Visual Resolution Flashcards
What is the Snellen fraction?
Test distance over the distance at which a letter subtends 5 min of arc
Describe the conversion of degree to minute it second.
1 degree= 1/360 of a circle
1 minute = 1/60 degree
1 second= 1/60 minute
Describe recognition acuity.
You need to be able to see the letter and name them to pass the test
Describe detection acuity.
Observing if a patient look towards an image indicating that they can see it.
Useful for infants when recognition acuity is not possible
Describe resolution acuity.
Tumbling E or Landolt C (each letter has same amount of difficulty)
Patient only need to be able to see the target
Describe localization acuity and vernier acuity.
The smallest gap between the lines that the patient can resolve
The smallest angle between the line that patient can resolve
Describe the relationship between viewing distance and angular subtense.
When you halve viewing distance, angular subtense of letter doubles
Double viewing distance, the angular subtense of the letter halves
Ex: 1/20th of distance, the letter will subtend 20x the angle
Describe the LogMAR chart.
5 letter on every line
Each line is 0.10 logMAR less than the previous line
Space between each letter is equal to the width of the letter
Space between each line is equal to the height of the letters in the lower line
Each letter 0.02 (every letter counts, subtract from total)
Lower score is better (start at 0.50)
What is normal V/A
20/20 is NOT normal VA in a healthy individual all the way up to 75 years of age
Does MAR change with viewing distance?
No
What can we do if the patient cannot read the top letter on the chart?
Move the chart or patient closer
Hand motion
Light projection
Light perception
No light perception
Describe the Jaeger chart for measuring near VA.
Letters range from J1 to J20
Not standardized
Describe the M notation near V/A chart.
M denotes the distant in meters which the letter subtends 5 minutes of arc (denominator of Snellen fraction)
Describe the point N near VA chart.
One point is 1/72 of an inch
(N5= 5/72 inches high)
Commonly used in computer fonts
Refers to size of block not letter
What 3 factors affect V/A?
Blur
Constrast
Retinal eccentricity
How can blur due to blur circle be reduced?
Reducing diameter of the blur circle with a pinhole aperture
What is the function of a pinhole?
Reduce diameter of blur circle no where the light focuses (same refractive error stays)
If pinhole does not help, another issue is present
How can we calculate contrast on a Snellen letter?
Contrast = (Lmax - Lmin) / (Lmax + Lmin)
Lmax- white
Lmin- black
What happens to V/A as contrast decreases?
V/A will decline with decreasing contrast
Increasing contrast once adequate contrast has been achieved will not improve visual resolution in normal, healthy eyes
What is the retinal eccentricity for V/A of 20/20, 20/30, 20/40 and 20/60?
Fovea - 20/20
1 degree - 20/30
1.5-2 degree - 20/40
3-5 degree - 20/60
