Contrast sensitivity

CONTRAST SENSITIVITY

PRESENTER; DR. LEENA VADHELMODERATOR: DR. MRUDULA BHAVE

Definition:Contrast:- defined as the degree of blackness to

whiteness of target.

Contrast threshold:-is the smallest amount of contrast required to be able to see the target.

Contrast sensitivity:- is the reciprocal of the contrast threshold.

WHAT IS CONTRAST SENSITIVITY

the ability to perceive slight changes in luminance between regions which are not separated by definite borders and is just as important as the ability to perceive sharp outlines of relatively small objects.

SNELLENS VISUAL ACUITY VERSUS CONTRAST SENSITIVITY

Snellen’s charts test VA at a very high contrast thus many patients have a good snellens VA but they may be visually handicapped in real life situations.

The VA drops in the situations with low contrast and the quality of vision is not good.

It provides the information that cannot be obtained from visual acuity, and it is often a better predictor of visual performance then VA.

CONTRAST SENSITIVITY CURVEC .S is presented

as curve which plots the lowest contrast level a patient can detect for specific size target.

The x-axis curve is for spatial frequency, y-axis is for contrast sensitivity.

CONTRAST SENSITIVITY FUNCTION

The contrast sensitivity function proposed by Manos and Sakrison.

A plot of CS over a range of spatial frequencies gives the contrast sensitivity function(CSF).

Contrast sensitivity depends on spatial frequency.

Spatial frequency of visual stimuli is given in cycles per degree.


Contrast sensitivity function (CSF) is a subjective measurement of a person's ability to detect a low contrast pattern stimuli, usually vertical stripes of decreasing shades of black to grey. The resulting measurement is said to give a more accurate representation of the eyes' visual performance.

CONTRAST SENSITIVITY FUNCTIONCSF has a peak value

of approximately 1 to 8 cycles/ degree.

Distinction of high frequencies not possible because of limited number of photoreceptors.


Neural and optical attenuation limits the high spatial frequency CS.

Point where the CSF cuts the x axis is called cut off frequency.

MICHELSON FORMULA:L max- Lmin/Lmax+LminLmax = Luminance on the lighter surfaceLmin = Luminance on the darker surface

WEBERS FORMULA: Defined as ( Lb – Lt ) / LbLb & Lt are the luminance of the background and

target respectively.Weber contrast used in calculating the contrast of

letters.

low medium high

object:100% contrast

image

spatial frequency

cont

rast

1

0

• The modulation transfer function (MTF) indicates the ability of an optical system to reproduce (transfer) various levels of detail (spatial frequencies) from the object to the image.

• Its units are the ratio of image contrast over the object contrast as a function of spatial frequency.

• It is the optical contribution to the contrast sensitivity function (CSF).

MTF: Cutoff FrequencyMTF: Cutoff Frequency

0

0.5

1

0 50 100 150 200 250 300

1 mm2 mm4 mm6 mm8 mm

mod

ulat

ion

tran

sfer

spatial frequency (c/deg)

cut-off frequency

57.3cutoffaf

Rule of thumb: cutoff frequency increases by ~30 c/d for each mm increase in pupil size

Gratings

One adjacent pair of light and dark bars makes up one cycle.

The thickness of the grating is described by their spatial frequency in cycles per degree of the visual angle at the eye..

Large number of grating can fit with in a degree of visual angle

Gratings

Gratings are broader and are of low spatial frequency.

Spatial phase defines its position.Change in spatial phase by 180 degrees

indicates that the grating is displaced by half a cycle.

Sine wave gratings

These gratings are lacking in sharp borders.

Sinusoidal gratings are used because they are not subject to defocus, aberration,diffraction, light scatter.

Only the contrast and spatial position are affected.

Square wave gratings

square-wave gratings have characteristics like a square such as sharp edges.

POSITION OF GRATINGS

TYPES OF CONTRAST SENSITIVITY:

1. Spatial contrast sensitivity

2. temporal contrast sensitivity

SPATIAL CONTRAST SENSITIVITY

Detection of striped pattern at various levels of contrast and spatial frequency.

