Contrast sensitivity

1. Contrast Sensitivity
Dr. Bebika Phuyal
1st year resident
Ophthalmology

2. Definition
•Contrast : defined as the degree of blackness to
whiteness of target
•Contrast threshold : It is a smallest amount of
contrast required to be able to see the target. It is the
reciprocal of contrast sensitivity

3. WHAT IS CONTRAST SENSITIVITY
• It is a measure of a ability of the visual system to distinguish
an object against its background
• The ability to perceive slight changes in luminance between
regions that are not separated by definite borders
• It is just as important as the ability to perceive sharp
outlines of relatively small objects

4. • A target must be sufficiently large to be seen, but must also
be of high enough contrast with its background. A light grey
letter will be less well seen against a white background then
a black letter
• Therefore, contrast sensitivity represents a different aspect
of visual function to that tested by the spatial resolution
tests, which all use high-contrast sensitivity
• Hence, if a patient with good visual acuity complain of visual
symptoms, contrast sensitivity testing may be a useful way of
objectively demonstrating a functional deficit

5. Symptoms of Contrast Sensitivity
• Have problems with night driving.
• May require extra light to read.
• Eyes becoming tired while reading or watching
television.
• Not able to see spots on clothes or dishes.
• Images appear washed out

6. Grating
• One adjacent pair of light and dark bars make up one cycle
• The thickness of grating is described by their spatial
frequency in cycles per degree of the visual angle at the eye
• Gratings are used to assess the contrast sensitivity.

7. Sine wave grating
• These gratings are lacking sharp borders
• It is used as they are not subjected to
aberration, diffraction, light scatter
• In this grating only contrast and spatial
position are affected

8. Square wave gratings
• It has characteristics like a square
such as sharp edges

9. Types of contrast sensitivity
1. Spatial contrast sensitivity-
• It refers to detection of striped patterns at various levels
of contrast and spatial frequencies
• To measure spatial contrast sensitivity, patient is
presented with sine wave gratings of parallel light and
dark bands ( Arden gratings ) and is asked to tell the
minimum contrast at which the bars can be seen at each
frequency.

10. •The width of the bars is defined as the spatial frequency
(which expresses the number of pairs of dark and light bars
subtending an angle of 1 degree at the eye.)
• A high spatial frequency implies narrow bars
• A low spatial frequency implies wide bars
•Spatial frequency of the
visual stimuli is given in
cycles per degree

11. 2. Temporal contrast sensitivity
• It is measured with gratings that reverse contrast at various
rates over time
• Here contrast sensitivity function is generated for the time-
related ( temporal ) processing in visual system by presenting
a uniform target field modulated sinusoidal in time
• Both spatial and temporal testing provide more complete and
systematic data on the status of visual performance than the
conventional tests

12. Variables in the measurement of contrast sensitivity
• Average amount of light reflected. It depends on
illumination of paper and darkness of ink
•Degree of blackness in relation to white background
•Distance between the grating periods

13. Measurement of contrast sensitivity
Michelson Formula
Contrast Sensitivity = Lmax-Lmin/Lmax+Lmin
L = Luminance recorded by photocells
scanning across the gratings.
L max = Luminance on brighter surface
L min = Luminance on darker surface

14. Measurement of contrast sensitivity
Webers Formula
• Contrast Sensitivity = (Lb - Lt)/Lb.
• Where Lb and Lt are the luminance of the background and
target respectively
• Weber contrast used in calculating the contrast of letters

15. Contrast Sensitivity Function
• Contrast sensitivity is given by a graph where CS over a range of
spatial frequencies is plotted.
• It assess sensitivity over a wide range of spatial frequency.
• It is a subjective measurement of a persons ability to detect low
contrast pattern stimuli, usually vertical stripes of decreasing
shades of black to grey.

16. CS plotted over a range of spatial frequency

17. Neural Mechanism Of Contrast Sensitivity
• Campbell and Green gave the concepts of different visual
channels for handling information about different bands of
spatial frequencies
• This concept indicates that retina is a non-uniform structure
• In retinal periphery is responsible for only low frequency
channels. Whereas,
• In central retina or fovea is responsible for high frequencies
channels
• For coarse grating, central and peripheral retina have equal
contrast sensitivity per unit area of retina, but larger the retinal
area stimulated greater is the sensitivity.

18. • Contrast sensitivity is due to a series of ganglion cells that
have receptive fields of different sizes so that they are
maximally sensitive to different spatial frequencies
• Since each ganglion is highly sensitive to some particular
spatial frequency, the visual world is broken into its separate
spatial frequency components and this information is then
passed in separate channels to the cortex, where it is
reconstructed.

19. Snellens visual acuity versus contrast sensitivity
• Snellens charts test VA has a very high contrast approximate 100%.
Thus many patients have a good snellens VA but they may be visually
handicapped in real life situations
• The visual acuity drops in the situations with low contrast and the
quality of the vision is not good

20. Methods of measuring Contrast Sensitivity
• Arden Gratings
• Cambridge Low-Contrast gratings
• Pelli-Robson Contrast Sensitivity Chart
• Solan letter low contrast flip chart
• Lea low contrast flip chart
• Vistech Contrast Sensitivity T
est
• Regan Low contrast letter charts
• CSV-1000E

21. Arden Gratings
• Introduced in 1978
• Booklet containing 7 plates one screening plate (No.1)
and six diagnostic plates (No. 2-7)
• The contrast changes from top to bottom of the
grating, lowest at the top and highest at the bottom
• The plates are studied at the distance of 57cm with
spatial frequency increasing from 0.2 to 6.4
cycles/degree, each being double the frequency of
previous one..
• A score is 1-20 is assigned to each plate
• Sum of six plates with upper limit of 82 was
established for normal subjects together.

