colour blindness

COLOUR VISION

Jagdish Dukre

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· Red cones· Green cones· Blue cones· Brightness = R + G· Color = R – G· Color = B – (R+G)· Red cones

outnumber green cones 2/1

· Red + Green cones outnumber blue cones 10/1

Retinal Cones–Normal Color Vision

Blue cones absent in

central fovea

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Red, green and blue cone sensitivity vs. wavelength curves

Retinal Cones–Normal Color Vision

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Types of Color Vision Deficiencies

• Trichromacy (“three-color vision”)– Normal Color Vision

1) Congenital colour blindness a) Dyschromatopsia• Anomalous Trichromacy (“unusual three-color vision”)

– See all three primary colors.– One color is seen weakly

• Protanomaly (L-cone defect) red-weak• Deuteranomaly (M-cone defect) green-weak• Tritanomaly (S-cone defect) blue-weak

• Dichromacy (“two-color vision”)– See only two of the three primary colors– One type of cone is totally absent or

nonfunctional.• Protanopia (L-cone absent)• Deuteranopia (M-cone absent)• Tritanopia (S-cone absent)

b) Achromatosia

• Cone monochromatism

only one primary colour.

• Rod Monochromatism

(no cones at all) (“no-color vision”)Sees no colors, only shades of grayDay blindness (VA 6/60)Fundus usually normal

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Some Views With and Without Color Vision

2) Acquired colour blindness

• Blue-yellow impairment retinal lesions• Red-green impairment optic nerve lesion• Blue colour defect old age

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What happens in hereditary color deficiency?

· Red or green cone peak sensitivity is shifted.

· Red or green cones absent.

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B RG

437 nm 564 nm533 nm

NORMAL CONE SENSITIVITY CURVES(TRICHROMAT)

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B RG

437 nm 564 nm

Deuteranomaly(green shifted toward red)

5% of Males

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B RG

437 nm 564 nm

Deutan Dichromat(no green cones; only red and blue)

1% of Males

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B R

437 nm 564 nm

Deutan Dichromat(no green cones; only red and blue)

1% of Males (there is no green curve)

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B RG

437 nm533 nm

Protanomalous (red shifted toward green)

1% of Males

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B RG

437 nm533 nm

Protan Dichromat(no red cones; only green and blue)

1% of Males

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B G

437 nm533 nm

Protan Dichromat(no red cones; only green and blue)

1% of Males (there is no red curve)

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Why do colors that look different to us appear the same to

color deficient individuals?

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B RG

Color Normal Individual

Large difference in

stimulation of green and red

cones

Small difference

in stimulation

Consider a green vs. yellow light…

The two spots appear

different in color because R-G is large for one, and small for the other.

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B RG

Deuteranomaly

Small difference

in stimulation

Small difference

in stimulation

Each spot produces the same R-G stimulation and thus looks the same!

(the green sensitivity curve is shifted toward the red)

Look the same!

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Hereditary Color Deficiency· 8-10% of males and 1/200 females (0.5%) are

born with red or green color deficiency.· Sex-linked recessive condition (X chromosome).

· Protanomaly—red cone peak shifted toward green (1%)

· Protan Dichromat—red cones absent (1%)· Deuteranomaly—green cone peak shifted

toward red (5%)· Deutan Dichromat—green cones absent (1%)· Hereditary tritan defects are rare (0.008%)

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Color Deficiency Males FemalesProtanopia 1% 0.01%

Deuteranopia 1% 0.01%Protanomaly 1% 0.01%

Deuteranomaly 5% 0.4%Overall (red-

green)8% 0.5%

Tritanopia 0.008%

0.008%

Tritanomaly Rare RareRod

monochromatism

Rare Rare

Cone monochromatis

m

Rare Rare

Tests for colour vision

• Pseudo-isochromatic chart test• Lantern test• Fansworth munsell 100 hue

test• Nagel’s anomaloscope• Holmgren’s test

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Pseudo-isochromatic chart test

• Commonly used are Ishihara plates

• HRR plates are also based on same principle.

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How to use the testThe plates are designed to be appreciated correctly in a

room which is lit adequately by daylight. The introduction of direct sunlight or the use of electric

light may produce some discrepancy in the results because of an alteration in the appearance of shades of color.

When it is convenient only to use electric light, it should be adjusted as far as possible to resemble the effect of natural daylight.