Sine wave gratings of parallel light and dark bands.

Measured with stationary gratingswidth of the bar is defined as spatial

frequencyHigh spatial frequency- narrow bars.low spatial frequency- wide bars.

The spatial frequency increases exponentially from left to right. The contrast also varies logarithmically from 100% at the bottom to about 0.5% at the top

TEMPORAL CONTRAST SENSITIVITYTemporal contrast sensitivity is measured with

gratings that reverse contrast at various rates over time.

here contrast sensitivity function is generated for the time related processing in the visual system by presenting a uniform target field modulated sinusoidal in time.

Both the system provides more complete and systemic data on the status of visual performance.

The shape of the contrast sensitivity function varies with factors:

LuminanceTarget sizeGrating motion Grating shape

Neural mechanism of contrast sensitivity:

Campbell and Green gave the concepts of different visual channels for handling information about bands of spatial frequencies.

This concept indicates that the retina is not uniform.

Fovea is specialized for high acuity and is responsible for high spatial frequencies.

In the retinal periphery, only low frequency channels are represented.

M and P------- pathways

Ganglion cells divded into p- Cells :- small,slow

conducting axons that input into parvocellular layers of the lateral geniculate body.

P-Cells have higher spatial resolution.

m-Cells :- large ,fast conducting axons that input into the magnocellular layers.

M- Cells have higher contrast sensitivity and temporal resolution and lower spatial resolution.

Channel theory

CSF is a function of several independent parallel detecting mechanisms.

Each channel is highly sensitive to some particular spatial frequency

Visual world is broken into its separate spatial frequency components and this information is then passed in separate channels to the cortex,where it reconstructed.

Visual system consists of 4 to 6 spatial frequency channels

ChannelsCSF channel could be due to a series of ganglion

cells that have receptive fields of different sizes so that they are maximally sensitive to different spatial frequencies.

Stimuli smaller than the center receptive field only produce a partial response from the ganglion cell.

Stimuli larger than the center receptive field also stimulate the surrounding area so that the overall response from ganglion cell is reduced.

SYMPTOMS OF CONTRAST SENSITIVITY

Have problems with night driving, including inability to see traffic lights

May require extra light to read

Their eyes may become

tired when they read or watch television

Not being able to see spots on clothes, counters, or dishes

Missing facial gestures

MEASUREMENT OF CONTRAST SENSITIVITY:

There are three variables in measurement of contrast sensitivity:

1. Average amount of light reflected depends on illumination of paper and darkness of ink.

2.Degree of blackness in relation to white background that is contrast.

3. The distance between the grating periods of cycles per degree of visual angle.

The various methods available to measure CS include

Pelli Robson test Bailey Lovie chart Vision contrast test system (VCTS) by Vistech Cambridge low contrast grating Regan charts FACT charts

PELLI ROBSON CONTRAST SENSITIVITY CHARTSletters of the same

size but with reducing contrast

two charts and two scoring pads.

Each chart has 6 letters in each row organized into two triplets of varying contrast.

illumination of the chart is 85 cd/mm2

1 meter distance

The score of the test is recorded by the faintest triplet out of which at least 2 letters are correctly identified.

The log CS value for this triplet is given by the number on the scoring pad.

CAMBRIDGE LOW CONTRAST GRATING

It is a rapid and simple screening test for contrast sensitivity.

Performed at a distance of 6m. It comprises of 12 pair of plates consisting of

stripes of varying contrast. First one is for demonstration and rest are for

the proper testing and are numbered from 1-10.

The plates are changed sequentially starting from plate 1 till the patient fails to respond.

Then a new series is begun starting 4 plates prior to where the patient failed to respond.

Four such series are completed and the score of each series is noted (numbered as per the number of plate read) and added.

The final total value is converted into contrast sensitivity from the provided table.

BAILEY LOVIE CHARTLow contrast acuity testing & acuity testing in

presence of glare.

Each row has same no. of symbols & constant spacing is used between rows & letters.

Chart is logarithmic based & visual acuity is based on log of minimum angle of resolution or logMAR.

