Low Vision Flashcards
4 main types of magnification
- RDM (D= O/N)
- RSM (S =N/O)
- Angular mag compares retinal image sizes
- Lateral mag compares original object size to new image size
Collimating magnifier
When exiting rays are parallel, like a HHM.
Accommodation/adds are not required.
Total magnification provided by a HHM =
Focal distance between object and eye x F
HHM is brought closer to the eye
What mag changes? What mag stays the same?
Angular mag does not change
Linear FOV will increase
How to calculate Linear FOV with HHM
Linear FOV = lens diameter / (F x distance between HMM and spec plane)
SM produce what type of image
Virtual, upright, magnified
How to calculate angular mag of a SM
Angular mag = lateral mag x distance
How to calculate magnification of telescopes
Mag = - F ocular/ F objective Mag = entrance pupil/ exit pupil
*ent pupil is commonly the objective lens, exit pupil is the ocular lens.
Telescopes are labeled AXB
What do these indicate
A is mag
B is entrance pupil/objective lens diameter
The FOV of a telescope is dependent on what
FOV is dependent on the size of the exit pupil of the telescope and the size of the entrance pupil of the patient
The FOV will continue to increase until the diameter of the exit pupil matches the diameter of the patient’s pupil. Then the FOV will be limited by the patient’s pupil.
Characteristics of a Keplarian telescope
+ Objective and ocular lenses
Image will be inverted unless you add an inverting lens. This will make the tube longer and heavier.
Exit pupil is outside the telescope, making the FOV larger.
However, the exit pupil is SMALL, making the image dim.
high mags
Characteristics of a galilean telescope
+ objective, - ocular lens
Image is upright
Tube is small and low weight
Exit pupil is located inside the telescope, making the FOV smaller
Exit pupil is large, making the image bright
Up to 4x mag due to limitations of the tube length.
A reading cap acts as a
HHM
A telescope can be changed to view near targets by
- Adjusting the tube length
- Adding a reading cap
- Viewing through an add power
2 types of spectacle mounted telescopes
- Center fit
2. Bioptic fit
2 primary types of peripheral VF loss and 2 primary optical devices that can relocate or expand the field
- Mirrors
- Usually for patients with temporal VF loss
- Always put the mirror nasal on the eye with the defect.
- Will induce a scotoma, may cause nausea. - Prism.
- Placed with base in the direction of the defect
- 2 pd cause 1 degree shift
__million americans are legally blind
__ million are visually impaired
- 1
4. 3
When patients require reading glasses greater than +4.00D, prescribe what type of prism? How much prism per eye?
Prescribe BI prism to move the image further away.
The amount per eye is 2 + D
Qualifications for legally blind
- Patient cannot read any letters on the 20/100 line in the better seeing eye
- Visual field diameter is 20 degrees or less in the better seeing eye.
1M is defined as
how tall is it in mm
The size of a symbol that subtends 5 arc min at 1 meter and is equivalent to a Snellen acuity of 20/20
It is 1.45mm tall.
Most newspaper print at 40cm is 1M. Equivalent to Snellen 20/50
How to calculate JND
The denominator/100
ex: pt can see 20/400? JND is 4.00D. Begin the trial frame refraction by showing the patient +/- 2.00D sphere lenses.
How to determine the amount of magnification required for a patient to view a distant object
Distance mag = Current distance BCVA/goal BCVA
2 formulas to determine a near add
- Kestenbaum’s rule: flip the distance BCVA
2. Lighthouse method = (current near VA/ goal near VA) (1/working distance)
3 ways contrast sensivity can be measured
Pelli Robson chart- each line reduces in contrast
Vistech- uses a series of sine wave gratings.
Bailey-Lovie: Consists of 3 charts, each at a different level of contrast
SM uses which 2 forms of magnification
Lateral
RDM
