L17,18 - Low Vision

Question 1

Registration of visual impairment

Answer 1

Sight impaired (SI) Partially sighted 
Severely sight impaired (SSI) blind

Question 2

Sight impaired

Answer 2

VA 3/60 to 6/60, full visual fields
VA up to 6/24, moderate VF constriction, medial opacities or aphakia
VA 6/18 or better with gross field defect

Question 3

Severely sight impaired

Answer 3

VA less than 3/60 or less with full VF
VA ranging from 3/60 to less than 6/60 with significantly contracted VF
VA of 6/60 or better with gross VF constriction, particularly lower field.

Question 4

Main causes of visual impairment

Answer 4

Unaddressed refractive error
Near vision impairment caused by unaddressed presbyopia
Cataract
Glaucoma

Question 5

A person with low vision (UK) …

Answer 5

Has an impairment in visual function which cannot be corrected by spectacles, CLs or medical intervention that causes restriction in that persons everyday life.

Question 6

Visual assessment for low vision

Answer 6

Visions - LogMAR distance, Baile- lovie N for near 
Visual fields 
Optical aids 
Non optical aids
Pupils, look for pathology 
Advice and management

Question 7

Distance vision and VA for low vision

Answer 7

Use LogMAR chart at 4m (or 2m/1m/50cm)
Binocular and monocular 
Record preferred eye 
Give time to respond 
Allow eccentric viewing
Place chart close enough to read one or two lines

Question 8

Near vision and NVA low vision

Answer 8

Bailey-lovie often used
Record speed
Record near WDs
Record if they use reading specs

Question 9

Contrast sensitivity and visual fields for low vision

Answer 9

Contrast sensitivity - use Pelli robson

Visual fields - adapt test to suit px (eg amsler chart)

Question 10

Near/ reading performance reference standard

Answer 10

X1 mag for object at 25cm required +4.00 add (or adequate accommodation)

Question 11

Magnification equation

Answer 11

M = F/4

F is dioptric power

Question 12

Divide by 3 rule

Answer 12

Gives an estimate of px near acuity based on snellen acuity
Divide the denominator by 3
(Doesn’t apply to central Scotomas)

Question 13

Two ways to determine starting magnification

Answer 13

Practical approach - increase mag by using lenses and shortening focal length

Theoretical approach - using formula
What px can see / what px wants to see

Question 14

4 types of magnification

Answer 14

1. Relative size
2. Relative distance
3. Transverse
4. Angular

Question 15

Relative size

Answer 15

Increasing the actual size of the object being viewed

Question 16

Relative distance

Answer 16

Reducing the distance between the object and the eye

M = old object distance / new object distance

Question 17

Transverse / linear magnification

Answer 17

Ratio of image to object length

Question 18

Angular magnification

Answer 18

Increasing angular subtense of the image being viewed (telescope magnifier)

Question 19

Hand magnifiers

Answer 19

Range from +4 to +50DS
Biconvex - offers larger lens diameters, more for low powered.
Aspheric - reduces weight and thickness and helps reduce aberrations for high powered

Question 20

Stand magnifiers

Answer 20

Fixed lens to object working distance

Question 21

Thick lens formula

Answer 21

Fe = Fm + Fa - (z x FmFa) 
Fe - equivalent power of the system 
Fm - equivalent power of the magnifier 
Fa - equivalent power of reading add 
Z - distance between eye and hand magnifier distance

Question 22

Field of view equation

Answer 22

w = y/(FmZ)

y = lens diameter 
Fm = equivalent power of magnifier 
z = distance from eye to magnifier

Question 23

Telescopes - types

Answer 23

Allow image even if far distance to be seen as larger by using Angular Mag.
There’s 2 types:
-Galilean
-Keplarian

Question 24

Telescopes - the objective Lens, the eye piece

Answer 24

Objective lens - lens furthest from the eye, always a converging lens (positive), must have longer focal length compared to the eye piece.

Eye piece - lens closest to the eye, can be positive (Keplerian) or negative (Galilean), must be positioned so image is formed by objective lens at primary focal point of the eye piece so must have shorter focal length compared to objective lens.

Question 25

Keplerian telescope and roof prisms

Answer 25

Two positive lenses cause an inverted image. Therefore roof prisms are added within the telescope to revert the image to an erect one.
But makes telescope longer and heavier

Question 26

Telescopes - exit pupil equation

Answer 26

Exit pupil = objective lens diameter / telescope mag

Question 27

The three magnification equations

Answer 27

M = angle subtended at the eye by image / angle subtended at the eye by object. 
M = F eyepiece (Fe) / Fobjective (Fo) 
M = size of entrance pupil / size of exit pupil

Question 28

Non optical aids definition

Answer 28

A non-optical aid is an item that helps the px to see without increasing the retinal image size.

Question 29

Two types of glare

Answer 29

Discomfort - px complains light within visual field is excessively bright causing discomfort.
Disability - also affects the visual acuity for example incase of a cataract causing light scatter.

Question 30

Typoscope

Answer 30

Black piece of card or plastic with portions cut out to allow lines of text to be highlighted.
Reduce back scatter of light, allows line of print to be highlighted, aids tracking of words

Question 31

Colour contrast - low vision

Answer 31

Using bright colours to distinguish. For example, a white mug for coffee.