Bifocal lens vertex power measurement with eyepiece lensmeter
Introduction
Bifocal lenses are used for correction of ametropia and presbyopia. Older mineral bifocal lenses contain segment for near with higher refraction index than is refractive index of basic lens. Newer plastic bifocal lenses contain segment for near with higher curvature than basic lens has. These lenses have on the front surface small touchable step. Sometimes we need special bifocal lenses for correction of accommodative strabismus called E-lines.
Goals
- Measure vertex power of the lens’ distance part
- Measure vertex power of the near segment and mark its optical center
- Draw the measured lens and mark its important points
Equipment
Hand mechanical lensmeter with eyepiece, bifocal lens
Methods
Measure vertex power of the lens’ distance part
Put the bifocal lens in the lensmeter’s rest place with concave surface down. Firstly, measure and note the vertex power of the distance part of the lens above the step. Finally mark the distance optical center with the instrument’s marking device.
Measure vertex power of the near segment and mark its optical center
Shift with the bifocal lens so you will place near segment in the instrument’s measuring axis. Find the near optical center, measure its vertex power and finally mark it with the marking device.
Draw the measured lens and mark its important points
Draw the picture of bifocal lens in ration 1:1. Draw exact diameter, shape and near segment position. Further measure distance between distance and near optical center, size of the near segment etc. Measure nasal decentration of the near segment.
Results
Measure vertex power of the lens’ distance part
SD =
Measure vertex power of the near segment and mark its optical center
SN =
Draw the measured lens and mark its important points
Discussion
Bifocal lenses are still very popular and often used. Its main advantage is wide visual field for distance and middle distance based on the rest of accommodation amplitude. Near segment reduce near visual field but help to rest of accommodation amplitude to focus near objects. In comparing with progressive lenses, it is interesting possibility how to correct presbyopia. But some patients complain on narrow visual field and jump in the visual field which is produced by different vertex power of the distance and near part.
Conclusion, notes, comments
Can it be negative addition?
How is near segment decentered?
Define minimal grinding high of the bifocal’s near segment?