Low Vision Flashcards
Angular Magnification
Ratio of the original retinal image size to the retinal image size viewed through an optical system
(object size and location does not change)
Equation for Power of a HHM
Store HHM label is for a standard 25cm WD so M=F/4
F = 4M
Types of Mag used in a HHM
RDM and Angular mag
Magnification of a HHM using original distance from target
M = (original distance from target)(F)
Feq when using a HHM or SM with an add.
How does moving the HHM change Feq
Feq = F1 + F2 - dF1F2
moving the HHM closer gives more + Feq
FOV through a HHM
-Where is it largest and what kind of image
FOV = (lens diameter) / (power) (lens distance from eye)
Largest FOV at lens primary focal pt -> creates enlarged virtual image at infinity
Types of mag in a SM
Lateral mag and RDM
Total mag is LM x RDM
Object and Image attributes for a SM
The object is located within the primary focal point so the image is upright, magnified and virtual and the pt will need to wear an add to compensate for the diverging light
Equation for total magnification of a SM/add combo
M = Fe/4
Max Mag = 1 + Fsm/4
Equation for Mag in a telescope
M = -Foc/Fobj
can also use Dent/Dex (diameter of entrance/exit pupils)
Interpret telescope notation 5 x 45
Mag is 5x
Objective lens diameter (entrance pupil) is 45mm
Factors for FOV in a telescope
Exit pupil diameter (can be increased by increasing entrance pupil/obj. lens diameter)
Diameter of entrance pupil of pts eye
Distance from telescope exit pupil to pts entrance pupil
Aspects of a Keplerian Telescope
Positive obj. and ocular
Forms an inverted image so a 3rd inverting lens is needed
Longer and Heavier but available in higher Mag
Exit pupil is outside scope so good FOV but it is small so the image is DIM
Aspects of a Galilean telescope
Positive obj. and negative ocular
Forms an upright image
Smaller and lighter but only available up to 4x
Exit pupil is inside scope so poor FOV but it is large so the image is BRIGHT and alignment is less critical
Ways to make a telemicroscope
Add a reading cap
Adjust tube length (technically not a telemicroscope)
magnification of a telemicroscope reading cap
Mc = F x (original distance from object to EYE)
-use standard distance of 0.25m if distance is not provided
Total magnification of a telemicroscope
Total = Mc x Mt
Ways to expand FOV
Minus lens (-5 to -10) Reverse Telescopes (usually used for spotting)
Describe ways to relocate FOV
Mirrors: for pts with temporal loss, induce a scotoma, create confusion/nausea due to the image in the mirror being reversed
Prisms: place base towards defect, reduces need for scanning
Spectacle Mounted Telescope Types
Center Fit: mounted in the line of sight, requires minimal training, good for continuous viewing but don’t walk around in them
Bioptic Fit: mounted in the upper part of the glasses, requires significant training, used for spotting
How many Americans legally blind/ legally impaired
- 1 mil. blind
4. 3 mil. impaired
Linear size of 1M
1.45 mm
How to reduce convergence demand on high powered glasses needed for near vision
BI prism (usually F+2)
- Also can use a HHM with a diameter as large as the pts PD
- pt with really high power can occlude an eye
Define legal blindness
Pt cannot read any letters on the 20/100 line in the better seeing eye with best correction
VF diameter is less than 20 degrees in the better seeing eye
