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At least 1,459 records · Page 81Linked to original sources
PubMed1989

A clinical evaluation of the BioPen.

We conducted a clinical trial on the Oculab BioPen, a portable, handheld applanation instrument designed to measure ocular axial lengths. We compared the measurements obtained from the BioPen with those obtained from the Ultrascan Digital B System IV from CooperVision. Accuracy and reproducibility were assessed in vitro by performing ten measurements with each instrument on a precalibrated 25.8-mm plastic test block. The in vivo reproducibility of the BioPen was evaluated by performing five serial readings on each eye of 58 patients. Keratometry measurements were also recorded to determine whether the BioPen provided consistent readings regardless of corneal curvature. We found the BioPen to be as accurate and reproducible as the Ultrascan Digital B in vitro and in vivo. Corneal curvature had no effect on the in vivo reproducibility of the BioPen.

Clinical Trials as Topic↗
PubMedDate not supplied

Problems of eye care delivery.

The past 50 years have seen the development of ophthalmology as a sophisticated specialty with improved training programs, increased output of ophthalmologists and growth of knowledge and technology. In spite of this progress, serious problems and conflicts impair the delivery of eye care. One solution may be found in a cooperative Screening Consultation Team system, such as the one currently being evaluated at the University of Florida.

Delivery of Health Care↗
PubMedDate not supplied

What is refraction?

Various concepts of "refraction," are illustrated. The value of refraction testing for evaluation of eye diseases and general functioning of the visual apparatus is considered. The question of whether ophthalmologists or other eye care personnel should be primarily responsible for performing refraction examinations is discussed.

Attitude to Health↗
PubMedDate not supplied

What can we learn from the visual care system of the British National Health Service?

The development of the British National Health Service over the past three decades is described, with particular emphasis on delivery of eye care. Differences between the British and American systems are discussed with consideration of the general political, philosophical and socioeconomic conditions of both countries.

Delivery of Health Care↗
PubMed1985

The magnitude of longitudinal chromatic aberration of the human eye between 458 and 633 nm.

The longitudinal chromatic aberration (LCA) of the human eye was determined between wavelengths 458 and 633 nm for ten observers by incorporating Argon and Helium-Neon Lasers into a Badal optometer system such that tonic (or "dark focus") resting positions of accommodation could be measured under darkroom conditions. A mean chromatic range of 1.87 D was found between 488 and 633 nm. The range increased slightly to 1.91 D when the experiment was repeated under cycloplegia on a subset of three subjects. Additional wavelengths (458 and 476 nm) for a further subset (four subjects) increased the range to 2.65 D. This magnitude of LCA would support recent predictions based on Abbe and Pulfrich refractometry analyses of the dispersion of the human crystalline lens and ocular media.

Accommodation, Ocular↗
PubMed1986

The longitudinal chromatic aberration of the human eye, and its correction.

The human eye suffers from longitudinal chromatic aberration, and this has been thought to average approximately 1.75 D between 420 and 660 nm. In many vision experiments the aberration may be a serious problem, and a number of lenses have been designed to correct it, two of which have recently been commercially available. These achromatising lenses produce an equal but opposite aberration to that of the average eye, and the power of the eye/lens combination should then be independent of wavelength. Recent measures of the eye's longitudinal aberration suggest that the average may be substantially greater than the above value, in which case lenses based on this figure should be inadequate. Using a Badal optometer we determined the far points for 20 subjects with each of the recent lenses, and without either, at nine wavelengths over the above range. Consistent with the early studies we found an average aberration of 1.82 D (SD 0.15 D). Both lenses performed as specified, and none of our subjects had over 0.48 D of residual aberration when corrected with either.

Adult↗
PubMed1986

Modulation dependence of the accommodation response to sinusoidal gratings.

The steady-state, monocular, accommodation response to sinusoidal grating targets (spatial frequencies 0.67, 2.0, 6.0 and 18 c/deg) at effective distances between 0.2 and 1.0 m was measured with a laser optometer, as a function of the grating modulation. In general, only a rather weak dependence of the response on modulation was observed. The results are discussed in terms of the theoretical modulation of the retinal image as a function of defocus. It is argued that the accommodative system usually works to produce a contrast in the grating image which is well above the detection threshold. Considerable inter-subject differences in response were observed, suggesting that a variety of strategies may be employed by different subjects when engaged upon the same voluntary accommodation task.

Accommodation, Ocular↗
PubMed1986

Dynamic interactions between accommodation and convergence are velocity sensitive.

Aftereffects of accommodation and vergence occur following approximately 1 min of adaptation to lenses and prisms respectively. This observation can be interpreted to mean that accommodation and vergence responses have phasic and tonic components. We have examined the role that these proposed subcomponents play in mutual interactions between accommodation and vergence. Both accommodative vergence (AV) and vergence accommodation (VA) were unresponsive to low temporal frequency variations (less than 0.1 Hz) in defocus and disparity respectively. However, both AV and VA were responsive to higher temporal frequency stimuli (up to 0.5 Hz). When negative feedback to the stimulated system was cancelled electronically, both AV and VA become responsive to low temporal frequency stimuli. The ratio or gain of accommodative vergence/accommodation (AC/A) and vergence accommodation/vergence (CA/C) increased nonlinearly with stimulus amplitude. Vergence aftereffects resulted from stimulation of AV and accommodative aftereffects resulted from stimulation of VA. These results are interpreted to mean there could be a complementary relationship between the amplitude of the AC/A ratio and proposed tonic adaptation of accommodation, and between the amplitude of the CA/C ratio and proposed tonic adaptation of vergence. A low saturation limit or stimulus window for tonic adaptation may account for the amplitude dependent nonlinearities of the AC/A and CA/C ratios.

Accommodation, Ocular↗
PubMed1986

Stimuli for accommodation: blur, chromatic aberration and size.

We investigated the frequency response of the accommodative system (0.05-1 Hz) using three stimuli: defocus blur, the effects of the chromatic aberration of the eye, and changing target size. A high-speed infrared optometer monitored accommodation while the subject viewed a target in a Badal optometer. Blur was provided by moving the target sinusoidally towards and away from the subject (1-3 D) and the size of the target was varied at the same frequency. Chromatic aberration was controlled by using either monochromatic (590 nm) or white light (3300 K). Gain and phase plots changed systematically as we varied the number of stimuli presented together. This suggests that besides defocus blur both chromatic aberration and changing size are involved in accommodative control.

Accommodation, Ocular↗
PubMed1987

Dioptric and non-dioptric stimuli for accommodation: target size alone and with blur and chromatic aberration.

The frequency response of the accommodative system (0.05-1 Hz) was determined for various combinations of stimuli: changing target size was presented alone, together with defocus blur, and with both defocus blur and chromatic aberration. A high-speed infrared optometer monitored accommodation while the subject viewed the target in a Badal optometer. Target size was varied sinusoidally and blur was provided by moving the target towards and away from the subject at the same frequency. Chromatic aberration was controlled by using either monochromatic (590 nm) or white (3300 K) light. The target was presented under open-loop conditions when size was the only stimulus. We find that besides the conventional dioptric stimuli, changes in target size that result in changes in apparent distance can have substantial effects on accommodation.

Accommodation, Ocular↗