of an isolated wavy fibril); (2) the undulations lead to a diffraction condition for determining the dependence of scattered intensity on scattering angle; (3) the relative orientations of fibril axes in different lamellae and the intrinsic electric susceptibility of a fibril determine the dependence of scattered intensity on azimuthal angle. The patterns predicted for anisotropic fibrils with a random distribution of lamella orientations or with distributions of lamella orientations that have one or two preferred directions superimposed upon a random background agree with the qualitative features of the experimental patterns observed with rabbit corneas. Experimental evidence in support of the distributions with preferred orientations is discussed.

A variational principle for scattering from rough surfaces

A variational principle is developed for scattering of waves by rough surfaces. It provides a prescription for improving the results of other methods and also provides a quantitative measure of their accuracy. This makes it possible not only to select the best available analytical treatment for any given application, but also to improve its accuracy.