ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00898.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

The relationship between corneal curvature measured by classical keratometry and refractive error raises the question of a possible systematic relationship between refractive error and the degree of corneal asphericity. A recomputation and analysis of previously published data leads to the conclusion that the different refractive groups have similar values of the parameter ‘p’ and differ only in the apical radius of the cor

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00899.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

A keratometric instrument is described which can be used to measure thep‐value of the corneal ellipse. The performance of the instrument is compared with that of the Guillbert Routit topographical keratometer and the Wesley Jessen P. E. K. The application of thep‐value to contact lens fitting is discus

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00900.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Although toric contact lenses have improved in design and development and their use is increasing, further consideration should be given to the alternative methods of astigmatism correction by contact lenses. Much can still be achieved by non‐toric lens, both for a wide range of astigmatism and for ease of supply. A number of these alternatives are examined, their advantages and disadvantages reappraised, and the merits of lens correction compared with surgical methods considere

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00901.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Various configurations have been suggested for the optical measurement of corneal thickness by slit beam doubling, and corresponding ray traces have been provided to relate actual thickness to apparent thickness. In this report, a set of equations is provided which apply to the general situation in which neither the angle of illumination nor the angle of observation are aligned with or symmetrical about the normal to the cornea. An explicit solution to these equations giving true corneal thickness in terms of apparent thickness cannot be established, and an iterative procedure is used to develop a relationship for a number of different pachometer configurations. The near linearity of these relationships suggests the existence of a simple equation to relate these entities. By making appropriate assumptions, a linear approximation relating the inverse of the true thickness to the inverse of the apparent thickness is developed. An equation for determining the location of measurements of peripheral corneal thickness is also derived. The equations presented here may be generally applied to all optical pachometer configurations, can be used to estimate the relative contributions of various sources of error in pachometry and provide a basis for obtaining uniform absolute derivations of corneal thickness.

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00902.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

The results are described of a 10‐year study of the longitudinal changes in the static response of an individual subject during the approach to absolute presbyopia. The amplitude of accommodation declines linearly with age. A new finding is that the magnitude of the slope of the linear portion of the response/stimulus curve also reduces with age, as does the level of tonic accommodatio

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00903.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Disturbances of binocular vision are described clinically by the Duane‐White classification in terms of the magnitude of the accommodative‐convergence ratio (AC/A). Convergence excess and convergence insufficiency are assumed to result from high and low AC/A ratios respectively. It is assumed that the abnormal AC/A ratio is an independent variable that underlies abnormal phorias. However, recent studies have demonstrated that the AC/A ratio is inversely related to the adaptability of tonic accommodation (lens adaptation) and directly related to adaptability of tonic vergence (prism adaptation). We have tested whether clinical categories of convergence excess and convergence insufficiency are associated with insufficient and excessive adaptation of tonic accommodation and tonic vergence. Results demonstrate greater amplitude and duration of accommodative after‐effects (lens adaptation) in the convergence insufficiency than the convergence excess group. Vergence after‐effects (prism adaptation) had the reverse trend for the two groups. These results indicate that adaptive disorders of accommodation and vergence may underlie binocular disorders in symptomatic patients categorized as convergence excess and convergence insuff

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00904.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

Asymmetrical optokinetic nystagmus (OKN) means that OKN has a lower gain (slow‐phase eye velocity/stimulus velocity) for monocular temporalward than nasalward visual field motion. OKN tends to be asymmetric in amblyopia, leading to suggestions of a link between OKN asymmetry and binocularity in the literature. The present study measured OKN in 13 amblyopes and five normal subjects. In an attempt to identify those binocular cells used in the OKN response, the degree of OKN asymmetry was compared with binocularity assessed by two different techniques: (1) stereopsis and (2) interocular transfer of threshold elevation (IOT). Horizontal monocular OKN was recorded for three different stimulus velocities in each subject. All the amblyopes were found to be stereoblind, although three amblyopes showed OKN asymmetries close to those found for the normal group. More association was seen between interocular transfer of the threshold elevation and OKN asymmetry; not all amblyopes demonstrated reduced IOT, but those amblyopes with no IOT all had OKN asymmetries more than 125% of the mean of the normal group. However, no association was seen between the amount of OKN asymmetry and the degree of IOT. The results are discussed in terms of the role of different groups of binocular neurones for OKN and the effect of the sensitive periods of development on these binocular neurone

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00905.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

The study examines to what degree the output of the Canon Auto‐ref R1 infra‐red (IR) optometer is affected by variations in pupil size for both static measurement of refractive error and for continuous recording of accommodation when switched to its modified mode of operation. In the normal static mode of operation the measurement of refractive state simply depends on the position of the peak photodetector output for each of three meridians: it does not depend on signal amplitude and hence pupil size. However, in continuous mode the actual output of a single photodiode is measured and the measurement is susceptible to changes in pupil size that occur within the area of the exit pupil utilized by the instrument. Using static and dynamic recording on model and human eyes the study shows that a 10% reduction in output signal relative to the maximum output level (for a constant stimulus) occurs for pupil sizes of 3.9 mm. The relatively small exit pupil has important advantages with regard to the use of mydriatic drugs and task‐induced changes in pupil dia

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00906.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

The relationship between refractive error and family history, age of onset and personality was investigated in 189 optometry students. Subjects completed a questionnaire requesting details of their refractive history, type of correction and their family history. Subjects also completed an Eysenck Personality Inventory (EPI). Subjects were categorized as hyperopes, emmetropes or myopes on the basis of their questionnaire responses. No significant relationship was found between refractive group and the prevalence of myopia in parents. Myopic subjects, however, showed a significantly higher prevalence of myopic siblings. Furthermore, late‐onset myopes showed a higher prevalence of myopic siblings than early‐onset myopes. No significant personality differences were found between the refractive gro

ISSN:0275-5408    

DOI:10.1111/j.1475-1313.1989.tb00907.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY