Pseudoisochromatic plates are a globally accepted and efficient method for the initial screening of color vision deficiencies, commonly known as color blindness. This standardized testing system provides a rapid way to distinguish individuals with normal color perception from those who struggle to differentiate specific colors. The simplicity of the test makes it a practical tool in various settings, including routine eye examinations, school screenings, and occupational health assessments.
Defining the Plates: Design and Purpose
A pseudoisochromatic plate is a printed card covered in a pattern of small, randomly sized, and varying-shade colored dots. The name itself is derived from Greek roots, meaning “false-same-color,” which describes the illusion created for individuals with impaired color vision. Within the background of multi-hued dots, a specific symbol, number, or path is embedded, composed of dots of a different color. The arrangement is constructed so that the figure and background colors appear different to someone with normal vision. For a person with a color vision deficiency, however, the target symbol’s color blends indistinguishably with the background colors.
The Underlying Mechanism of Color Confusion
The effectiveness of pseudoisochromatic plates relies on the biology of the retina, specifically the cone cells responsible for daytime color vision. Normal color vision, known as trichromacy, depends on three types of cone cells, each sensitive to long (red), medium (green), or short (blue) wavelengths of light. Color vision deficiencies arise when one or more of these photopigments are missing or function abnormally.
The plates are designed to target the most common deficiency: the inability to distinguish between red and green hues. The dots used for the figure and the background are carefully selected to lie along a “confusion line” in the color spectrum. This means the colors have similar perceived brightness, or luminance, but possess different hues, or chrominance, to a person with normal vision.
Individuals with a red-green defect struggle to perceive the chrominance difference because their affected cone type cannot properly discriminate between the wavelengths. Since the plates minimize the luminance contrast between the figure and the background, the color-deficient person loses the primary cue for separation, causing the symbol to vanish.
Administering and Interpreting Standardized Tests
The practical application of pseudoisochromatic plates is achieved through standardized test systems, the most recognized being the Ishihara test and the Hardy-Rand-Rittler (HRR) test. Test administration requires strict control over the viewing environment to ensure accurate results, typically demanding illumination that closely simulates natural daylight. The plate is presented to the patient at a specific distance, and they are usually given only a few seconds to identify the embedded figure.
Common Test Systems
The Ishihara test primarily screens for congenital red-green defects. It uses plates that include vanishing figures, which are only seen by those with normal vision, and transformation figures, which appear as different numbers to normal and color-deficient individuals. The HRR test offers broader screening, as it also includes plates designed to detect blue-yellow color deficiencies, known as tritan defects. The HRR often uses geometric shapes, which is beneficial for testing non-verbal patients or young children.
Interpretation of Results
Interpretation involves classifying the type and severity of the defect based on the pattern of errors. For red-green defects, the test can differentiate between protan-type deficiencies (long-wavelength cones affected) and deutan-type deficiencies (medium-wavelength cones affected). The specific plates missed determines whether the patient has a complete absence of the cone type (dichromacy, such as protanopia or deuteranopia) or an anomalous function (anomalous trichromacy). The total number of errors made on progressively difficult plates helps to grade the severity of the deficiency.