Biomedical subjects
Publications and source records attributed to C W Fowler.
Apparatus is described for the measurement of lens power distribution across the surface of a progressive addition spectacle lens. Although the method used is different to that specified in BS 2738-7:1996, the measurements are nevertheless comparable, and are considered sufficiently accurate for making basic comparisons of lens type.
A clinical trial was performed on 20 optometry students in order to evaluate the suitability of the SUN SK-2000 (SUN contact lens company, Japan) computer-assisted videokeratoscope for use in clinical practice. Suitability was assessed by observing three factors: the accuracy, repeatability and agreement for central mean corneal power (MCP) with a keratometer (Javal Schiotz). Measurements of eight peripheral points and the central radius of curvature were made on 20 optometry students and on an 8 mm calibration surface using the SUN SK-2000. The SUN SK-2000 accurately determined the 8 mm calibration surface (supplied by manufacturers) to be spherical (SD = +/- 0.013, P = 0.776). Careful alignment was found to be an important factor in determining the actual radius of the calibration sphere. Unfortunately, the system required measurements to be taken at a point which was not coincident with the line of sight. A simple improvement to this has been suggested. Readings were most repeatable for central corneal radii. Peripheral radii showed the most repeatability for the horizontal and inferior corneal radii. It has been suggested that the ocular adnexa may be a possible cause for the poor repeatability in the superior area of the cornea. The level of agreement between the Javal Schiotz keratometer and the SUN SK-2000 in terms of mean refractive error (MRE) was not found to be sufficient for the two instruments to be interchangeable (P = 0.011). The C0 and C45 components of corneal astigmatism were found to have little association with the keratometer.
This paper reviews the literature on the effects of luminance and chromatic contrast on reading and object recognition in low vision. The general consensus is that whereas luminance contrast can enhance reading and object recognition for some low vision subjects, chromatic contrast does not aid low vision reading but may be useful in low vision object recognition. Although providing useful theoretical results, the experimental designs have so far been remote from the conditions that low vision patients may experience in the home and work environment. Suggestions are made for an 'ideal' experimental design and details of current research, using conditions more likely to be experienced by low vision subjects in everyday life, are given. This research makes use of a recently developed instrument, the Intuitive Colorimeter.
Various methods of measuring corneal topography are described. The advantages and disadvantages of the principles used in the measurement of corneal shape by the various techniques are discussed. The corneal surface may be described in a number of ways; some researchers have used conic sections while others have used more complex polynomial expressions. Computer algorithms have also been developed to calculate quantitative measures of corneal topography to augment the information obtained from topographical maps. These descriptors are discussed in this article.
Although there are many designs of progressive power multifocal lenses commercially available, they all follow the same basic design concept, with the lens power varying across their aperture as a result of using one aspheric lens surface. An alternative approach was tried on an experimental basis, where the variation in power is produced by a liquid-filled deformable lens.
The optico-physical characteristics of four different designs of progressive addition lenses(PALs) were assessed using a computer-controlled automated focimeter. For each design and for the four designs taken together, reading addition power was shown to correlate positively with PAL progression corridor length and negatively with PAL progression corridor width. Empirical measurements from the four PALs measured showed each design to have more favourable characteristics when compared with a theoretical single power law model.
Liquid crystal lenses are possibly the spectacle lenses of the future for presbyopic corrections. They present a variable refractive index which can be used to produce the necessary addition needed for presbyopia. The variability of the refractive index is due to the fact that these materials are optically anisotropic. This effect is obtained by the application of an electric or magnetic field through the liquid crystal layer. A review of the literature relating to the evolution, the preparation and the operation of these lenses is given. A discussion is also included of the practical problems and drawbacks, which at present limit their use, and possible solutions.
Although the Varilux type varifocal has been and still is very popular, research has been carried out on other methods of providing a progressive addition lens. This paper deals with the field of gradient-index optics. It includes a review of the literature related to the production of a gradient-index element and the theoretical analysis of concentric varifocals. A procedure is described to construct a varifocal lens, after that described by Wood in 1905, to produce lenses known as 'non-homogeneous cylinders' or 'pseudo-lenses'. The method is based on a diffusion process, taking place in half of the lens, and creates steps in the refractive index of the material. Experimental data are also given on the change of the spherical and cylindrical power, which indicate that the lens constructed is an optical system having a rotational symmetry in power, around a fixed point.
