Chromatic analysis of photorefraction: a model for the Otago photoscreener.

A model is presented for photorefractive testing of the living eye with the circular knife-edge Otago photoscreener. Wavelength-dependent fundal remittance and line spread functions are introduced and used to account for the yellow-orange-red range of colours observed in pupillary images. The response of the photoscreener to errors of focus is described in terms of luminance and dominant wavelength profiles.

Reflected line spread function of the human ocular fundus.

Characterisation of the reflection from the fundus of the eye is important for the development of an improved theoretical description of photoscreening. Measurement has been made of the reflected line spread function of the human fundus in vivo using a slit-lamp microscope with video camera and frame-grabber. Line spread functions representative of red, green, and blue light were obtained from fundus sites just temporal to the macula, 10 degrees temporal to the macula, away from the macula temporal, and 30 degrees temporal to the macula. The results show that the lateral spreading of light is greatest for red light, and support the view that red light penetrates further into the fundus before being reflected than green or blue.

Pupillary irradiance distributions for light backscattered in the human eye.

An investigation of linear knife-edge photoscreening devices for detecting focusing errors, refractive defects and other optical artifacts in the human eye is described. Pupillary irradiance distributions calculated for a model eye are shown to agree qualitatively with distributions recorded experimentally using video frame grabbing equipment. The form of the pupillary irradiance distribution from a cyclopleged human eye supports a diffuse-like characterization for the reflection/scattering processes at the retina. The extreme sensitivity of the instruments enables real-time detection of refractive effects from tear films on the cornea and real-time tracking of floaters.

Optical anisotropy in thin films deposited obliquely: in situ observations and computer modeling.

Normal incidence optical anisotropies are monitored during the growth of dielectric and metal films deposited at 45 degrees by logging the quantities R( perpendicular),R( ||), T( perpendicular) and T( ||) as functions of mass thickness d. For both ZrO(2) and Ti(2)O(3), the experimental profile of (R( perpendicular) - R( ||)) vs d is consistent with a refractive anisotropy that remains nearly constant during deposition. Gold, silver, and aluminum (deposited in the absence of oxygen) are shown to exhibit characteristic anisotropy vs d profiles that can be associated with the refractive index resonance in a globular metal film. The anisotropic effects observed in metals, both during deposition and during subsequent argon-ion sputter etching, are modeled by considering the film to comprise a mixture of crystallite-and void-defined materials and by adopting simplified structural hysteresis loops.

Compensation of fabry-perot surface defects. 2: silicon oxide compensating layers.

The properties of a Fabry-Perot etalon, with surface defects compensated by irradiating a silicon oxide film, are computed for the case in which the initial surface defects function is rectangular. As a result of absorption, the compensating layer of silicon oxide must be deposited before the reflecting coating if the etalon is to be used at uv wavelengths. For visible wavelengths, the compensating layer may be a half-wave silicon oxide film deposited over the reflecting coating. An example is discussed in which the rms deviation from the mean thickness of an etalon is reduced from 2.37 nm to 0.80 nm. An automatic self-compensating process is proposed in which the radiation transmitted by an etalon may be used to produce an appropriate change in the thickness profile of a silicon oxide film.

Fourier description of analysis and synthesis operations for a stack of thin films of equal optical thickness.

Analysis and synthesis operations for a stack of dielectric layers with equal optical thickness are described in terms of the Wiener-Khintchine theorem with variables reflectance R and distribution of optical paths f. The method yields an infinite sequence of refractive indices that converge to a substrate index. An iterative process is used to determine the minimum phase solution, and all practical solutions are constructed from it by a root-shifting procedure. Realizability and practicability conditions are discussed.

The change in thickness and other optical properties of ultraviolet irradiated silicon oxide films.

Many of the optical properties of irradiated silicon oxide films, relevant to the employment of the films as phase compensating layers for Fabry-Perot surface defects, are determined. Ultraviolet radiation induces complementary changes in thickness and refractive index but does not increase the surface roughness of the films. Partially irradiated films exhibit inhomogeneity of refractive index, and a film model is proposed. The optical properties change with age, but the effects are sometimes small. For example, a step of height 8 nm, induced by irradiating a 143-nm film, changed height by about 0.2 nm over a period of three months.

Compensation of fabry-perot surface defects. 1: deposition through a moving slotted mask.

The compensation of Fabry-Perot plate surface defects by the deposition of a dielectric film of appropriate thickness profile is discussed. The method of depositing a film over the reflecting coating on an etalon plate is often unsatisfactory, because the optical thickness of the etalon is insensitive to change in thickness of the dielectric film when the reflectance is a maximum. A dielectric film deposited before a reflecting coating produces a change in the optical thickness of the etalon equal to the thickness of the film and barely changes the reflectance. A method for depositing dielectric layers of suitable thickness profile is described. A slotted mask, prepared from an interferogram recording the surface defects, is used to produce an appropriate attenuation of evaporated dielectric material.

A method for mapping and determining the surface defects function of pairs of coated optical flats.

A new photographic technique for mapping pairs of coated optical flats, applicable to surfaces matched to lambda/10 or better, is described. By slowly changing the separation between the flats during exposure, interferograms are produced in which surface error is represented almost linearly by photographic transmission, and the surface defects distribution is determined from a large number of samples of the transmission of an interferogram. An illustrative example is discussed in which the surface defects distribution of a pair of Fabry-Perot plates is found to be asymmetric.

