MAGNIFICATION Flashcards
Method of increasing the retinal image so that enough of the retina is stimulated to send an impulse through the optic nerve to the brain, allowing an object to be perceived.
Magnification
Ways of enlarging retinal image
*Moving the object closer
*Enlarging the object itself
*Optical magnification
*Electronic magnification
4 Types of Magnification
*Relative distance magnification (RDM)
*Relative size magnification (RSM)
*Optical/Angular magnification
*Electronic/Projection Magnification
Moving the object closer or a person approaches the object
Relative Distance Magnification (RDM)
It is necessary to focus the image
with lenses or accommodation.
The object becomes magnified by two times if we move it half-way closer to the observer’s eye from the original distance.
Relative Distance Magnification (RDM)
Formula of Relative Distance Magnification (RDM)
reference distance/new distance (r/d)
Increasing the size of an object.
Relative Size Magnification (RSM)
Formula of Relative Size Magnification (RSM)
New size/reference size
Makes an object at a distance appear closer to the eye, through the use of various lenses or lens systems.
Angular Magnification
Increases the size of the image to be through the projection process.
Electronic/Projection Magnification
Determination of the Needed Magnification
*What is the achieved visual acuity of the client?
*What is the goal or desired visual acuity of the client?
Formula of magnification
Achieved VA/desired VA
Formula of magnification
a/r
80 wpm
To spot
enables identification of a single word
To spot
160 wpm
To be fluent
enables reading accurately
To be fluent
320 wpm
Maximum
enables reading accurately at a high speed
Maximum
Consider threshold rather than the goal acuity or print size
Reserve or Threshold Acuity
one line smaller
Spot
three lines smaller
Fluent
fives line smaller
Maximum
Kestenbaum’s Formula
Take the reciprocal of the best Snellen acuity distance to derive the initial power required to read 1M or standard news print
