The anomaloscope is a precise test device that quantitatively determines the type and severity of color vision deficiency by using color-light mixing and monochromatic light color matching. It is used when an accurate type diagnosis is needed or when the presence of color vision deficiency must be confirmed.
The main purposes of the anomaloscope are the following three points.
In 1907, Willibald Nagel of Germany developed the Nagel anomaloscope based on the principle of Rayleigh matching. Since then, it has been used as the gold standard for the definitive diagnosis of color vision deficiency. Because congenital red-green color vision deficiency reflects abnormalities in the cones involved in distinguishing red and green light (L cones and M cones), the Nagel type, which adjusts the same wavelength range, is considered optimal for definitive diagnosis.
A stepwise testing flow is used from screening to definitive diagnosis of color vision deficiency.
| Step | Test | Purpose |
|---|---|---|
| 1 | Ishihara pseudoisochromatic plates | Screening (detecting whether color vision deficiency is present) |
| 2 | Panel D-15 | Severity assessment (rough evaluation of severe, moderate, and mild) |
| 3 | anomaloscope | Definitive diagnosis and precise type classification |
The anomaloscope is not suitable for screening, but for confirming the type and quantitatively assessing severity, it has unmatched accuracy among color vision tests.
After color vision abnormality is suspected with the Ishihara test or D-15, it is indicated when precise distinction between type 1 and type 2 is needed, or when deciding whether it is dichromacy or anomalous trichromacy. It is also an essential test when precise color vision evaluation is required for legal or occupational reasons, such as for airline pilots and train drivers, and when differentiating acquired and congenital color vision abnormalities is necessary.
The main indications for an anomaloscope are as follows.
The characteristics of each color vision test are shown below.
| Test | Screening | Type diagnosis | Severity assessment | Notes |
|---|---|---|---|---|
| Ishihara pseudoisochromatic plates | ◎ | △ | × | For school health checks and outpatient screening |
| Panel D-15 | ○ | ○ | ○ (mild to moderate) | Useful for rough assessment of severity |
| 100-hue test | ○ | ○ | ◎ | Excellent for detailed severity assessment |
| Anomaloscope | × | ◎ (most reliable) | ◎ | Gold standard for definitive diagnosis |
Because an anomaloscope takes time to perform and the device is limited to specialized facilities, it is not suitable for screening purposes3).
An anomaloscope is based on the principle of color matching: the person being tested adjusts the light-mixing ratio until the colors match (equal color).
The eyepiece of the Nagel-type anomaloscope has a circular field of view divided into two halves.
The person being tested looks for the setting where both sides appear the same color and equally bright while changing the red/green ratio. The matching point (equivalence point) and its range (equivalence range) are recorded.
Rayleigh match (red-green match)
Target: congenital red-green color vision deficiency (types 1 and 2)
Light source: yellow light (589 nm) vs. mixed red (670 nm) + green (546 nm) light
Device: Nagel-type anomaloscope
Principle: Matching between the red/green mixing ratio and yellow light. If the sensitivity ratio of the L and M cones differs from normal, the matching point and matching range change.
Moreland matching (blue-green matching)
Target: Congenital blue-yellow color vision deficiency (type 3 color vision)
Light source: blue-green monochromatic light vs mixed light of blue light + green light
Device: an extended device compatible with Moreland matching
Principle: Matching between the blue/green mixing ratio and blue-green light. Reflects abnormal S-cone sensitivity. With the Nagel type, type 3 color vision cannot be evaluated.
Rayleigh matching is a color-matching method that matches the brightness and color of yellow monochromatic light (589 nm) with red-green mixed light. In normal color vision, matching is achieved only at a certain red/green ratio, but if there is an abnormality in the L or M cones, the range over which matching is achieved becomes much wider. In dichromacy, matching is achieved across the entire mixing-ratio range, and in anomalous trichromacy, the matching range is wide but finite. By quantifying these differences, the type and degree of color vision can be assessed numerically.
In people with normal color vision, color matching is achieved only within a narrow range around 1:1 (green:red). When color vision is abnormal, the matching range widens, and in dichromacy, matching is possible across the entire range.
