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Biomedical subjects

I J Hodgkinson

Publications and source records attributed to I J Hodgkinson.

10 recordsLinked to original sources
PubMed1994

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.

Choroid↗
PubMed1994

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.

Accommodation, Ocular↗
PubMed1993

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.

Amblyopia↗
PubMed1993

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.

Adult↗
PubMed1992

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.

Amblyopia↗
PubMed1991

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.

Child, Preschool↗
PubMed1990

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.

Eye Diseases↗