Colour Vision

• • light is a small part of the electromagnetic spectrum with the human eye detecting wavelengths between 380 and 780 nanometres
  • all visible hues are seen by the combination of three colours taken from long red wavelengths, medium green wavelengths and short blue wavelengths
  • newtons prism experiment showed that white light can be dispersed through a prism and can be recomposed with a second prism
• young-Helmholtz theory or trichromatic theory states that each point of the retina could be put in motion more or less by three primary colours
• opponent colour theory states that there are 4 primary colours (red, green, blue and yellow and they are arranged into opponent pairs which are red vs green and blue vs yellow. there is also a luminance mechanism which is black vs white
• • zone theory is the basis of the modern colour vision theory
  • zone theory states that colour vision is a combination of vision being trichromatic and mediated by cones. colour vision is also coded into opponent channels by ganglion cells level and the luminance channel derives input from long and medium wavelength channels and a small input from a short wavelength channels
• • colour vision is formed from three classess of cone photopigments and is trichromatic at the receptor level
  • electrical signals from the three cone types are coded in the plexiform layers of the retina so that the three opponent signals can be detected at the ganglion cells. the horizontal and muller cells connect receptors and ganglion cells laterally within the plexiform layers and give rise to receptor fields and colour adaptation effects
• • the retina contains two types of photoreceptor: rods and cones
  • rods function at low levels of light and cones at high levels of light
  • rods are not present in the fovea and cones populate the central 2 degrees of view. Both reduce in density at the peripheral retina
  • cones sensitivity diminishes if light enters the eye obliquely, monochromatic light appears to change in hue and saturation but rods have no directional sensitivity
• • the proportion of cones in the central fovea is 40:20:1 (red:green:blue) and near the fovea the red:green ratio ranges from 1:2:1 to 3:9:1
  • due to the unequal representation of the three pigments blue receptors are almost absent from the central fovea giving rise to small field tritanopia if colour matches are made within the central area subtending less than 0.5 degrees. so the field of view is very important in colour vision exams. the field size of 2 degrees ensures that very few rod receptors are stimulated
• Red green colour vision remains stable throughout life but blue threshold sensitivity decreases with age because of the greater absorption of shorter wavelengths within the crystalline lens. differences in macula pigment and retinal lutein levels also lead to variation in blue perception in people with normal colour vision
• Congenital colour vision deficiencies are caused by abnormal cone pigments in the retina.
• Abnormal absorption characteristics in the retina can be caused by a lack of photopigments.
• Trichromacy is when all cones and pigments are present and functioning.
• Anomalous trichromacy is further classified into protanomaly if there is an abnormal red long pigment, deuteranomaly if there is an abnormal medium green pigment, and tritanomaly if there is an abnormal short blue pigment.
• Dichromacy is when two pigments are present, but the amount absorbed by the two photopigments is different, limiting the distinction between photons by wavelength.
• Dichromacy is further classified into protanope if the long red pigment is missing, deuteranope if the medium green wavelength is missing, and a tritanope if the short blue pigment is missing.
• Monochromacy is when no cones or only one cone is functioning, meaning monochromats are unable to distinguish between objects on the basis of wavelength alone.
• congenital colour vision is a recessive x chromosome linked trait meaning the mother carries the defective gene but usually has normal vision. