## 1 Introduction
Alterations in the genetic coding of these photopigments result in colour vision deficiencies. Colour vision defects are generally classified as congenital or acquired [19; 24]. Congenital disabilities arise from genetic alterations of photopigments and are present from birth, remaining stable throughout life. These defects are binocular and predominantly affect M and L mechanisms, leading to red and green deficiencies [25]. Due to their X-linked inheritance, congenital disabilities are more common in males (8%) than in females (0.5%) [26]. Acquired colour vision defects typically affect the S mechanism, leading to blue-yellow deficiencies [27]. These defects can develop due to external factors and are not sex-dependent. They can vary over time and manifest monocularly or binocularly. Factors causing acquired colour vision deficiencies include diseases like Parkinson's [28; 29; 30], Alzheimer's [31], diabetes [32], immune system infections [33], retinal damage, antibiotic use, hypertension medications, optic neuropathies [34], exposure to industrial chemicals [35], and hypoxic environments [36]. Given the diverse aetiology of acquired colour vision deficiencies, it is plausible that COVID-19 infection and prolonged exposure to digital screens could increase their occurrence. To identify and classify congenital and acquired vision deficiencies, various colour vision tests are available. These tests can be categorised into three groups: discrimination tests, matching tests, and detection tests [19]. Discrimination tests include:  
Pseudoisochromatic Plates Tests: These use sets of plates with coloured figures that are perceived differently depending on the colour vision defect. They detect and differentiate various congenital colour vision deficiencies. Arrangement or Hue Discrimination Tests: These involve arranging colours in a gradual progression, detecting both congenital and acquired colour vision defects, and assessing a subject's ability to discriminate tones across different spectral ranges. To assess congenital disabilities, the most commonly used test is the Ishihara-32 Plates Test [37; 38; 39]. This test accurately identifies congenital colour vision deficiencies, detecting total weakness and red-green deficiencies. Normal subjects see the indicated figure, while colour-deficient individuals see a different figure, identifying Proton (red deficiency) and Deutran (green deficiency) subjects. For detecting acquired colour vision deficiencies and assessing chromatic discrimination ability, the Farnsworth-Munsell 100 Hue Test is highly recommended [19; 40]. This test consists of 93 Munsell colour samples arranged in a sequence. Subjects arrange the caps by hue between fixed reference samples. The Total Error Score (TES) and its derivatives are used to evaluate outcomes [41]. The Square Root of the Total Error Score (\(\sqrt{\text{TES}}\)) provides a normal distribution of errors. A radial diagram chart represents individual cap scores [42]. \(\sqrt{\text{TES}}\) values categorise colour discrimination ability as Superior (0-16), Average (20-100), and Low (>100) [43]. The orientation of axes in the radial diagram determines the type (Protan, Deutran, or Triton) and severity of the deficiency. This study focused on analysing acquired colour vision deficiencies caused by prolonged screen use or COVID-19 infection, selecting only normal trichromats (without congenital disabilities). The Ishihara-32 Plates Test was used to exclude subjects with congenital colour vision defects, and the Farnsworth-Munsell 100 Hue Test was employed to detect acquired deficiencies. A literature review of previous scientific studies on colour vision and international standards for colour vision assessment informed the selection of these tests. ASTM Standard E1499-16 provides robust procedures for selecting and evaluating observers, recommending the Ishihara Test for detecting congenital vision defects and the Farnsworth-Munsell 100 Hue Test for detailed colour discrimination measurement.