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28 Cards in this Set
- Front
- Back
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What is magnification ? |
Magnification is needed when the rx is not improved. So retinal image size (RIS) is increased and is easier to see Magnification is defined as: after retinal image size / before retinal image size RIS depends on angle subtended at nodal point of the eye |
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What 4 methods of making magnification? |
1) Increase print size (m= new obj size / old object size) 2) Decrease working distance (m=old wd/new wd) 3) Angular magnification aka telescopic magnification (m=angle subtended by telescope / angle subtended by original object) 4) Tranverse aka real magnification (m=size of image / size of original object) |
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How do we get total mag by using two or more methods of mag? |
Change print size from 6 mm to 15 mm & WD from 30 cm to 10 cm
Mag= new obj size / old obj size 15/6=2.5 x
Mag= old WD / new WD 30/10 = 3x
Thus, 2.5 x 3 = 7.5 x
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Describe increasing the size of the object; what is this done for? |
Magnification = new object size / old object size For near tasks eg large screen TVs, large tints and computer software |
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Describe large print books .. |
Available RNIB National Library serve Modern production techniques make these feasible Print size 1.5-2.5x of normal size Concentrated on popular fiction |
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Describe ➕ages & ➖ages of large print books |
➕ages: Don't have to alter reading petite Can keep normal WD Binocular viewing
➖ages: Large and big books Expensive Thin paper often see words on reverse Limited availability; 1.5% of 2 million books |
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Describe giant print productions |
Giant print productions available from loan from RNIB 24 point Limited Concentrated on children books |
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Describe newspaper / magazines available in large print .. |
Can get these from subscription: Big print and Sidecirw |
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What other types of appliances can you get large prints on? (7) |
Dymo labelling machine Universal remote control Telephones
Utility bills (on request) Music Informational leaflets
Knitting patterns |
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Describe large print on E books |
Electronic readers aka e readers eg Amazon Kindle Variable line spacing and print size:10 point but max 20 point High contrast screens No spec reflection but will get 60% in well lit office environment "Read to me" features available |
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Describe large print features on keyboard/large monitors and computer software |
Keyboard and large monitors - contrast reserve
Computer software: Ai squared zoomtext 9.0 and Dolphin Supernova: 32 x mag. Colour and contrast changes and get screen reading More polished & less pixelated than Ease of Access and Accessibility Wizard on Windows 7. This is magnifier at top of the screen |
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What is disability? |
Disability is defined by Disability Discrimination Act 1993. Now incorporated into Equality Act 2010
Disability is physical or mental impairment that has substantial / long term adverse effect whereby it affects the persons ability to carry out normal day to day functions |
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What implications does disability place on service providers ? |
Service providers must not unlawfully treat disabled people as unfavourably. They have to make reasonable adjustments eg large prints in shops, bills /banks statements and PPC. Photographic enlargement, photocopying and chip and signature cards. Reduced contrast with multiple copies of paper |
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Describe decreasing the working distance |
M= old viewing distance / new viewing distance For D: Tv moved from 6 feet to 12 feet so 2 x mag ie 12/6 For N: print size read from 30 cm to 5 cm so 6 x mag ie 30/5
This method mainly done by young adults and children Older adults find accommodative demand too severe Myopes read w out specs; myopic far point. |
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How can we decrease working distance? |
Use PLUS lenses! These allow closest WD w/o any effect on accommodative demand So: object @ focal point & image at infinity; parallel light rays coming in. As power of + lens increases get shorter focal length and bigger angle subtended at nodal point of eye |
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What is formula for plus lens? |
M=F/4
We get to this because we take standard lens to be +4 and WD 25 cm or focal length |
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➖ages of formula for plus lens |
Lens compared to a stand.lens +4 taken to be the standard should be +3 Not all reading adds are +4 |
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Describe plus lens magnifiers |
Plus lens magnifiers are hand, stand or spec mounted All work in same way, have NO effect on accommodative demand, small FOV b/c of lens diam/aberrations at 1.25 cm WD
Nb: hand, stand or spec mounted .. these objects be held closer to eye Have practical limit on magnification of 20 x (F= +80. So M=F/4 ➡️ 80/4=20x) |
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Describe angular magnification aka telescopic magnification |
M= angle subtended by telescope / angle subtended by original object
Optical method of magnification better than plus lens magnifiers because can be used at any WD thus object size doesn't have to be changed
Can use at D: street signs, bus numbers, blackboards, theatres I: computer, laptop, music, playing cards N: reading and writing |
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Advantages and disadvantages of angular aka telescopic magnification |
Can be handheld: binoc/monoc or spec mounted If spec mounted only D or N Binoc done for practicality ie balanced the weight and when used at N tubes inverted in
-age: small FOV for D thus can't use when mobile only for spotting tasks |
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Describe optics of Astronomical telescope (Angular mag/telescope) |
Uses 2 plus lenses The objective makes image at the distance at its focal length ie Fo' image is real and inverted Eyepiece acts as magnifier as makes divergent light rays from focal length parallel
2 plus lenses = focal length is long ie long telescope bc fE' is +I've but compensating lens must be powerful to decrease telescope length
Focal point at eyepiece/objective at same place Exit pupil is outside entrance pupil |
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Describe optics of Galilean telescope |
1 plus 1 minus lens
Rays of light converge to objective lens ie plus lens which is intersected by minus lens makes rays of light parallel
Telescope is short cw Astronomical bc fE' is -ive but compensating lenses still need to be strong |
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Advantages/disadvantages of astronomical telescope |
Astronom: t= fo' + fE' (length of telescope; long)
Better image quality and big fov bc persons pupil close to exit pupil of the telescope
12 x
Image always inverted ie upside down bc telescope magnification is minus so erecting prisms needed
Hard to spect mount |
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Advantages/disadvantages of Galilean telescope |
t=fo' - fE' (length of telescope; short)
Poor image quality and small FOV
3x
Image always erect ie right way up bc telescopes magnification is +ive
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Formula for magnification for either telescope (Astronomical or Galilean) |
m= h'-fo' / fE' x h' = -fo' / fE' = -Fe / Fo |
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What is the exit pupil? |
Exit pupil is the image of the objective lens which is seen through the eyepiece The rays of light that pass through objective lens pass through exit pupil Exit pupil movement can be used to identify telescope: as observer moves head Astronomical gives against Galilean gives with |
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Describe Tranverse aka real magnification |
M= size of image / size of original object Get this magnification by EVES: Electronic Visual Enhancement Syetem
1st EVES ➡️CCTV: side by side, in line or fixed CCTV portable camera clip on laptop or computer Also can have pocket, TV or Optlec Fairview EVES |
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What are characteristics of five magnifier types? |
Back (Definition) |
