Generality
The tables of Ishihara allow to assess the presence of visual alterations, as regards the perception of colors.
The test is composed of a sequence of pseudo-isochromatic tables, in which a matrix of points of variable size, defines a number or symbol (star, circle or triangle), easily recognizable to people with a normal color perception. On the contrary, color-blind or color-blinded patients cannot see some or all of the figures hidden within the table.
The Ishihara test is commonly used for the diagnosis of color blindness and to define the extent of defects in color sensitivity.
What are
The interpretation of Ishihara tables is a rapid test that allows us to highlight the deficient vision of color.
This assessment involves the use of 38 pseudo-isochromatic tables, contained in sequence in a book, specifically designed for screening and diagnosis of dyschromatopsias ; the Ishihara test consists of recognizing numbers that tend to blend in with the colored background or, for children, identifying paths or symbols.
In the presence of certain pathological conditions (color blindness or acquired deficits of color vision), the set of colored dots may not be displayed or be confused with each other.
The tables of Ishihara are valid above all to diagnose the congenital defects (ie present from birth) of the color vision of the red-green axis .
Color vision and color blindness
- Color blindness is a visual deficit associated with a functional anomaly of the cones. When one or more types of these specialized retinal cells are defective, perception and color discrimination is altered.
- The ability to perceive colors is based, in particular, on the presence of three types of cones, which respond to particular wavelengths in the field of visible light . In these photoreceptors, in fact, there are three types of proteins (opsins), which are respectively sensitive to a stimulus of about 420 nm ( blue spectrum), 530 nm ( green ) and 560 nm ( red ).
- The trichromatic vision and the distinction of the various shades derives from the activation of the three types of cones, in various combinations and percentages, in response to the radiation emitted by the observed object. The contemporary and maximum stimulus of the cones provides the perception of white.
What are they for
The Ishihara boards are a verification test for dyschromatopsies, so they are submitted to the patient to evaluate if he sees the colors correctly. This exam allows to detect and diagnose any defects in color perception (congenital or acquired), establishing the characteristics (which colors are not defined by sight).
Did you know that…
The interpretation of Ishihara tables is the most commonly used test for color blinds in clinical practice. This exam is useful for screening and qualitative investigations of defects in color perception.
Ishihara's series of tables is designed to perform a quick color recognition test .
More in detail, numbers or symbols are hidden within a colored background (often consisting of beige-green dots). These images are easily visible for those who have a normal chromatic sensitivity, while color-blind or acquired color vision deficiency patients fail or are unable to see some or all of the hidden figures. A table attached to the Ishihara tables allows the identification of the type of chromatic alteration possibly present.
When is Ishihara tested?
The tables of Ishihara are among the so-called color blind tests .
These non-invasive examinations are performed when it is assumed that the patient suffers from an altered vision of colors, absolute or partial.
The tables are made with a figure and background of different colors, but with the same brightness, therefore not recognizable if observed by the subjects presenting dyschromatopsias.
Curiosity: why is it called that?
The Ishihara test owes its name to the creator of the tables, namely the Japanese doctor Shinobu Ishihara . In principle, the evaluation was reserved for the exclusive use of the army (reason for which the examination was created): the test was submitted to military recruits, in order to detect the presence of anomalies in color vision. The early versions of the Ishihara plates were initially based on the interpretation of the Japanese hiragana and katakana characters.
The re-release of the test (with Arabic numerals and instructions in English) for diffusion in different parts of the world took place, then, in 1917.
Who does the test run from?
The tables of Ishihara are submitted to the patient during an eye examination . The specialist doctor is able to interpret the results, confirm the existence of the problem and establish the correct diagnosis.
Associated examinations
To further investigate the anomalies of the chromatic sense, in addition to the tables of Ishihara, the patient undergoes the Farnsworth test . This consists in sequentially ordering, in the correct tonal succession, a series of diskettes or other objects of the same brightness and color saturation.
