Contrast Sensitivity Flashcards
What is the significance to doing contrast sensitivity test
evaluates the patients ability to see a range of target sizes at various contrast levels under optimum conditions
What is the most common equation that defines contrast
Michelson Contrast
contrast = (Lmax - Lmin) / (Lmax + Lmin)
What is the less common equation that defines contrast
Weber Fxn
Contrast = (Lt -Lb) / Lb
Lt - luminance of target
Lb = luminance of background
What does visual acuity test in terms of contrast, how ideal is this to the real world ?
Measures sharp edges at high contrast
not ideal to real life scenarios
Define contrast sensitivity in terms of threshold
CS = 1 / threshold
what is threshold
the minimum amount of contrast needed to see a letter
what is the relationship between contrast sensitivity and threshold
high threshold = low contrast sensitivity
indirect relationship
if a pt has contrast threshold = 10%, find their contrast sensitivity
1/ 0.10 = 10
what are the contrast sensitivity and threshold
none
if a pt has contrast threshold = 50%, find their contrast sensitivity
1/ 0.50 = 2
What does spatial frequency tell us
measuring the width of stripes in cycles per degree
What are the two most common spatial frequency gratings used ?
square wave and sine wave
what is square wave frequency
the change between black and white shades are abrupt, producing sharp edges on the graph
what is sine wave frequency
where the change in black and white shades are smooth, producing a gradual change on the graph
what is the definition of one cycle
one cycle includes one light bar (peak) and one dark bar (trough)
how many stripes per degree appear for each cycle per degree
1 cycle = 2 stripes
if a pt has A SF of 20cpd, what level of snellen VA does this correspond to
20 cpd = 40spd
40 stipes per 60min
1 stripe = 60min/40 stripes = 1.5 min of arc
MAR = 1.5, VA = 1/MAR
VA = 20/30
What is the grating acuity limit
represents the theoretical maximum VA at 100% contrast
which target size is a patient most sensitive to
medium size
what is type 1 CS loss
challenges with HIGH SF, small targets
what is the most common reason to have type 1 CS
uncorrected myopia
What is type 2 CS
challenges with LOW and HIGH SF
what is the most common reason to have type 2 CS
cataract
What is type 3 CS
unable to see LOW SF, large targets
