Contrast Sensitivity Flashcards
3 reasons why we measure CS
Management of ocular disease (eg cataracts)
Management of people with age related macular degeneration
Management of bifocal or multi focal contact lenses and IOL patients
What are the 4 parameters and what do we modify these to do
Phase- not rlly used clinically- position of waveform related to its position on a circle
orientation,- horizontal vertical or optotypes R eg
contrast- worked out by luminance of sine wave (modulation)
spatial frequency- measured in cycles per degree cpd. How far apart the bars are
We modify these in order to determine a particular resolution for a px
Why do clinical test gratings spatial frequencies vary logarithmically
Clinical test gratings don’t have a uniform spatial frequency, these may vary logarithmically to get a more even spread of SFs
Info ab pxs vision over a range of spatial frequncies to determine where vision is most sensitive to contrast
What do sinosuidal gratings vary in and where are these used
What are sinosuidal gratings threshold of recognition affected by
Sinosuidal gratings vary in brightness and these are used clinically to measure cs and sf.
Threshold of recognition for sinosuidal gratings is affected by both its cs and sf
What does csf provide info about
Provides info about a given pxs vision over a range of spatial frequencies.
If it only measured sf= snellen if only measured contrast=cs. So this combines them both to see what they can see at what contrast levels.
What is used to measure contrast sensitivity
what gratings
And what is the equation
Sine wave gratings are used
But square wave gratings could also be used
Y=a sin(bx)+c
What is contrast threshold and contrast sensitivity
Grating is changed till the bars can just be seen so this is the contrast threshold. Px is shown graphs of certain SFS and contrast is adjusted till the bars are seen= contrast threshold
Contrast sensitivity= 1/ contrast threshold.
Then plot this Ona. Csf graph
equation to work out luminance contrast or modulation
Lmax-Lmin/ Lmax+Lmin
= contrast threshold
1/ans= contrast sensitivity
The resultant csf provides info ab a given pxs vision over a range of sfs and contrasts.
Describe the graph to me- y and x axis
On the left hand side on the y axis we have contrast sensitivity from 1-1000. This is a reciprocal so higher numbers are actually reduced contrast as 1/1000 is lower than 1/1 so that means their contrast threshold is smaller at the top but that means their contrast sensitivity is higher at the top which means they can see low contrast targets. And bottom is high contrast targets. So 1000= low contrast target.
On the x axis we have spatial frequencies from 0.1-100 and 0.1 is large and the right hand side is smaller as the bars ae closer together. So 5/6/7 cpd smaller as bars are closer together. More cycles per degree. Less cycle per degree bars are further apart= larger.
Describe the actual graph curve
The peak is where the vision is the most sensitive
Below the graph is seen by the px above the graph isnt
Goes from left to right till px cant distinguish between light and dark. Where it hits x axis= limit of contrast sensitivity. Younger people have a higher spatial frequency cut off than older people. So where the curve intersects is where we measure va bc this is the smallest they can see with full contrast so like snellen.
Pelli Robson is measured along the peak of csf. As this is where their vision was most sensitive
We want the px to be as far up the graph peak higher= they can see with lower contrast levels. If the peak shifts right this means that they can see smaller targets with lower contrast so their contrast is more sensitive to those smaller targets. Shift left means they can no longer see smaller optotypes with the same level of contrast.
4 methods to measure contrast sensitivity summarised
Vistech- measures contrast sensitivity but varies spatial frequency, contrast and orientation.
Low contrast and high contrast charts- measure sf at same level of contrast either 10 percent or 96 percent but changes spatial frequency and there is constant crowding and logmar progression.
Pelli robson- measures contrast sensitivity at the same spatial frequency but changes the contrast levels.
Quick summary of csf
Cs reciprocal version meaning
Cs is measured in cycles per degree
The more cycles the smaller the target
Cs= reciprocal version so really sensitivity is higher to that lower contrast as it goes up
Vistech
Distance from viewer
How do you carry it out and plot and compare the results
What does vistech consist of
3M from viewer
Ask the px what orientation the line is (left, right, or straight) sf, contrast and orientation (90,105 (left), 75 (right)) is changed.
Values are plotted and compared with normal values to generate a percentile rank or score. (Plotted on a curve and then on a graph and grey area is normal)
Circular grating patterns are used, 5 rows and 9 columns with decreasing contrast as you go along. Plot faintest line read by px. Grey area you should see in,
Low contrast vision charters
Distance
How is it scored
What does it consist of
4M from viewer
Each letter is worth -0.02 log are. Score= -0.02 times no of optotypes read. Analysis of results at the end, basically the more negative the smaller the numbers that are read so more negative the better.
Constant contrast at either 96 or 10 percent Michelson contrast, vary spatial frequencies and there is constant contrast and logmar progression and crowding.
Externally illuminated= 85cd/m^2
Pelli Robson
distance from viewers
Why can we also use a different distance and what is this distance
1m from viewer, or 3M as an alternative (adjust near add. 1/distance as positive power)
3M can be used so letters subtend 3cpd 6/60 so are bigger. Adjust near add and this should reduce accom and improve the reliability of the test. Add positive add to address difference in vergence.
Facts about Pelli Robson
Then how is Pelli Robson scored
16 letter triplets on 8 lines
Externally illuminated 85cd/m^2
Constant sf but contrast decreases by 0.15logsteps down chart so each triplet has the same contrast. Dont count 1st triplet
Each letter is worth 0.05 so triplet= 0.15
Pelli Robson charts are measured down the centre of the csf. Takes csf at its peak. Visual system is the most sensitive to contrast here.
Mark triplet right if c=o=d
Inverse log score= contrast sensitivity then 1/ans= contrast threshold. Ct as a percentage= times by 100
Lower contrast threshold means higher contrast sensitivity
What is glare
Unwanted light within the visual field. Light is scattered in the eye to create a veiling luminance which decreases contrast and contrast sensitivity. This is superimposed on the retinal image.
Two types of glare
Disability glare-hurts to see, and cant see. Caused by light scatter in the eye which leads to veiling luminance and decreased contrast
Discomfort glare- uncomfortable, doesnt really affect ability to see