The interpretation of the Ishihara tables can be supplemented by the HRR test, useful for investigating the defects in the blue-yellow axis.
Causes
An altered perception for a given color (dyschromatopsia) may be congenital or acquired, ie secondary to other pathologies.
The tables of Ishihara help to diagnose the following conditions:
- Color blindness : a hereditary visual anomaly that involves an altered perception of colors. Color blindness is caused by a genetic defect responsible for the dysfunction of the retinal cones. In most cases, the disorder manifests itself as a deficit in the perception of red and green. In children, color blindness can be suspected when they start drawing, using some colors abnormally. Sometimes, however, the defect goes unnoticed and is detected only after a normal inspection by the ophthalmologist.
- Other forms of dyschromatopsia : different pathologies can lead to deficiencies in chromatic vision. This means that the abnormality may depend on disorders such as retinopathies, optic neuropathies, head trauma or stroke with the involvement of visual centers. In patients suffering from a form of acquired dyschromatopsia, the Ishihara tables make it possible to ascertain which shades are not able to be defined, the severity of the disorder and how the perception of colors can influence everyday life.
How to do it
The Ishihara test involves submitting to the patient the 38 tables contained in a book of pages with a black or white background. These boards should be read at a reading distance of 30-40 cm and with adequate lighting.
The examined subject must recognize numbers, letters or signs, made with the use of circles of different shades, which are evident to those who have a normal chromatic sense, but difficult or impossible to recognize for those who do not distinguish the colors well.
The tables of Ishihara are presented one by one to the subject and the ophthalmologist notes the answers.
The patient who suffers from color blindness or other forms of dyschromatopsia will have, depending on the severity of his disorder, minor or more serious difficulties in recognizing primary colors and their nuances.
With children, the same type of test is used with a trick: in the Ishihara tables, the numbers are replaced with a drawing (eg star, circle, etc.) or a path to follow with the finger . In the case of a chromatic anomaly, the "path" to be recognized on a background of circles is indistinguishable.
To complete the Ishihara test, it is sufficient to present to the patient other tables in which only those with dyschromatopsia recognize an image, while normal subjects do not see anything, except a disordered set of colored circles.
The tables of Ishihara are specially graduated, therefore they allow to assess the extent of the disorder: some shades are not completely invisible to the eyes of those affected by dyschromatopsia, but are only distinguished if they are very bright and saturated. In this regard it should be noted that the anomalies of the chromatic sense are more frequently only partial, than absolute.
Interpretation of results
For the purpose of the Ishihara test, the interpretation "key" attached to the tables allows the ophthalmologist to identify the errors committed by the patient and to make the related diagnosis.
- To those who have a normal chromatic sense, the numbers and letters in each Ishihara table are evident;
- For subjects with a deficit in color perception, however, recognizing images in Ishihara tables is difficult or impossible.
The Ishihara test can be performed either in mono or binocular vision, in consideration of the fact that:
- Congenital defects, such as color blindness, are always binocular;
- Acquired defects can be monocular.
Reading of the Ishihara tables: notes
- Table 1 : contains the number 12 and is not pseudoisochromatic; in practice, this table by Ishihara is demonstrative and is also read by those who see colors badly.
- Tables 2-17 : the symbols are not read or create confusion in those who present anomalies of the color vision in the red-green axis (as in the case of color blindness).
- Tables 18-21 : numbers are not contained, which are read, instead, by those who present a red-green deficit.
- Tables 22-25 : they allow to distinguish the true blindness for a color (protanopia or deuteranopia depending on whether it is red or green) from partial blindness (protanomalia or deuteranomalia).
- Tables 27-38 : numbers and letters are replaced by easy-to-read routes or symbols for children.
Limits of the tables of Ishihara
The Ishihara test is ideal for rapid identification of congenital defects in the red-green axis . However, this evaluation does not provide complete information on the chromatic-perception deficits affecting the blue-yellow axis . Therefore, the Ishihara tables contribute to knowing the problem, but the results should be supplemented with other tests for colorblinds.