Numerous reasons have been cited for patient non-tolerance to Progressive Addition Lenses (PALs). This paper considers two presbyopic subject populations - those who successfully wore PALs (n = 20), and those who were non-tolerant to them (n = 18). The dispensing accuracy of the PALs worn by subjects was assessed and the surface quality of those lenses worn by unsuccessful patients studied. These factors do not account for any significant differences between the two groups, suggesting that other causes must separate the subject populations.
An instrument is described for the automatic measurement of lens power across the whole aperture of a varifocal lens. The apparatus is based on an electronic focimeter controlled by an external computer, which also positions the lens by two stepper motors. Methods of display of the data are discussed, and the repeatability of measurement is illustrated.
Progressive addition lenses (PAL's) are subject to objectionable astigmatism due to the aspheric nature of their anterior surfaces. Optico-physical assessment of PAL's is commonly undertaken but these measures should be related to the psychophysical responses of patients whilst wearing PAL's. A technique previously used for the psychophysical assessment of contact lenses and spectacle lenses is adapted and demonstrated for the measurement of grating visual acuity (VA) through the aspheric portion of PAL's. The apparatus encompasses an astronomical telescope with magnifying power of unity designed to stabilize accommodation. This paper considers the astigmatism present with three different PAL designs along a horizontal section cutting orthogonally through the umbilical line of each lens. VA was measured, using high contrast vertical and horizontal gratings, along the same horizontal sections of the PAL's studied. A reduction in VA was noted with an increase in the angle of eccentricity from the umbilical line.
A method is described where the surface curvatures and axis orientation of a toroidal surface can be determined by measuring the lens in three fixed orientations. This method also has possible applications in the measurement of progressive addition lenses.
Three methods of measuring aberrational astigmatism in progressive addition spectacle lenses are described. Measurements are presented from each method made on an Essilor Varilux 2 lens, which illustrate the small differences between the methods for low power lenses.
Analysis of 98 progressive addition lens (PAL) wearers was undertaken using both mailed questionnaires and by assessing patient records. Statistical analysis was undertaken to investigate the nature and dependent variables of a PAL population. The PAL patients were from the Aston University undergraduate optometric clinic. Social survey techniques, such as the use of personalized mail, first-class stamps, and follow-up letters, were employed to produce a high return rate for the questionnaire. The number of returned questionnaires reached 89.9% of those originally sent which may be considered to be highly representative. The clinical survey showed the success rate for the undergraduate clinic to be either 85.7% or 80.6% depending upon the criterion for successful wear.
An instrument is described for the assessment of varifocal lens surfaces based on a reflection technique. Results are also presented of measurements made on a series of lenses of the same design, illustrating some of the differences that exist between glass and plastics versions of the same lens.
This paper reviews recent developments in the field of progressive addition spectacle lenses and variable focus lens systems design. The study is a sequel to the previous analytical work of von Rohr (1916), Graham (1942), and Bennett (1970). A brief history of the work documented by the aforementioned authors is presented. Comparative assessment is undertaken of the most recent patents of progressive addition and variable focus spectacle lenses. A largely verbal description is employed because patent literature is often devoid of mathematical models.
Smith and Atchison (1983) give an optical model of a "zonal aspheric" aphakic lens, but note that lenses mass-produced to this design philosophy do not show similar optical properties to the model. This paper offers an explanation for this discrepancy, supported by lens measurements.
Katz described a design for a postcataract spectacle lens with two aspheric surfaces. Previously, von Rohr and Jeffree had patented designs with similar optical properties, but using steeper lens forms than the Katz design. These three design studies are compared with two aspheric spectacle lenses that have been produced commercially in order to demonstrate the differences in calculated aberrations and their significance on the practicality of a lens design.