Chapter 13: Optics of low vision aids

Question 1

What are low vision aids?

Answer 1

magnifing devices of several kinds to assist poorly sighted patient usually for reading aids i.e. near vision

Question 2

How do low vision aids work?

Answer 2

present pt with magnified view of object - act by increasing angle subtended by the object at the eye, producing an enlarged retinal image

Question 3

What is the equation to calculate magnifying power of a low vision aid system?

Answer 3

Magnifying power, MP = retinal image size with use of instrument/ retinal image size without use of instrument

Question 4

What are projection systems and one example?

Answer 4

enlarged image of object presented to the patient on a screen, can view from convenient distance. e.g. cloesd circuit television

Question 5

What type of lens is used as a magnifying loupe?

Answer 5

convex lens - high power simple lens mounted in spectacle frame

Question 6

What type of lens is used as a magnifying loupe?

Answer 6

convex lens - high power simple lens mounted in spectacle frame or hand-help magnifiers/mounted on legs

Question 7

How does convex lens work as low vision aid?

Answer 7

allows eye to view the object at closer range than would be possible unaided or with a standard presbyopic reading correction

Question 8

How does the power of a magnifying lens compare with that of a standard loupe?

Answer 8

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

Question 9

In what position in relation to optical ray diagrams is the object situated for low vision aids?

Answer 9

located between the first principal focus and the lens

Question 10

What happens as the object moves nearer to the first principal focus of the low vision aid (i.e. away from eye)?

Answer 10

the virtual image becomes larger + further from eye

Question 11

What is a variant of the convex lens low vision aid?

Answer 11

‘paper weight’ plano convex lens - thick, rests directly on page

Question 12

What 2 things does the field of vision obtained with a convex lens as a magnifier depend on?

Answer 12

1. size/aperture of lens
2. eye-lens distance

Question 13

What is the relationship between the eye-lens distance of the magnifying aid and the field of vision?

Answer 13

the greater the eye-lens distance, the smaller the field of vision (but greater image size)

Question 14

How do convex cylindrical lenses work as reading aids?

Answer 14

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

Question 15

What types of lenses is the Galilean telescope formed of and how are they arranged?

Answer 15

convex objective and concave eye-piece lens, separated by the difference of their focal lengths

Question 16

What are the properties of the image produced by a Galilean telescope?

Answer 16

erect, magnified image

Question 17

Is the image from a Galilean telescope distorted by curvature of field or astigmatism?

Answer 17

no

Question 18

What makes a Galilean telescope useful as a magnifying/low vision aid?

Answer 18

system is compact and light, can be mounted in spectacle frame

Question 19

What are 2 things the Galilean telescope can be adapted for and can they be combined?

Answer 19

1. viewing near objects
2. viewing distant objects

difficult to combine the 2 in the same instrument

Question 20

How does the Galilean telescope magnify images of objects?

Answer 20

increasing angle subtended by object at the eye

Question 21

How do you calculate the angular magnification from a low vision aid? 2 possible equations

Answer 21

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

Question 22

What are 3 factors that limit practical usefulness of optical magnifiying devices?

Answer 22

1. high magnification results in reduced field of view, so rapid scanning of print is impossible and limits use as distance low vision aid
2. object to be viewed must be held close to eye
3. magnification means depth of focus is reduced - object-lens distance is critical

Question 23

What magnification should be prescribed for low vision aids and why?

Answer 23

lowest magnifications that is adequate - unpleasnt instability of field and focus caused by any unsteadiness of hand/head