3.2 Color vision & contrast sensitivity testing

Why Test Color Vision

Color vision testing screens for both congenital and acquired defects. The retina's cones come in three types, sensitive to long (red), medium (green), and short (blue) wavelengths. A defect in any of these pigments shifts color perception. As a COT you administer standardized plate or arrangement tests under proper lighting and record the result accurately.

Congenital versus acquired defects

Congenital color deficiency is present from birth, stable over life, usually red-green, symmetric (the same in both eyes), and inherited in an X-linked recessive pattern, which is why it affects about 8% of males but well under 1% of females. Patients often do not know they have it. The classic types are:

• Protan defects: reduced or absent red (L-cone) response (protanomaly is weak, protanopia is absent).
• Deutan defects: reduced or absent green (M-cone) response (deuteranomaly, deuteranopia). Deutan is the most common type.
• Tritan defects: blue-yellow (S-cone) loss, which is rare congenitally and more often acquired.

Acquired color deficiency develops later from ocular or systemic disease or drug toxicity. It is often asymmetric (different between the two eyes), can change over time, and frequently produces blue-yellow (tritan) loss. A helpful clinical rule (Kollner's rule, with exceptions): optic nerve disease tends to cause red-green loss, while retinal/macular disease tends to cause blue-yellow loss. Because acquired defects can be asymmetric, always test each eye separately.

Pseudoisochromatic Plate Tests

Ishihara plates

The Ishihara test is the most widely used screen for red-green defects. Each plate shows colored dots forming a number (or a winding trail for young children) that is visible to normal observers but not to those with a red-green defect, or the reverse on "hidden-digit" plates. Administer it as follows:

1. Use daylight-balanced illumination; incandescent light shifts the hues and invalidates the test.
2. Hold the plate at about a comfortable reading distance, perpendicular to the line of sight.
3. Allow only a few seconds per plate.
4. Test one eye at a time, with the patient wearing near correction.

Record the number of plates read correctly, for example "OD 13/14 Ishihara plates." Ishihara does not detect blue-yellow defects.

HRR plates

The Hardy-Rand-Rittler (HRR) plates use colored symbols (circle, cross, triangle) and, unlike Ishihara, screen for both red-green and blue-yellow defects and can grade severity (mild, medium, strong). This makes HRR useful when an acquired tritan defect is suspected.

Arrangement Test: Farnsworth D-15

The Farnsworth D-15 asks the patient to arrange 15 colored caps in order of hue, starting from a fixed reference cap. Someone with normal color vision places them in a smooth chromatic sequence; a color-deficient patient makes characteristic crossing errors. When the results are plotted, the direction of the crossing lines identifies the defect type along a protan, deutan, or tritan axis. The D-15 separates clinically significant "fail" defects from mild ones and is valuable for occupational screening (for jobs that require color discrimination). A longer version, the Farnsworth-Munsell 100-hue test, grades discrimination more finely but takes much longer.

Contrast Sensitivity Testing

Visual acuity measures the resolution of high-contrast black-on-white letters, but real-world vision also depends on seeing low-contrast targets. A patient can have "20/20" acuity yet struggle with faded, foggy, or dim images, a complaint common in cataract, glaucoma, optic neuropathy, and amblyopia. Contrast sensitivity testing fills this gap.

The Pelli-Robson chart

The Pelli-Robson contrast sensitivity chart displays letters of a single large size whose contrast decreases in triplets down the chart, from near-black to near-invisible gray. The patient reads down until the letters fade out; the faintest triplet read correctly gives a log contrast sensitivity score (higher is better). Test each eye separately at the specified distance (usually 1 m) with best correction and good, glare-free lighting. Because it can reveal functional loss when Snellen acuity looks normal, contrast sensitivity is valuable for:

• Documenting cataract impact when acuity underestimates disability.
• Monitoring glaucoma and optic nerve disease.
• Assessing low-vision patients and quality-of-vision complaints.
• Evaluating post-refractive-surgery symptoms.

Record the score with the eye tested, correction worn, and chart used so results stay comparable over time.

Practical testing tips

• Correct lighting is essential: daylight-balanced, consistent, and glare-free.
• Ensure the patient wears near/reading correction for plate and near tests.
• Test monocularly to catch asymmetric (acquired) defects.
• Document the specific test used, because different tests are not interchangeable.

Cone Physiology and Severe Deficiency

Normal color vision is trichromatic, relying on all three cone types working together. A person missing one cone type is a dichromat; someone who has all three but with one anomalous pigment is an anomalous trichromat, the milder and more common state. Rare monochromatism (achromatopsia) means little or no functional cone color vision and is usually accompanied by poor acuity, nystagmus, and light sensitivity.

Drug and disease causes

Acquired color loss can follow drug toxicity, so note relevant medications when an acquired defect appears. Ethambutol (used for tuberculosis) and hydroxychloroquine can damage color vision, and digoxin classically causes xanthopsia, a yellow tinge to vision. Occupational screening matters here as well, because aviation, maritime, military, and electrical trades set color-discrimination standards a candidate must meet.