4.2.2 Understands the application of complex low vision aids

Question 1

Adaptations of phone use:

Answer 1

• Bigger - larger text, zoom in, magnifier setting or app
• Contrast - bold text, invert colours, colour contrast
• Brightness - increase it, increase saturation
• Audio - text to voice or audio description

Question 2

Complex Low vision Aids:

Answer 2

• Spectacle mounted telescopes
• Field expanders
• CCTVs
  
  • X-Y table
  • Television readers
  • Head mounted
  • Portable
• Intraocular telescope (not really required but may impress assessor)

Question 3

Others Aids

Answer 3

• Bright field/flat field
• Bar magnifier
• Computer LVAs
  
  • Magnifying screen filter
  • Enlarged keyboard
  • Screen reading software - text to speech
• Sight substitution aids
  
  • Postural aids - clipboards, reading stands, double clamps
  • Audio - talking books, newspapers, labels
  • Mobility aids - guide dogs, sticks and canes
  • Braile

Question 4

CCTV Advantages:Disadvantages

Answer 4

• CCTV Advantages:
  
  • High Mag - up to 70X, zoom in & out
  • Contrast reversal & enhancement - enhances from 70 to 100%, reverse polarity, colour options & margins to use as typoscope
  • Better posture - not looking downward
  • Socially acceptable - at work & school
  • Binocular
  • Good depth of field
• CCTV Disadvantages:
  
  • Expensive!
  • Hard to use - need practice for all the controls!
  • Not very portable although TV readers are
• Nystagmus for telescopes

Question 5

Electronic

Answer 5

 Most common of real image magnification is the use of a closed-circuit television (CCTV)
 Mag is calculated as M = size of image/size of original object
 Image quality depends on screen resolution)
 Maximum mag achievable by optical magnifiers is about 20X (80D), this can be up to 70x with electronic aids, but limit of about 30x is common
 Variable magnification may also be useful if the patient’s eye condition changes

 Electronic features (advantages)
o Foreground/background colour options, windowing etc
o Contract, luminance, enhancing
o Contrast reversal (50% of VIP prefer white on black)

Question 6

Electronic : advantages vs disadvantages

Answer 6

Advantages of EVEs
- Aberration free
- Max mag up to 70x higher (limit of 30x is common)
- Zoom controls allow variable magnification
- Foreground/background colour options, windowing etc
- Contract, luminance, enhancing
- Contrast reversal (50% of VIP prefer white on black)
- Binocular viewing
- Normal viewing distance/posture
- Variable camera-to-task and eye-to-screen distances
- Reading duration > optics LVA
- May be more psychologically acceptable than optical aids
- May be useful if a physical handicap prevents use of aids

Disadvantages
- Cost: expensive to buy and service/repair
- Size depending on type/model
- Image quality – depends on screen
- Depth of field is limited by the focal length of the camera and can cause problems with thick books (use glass sheet to flatten out)

Question 7

CCTV

Answer 7

• Stand mounted
• Still one of the most commonly used designs
• Each component fixed and mounted vertically “in-line”
• Working space fixed or limited
• For distance need second camera to be added
• Reading material needs to be flat – depth of focus
• Advantages
• Good of physical handicap
• Good for extensive VF defects, SES
• High Mag - up to 70X, zoom in & out
• Contrast reversal & enhancement - enhances from 70 to 100%, reverse polarity, colour options & margins to use as typoscope
• Better posture - not looking downward
• Socially acceptable - at work & school
• Binocular
• Good depth of field
• Disadvantages
• Expensive, bulky, servicing and repair
• Practice required
• Not very portable although TV readers are
• Depth of field

Question 8

• Hand help/portable (P-EVES)

Answer 8

o Greater versatility than CCTV
o Most common type and held at any desired distance
o Cheap, lightweight and portable
o Convenient to carry for short duration
o Usually used with px with distance rx however near can be worn if use a shorter mag-eye distance
o Field of view is directly proportional to diameter of lens and indirectly prop to lens power and eye-lens distance
o May be helpful for px with reduced peripheral fields
o Further lens held from eye- increased distortions and poorer field of view
o Ineffective if used incorrectly with reading RX and difficult to maintain correct position and steady eye strategy for extended times

Question 9

Who benefits/EVES compared to optical LVAs

Answer 9

• Usually young and highly motivated
  o May be linked to congenital visual impairment – poorer VA
  o Distance VA better than 6/90 may gain little benefit from using EVEs
• More useful than magnifiers for very small print only
• Not yet available on NHS
• Available through local libraries/charities, no special schemes for education/employment

Question 10

Telescopes

Answer 10

• A telescope allows magnification of an object without having to change the working space. This is achieved by a combination of lenses separated by a finite distance, plus lenses in an astronomical or Keplerian system, or a negative eyepiece and positive objective in a Galilean system.

Pros

• Offer distance magnification.
• Near telemicroscopes offer an increased working distance over spectacle magnifiers. They are sometimes useful for hobbies or tasks where a tool or implement needs to be between the viewer and the task.
• Variable focus. Most distance telescopic systems may be adjusted to allow for some intermediate and near viewing. Some allow different caps to be placed over the basic unit.
• Spectacle-mounted telescopes may be used for longer viewing tasks, such as television viewing, if spectacle mounted.
• Portability. Hand-held and finger-ring designs are easily carried about.
• Binocularity maintained. This is possible with either spectacle mountings (the tubes converged for near viewing tasks) or hand-held binoculars. Careful focusing is required to maintain comfortable binocularity.
• Adaptable. Many near telescopes may be trephine or advanced mounted and often may be interchangeable with near spectacle magnifying devices.
• Wide range available. There are many devices available over a wide range of magnifications, including some contact lens aids (high minus contact lens with various high-plus spectacles over the top).
  Cons
• Restricted field of view. The further the objective from the eye, the greater the restriction.
• May be expensive.
• Poor cosmesis.
• Often heavy.
• Training often required for focusing.
• Spectacle form may need patient fitting to avoid diplopia or distortions.
• Hand or head movements magnified in the view (this author has heard a patient to whom he has prescribed spectacle mounted telescopes complaining of the TV moving as they breathe or their heart beats).

