Lecture 17 - Electronic vision enhancement systems

Question 1

Methods of producing magnification:

Answer 1

1. Increase size of object
2. Decrease viewing distance
3. Telescopic magnification
   4. Real image / transverse magnification (electronic magnification)
   - which we will study now

Question 2

Types of Camera based Electronic vision enhancement systems (EVES):

Answer 2

• Stand mounted
• Mouse; portable
• Head mounter
• Hand held

Question 3

Advantages of EVES:

Answer 3

• Aberration free, but image quality depends on screen resolution
• Max mag up to 70x or higher, limit of 30x common
- Optical magnifiers is about 20x this can be up to 70x or higher with EVES
• Zoom control allows variable magnification
- Page navigation
- Approx. only 4 characters required for reading but up to 15 requires for optimum page navigation
• Electronic contrast reversal
- >50% px prefer white on black (reduce scatter)
- Colour monitors allow diff color combos
• Other electronic features
- Text can be underlined + windowed
- Split screen presentation for both high+low mag simultaneously
• Binocular viewing
• “normal” viewing distance/posture
- reading duration greater than with high magnification optical aids
- but reading speed likely to be similar
• Variable camera-to-task and eye-to-screen distances
• May be more psychologically acceptable than optical aids?
• May be useful if a physical handicap prevents the use of optical aids?
- good for patients with extensive field defects
- easier to adapt to “steady eye strategy”

Question 4

Disadvantages of EVES:

Answer 4

• Cost - expensive to buy and service/repair
• Size depending on type/model
• Image quality - depending on screen
- The persistence of the image on the screen’s phosphor causes blurring/fading if the reading material is moved too quickly. Not an issue with LED/LCD.
• Depth-of-field
- This is limited by the focal length of the camera and can cause problems with thick books (use a glass sheet to flatten it out)

Question 5

CCTV:

Answer 5

• probably still one of the most commonly used designs :
• each comparent xed and mounted vertically “in-line”
• for distance need second camera to be added
• !! reading material needs to be flat - depth of focus !!

Question 6

CCTV: Advantages and Disadvantages

Answer 6

Advantages
• Good if physical handicap
- (Parkinson’s Disease)
• Good for extensive VF defects
- “steady eye strategy”

Disadvantages
• Expensive, bulky, servicing/repair
• Practice required (reading speed)
- 15-20 sessions
• Control positions
• Depth-of-field

Question 7

Other electronic magnification systems:

Answer 7

• Hand held/portable (P-EVES)
- greater versatility
• Television/USB readers
- low cost
• Head mounted video magnification devices
- ?ability to wear permanently?
- Distance and near magnification
- Independent of external illumination/reduced glare

Question 8

Visual performance with EVES compared to optical LVAs:

Answer 8

• Suggested that more practice required to achieve optimum reading speed than with optical aids, but small print is read better and faster with EVES than optical magnifiers
• Visual performance is very task specific, with tasks like “map tracking”, navigating columns and ‘“label identification” better with optical magnifiers

Question 9

Head mounted EVES:

Answer 9

In general, visual performance achieved with head mounted EVES does not justify their high cost. Although many systems have cameras intended for viewing distant objects, the main uses are:
- reading
- dealing with correspondence/filling in forms
- school tasks
- viewing photographs

LVES (Low Vision Enhancement System)
• Concept 1988
• All distances
• Stereoscopic
• Not dependent on external illumination
• Variable magnification/ focus

Question 10

Performance between EVES:

Answer 10

• Head mounted Reading speeds are similar to standard CCTV’s or large print
• Younger and highly motivated more successful with EVES
• Older Px more successful with optical aids

• HMD - limited performance benefit
• EVES - good for very small print
• HMD - greatest benefit for early-onset eye conditions

Question 11

How does the Px obtain an electronic LV aid?

Answer 11

• Optical aids provided for free through HES
• No equivalent in GOS; but local schemes may have LVA assessment and health authority pays
• No general scheme for electronic aids, although argued could be cost effective
• If bought privately, VIP’s are exempt from VAT
• May be borrowed/used in public services, or charities

Question 12

What factors affect success using EVES?

Answer 12

General EVEs
• Age
- Onset of eye condition
• VA; CSF
- better than 6/90 = little benefit
• Motivation
• Muscular strain
• Individual preference/performance
• Cost; Not available on NHS

Portable EVEs
• no replacement for optical LVAs, best used in conjunction
• most useful for extended reading tasks
• may be a helpful addition
- if optical LVAs are unsatisfactory;
- for tasks that cannot be done with optical LVAs
• a significant minority of patients seen in NHS Low Vision Clinics could benefit from p-EVES