Farnsworth D-15

What it measures

The Farnsworth D-15 measures hue discrimination. It separates normal or near normal colour vision from a significant defect, and indicates the axis of that defect: protan, deutan or tritan.

The caps sit on one circle of roughly constant lightness and saturation, so only hue distinguishes them. A patient who cannot discriminate along one direction of colour space confuses the caps lying opposite each other on it, and places them out of sequence. That pattern gives the axis.

Ishihara plates screen for congenital red-green defects. The D-15 grades severity, reports an axis, and responds to acquired defects, including the blue-yellow loss of some retinal and optic nerve disease.

How it is administered

The patient wears habitual near correction. Tinted lenses and colour correcting filters are removed, because they change the arrangement.

A reference cap is fixed at the start of the row. The patient places each remaining cap next to the last one, always choosing the cap closest in colour, working steadily and without long hesitation. Each eye is tested separately, the other occluded.

Room lighting affects the result. The original test is specified under a daylight illuminant. On a screen the display replaces both the caps and the illuminant, so the room should be moderately lit, without coloured light or reflections. Fix display brightness before starting and leave it unchanged.

Reading the result

VisReed compares the finished arrangement with the correct hue sequence and reports a pass or a fail. It keeps the arrangement itself and the time taken in seconds.

The comparison is exact, so one cap out of place gives a fail. A single isolated transposition is common and is not usually read as a defect, so read the arrangement rather than the flag.

The printed sheet plots the arrangement on the circular diagram, with the protan, deutan and tritan lines drawn at fixed reference positions. The crossings, their number and their direction are read off that plot. An arrangement with no crossings is within normal limits. Crossings running parallel along one direction indicate protan, deutan or tritan loss, and the clinician records the severity and the axis in the notes.

A scattered plot with no consistent direction is not an axis defect. It more often reflects poor discrimination overall, reduced acuity, or low attention.

When it is used

The D-15 is used after a plate screen suggests a defect, to grade its severity and name its axis. It also serves occupational assessment, where the question is whether a defect is moderate or severe.

It also follows acquired colour vision loss: optic neuropathy, toxic and nutritional causes, drug monitoring such as ethambutol or hydroxychloroquine, and macular disease. As a broad tendency with many exceptions, outer retinal disease shifts the axis towards blue-yellow and optic nerve disease towards red-green. Repeat testing is more informative than one result.

Limitations

Display colour accuracy decides whether this test is valid at all. The caps are defined by their colour. On a display that is not profiled and calibrated, the patient sees colours the test does not specify, and the arrangement they produce says nothing about their hue discrimination. Do not run it on an uncalibrated panel, with automatic brightness, under a blue light mode, or through a filter.

Ambient light is not controlled, and viewing distance is not fixed, so the angular size of the patches varies.

A screen version does not replace the physical D-15 under a standard illuminant. For occupational certification, use the physical set.

The D-15 is a coarse grading. It passes many mild anomalous trichromats, so a pass does not rule out a mild defect. Where finer discrimination is needed, use a desaturated panel or an anomaloscope.

Results stay in the browser on the machine used for the test. They do not reach a patient record, and clearing the browser data removes them.

Validated status means this implementation is considered ready for clinical use on this platform. It does not mean a published study has shown equivalence to the physical test.