Question 11

Telescope definition:

Answer 11

Here are the main points in bullet form:

• Telescope definition:
  • Composed of two or more lenses separated by a gap.
  • Designed to refract incident light to increase its angle of incidence on the retina.
  • Provides angular magnification.
• Distinction from spectacle magnifiers:
  • Telescopes: Multi-lens systems offering angular magnification.
  • Spectacle magnifiers: Single piece of refracting material (e.g., strong lenses in glasses).
• Afocal (normal adjustment) systems:
  • Both object and image are at infinity.
  • Offers angular magnification only.
  • Has no focal length (afocal).
• Focal systems:
  • Image is formed closer than infinity.
  • Offers relative distance magnification — object appears closer.
  • Has a finite focal length.
• Telemicroscope definition:
  • A telescope adapted for intermediate or near viewing.
  • Also referred to as near-telescope or by other similar terms.

Question 12

Two key uses of telescopes

Answer 12

Here are the main points in bullet form:

• Two key advantages of telescopes over other optical aids:
  1. Angular magnification for distance:
     • Telescopes provide non-electronic magnification of distant objects.
     • Useful for people with visual impairment needing to see far-away objects more clearly.
  2. Telemicroscopes for near tasks:
     • When focused for near, telescopes become telemicroscopes.
     • Offer a longer working distance compared to spectacle magnifiers of similar magnification.
     • This extended distance can be advantageous for certain near tasks.

Question 13

Galilean Telescopes

Answer 13

The combination of a positive objective lens and a negative eyepiece is known as a Galilean system. For light coming from infinity, the objective lens focuses upon a point which is coincident with the primary focal point of the eyepiece. Note:

• The eyepiece is negative.
• The eyepiece is numerically stronger than the objective.
• The length of the system is the sum of focal lengths of the objective lens (f’1) and that of the eyepiece (f2) and, as the eyepiece is negative, this represents a lower numerical value than if the two lenses were positive.
• The image formed is erect.
• The exit pupil, which is the image of the objective as seen through the eyepiece, is located within the tube.

Question 14

For afocal systems, the magnification is found to be the ratio of the power of the two lenses;

Answer 14

M = - F1/F2

Or M = - Feyepiece/Fobjective

Example

A Galilean telescope is formed of a +20.00DS objective lens and a -50.00DS eyepiece. What is the magnification?
M = Feyepiece/Fobjective
= - (-50)/20
= 2.5 x
The answer is a positive number so implying an erect image and therefore confirming a Galilean system.

Question 15

Keplerian telescopes

Answer 15

Here are the main points on Keplerian telescopes in bullet form:

• Definition:
  • Both the objective and eyepiece lenses are of positive power.
  • The secondary focal point of the objective aligns with the primary focal point of the eyepiece.
• Design characteristics:
  • Eyepiece is stronger (numerically) than the objective.
  • Tube length = sum of both focal lengths → longer than Galilean systems.
  • Magnification is negative, indicating an inverted image.
• Image orientation:
  • Without correction, image is inverted → system is astronomical.
  • For everyday use, prisms are added to erect the image → becomes a terrestrial telescope.
• Exit pupil:
  • Located just outside the eyepiece.
  • Aligns well with the viewer’s entrance pupil (iris) → more light enters the eye, improving image brightness and clarity.
• Light path:
  • Light rays cross over within the tube.
  • A stop placed at this crossover point can reduce spherical aberration, further enhancing image quality.
• Summary advantage:
  • Generally provides better image quality than Galilean systems due to exit pupil position and potential for aberration control.

Question 16

A Keplerian telescope is formed of a +20.00DS objective lens and a +50.00DS eyepiece. What is the magnification?

Answer 16

M = Feyepiece/Fobjective
= - 50/20
= - 2.5x
The answer is a negative number and therefore inverted.

For both types of telescope, uncorrected ametropia will have an impact on the expected magnification. The best way of thinking about this is that the uncorrected ametropia modifies the power of the eyepiece.

Question 17

Uses of telescopes

Answer 17

Here are the main points on uses of telescopes in bullet form:

• Common misuse:
  • Prescribing telescopes for “spotting the bus” is generally ineffective.
  • Reduced field of view makes tracking moving targets difficult.
• Best suited for static targets, such as:
  • Shop signs
  • Aisle markers in supermarkets
  • Train timetables
  • Clocks
  • Display screens
• Device types and usage:
  • Hand-held telescopes: Best for short-term spotting.
  • Spectacle-mounted telescopes: Better for extended viewing.
• Bioptic telescopes:
  • A spectacle-mounted distance telescope on a moveable holder.
  • Used in some countries to allow visually impaired individuals to drive.
  • Users switch between normal vision and telescope view as needed.
• UK context:
  • Visually impaired people cannot legally drive using bioptics.
  • Driverless cars offer future promise for increased independence.