12.2 Colour Vision: Pseudoisochromatic Plates, Farnsworth-Munsell D-15 & the 100-Hue Test

Physiology and classification

Normal human colour vision is trichromatic, based on three cone photopigments: S (short, blue-sensitive, peak about 420 nm), M (medium, green, about 530 nm) and L (long, red, about 560 nm).

Congenital colour deficiency is overwhelmingly red-green (protan and deutan), X-linked recessive, present in about 8% of males and 0.5% of females. It is stable, bilateral, symmetrical and present from birth, and the patient is often unaware. Deuteranomaly is the most common single type.

Acquired colour deficiency is a different animal: it is often unilateral or asymmetrical, progressive or fluctuating, accompanied by other visual symptoms, and the patient notices it.

Köllner's rule is the interpretive key:

• Outer retinal disease (macular degeneration, central serous chorioretinopathy, diabetic maculopathy) → blue-yellow (tritan) defects.
• Optic nerve disease (optic neuritis, compressive optic neuropathy, toxic optic neuropathy) → red-green defects.

The rule has exceptions — glaucoma produces blue-yellow loss despite being an optic neuropathy, and some hereditary maculopathies produce red-green loss — but it is the framework the examination expects.

Drug and toxin causes worth knowing: ethambutol (red-green, dose-related, reversible if caught early), digoxin (yellow-green vision, xanthopsia), hydroxychloroquine, sildenafil-class drugs (transient blue tinge), and methanol.

Pseudoisochromatic plates

Ishihara plates are the standard screening test. Each plate presents a figure of coloured dots against a background of dots that differ in hue but match in brightness and saturation, so the figure can only be seen by hue discrimination.

Plate types: demonstration (seen by everyone, including malingerers), transformation (normal and deficient see different numbers), vanishing (normal sees a number, deficient sees none), hidden digit (deficient sees a number, normal sees none) and classification plates (separate protan from deutan).

Technique, and the errors that invalidate it:

1. Use natural daylight or a calibrated daylight illuminant — approximately 6500 K. Incandescent light shifts the spectrum and produces false results. This is the most common technique error.
2. Test one eye at a time — essential for detecting acquired unilateral defects, and routinely skipped.
3. Hold the book at about 75 cm, plates perpendicular to the line of sight.
4. Allow about 3 seconds per plate; delayed or hesitant responses are recorded.
5. Patient wears their near correction.
6. Record as plates read correctly over plates presented, for example "12/14 OD, 6/14 OS."

Limitations: Ishihara detects red-green deficiency only. It will not detect a tritan defect and therefore cannot be used to screen for acquired retinal disease. Other plate sets include Hardy-Rand-Rittler (HRR), which does test blue-yellow and grades severity, and the Standard Pseudoisochromatic Plates (SPP-2), designed for acquired defects.

Farnsworth D-15

The Farnsworth Panel D-15 is an arrangement test: 15 coloured caps plus a fixed reference cap, which the patient arranges in order of nearest colour resemblance in a tray.

Purpose. It separates mild (pass) from moderate to severe (fail) defects and classifies the axis of the defect. It is not a screening test for mild anomalous trichromacy — many anomalous trichromats pass — which is exactly its value: a failed D-15 means a defect large enough to affect colour-dependent occupations.

Technique: daylight illumination, caps handled by their rims to avoid soiling, one eye at a time, no time pressure but typically a 2-minute guide, and the cap order recorded from the numbers on the underside.

Scoring. The cap order is plotted on a circular diagram and the sequence is joined with lines.

A desaturated D-15 (Lanthony) uses paler caps and is more sensitive, catching milder acquired defects that the standard panel misses.

Farnsworth-Munsell 100-Hue

The FM 100-Hue is the most sensitive clinical colour test. Despite the name it uses 85 caps in four trays, each tray covering a portion of the hue circle, which the patient arranges in order between fixed end caps.

Scoring. Each cap receives an error score based on how far its neighbours differ from the correct sequence; the scores are plotted on a polar diagram and summed as a total error score (TES).

The shape of the polar plot identifies the axis: a bipolar cluster of errors along the protan, deutan or tritan meridian. Scores worsen naturally with age and with reduced acuity, so results must be interpreted against age-matched norms.

Practical drawbacks: it takes 15 to 30 minutes per eye, demands sustained concentration, and is fatiguing. It is reserved for quantifying and monitoring a known defect — serial testing in a patient on ethambutol or hydroxychloroquine, or tracking recovery in optic neuritis — rather than for screening.

Choosing the right test

The bedside red cap test deserves its own mention: hold a red object, ask the patient to compare its redness between the two eyes, and ask what percentage of the good eye's red the affected eye sees. A report of "70% as red" is a sensitive early sign of optic nerve dysfunction and takes ten seconds.