visual acuity Flashcards
(14 cards)
spacial frequency
measured in cycles per degree
the number of cycles that fit into a single degree of visual angle
temporal frequency
measures in cycles per second HERTZ
what is visual acuity expressed as
snellen fraction
is 6/9 vision worse than 6/6
yes because larger denominated means poorer VA
top number always 6
arcmin
1/60th of a degree
alternative way to express visual acuity
LOGMAR
logarithm of minimum angle of resolution
convert snellen to logmar
- resolve fraction 6/12 = 0.5
- log10(0.5) = -0.30
- change sign = 0.30 logMAR
convert logMAR to snellen
- change sign 0.30LogMAR = -0.30
- 10to the minus0.3 (inverse log) = 0.5
- 6/0.5 =12 = 6/12
other charts
ETDRS charts
same number of letters per line
crowding
look at pic on phone
vanishing optotype
look at pic on phone
detection acuity
smallest stimulus that can be seen without needing to identify or resolve its fine details
eg seeing distant star in night sky
resolution acuity
smallest seperation between two stimuli that can be seen
eg seeing 2 neighbouring stars at separate
pinhole acuity
tests weather refractive error is cause of reduced vision acuity, you can see better through pin hole as central light rays reach retina
if doesn’t improve then more serious
