hand and stand magnifiers Flashcards
equivalent viewing power:
another term for magnification
magnification depends on how the lens is used
calculated using diff component of optical system
equivalent viewing distance:
reciprocal of equivalent viewing power
hand magnifiers
low cost easy to use/ maintain portable can have some internal illumination large variety in cost and quality, doesn't need professional consultation, can be used in combination with rx, good for variable head psotions
disad- use hand, wot if parkinsons? text has to be on flat surfaces. hand gets tired, lack of patience, has to be at correct position
how do u work put enlargement for a bar/flat field magnifier-
nr/ (t(1-n)+nr)
bar and flat field magnifiers
lens acts as a collector of light- lens that lives bar field is diverging
provide real enlargement
whats the aim of low vision
MAGNIFICATION
aka image enlargement
what are the four main types of image enlargement
- relative size enlargement
- relative distance enlargement
- electronic-transverse- enlargement
- angular enlargement
whats relative distnace enlargement
half wd
then u double retinal image size
if hey cannot accommodate- then u may need a add
when starting any low vision assessment- get a REFERENCE point-
whats the reference point in a low vision assessment
what can the px read at 25cm with a +4.00 add
how do u work out relative distance enlargement
u figure out what they can read at 25cm using a +4.00D add
then u divide this by what they want to read
this gives u the ENLARGEMENT RATIO
the new WD is now the old WD/ENLARGEMENT RATIO
new MAG power- 1/new WD * 100
what’s acuity reserve
when in consulting room u have optimal conditions, with easy to read text, and they read for a short period of time
therefore u assume an acuity reserve of 2:1/ 3:1
Whats relative size enlargement
increase the retinal image size
by keeping wd same
u enlarge size of print
u work out ENLARGEMENT= new print size/ old print size
you may need to combine relative size and relative distance enlargement :
as a lower powered lens- better image quality an digger field of view
electronic enlargement
use camera? to introduce colour, improve contrast, or increase size
electronic enlargement- image on screen/ original image
adv- can get high levels of mag- more comfortable WD- can underline-maintains binocular view- looks good- larger fov
but well expensive,lack of portability? disadvantaged by quality of camera and screen
what’s angular magnification
use of telescopes
d and N
for N u need a lens cap
ENLARGEMENT- angle subtended through optical system/ angle subtended when viewed directly
whats the max and min wd
max is focal length of lens
min depends on accom
each lens has 2 functions; i.e.
focusing power
magnification power
whaats nominal mag
F/4
if px is wearing glasses/ using accom with magnifier. u need to use equivalent power formula
Fe= Fm + Fa - (dFmFa)
d- distance between eye and magnifier
what mag is labelled on a magnifier
trade
what do u assume for trade mag
the px is using +4.00 add or +4.00 accomm
and also magnifier is held up to the eye
easy way to work out is fe=fm/4+1
The enlargement of a simple magnifier will remain the same, regardless of reading add/accommodation, if
the lens is held at its focal length away from the eye
Holding the magnifier closer than its focal length
INCREASES the equivalent power
Holding the magnifier further away than its focal length
DECREASES the equivalent power
