Anomaloscope Principles and Clinical Procedures for Diagnosis of Color Vision Defects

The anomaloscope is the only clinical test capable of providing a positive diagnosis of congenital color vision defects.
The instrument utilizes a bipartite field, where the subject views a circular field split into two halves that must be matched for both color and brightness.
The most familiar matching task used in this instrument is the Rayleigh Match.
The Rayleigh Match equation is defined as: $R + G = Y$
Classification of results based on matching behavior:
- A requirement for more green than normal in the mixture indicates a "deutan" defect.
- A requirement for more red than normal in the mixture indicates a "protan" defect.

The NAGEL anomaloscope specifically employs pure spectral lights for the matching task.
A bright white illuminated field is provided for the subject to view between matches to maintain adaptation states.
For optimal testing conditions, it is preferable for the subject to look into a darkened room.

It is critical to recognize that each individual instrument, even those of the same design or model, will exhibit slight variations in performance and calibration.
The instrument features two primary controls for the user to adjust:
- Yellow luminance control: Scaled from $0$ to $87$ .
- Red/Green mixture control: Scaled from $0$ to $73$ , where $0$ represents pure green and $73$ represents pure red.

Step 1: Calibration and Initial Setting
- Set the luminance control to the standard value of $14$ .
- Instruct the subject to turn the Red/Green (R/G) mixture control dial to achieve a color match.
- Note: The examiner does not need to ask the subject to name the color, nor should they specify what color is being matched.
Step 2: Trial Repetition
- Perform $2-3$ matches.
- For each match, the examiner should start by setting the mixture dial to an extreme end of the scale.
Step 3: Range Analysis
- Note the matching range. In normal subjects, this range is typically around $4$ units.
- Determine the mid-point of the subject's matching range.
Step 4: Verification of Anomalous Matches
- Note whether "anomalous" matches are being given serious consideration by the subject.
- Check several specific points within the subject's matching range to confirm both color and luminance matches.
- Confirm whether the subject can match points outside their identified range simply by changing the luminance; if they cannot, the range is confirmed.

Results from the red-green color equation are used to classify individuals into specific categories based on characteristic matches compared to normal trichromats.
Normal Trichromats:
- Classified as "Normal."
- Associated with an Anomalous Quotient (AQ) centered around $1.0$ .
Red-Green Deviant:
- Classified as "Color-Weak."
Simple Anomalous Trichromats:
- PA (Protanomalous): Protan-type anomalous trichromacy.
- DA (Deuteranomalous): Deutan-type anomalous trichromacy.
Extreme Anomalous Trichromats:
- EPA (Extreme Protanomalous): A more severe form of protanomalous vision.
- EDA (Extreme Deuteranomalous): A more severe form of deuteranomalous vision.
Dichromats:
- P (Protanope): Total lack of one red-green photopigment.
- D (Deuteranope): Total lack of one red-green photopigment.
Statistical Parameters:
- Critical Anomalous Quotients (AQ) values include markers at $-3\,SD$ and $+3\,SD$ , and specific thresholds at $0.35$ , $1.0$ , and $3.0$ .

Anomalous trichromats may produce match settings that are highly repeatable (consistent), yet these settings are not located in the "normal" position on the mixture scale.
Their wavelength discrimination for these specific lights typically remains normal at approximately $1\,nm$ .
Conversely, some individuals identified as "normal" trichromats may actually show significant variability in their repeated readings, indicating poor wavelength discrimination despite having a normal midpoint.

Both types of dichromats (Protanopes and Deuteranopes) appear to have the same matching range, which often encompasses the complete range of the instrument (they can match pure red or pure green to the yellow stimulus by adjusting luminance).
Dichromats are distinguished based on their brightness (luminance) matches:
- Protanopes: The red stimulus appears very dark to them. Consequently, they match the red stimulus with a very low intensity yellow.
- Protanopes vs. Greens: They will match the green stimulus at a normal intensity level.
- Deuteranopes: Unlike Protanopes, they do not show this specific darkening of the red end of the spectrum.