Berkely Glare Test provides 10% of contrast & is provided with glare source

FUNCTIONAL ACUITY CONTRAST TESTING(F.A.C.T)

FACT charts were developed by Dr. Arthur Ginsburg.

FACT comprises of a chart with sine-wave gratings of varying frequencies.

The chart tests five spatial frequencies (sizes) and nine levels of contrast

The Contrast varies in a row, decrease from left to right

And the spatial frequencies increase as one move down the various columns from top to bottom.

This test is performed at a distance of 10 feet.The patient determines the last grating seen

for each row (A, B, C, D and E) and reports the orientation of the grating: right, up or left.

The last correct grating seen for each spatial frequency is plotted on a contrast sensitivity curve.

THE ARDEN PLATE TEST Introduced in 1978.Consists of a booklet

containing several sine wave gratings patterns

Each grating is oriented vertically

The contrast varies from the top to the bottom of the grating, lowest at the top and highest at the bottom.

VISTECH CONTRAST SENSITIVITY TEST

These charts consist of sine wave gratings. Each chart contains five rows and nine columns of circular photographic plates (disc) on gray background.

Each row has different spatial frequency (1.5 to 18 c.p.d. at three meters) and contrast within the row reduces from left to right.

The gratings are presented in three orientations: vertical 90 degrees, 15 degrees clockwise or anticlockwise.

Two separate Vistech charts are available: the VCTS-6500 for distance testing and the VCTS-6000 for near testing.

A projector slide the VCTS-500S is also available.

A luminance of between 103 and 240cd/m2 is recommended.

It can be used to document the performance of low vision patients.

REGAN LOW CONTRAST LETTER CHARTS

Consists of three letter charts, printed on white cardboard having contrast of 97%, 7% and 4%

3 meter with eight letters in each linePatients are 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 there chief role in detecting early visual loss in diabetes and glaucoma.

It is widely used in the worldThis test provides four rows of

sine-wave gratingsThe test distant 2.5 meters,

these gratings test the spatial frequencies of 3,7,12,and 18 cycles/degree

Good for evaluation eye disease

Use for screening refractive surgery patients

Used for in or out of office screening and for cataract evaluation

Used to evaluate both contrast sensitivity and low contrast acuity

Contain 3 contrast levels

Provided the same tests as standard CSV-1000-S expect that acuity is presented in landolt c format

Useful when patient cant read English alphabet

It is special test designed specifically for food and drug administration clinical trials

FACTORS AFFECTING CONTRAST SENSITIVITY1. refractive errors.

2. age3.cataract4.contact lenses5.refractive surgery.6.glaucoma.7.diabetic retinopathy.8.optic neruopathies9.pituitary adenoma.10.drugs.11.toxic chemicals

Refractive errors:Uncorrected refractive disorders manifest

themselves as a decline in contrast sensitivity first at smaller grating sizes or higher spatial frequencies for mild refractive disorders.

As the degree of refractive disorder increases, contrast sensitivity declines at the middle and then larger grating sizes (middle to lower spatial frequencies).

STUDY

Measurements were made at distance (with Vistech charts), at near (with Arden gratings), and with a pinhole to control pupil size.

Under all conditions, when visual acuity was reduced by spherical lenses, there was a loss of contrast sensitivity over a broad range of spatial frequencies.

Ophthalmology 1988 Jan;95(1):139-43. Eeffectof visual blur on contrast sensitivity. Clinical implication Marmor MF, Gawande A

Age:There occurs a definite decrease in contrast

sensitivity with increasing age.

The pathogenesis of this decline in vision is likely involves changes in the spherical aberration of the crystalline lens

As the aperture of lens increases the average focal point move towards the lens so that a larger pupil produces greater spherical aberration

Spherical aberration of cornea changes little

with the age of the people.

Total wavefront aberration of the eye increases more than threefold, between 20 to 70 years of age.

the aging lens loses its balance with the cornea, because both the magnitude and sign of its spherical aberration change significantly.

A loss of balance between corneal and lenticular spherical aberration causes the degradation of optical quality in the aging eye.

STUDY

Monocular and binocular contrast sensitivity function for a range of spatial frequencies was measured in two groups of subjects with normal vision.

