Visual Resolution Flashcards
What is the Snellen Fraction?
Test Distance/ Distance at which the letter subtends 5 minutes (5/60*) of arc
What does VA 20/40 mean?
The smallest letter the pt could read subtended 5 minutes of an arc at 40 feet
What is recognition acuity?
Snellen Chart
Must be able to see and name the letters
What is detection acuity?
The direction the child moves it eyes to the area with more stimulus
What is resolution acuity?
The direction the letter points
What is localization acuity?
What is the smallest gap between the lines that a patient can resolve
What is vernier acuity?
The smallest angle between lines that a pt can resolve
How do you find the height of a 20/200 letter?
20/200 letter subtends 5 mins of arc at 200ft thus half the letter subtends 2.5 mins at 200 ft
How much does a 20/100 letter subtend at 20ft?
20/100 subtends 5’ at 100ft thus at 20ft it would subtend 5 x5= 25’
How much does a 20/15 letter subtend at 20ft?
20/15 subtends 5’ at 15ft thus at 20ft it would subtend 5 x(15/20)= 3.75’
What happens when you halve or double the viewing distance?
When you halve the viewing distance the angular subtense doubles.
When you double the viewing distance the angular subtense halves
What does vernier acuity give you?
Vernier acuity displays the smallest angle between the lines that the patient can resolve thus you must solve for the angle theta= Tan(x)
Can you average snellen VA?
NO you can only average MAR
What is MAR?
The minimum angle of resolution which is the angle that must be resolved for the patient to be able to identify the letter correctly
What does each limb of a letter subtend?
Snellen letters are made on 5x5 thus each limb subtends 1’
What does a 5x4 non serif subtend?
Each limb is still only 1’ because it is on 5x5
How do you calculate MAR?
Denominator/ Numerator ie: 20/100= 100/20= MAR=5
logMAR= 1.0
What does each limb of a 20/40 letter subtend at 20ft?
Each limb would subtend 2’ each thus at 20ft it is 5x2= 10 minutes
What is the space between each letter on a logMAR chart equal to?
The width of a letter
How much is each letter on a logMAR chart worth?
0.02
Is logMAR 20/20= 0.0 good?
No, 75yo can see this.
What is the decimal equivalent of 20/100 letter?
0.2 (Numerator/ Denominator)
If the patient can’t read the letters what is the order of tests your perform?
- Hand motion
- Light Projection
- Light perception
- No light perception
What is near VA performed at?
16in or 40cm
What is near Jaeger?
Letter ranging from J1- J20 but they are not standardized size
What is M notation?
M denotes the distance in meters which the letter subtend 5’. M= denominator of a Snellen fraction (in meters)
If the smallest print a pt can read is 0.5M what is their VA at 40 cm?
0.4/0.5
What is N notion?
Used for near VA.
One point= 1/72 of an inch. The smallest size is N5 or 5/72.
What does point size refer to?
It refers to the size of the block the letter is on. Ie: 10 point means the block was 10/72 in high.
How big are lower case letters with or without ascenders considered to be?
5/72in. high
Which factors affect VA?
Blur
Contrast
Retinal Eccentricity
What happens if blur circles are overlapping?
The patient will be unable to resolve a letter
What does a pinhole od?
It reduces the diameter of the blur circle. It does not change where the light focuses thus the eye still has the same refractive power
What is the contrast equation?
Lmax-Lmin/ Lmax + Lmin
At which point in contrast does VA change?
Less about 40% contrast do we see a drop off in VA
What is retinal eccentricity?
Retinal eccentricity describes how far a point on the retina lies away from the fovea (center of macula)
How is retinal eccentricity expressed?
It is expressed as an angle. In this picture point X lives alpha degrees away from the fovea.
What is the VA for retinal eccentricity on the fovea?
20/20
What is the VA for retinal eccentricity on 1 degree off the fovea?
20/30
What is the VA for retinal eccentricity 1.5-2 degrees off the fovea?
20/40
What is the VA for retinal eccentricity 3-5 degrees off the fovea?
20/60
