Real Image/Transverse Magnification

M = SIZE OF IMG (on screen) / SIZE OF ORIGINAL OBJECT

CCTV is the most common way of producing this kinda magnification. It's aberration free (with magnification edge of magnifier view = rubbish) but the resolution of the screen determines image quality. Up to 70x mag is possible but usually about 30x is limit. The magnification is variable with a zoom control in a way that just ain't possible with an optical aid. This means if the px requires a change in magnification it remains useful. It has variable camera/task and eye/screen distances so allows for more normal viewing and extended reading. Reading speed is about the same though. Contrast etc is variable too.

Components

• Camera
• Light Source
• X-Y Platform
• Monitor screen (bigger screen = larger FOV - only 4 characters required for reading but up to 15 required for optimum page navigation)
• With the most common design (PORTABLE CCTV) each component is fixed and mounted vertically 'in line', working space is fixed or limited, all the controls are together so easier to set up. Also it's more portable duh.
• Can also add 2nd camera for distance

Advantages

• Zoom on magnification - overall view at low mag then increase for detail
• Adjustable mag if vision changes
• Monochrome/colour monitor allows for real life colour or different combinations
• Contrast reversal 0 >50% of px prefer white on black as it reduces scatter in media opacities
• Reading duration much increased but speed the same
• Binocular viewing from a normal distance
• More psychologically acceptable than a magnifier? Just looks like a computer if it's a kid@school
• Good if Parkinsons or something prevents optical aid usage
• Good if px has extensive vfd. Easier to adapt to steady eye strategy with it
• Split screen for simultaneous tasks

Disadvantages

• Expensive & need continuing service & repair
• Persistence of image on phosphor if CRT style screen
• Requires more practice than optical aids (15-20 sessions of daily practice)
• Bulky/obtrusive/difficult to move around - would need one for home and one for work
• Control posns often better for right handed
• Depth of field is limited (probs with thick books)

A head mounted device works in basically the same way. There are also TV readers which attach to the px's TV. The users of EVE aids are usually young & motivated. Px w/ VA>6/90 seem to gain little benefit from their use (mainly for reading, filling in forms, school tasks and viewing photos.

As regards to obtaining the visual aid, optical aids are usually provided by the HES - free to any px on permenant loan. There's no general formal scheme for electronic aids - they are bought privately. A registered VIP doesn't pay the VAT. Local charities or 'electronic aids for the blind' also supply but most are bought privately. They are available to use in libraries.

Tapermag

• Real image magnifier consisting of a bundle of transparent glass optical fibres
• Each fibre is narrow at the end which rests on the text and expands towards the top edge. The magnification is mainly real image magnification with a bit of relative distance magnification (the image is closer to the eye than the reading material)
• Aberration free but has a limited range of magnification (2.1-2.3x mag)
• Expensive compared to plus lens magnifiers.

LV: Flat Field/Bar Magnifiers

Flat Field Magnifiers

• Self Illuminating - if the light path of obj to eye is reversed can see that all incoming light is focused at the plane of the magnifier
• Height of a flat field magnifier can be less than, equal to or greater than r (radius)

Magnification of Flat Bar Magnifiers

M = nr / nr + l (1-n)

for a hemi-spherical magnifier where thickness = radius of curv

t = l = r

The thicker a flat field magnifier is in relation to r the greater the magnification. The image is formed very close to the object so there is no relative distance magnification. Patient must wear near correction. Also you can have bar magnifier with magnification in one direction only - plus cyl lens

Advantages

• Normal reading posture
• Very bright - gathers light
• Good for px w/hand tremor
• Clear image across the lens w/minimal aberrations
• Combine w/spec mounted to get good mag while maintaining fairly distant viewing

Disadvantages

• Large lenses - heavy
• Magnification typically low
• Reading Material has to be flat and on firm surface
  

LV: Magnification

Magnification is "after" retinal image size/"before" retinal image size. It is needed in LV when the rx doesn't create enough improvement for the patient to do visual tasks. It can be achieved by

1. Increasing the size of the object M=new obj size/old obj size
   
2. Decreasing viewing distance M=old viewing distance/new viewing distance
   
3. Real image/transverse magnification (eg CCTV) M=size of image/size of orig.obj
   
4. Telescopic magnification = angle subtended by telescopic image (theta')/angle subtended by original object (theta)

Different methods of magnification can be combined - in this case the total magnification is found by multiplication. For example - "What is the mag obtained by changing print size from 6mm to 15mm, and visual distance from 30cm to 10cm?"

M1 = new/old = 15/6 = 2.5x mag
M2 = old dist/new dist = 30/10 = 3x mag
Total = 2.5x3 = 7.5x mag

Increasing Size Of Object

• Large screen TV
• Large print stuff - books, typewriter, telephone, bank statements
• Photographic enlargement/photocopying
• Computers - large monitors/keyboards/enlarged screen display

Changing Viewing Distance

• Simple and effective for distance or near vision - sitting closer to TV
• eg moving from 12ft to 6ft away from TV = 2x magnification
• Bringing print from 30cm away to 5cm away = 6x magnification

Real Img/Transverse Magnification

• Mainly electronic magnification systems
• Mainly NV but also systems for distance and intermediate tasks (eg video systems with autofocus)

Telescopic Magnification

• Versatile optical method for D, I & N