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
Inverse square law
LV= 1/r^2
Lv= veiling luminance and r= distance to light source
As distance increases LV decreases so closer to eye= glare
But as luminance increases va should also increase as smaller pupils= less spherical aberrations
Also illuminance is directly proportional to intensity of source/ distance^2
So 1/d^2 or intensity of source/d^2
How to measure glare
Back tester/ brightness acuity tester (illuminated hemisphere)
Or pen torch or angle poise lamp
Brightness acuity tester
Illuminated hemisphere
Place it over px eye, pull down shutter with aperture (60mm diameter and 12mm aperture)
3 setting and modify glare source and work out how well they can see
-Bright indoor lighting (low) overcast day (medium) sunlight (high)
Advise px eg wide rimmed hat or dont go driving when sun is low on horizon etc.
Why is a pen torch or angle poise lamp not great
Low tech approach
Not reproducible
Lack of standardisation and comparison
60yo vs 20yo
60yo’s retina receives 1/3 of the light of a 20yo
Diseases affecting csf- corneal
Refractive surgery
Keratoconus
Corneal dystrophies
Corneal oedema
Contact lens wear
Diseases affecting csf- crystalline lens
Cataracts
Pseudophakia
diseases affecting csf- retina and beyond
Glaucoma
Ocular hypertension
Optic neuritis
Multiple sclerosis
Papilloedema
Amblyopia
Why do we measure csf
For diagnosis, screening, determination of change and assessment of visual function
Csf graphs for normal vs post surgery or cataract and cataract and glare
Lower peaks for post surgery and cataracts bc less contrast
For retina and beyond describe the csf graphs for strabismic vs anisometropic amblyopia vs Normal and a reference
Anisometropic is worse, this affects ALL sfs. Aniso for all. Difference in prescription in both eyes. The graph is low compared to normal the entire graph is lower
Strabismic- eyes are not aligned properly characterised by a high frequency decrease in cs. So normal till the end and it shifts to the left compared to normal.
(M Abrahamsson, 1998)
What does a drop in middle sfs mean
Means the px may not be able to see objects around bends easily
Results letter charts for the 96 percent one and 10 percent one for young vs old normals
96 percent chart
Young normals=. -0.20+- 0.06logmar (6/4)
Old normals= -0.11+- 0.08logmar (6/5)
10 percent chart
Young normals= 0.05 +-0.08 logmar
Old normals=. 0.19+- 0.10 logmar
Pelli Robson young normal vs old normal
Young normal= 1.86+- 0.09 log cs units
Old normals=. 1.80 +- 0.11 log cs units
Pelli Robson young vs old with glare
Young with glare= 1.80+- 0.09 log cs units
Old with glare= 1.65 +- 0.11 log cs units
Young vs old subjects for low contrast
Young subjects lose 2 lines from 90 to 10 percent
Old subjects lose 3 lines
Normal score for Pelli Robson
2= normal
Less than 1.5= visual disability.
Why does the Elliot method for Pelli Robson improve reliability
Originally it was 2/3 of triplet right= right
Elliot said C=O=D
What is glare
Just unwanted light within the visual field
What is disability glare and name 2 effects of disabuility glare
Light scatter etc produces veiling luminance light scatter within eye superimposes on retinal image leads to reduced contrast and contrast sensitivity.
2 effects- change in adaptation, production of veiling luminance
Discomfort is just cant see properly
Angle poise lamp dont allow it to become a what
And acuity increases when illumination increases for what type of people…
To become a photo stress test
For elderly and people with armd
Conventionally what are clinical assessments of VA related to and what is another method
Conventionally clinical assessments of VA are related to the eyes resolving power
Another method is based on the eyes sensitivity to luminance contrast
Why is Elliot’s method better and why does it improve reliability
Original scoring was 2/3 in a triplet right
Elliot said allow C=O=D and he proposed letter by letter scoring with each letter worth 0.05 units. Increases reliability
Pelli Robson chart what is the working distance and how many cpd does this subtend
1m working distance at 1cpd. Add a +1.00 DS add for older presbyopic patients.
3M suggested as alternative so letters subtend 3cpd. (6/60) as an alternative to 1m. Remember to adjust near add
When does acuity improve and especially with what type of patients
Acuity improves w illumination
Especially in elderly and patients with ARMD
4 reasons why we measure cs
Diagnosis
Screening- determination of normal and abnormal
Determination of change- evaluating procedures
Evaluating visual function- assessment of function
graph with normal refractive error and light scatter
Normal highest. Light scatter lowest whole curve shifts down
Refractive error in middle. =
Starts off normal then around the peak whole curve starts shifting down and it ends in the same place as where light scatter ends off at the same spatial frequency cut off