In the age range 50-87 years there was a linear decline in contrast sensitivity with age for medium and high spatial frequencies, but sensitivity for low spatial frequencies was independent of age.

Effect of age on contrast sensitivity function: uniocular and binocular findings J E Ross D D Clarke and A J Bron.

Cataract:Early cataracts generally cause contrast

sensitivity losses similar to refractive disorders at higher spatial frequencies.

The addition of glare source will exacerbate the results for patients with cataract, producing lower contrast sensitivity at some or all grating sizes

STUDY

significant loss of contrast sensitivity at all spatial frequencies with increasing cataract severity. Contrast sensitivity was significantly reduced at high spatial frequency (18 cpd) in cortical cataracts in the presence of glare in day light and at low spatial frequency (3 cpd) in night light.

Contrast sensitivity scores may offer additional information over standard VA tests in patients with early age-related cataracts.

Ophthalmic Vis Res. 2011 January; 6(1): 26–31. Effect of Cataract Type and Severity on Visual Acuity and Contrast

Sensitivity Javad Heravian Shandiz MSc, PhD,1 Akbar Derakhshan MD,2

Contact lenses:to ensure proper contact lens fit as well as

determining when replacement is necessary.Uncorrected residual astigmatism from soft

contact lens can result in decrease contrast sensitivity at the middle and higher spatial frequencies when compared to hard contact lenses.

study concluded that those female professional and collegiate tennis players wearing no corrective lenses were significantly higher in CS at intermittent and high spatial frequencies than those who wore daily-wear soft contact lenses in competition.

Kluka, D.A. and Love, P.A. “Effects of Daily – Wear Contact Lenses Upon Contrast Sensitivity in Selected Professional and Collegiate Female Tennis Players,” UBA at Birmingham

STUDYGlaucoma: Glaucoma can reduce contrast sensitivity

for all grating sizes, a number of studies shown reduce contrast sensitivity mostly at the middle spatial frequencies.

study was carried out to explore the effects of glaucoma on low-spatial-frequency contrast sensitivity by using tests designed to assess performance of either the magnocellular (M) or parvocellular (P) visual pathways. Sensitivity was reduced foveally and in the midperiphery across the spatial frequency range. In the area of early visual field loss, the glaucoma group demonstrated further sensitivity reduction across the spatial frequency range.

The low-spatial-frequency-sensitive channels of both pathways, which are presumably mediated by cells with larger receptive fields, are approximately equivalently impaired in glaucoma.

Contrast sensitivity changes due to glaucoma and normal aging: low-spatial-frequency losses in both magnocellular and parvocellular pathways.

McKendrick AM Sampson GP, Walland MJ, Badcock DR Invest Ophthalmol Vis Sci.2007 May;48(5):2115-22

STUDY

IOLsA study was carried out to evaluate the

contrast sensitivity at near and distance after MFIOL implantation in 21 patients. A control group with monofocal iol was also studied. CS measured after 1,3,6,12,and 18 months after IOL.

CONTRAST SENSITIVITY

MULTIFOCAL MONOFOCAL

CONTRAST SENSITIVITY AT DISTANCE(1 MONTH) (3 MONTHS)

(6, 12 AND 18)CONTRAST SENSITIVITY AT NEAR

L Cataract refractive surgery.2003apr;29(4):703-11Montes mico R

MONOFOCAL(ALCON ACRYSOF)

ACCOMODATING(HUMAN OPTICS)

DIFFRACTIVE MFIOL(TECNIS)

REFRACTIVE MFIOL(AMO REZOOM)

NEAR VISUAL ACUITY

- - BETTER -

DIST. CS HIGH HIGH - LOWNEAR CS

HIGH SFMONOCULAR AND BINOCULAR CS

HIGH

HIGH

HIGH

HIGH

HIGH

LOW

LOW

LOW

multifocal In patients with unilateral cataract, monofocal, accommodating and partially diffractive multifocal IOL provided higher CS scores when compared with refractive IOL.

