Chapter 13: Optics of low vision aids Flashcards
What are low vision aids?
magnifing devices of several kinds to assist poorly sighted patient usually for reading aids i.e. near vision
How do low vision aids work?
present pt with magnified view of object - act by increasing angle subtended by the object at the eye, producing an enlarged retinal image
What is the equation to calculate magnifying power of a low vision aid system?
Magnifying power, MP = retinal image size with use of instrument/ retinal image size without use of instrument
What are projection systems and one example?
enlarged image of object presented to the patient on a screen, can view from convenient distance. e.g. cloesd circuit television
What type of lens is used as a magnifying loupe?
convex lens - high power simple lens mounted in spectacle frame
What type of lens is used as a magnifying loupe?
convex lens - high power simple lens mounted in spectacle frame or hand-help magnifiers/mounted on legs
How does convex lens work as low vision aid?
allows eye to view the object at closer range than would be possible unaided or with a standard presbyopic reading correction
How does the power of a magnifying lens compare with that of a standard loupe?
power used e.g. +5.0 DS is less than that of a standard loupe (x8 = +32 DS) - can be regarded as enhanced presbyopic prescription
In what position in relation to optical ray diagrams is the object situated for low vision aids?
located between the first principal focus and the lens
What happens as the object moves nearer to the first principal focus of the low vision aid (i.e. away from eye)?
the virtual image becomes larger + further from eye
What is a variant of the convex lens low vision aid?
‘paper weight’ plano convex lens - thick, rests directly on page
What 2 things does the field of vision obtained with a convex lens as a magnifier depend on?
- size/aperture of lens
- eye-lens distance
What is the relationship between the eye-lens distance of the magnifying aid and the field of vision?
the greater the eye-lens distance, the smaller the field of vision (but greater image size)
How do convex cylindrical lenses work as reading aids?
bar-shaped lens with no refractive power / only low converging power in its long axis and high converging powe in cross section is laid on line of print, produced vertical magnification of letters
What types of lenses is the Galilean telescope formed of and how are they arranged?
convex objective and concave eye-piece lens, separated by the difference of their focal lengths
What are the properties of the image produced by a Galilean telescope?
erect, magnified image
Is the image from a Galilean telescope distorted by curvature of field or astigmatism?
no
What makes a Galilean telescope useful as a magnifying/low vision aid?
system is compact and light, can be mounted in spectacle frame
What are 2 things the Galilean telescope can be adapted for and can they be combined?
- viewing near objects
- viewing distant objects
difficult to combine the 2 in the same instrument
How does the Galilean telescope magnify images of objects?
increasing angle subtended by object at the eye
How do you calculate the angular magnification from a low vision aid? 2 possible equations
Angular magnification,** M = ae / ai**
where ae is angle of emergene, ai is angle of incidence
or M = Fe / Fo
where Fe is power of eye piece lens in dioptres, Fo is power of objective lens in dioptres
What are 3 factors that limit practical usefulness of optical magnifiying devices?
- high magnification results in reduced field of view, so rapid scanning of print is impossible and limits use as distance low vision aid
- object to be viewed must be held close to eye
- magnification means depth of focus is reduced - object-lens distance is critical
What magnification should be prescribed for low vision aids and why?
lowest magnifications that is adequate - unpleasnt instability of field and focus caused by any unsteadiness of hand/head