Materials for birefringent coatings.

We discuss recent developments in methods for monitoring the anisotropic growth and properties of tilted columnar films and present results for the perpendicular incidence birefringence of aluminum oxide, Merck Substance Hi, silicon oxide, tantalum oxide, titanium oxide, and zirconium oxide.

Birefringent filters for tuning flashlamp-pumped dye lasers: simplified theory and design.

The theory of a birefringent filter in a laser cavity simplifies when the Jones matrix of a tilted birefringent plate is reduced to its most fundamental form. Theorems relating to stacks of plates are deduced. For example, certain periodic stacks are shown to have no secondary peaks. Approximate expressions are derived for the important parameters of the filters. Designs with maximum tuning range and minimum total number of plates are achieved by tailoring the secondary peaks to meet the inverse of the laser gain curve.

Photorefraction of the living eye: a model for linear knife edge photoscreening.

A computer model is developed to explain colored artifacts in pupil images of the living human eye formed by the linear knife edge photoscreener. An approximate expression for the RGB irradiances recorded by the photoscreener is derived by using the statistical properties of the line spread functions that characterize the source, the aberrations of the eye, and the reflecting properties of the retina. The pupil image for each color is shown to be very much a map of the deviations perpendicular to the knife edge imposed on rays by an equivalent aberration plate, and the overall response of the photoscreener to specific aberrations of the eye is cataloged for errors of accommodation and chromatic aberration, spherical aberration, astigmatism, and tear bands.

Characterization of the fundal reflectance of infants.

Pupillary images recorded by a linear knife-edge photoscreener for infants accommodating more or less within the bounds of chromatic aberration show three smooth offset pupillary irradiance ramps when analyzed with a color video camera and frame-grabbing hardware. Both the shape of the ramps and the lack of a second image in a shadowing experiment support the view that the retina acts as an angularly diffuse partial reflector in photorefraction. We propose a lateral image-spreading characteristic to account for observed color-dependent ramp heights and offsets and consider possible physical mechanisms for lateral spreading.

Point-spread function for light scattered in the human ocular fundus.

A model based on the framework of fundal reflectance model III is developed for the lateral spreading of light that emerges from the human ocular fundus after scattering in the choriocapillaris and the choroidal stroma. Blood in these layers is modeled as a forward scatterer, and the Kubelka-Munk diffuse radiation environment used for the choroid in model III is replaced by an arrangement of embedded reflectors that gives the same remittance. An equation is derived for the point-spread function, and sample calculations illustrate the dependence of the spreading function on retinal site and wavelength and identify the characteristic influences of the long-wavelength edge of the 575-nm oxyhemoglobin absorption band and the choroidal melanin.

Scatter from tilted-columnar birefringent thin films: observation and measurement of anisotropic scatter distributions.

We show that the angular distribution of scattered light (haze) from tilted-columnar birefringent thin films is highly anisotropic. When a narrow laser beam is incident normally upon a substrate coated with a tilted-columnar film, such as titania deposited at 60° to a thickness of ≈2 µm, the distribution of light scattered into the surrounding transmission and reflection air spaces has the form of one or more arcs. Scatter of light into the substrate is also highly anisotropic, and secondary scatter from the trapped substrate flux produces characteristic bright patterns that appear superposed on the coating.

When viewed in transmission the bright pattern typically consists of two strong lobes and in reflection four weaker lobes that make a cross. The various anisotropic distributions are shown to be consistent with interference of light from scattering centers that are correlated along the column direction and hence can be described as reflections from the tilted-columnar thin-film microstructure.

The development of fixing and focusing behaviour in normal human infants as observed with the Otago photoscreener.

The Otago photoscreener is an optical instrument which gives a very sensitive indication of the accuracy with which a subject's eyes are fixing and focusing. Early experience suggested that this instrument could be used effectively to screen for the presence of amblyogenic factors in pre-verbal infants. This communication describes the development of ocular fixation and focusing in 137 normal infants who were followed at regular intervals during the first year of life. Accurate fixation and focusing was found in 13% of three-month-old infants, in 68% of six-month-old infants and in 76% of one year olds. The levels of 'focusing' visual acuity obtained from the photoscreen data are considerably better than the levels of cortical acuity measured by the standard behavioral and electrophysiological methods. This suggests that human infants fix and focus accurately for a considerable period before they are able to perceive all the details obtained in their retinal images.

Reliability of the Otago photoscreener. A study of a thousand cases.

The Otago photoscreener is a 35 mm single len reflex camera in which the flash light comes from a narrow ring around the outer margin of its lens. The margin is also the limiting aperture of the optic system and in the centre of the lens is a flickering fixation light. In a colour photograph taken at a distance of 66 cm from the face of the subject who is accurately focusing on and fixing the camera fixation light with both eyes, the fundus reflex in each pupil is very dark red and the corneal light reflexes are symmetrical. If either or both eyes are not appropriately focused or fixing, the fundus reflex is brighter and yellow or white. This article describes a prospective trial of the performance of the Otago photoscreener in a series of 1000 infants with actual or suspected amblyopia, refractive error or strabismus. In this study photoscreening showed a sensitivity of 94% and a specificity of 79%. Photoscreening also identified some cases of esotropia and of refractive error which were missed on clinical examination. In this series photoscreening passed as normal three children with mild to moderately severe amblyopia. This represents les than 1% of the clinically abnormal children.