Subjective refraction: the mechanism underlying the routine

Factors affecting the academic performance of optometry students in Mozambique

PURPOSE

The Mozambique Eyecare Project is a higher education partnership for the development, implementation, and evaluation of a model of optometry training at UniLúrio in Mozambique. There are many composite elements to the development of sustainable eye health structures, and appropriate education for eye health workers remains a key determinant of successful eye care development. However, from the first intake of 16 students, only 9 students graduated from the program, whereas only 6 graduated from the second intake of 24 students. This low graduation rate is attributable to a combination of substandard academic performance and student dropout. The aim of this article was to identify factors affecting the academic performance of optometry students in Mozambique.

METHODS

Nine lecturers (the entire faculty) and 15 students (9 from the first intake and 6 from the second) were recruited to the study. Clinical competency assessments were carried out on the students, semistructured individual interviews were conducted with the course lecturers, and a course evaluation questionnaire was completed by students. The results were combined to understand the complexities surrounding the optometry student training and performance.

RESULTS

One student out of nine from the first intake and three students out of six from the second were graded as competent in all the elements of the refraction clinical competency examination. Analysis of data from the interviews and questionnaire yielded four dominant themes that were viewed as important determinants of student refraction competencies: student learning context, teaching context, clinic conditions and assessment, and the existing operating health care context.

CONCLUSIONS

The evaluations have helped the university and course partners to better structure the teaching and adapt the learning environments by recommending a preparatory year and a review of the curriculum and clinic structure, implementing more transparent entry requirements, increasing awareness of the program, and improving Internet infrastructure.

Chromatic aberration in heterocentric astigmatic systems including the eye

PURPOSE

There is inconsistency in the literature in the definitions of longitudinal and transverse chromatic aberration, and there appear to be no definitions that make allowance for astigmatism and heterocentricity. The purpose is to propose definitions of longitudinal and transverse chromatic aberration that hold for systems which, like the typical eye, may be heterocentric and astigmatic and to develop the associated optics.

METHODS

Common definitions of longitudinal and transverse chromatic aberration based on Gaussian optics are generalized naturally in terms of linear optics to accommodate heterocentricity and astigmatism.

CONCLUSIONS

The definitions offered here apply to systems in general, including the visual optical system of the eye, and hold for homocentric stigmatic systems in particular. Care is advocated in the use of the terms longitudinal and transverse chromatic aberration.

Bounds and intervals around nonzero cylinder powers in symmetric dioptric power space

We seek to analyze the geometry and explain how bounds and intervals of nonzero purely cylindrical powers are obtained and applied in symmetric dioptric power space and envisaged in the clinic. The principal powers at zero and at the focus at the cylinder power of a lens are subject to the same uncertainty when measured. Accompanying these uncertainties is an error in axis position. Error cells are constructed for typical cylinder axes and an associated power. The geometry contains an elegant clinical determination for cross-cylinder compensation of astigmatism in terms of calculation friendly quantities. The extreme positions in the error cells define bounds for the cross-cylinder powers and their meridians. When clinical powers in a chosen error cell are transposed, the new powers are within a different cell. This ambiguous cell pair maps to a single cell in an antistigmatic plane around cross-cylinder powers.

Flipped optical systems

In the standard refraction routine, a Jackson cross-cylinder is reversed in front of the eye by the process of flipping it about its handle. Reverse telescopes have applications in low vision. In ophthalmoscopy, the practitioner views a patient's retina using light that traverses the eye in a direction opposite to that involved in vision. These represent examples of flipped systems that are involved in optometry. The purpose of this paper is to examine what happens to the optical character of an arbitrary optical system when it is flipped in this manner. The analysis is in terms of the ray transference of the system and, because the transference completely characterises the first-order optics of a system, is complete within the limitations of linear optics. It allows for elements in the system to be astigmatic and decentred. An expression is derived for the transference of the flipped system in terms of the transference of the system itself. Expressions are also obtained for the fundamental first-order optical properties and the dioptric power of the flipped system. Three numerical examples are given in the Appendix.