lecture 7

• Convex lens

  A lens that is thicker in the middle than at the edges, causing light rays to bend inward and converge
• Convex lens

  • May be single, or a compound lens system designed to minimize selected aberrations
  • Plus lenses may be as powerful as +80.00 D or more
• Forms of plus lenses

  • Spectacle-mounted "microscopes"
  • Hand-held magnifiers (HM)
  • Stand magnifiers (SM)
• Hand-held magnifier

  A handle attached to the lens
• Stand magnifier

  The lens is supported by a stand, fixed distance from the object/page
• Microscope (MS)

  Gives relative distance magnification (relative distance enlargement – proximal enlargement)
• Children/young adults can obtain relative distance magnification using accommodation, but adults can't do this
• Microscope (MS) types

  • Standard spectacle lenses (single vision, bifocals)
  • Paired lenses (with base-in prism)
  • Clip-on bar-mounted or head-band mounted magnifiers
• Advantages of microscopes

  • The PX has both hands free to hold, carry out, the special task
  • Large field-of-view compared to other devices of equivalent power. With practice, reading speeds are generally faster with microscopes than with other optical aids of the same power
  • Similar cosmetic appearance to regular glasses – spectacle frame of choice
• Disadvantages of microscopes

  • Reduced working distance
  • The position of the head, neck, and arms may result in fatigue and postural discomfort
  • Task illumination difficult
  • The user must make arm movements rather than eye movements in order to scan text. This new relationship must be learned
  • May be heavy and physically constricting
• Binocular spectacle magnification

  • Over the shelf prism high powered near addition could be successful
  • To achieve binocularity with spectacle magnification (custom-made MS), the optical centers of lenses should be coincident with the object's WD, achieved by decentration or adding base-in prism
• Decentration

  Half of the pupillary distance (PD) = (Back vertex distance (BVD) + Ocular radius (OR) + Working distance (WD) of object) / (BVD + OR)