lecture 7

Cards (12)

  • 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)