Prescribing Telescopes Flashcards
What are the first steps to prescribing
- identify aids that provide the required magnifiying power power
- identify aids that are suited to the task
What to do if Px VAs does not improve
- find the issue
- focusing
- baseline VA/M-Calculation
- Contrast sensitivity
- scotoma
- cosmesis
Factors affecting VA on testing
- illumination
- visual field status
- measuring technique
- contrast, crowding, distance
Equation for simple contrast
= Lb - Lt/Lb
- Lb = background luminance?
- Lt = target luminance?
What is the minimum contrast required
10:1
What is the optimum contrast threshold
20:1
What is the contrast reserve, and the formula
Ratio between px contrast threshold and target contrast
Ability to read or scan a scene relies on contrast reserve
- TC/CT
- TC - Target contrast
- CT - Contrast threshold
Equation for calculating distance magnification
M= TA/BVA
Options for improving distance vision
- spectacle lenses
- contact lenses
- coatings and tints
- telescopes
- multiple pinhole lenses
Steps for magnification of near tasks
- calculate magnification
- try predicted M
- Modify
- choose LVA for tasks
- issue trial LVA
What is divide by 3 rule
- dist VA = 6/x
- Near VA = X/3
- BVAdist = 6/18, =VAnear = 18/3 = N6 at 25cm
Reasons for not using distance VA to estimate near VA
- accommodation at near
- pupil constriction
- different lighting
- myopia at near
- cataract, posterior subcapsular bad at near, nuclear better at near
Types of charts to measure NVA
- N-point chart
- Sloan N scale
- Keeler A chart
- near Snellen equivelant
- near logMAR chart
What is acuity reserve
- print size: threshold
- N12:N6
- AR = 2:1
Equation for equivalent viewing distance
Required EVD = (TA/CPS) x current viewing distance