The plates are held 75 cm. from the subject and tilted so that the plane of the paper is at right angles to the line of vision.

• The correct position of each plate is indicated by the number which is printed on the back of the plate.

• The numerals which are seen on plates 1-17 are stated, and each answer should be given without more than three seconds delay.

• If the subject is unable to read numerals, plates 18-24 are used and the winding lines between the two X’s are traced with the brush.

• Each tracing should be completed within ten seconds.

• It is not necessary in all cases to use the whole series of plates.

• Plates 16 and 17 may be omitted if the test is designed merely to separate the color defectives from those with normal color appreciation.

• In a large scale examination the test may be simplified to an examination of six plates only; – No 1, – one of the Nos 2, 3, – one of Nos 4, 5, 6, 7, – one of Nos 8, 9, – one of Nos 10, 11, 12. 13 and – one of Nos 14, 15.

• It may be necessary to vary the order of the plates if it is suspected that there is a deliberate decetion on the part of the subject

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Correct number is ? 25

Correct number is ? 29

Correct number is 45

Correct number is ? 56

Correct number is ? 6

Plate no. Normal person

RED-GREENDeficient

Total colour blind or weakness

1 12 12 12

2 8 3 X

3 29 70 X

4 5 2 X

5 3 5 X

6 15 17 X

7 74 21 X

8 6 X X

9 45 X X

10 5 X X

11 7 X X

12 16 X X

13 73 X X

14 X 5 X

15 X 45 X

Protan Deutan

strong mild strong mild

16 26 6 (2) 6 2 2 (6)

17 42 2 (4) 2 4 4 (2)

The mark X shows that the plate cannot be read. Blank spce denotes that the reading is indefinite. The numerals in parenthesis show that they can be read but they are comparatively unclear.

Analysis of the results

• As assessment of the readings of plates 1 to 15 determines the normality or defectiveness of color vision.

• If 13 or more plates are read normally, the color vision is regarded as normal.

• If only 9 or less than 9 plates are read normally, the colour vision is regarded as deficient.

• However, in reference to plates 14 and15, only those who read the numerals 5 and 45 and read them easier than those on plates 10 and 9 are recorded as abnormal readings.

• It is rare to find a person whose recording of normal answers is 14-16 plates.

• An assessment of such a case requires the use of other colour vision tests, including the anomaloscope.

• In the assessment of color appreciation by the short method involving 6 plates only as described on page 4, a normal recording of all plates is proof or normal color vision.

• If there is a discrepancy in any of the recordings, the full series of plates should be used before diagnosing a red-green deficiency.

Nagel’s anomaloscope

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The Nagel anomaloscope is the goldstandard.

Advantages: test has a long and hallowedhistory that is well respected.

Disadvantages: expensive instrument thatrequires an experienced examiner’s skills.

Validity: validation measures of other colourvision tests are based on this instrument.

Calibration: requires spectroscope to calibrate.

Farnsworth-Munsell 100 Hue Test (FM-100)

In its present form, the test consists of eighty-fivecaps in four boxes, used one box at a time. There are no confusion colours in a box and

therefore thetest is a hue discrimination metric.The caps are in equal steps of hue around a huecircle.

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• The results are quantitative, can be subjected to age correction, and are amenable to statistical evaluation.

• Validity - since this is the only test of its sort (hue discrimination) cross validation with other metrics is not possible.

• The test has become the second gold standard for assessing a wide variety of hereditary and acquired conditions

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• Procedure- Test is done at 35 cm at day light at right angle of the visual plane.

- It consists of 10 plates each contains four peripheral colored dots with one on the centre.

- The patient is asked to select the peripheral that most closely matches the central one

- Results are written as Top(T), Bottom (B),Right (R),Left (L) and score paper is present to analyze defect due to patient response.

The City University test TCU

Holmgren’s wool test

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• The patient had to match one piece of wool to the samples in the box in this colour blindness test.

• There are light and dark shades to confuse the patient. This helped detect problems.

• The numbers on the pieces of wool were codes.

• Swedish Holmgren's coloured wool test for colour blindness, Europe, 1871-1900 physiologist Alarik Frithiof Holmgren (1831-1897) devised this test in 1874.

• He pursued his investigations following a railway accident in Sweden in 1876.

• The accident was believed to be caused by a colour blind train driver.

• Following Holmgren’s research, colour blindness tests were made compulsory for railway and shipping workers in Sweden.

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THANK YOU….

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