The anomaloscope is considered the most accurate method of classifying congenital red-green color defects. For this test, the patient views a split screen and is asked to match the yellow appearance of one half by mixing different proportions of red and green light on the other half of the screen (see image below). It is also called the "Nagel anomaloscope" and makes use of the "Rayleigh equation" in testing for color deficiency.
The pseudoisochromatic plates (e.g. Ishihara or Hardy-Rand-Rittler) are typically the most common devices used to screen for color-deficient people. They consist of a series of colored numbers or figures that stand out from a background of colored dots. Color vision patients either see no pattern or an alternative pattern based on brightness rather than hue. Though good as quick screening tests, they cannot be used to classify the type of color deficiency.
The Farnswell D-15 and 100 are much more accurate in classifying color deficiency states. These tests consists of arranging colored tablets sequentially according to their varying hues. As their names imply, the Farnswell 100 test consists of arranging 100 different colored tablets while the D-15 consists of arranging 15 tablets. As you might expect, the D-15 is much easier to take as a patient.
Neutral density filters reduce intensity of all wavelengths or colors of light equally, giving no changes in hue. They are not used to determine color deficiency. Instead, they find applications in quantifying the level of an afferent pupillary defect, assessing the presence of amblyopia, and as a type of occlusion therapy for amblyopia.