The anomaloscope findings for each color-vision type are shown below.
| Color vision type | Matching range (Rayleigh match) | Luminance adjustment | Judgment |
|---|---|---|---|
| Normal color vision | Narrow range near 1:1 | Slight | Normal |
| Type 1 dichromacy (protanopia) | Matches across the entire range (0–73) with red only | Dim the red light | Type 1 dichromacy |
| Type 1 anomalous trichromacy (protanomaly) | A broad range toward red | Dim the red light slightly | Type 1 anomalous trichromacy |
| Type 2 dichromacy (deuteranopia) | Matches across the entire range with green only | Almost no brightness adjustment | Type 2 dichromacy |
| Type 2 anomalous trichromacy (deuteranomaly) | Wide range shifted toward green | Slight luminance adjustment | Type 2 anomalous trichromacy |
A characteristic of type 1 color vision deficiency is that, because of absent or reduced sensitivity of the L cone, the relative visual sensitivity of red light (perceived luminance) is decreased, so a luminance adjustment that makes red light darker occurs during color matching. Whether this luminance adjustment is present is the most important distinguishing point between type 1 and type 2.
In dichromacy, the matching range extends across the full scale (0–73), whereas in anomalous trichromacy the matching range is wider than normal but does not extend across the full range. The degree can be assessed by whether the anomalous trichromacy matching range includes the normal matching point4).
The most important distinguishing point is the difference in luminance adjustment (relative visual sensitivity). In type 1 (protan type), abnormal sensitivity of the L cone makes red light seem darker, so an adjustment that lowers the luminance of red light occurs during color matching. In type 2 (deutan type), abnormal sensitivity of the M cone has little effect on luminance perception, so color matching is achieved with almost no need for luminance adjustment. In addition, the bias of the matching range also differs: type 1 tends to be shifted toward the red side, and type 2 toward the green side.
Some occupations have legal standards related to color vision, and precise type classification is required. Examples include airline pilots, train drivers, ship operators, police officers, and Self-Defense Force personnel2). For these occupations, screening tests such as Ishihara plates alone are not sufficient, and numerical evaluation of the matching range with an anomaloscope may be necessary.
In acquired color vision deficiency (caused by optic nerve disease, macular disease, drug-induced color vision deficiency, and similar causes), the fact that the color-matching range changes over time is an important point that distinguishes it from congenital color vision deficiency. Congenital color vision deficiency shows a stable color-matching range throughout life, whereas in acquired color vision deficiency the color-matching range changes with the activity of the underlying disease5). For this reason, repeated anomaloscope testing is useful in suspected cases of acquired color vision deficiency.
Accurately recording the type and severity of congenital red-green color vision deficiency is useful for genetic counseling based on the pattern of X-linked recessive inheritance. Some carrier women show a slight widening of the color-matching range, and detailed evaluation with an anomaloscope can sometimes help support carrier diagnosis6).
Because the equipment is expensive and requires skill to use, it is limited to university hospitals and specialized ophthalmology facilities. Many general eye clinics do not have an anomaloscope.
The global prevalence of congenital red-green color vision deficiency is about 8% in men and about 0.5% in women, with differences among populations1). Prevalence varies by ethnicity and region, and in Japanese men about 5% and women about 0.2% have been reported. Because of this high prevalence, it is considered important to establish appropriate color vision testing systems in school health screenings and pre-employment examinations7).
Conventional optical anomaloscopes use halogen lamps and interference filters, but in recent years development has advanced for LED- and monitor-based digital anomaloscopes3). Digitalization is expected to improve portability and make it possible to perform the test outside specialist facilities.
Research is being conducted on simple color matching tests using the display of smart devices. However, because they are affected by the display’s color reproduction characteristics, calibration, and ambient lighting, they are not yet a substitute for the Nagel-type anomaloscope.
Combining genotype analysis of the L and M genes by next-generation sequencing with phenotypic evaluation using an anomaloscope is advancing research to precisely analyze the relationship between the types of hybrid genes and the color matching range6). Clarifying the correspondence between genotype and phenotype is expected to help improve the accuracy of genetic counseling.