22. Cambridge Low-Contrast gratings
• Consists of a set of 10 plates containing gratings in a spiral bound
booklet
• To perform the test, the booklet is hung on the wall at a distance
of 6 m
• The pages are presented in pairs, one above another. One page in
each pair contains grating and the other is blank but have the
same reflectance
• The plates are changed sequentially in descending order of
contrast and are stopped when the first error is made

23. • Then a new series is begun starting from 4
plates prior to where the patient failed to
respond
• Four such descending series are completed
and the score of each series is noted ( the
score is numbered as per the no. of plates
read ) and added
• The final total value is converted into
contrast sensitivity from the score sheet
• When no error is made at plate 10, then the
score of 11 is given.

24. Pelli-Robson Contrast Sensitivity Chart
• The chart consists of letters and is printed on
both the sides. The two sides have different
letter sequence but are otherwise identical
• The letters on the chart are organized as
triplets. Two triplets in each line
• The contrast decreases from one triplet to the
next
• The log contrast sensitivity varies from 0.00 to
2.25

25. Procedure:
• The chart is hung on the wall, patient sits directly in front of the chart at 1
m distance so that the center of the chart is approximately at the level of
subject’s eye
• The chart is illuminated as uniformly as possible.
• The patient is made to name each letter in the chart, starting from the
upper left corner and reading horizontally across the line.
• Patient is made to guess, when he/she believes that the letters are
invisible. The test is concluded when the patient guesses two of the three
letters of the triplet incorrectly

26. Solan letter low contrast flip chart
• Easily measures, records and detects changes in the transfer of visual
information when the change affects visual acuity at the low contrast
levels.
• Each page measuring: 22.9 x 8.25cm
• Distance: 3 m
• Each eye tested separately
• Charts include following contrast levels: 25%, 10%, 5%, 2.5% and 1.5%
• Each chart includes recording forms and instructions
• The Lowest contrast level at which the patient is able to read the
letters correctly is recorded in percentage.

27. Lea low contrast flip chart
• Test contrast sensitivity with lea numbers.
• Each page measuring (23 x 8.25 cm )
• Distance: 3 m
• Each eye tested separately.
• Charts include following contrast level: 25%,
10%, 5%, 2.5%, 1.25%.
• The patient is asked to read the numbers at
each contrast levels.
• The lowest contrast level at which the patient
is able to read the numbers correctly is
recorded in percentage.

28. Vistech Contrast Sensitivity Test
• Consists of sine wave gratings.
• Done at distance of 3 m.
• Each chart contains five rows and nine columns
of circular photographic plates on grey background.
• Each row has different spatial frequency and contrast within the rows
reduces from left to right.
• The gratings are presented in three orientations: vertical, 15 degree
clockwise,or anti clockwise.
• The patient have to report the last gratings seen for each row and the
orientation of grating.

29. Regan Low contrast letter charts
• Consists of three letter charts, printed on
white cardboard with contrast of 97%
7% and 4%
• Patient is instructed to start at the top
and to continue reading until they can
correctly identify no letters on a line
• For one chart letter size goes on
reducing and contrast remains constant
• Regan in 1988 suggested its chief role in
detecting early visual loss in diabetes
and glaucoma

30. CSV-1000E
• It is widely used in world.
• This test provides four rows of sine-wave gratings.
• T
est distance is 2.5 meters
• These gratings test the spatial frequencies of 3,
7,12 and 18 cycles per degree.

31. Factors affecting Contrast Sensitivity
• Refractive errors
Visibility of low spatial frequencies is not limited by refractive
property of the eye; the refractive errors affect only the higher
frequencies
• Age
It has been noted that from the age of 20, CS decline with the age by
about 10% for each decade of life
• Lenticular changes
Early lens change can reduce contrast sensitivity essentially for low
spatial frequencies (immature cataract)

32. • Ocular diseases: Retinal, Optic Nerve and visual pathway diseases,
Glaucoma, Diabetic retinopathy, Ocular hypertension, Retrobulbar
neuritis, Amblyopia, Age related macular degeneration
• Systemic diseases: Multiple Sclerosis, Pituitary Adenoma

33. Importance of assessing Contrast Sensitivity
• Many conditions reduce both contrast sensitivity and VA, but under some
circumstances ( eg. amblyopia, optic neuropathies, incipient cataracts), visual
function measured by contrast sensitivity can be reduced even with a normal
Snellen visual acuity. Retinal pathology may affect contrast sensitivity more (as
with retinitis pigmentosa or central serous retinopathy) than it does Snellen
visual acuity.
• For patients who have visual problems despite a normal visual acuity, loss of
contrast sensitivity is more disturbing than decreased visual acuity. Hence,
contrast sensitivity testing may be useful in identifying the visual problem.
• To assess and treat the patients who have undergone refractive surgery.
Example: LASIK.

34. Reference
• American Academy of Ophthalmology. (2001). Optics, Refraction and
Contact Lenses (Basic & Clinical Science Course) (Rev Ed). American
Academy of Ophthalmology.
• Kanski, J., n.d. Clinical ophthalmology. 9th ed. Edinburgh: Butterworth
Heinemann/Elsevier
• S., & Tandon, R. (2019). Parsons’ Diseases of the Eye (23rd ed.).
Elsevier India.

35. Thankyou