contrast sensitivity.pptx

Contrast sensitivity

1. Contrast Sensitivity Dr. Bebika Phuyal 1 st 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. Contras t Sensit i vity F un c tion 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. C S p l otted over a range of spa ti a l 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 Ca m bridge Lo w - Con t rast grati n gs P ell i- R obson Con t rast Sens i ti v ity Ch a rt Solan letter low c o ntra s t flip ch a rt Le a low contra s t flip ch a rt V i s tech Con t rast Sens i ti v ity T est R egan Lo w co n tra s t letter ch arts 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. Cambridg e Lo w - Contras t g r atin g s 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. P el l i - R obson C o ntras t 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. P rocedure: 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. Le a low contras t 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. Functional acuit y contras t tes t in g ( F .A.C. T ) ‡ FACT comprises of a chart with sine wave gratings of varying frequencies. The chart tests five spatial frequencies and nine levels of contrast. The contrast varies in a row, decrease from left to right and the spatial frequency increase down the various columns from top to bottom. The test is performed at 3m distance. The patient reports the last grating seen for each row (A,B,C,D,E) also, he has to report the orientation of the gratings: right, left, up. The last corrected grating seen for each spatial frequency is then plotted.

30. R ega n Lo w contras t le t te r char ts 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

31. CS V -1000E It is w idel y used i n world. T h is te s t provide s fo u r rows of s i n e -wave grat i ng s . T es t dis t an c e is 2. 5 meters The s e grati n gs tes t the spat i a l frequenc i e s of 3 , 7,1 2 and 18 cy c les pe r degree.

32. Factors affec t in g Contras t 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)

33. 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

34. Importanc e of assessin g Contras t 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.

35. 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.