Clinical and experimental ophthalmology Differences in contrast sensitivity between monofocal,

multifocal and accommodatingintraocular lenses: long-term results Cem Mesci MD, Hasan H Erbil MD, Ali Olgun MD, Nihat Aydin MD, Article first published online: 3 NOV 2010 DOI: 10.1111/j.1442-

9071.2010.02357.

Diabetic retinopathy:In diabetic retinopathy patients, contrast sensitivity

loss may occur for all grating sizes.a study was carried out to evaluate contrast

sensitivity (CS) and visual acuity (VA) in detecting functional losses in participants with type 2 diabetes.

LogMAR VA and letter CS were measured on 20 type 2 diabetic and 24 age-matched control participants. The diabetic participants were sub-grouped according to the level of retinopathy present.

RESULTS: Letter CS was able to distinguish those

participants with and without macular oedema and those with no or minimal diabetic retinopathy from the control group, whereas VA was not.

Clinical optom 2003 May;86(3):152-6 Letter contrast sensitivity changes in early diabetic retinopathy Stavrou EP, Wood JM

Optic neuropathies:A variety of optic neuropathies including

afferent pupillary defect, optic neuritis, and multiple sclerosis will affect contrast sensitivity losses over some or all grating sizes.

Multiple sclerosis can sometimes affect only middle sizes.

Pituitary adenoma:Pituitary adenoma has been shown to cause

contrast sensitivity loss at the middle grating sizes.

STUDY

Cases report was made by neuroophthalmological examination in two patients with parasellar pituitary adenoma before and after surgery.

The twelve months follow-up of visual dysfunction includes standard Snellen visual acuity and low contrast number visual acuity.

The presented cases indicate that the low contrast acuity tests in patients with parasellar pituitary adenoma are sensitive indicators.

The loss of low contrast visual acuity improved after surgery as did other visual measures, except in cases with significant low contrast deficit at 1.5% contrast.

Kruzer, A.R. (1986) “ Contrast Sensitivity Signals Pitutary Adenoma,” Rev. Of.Opt. 123(4), 119

Toxic Chemicals:Several occupational studies have indicated that

chronic, airborne exposure adversely affects neurobehavioral functions in workers, particularly visual color discrimination and tasks dependent on rapid visual-information processing

Frenette, B.,Mergler, D. And Bowler, R.(1991) “Contrast Sensitivity Loss in Group of Former

Microelectronics Workers with Normal Visual Acuity,” Opt. and Sci.,68(7),.

CONTRAST SENSTIVITY AND LASIK

Why refractive surgery can affect the quality of vision.

there are other factors in the eye, which affect vision quality, that are unrelated to eye focus. These factors can cause the quality of vision to vary greatly, even when a patient has 20/20 or better acuity. Contrast sensitivity is the best method for quantifying the quality of vision

Examples of the change in contrast sensitivity with lasik refractive surgery:

The graphs show the pretreatment level of contrast sensitivity (with the patient wearing his or her glasses) and the contrast sensitivity after 1 day, 1 week and 1 month of surgery (with the patient wearing the best correction).

the patient contrast sensitivity does not recover to the presurgery levels. The following graph shows an example of this response in vision quality

Several surgeons have noted that some patients with high levels of myopia (> - 9 diopters) actually improve contrast sensitivity after LASIK surgery. The graph below shows an example. The reason for this improvement in the quality of vision is not completely understood, but it is thought to be due to the poor quality of vision that some patients have through their glasses or contact lenses before LASIK.

Low contrast acuity after lasik

glare testing, which shines a bright light in the patient's eye, reduces pupil size and typically causes vision to improve. The graph below shows the results of a patient tested for contrast sensitivity in darkness and then again with glare. the glare condition dramatically improves the contrast sensitivity

With glare, the yellow filter also improved contrast sensitivity in LASIK patients, but not significantly.

J Refract surgery2005 Mar-Apr;21(2):158-65. Effect of a yellow filter on contrast sensitivity and disability glare

after laser in situ keratomileusis under mesopic and photopic conditions.

Pérez-Carrasco MJ, Puell MC, Sánchez-Ramos C, López-Castro A, Langa

A.

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Contrast sensitivity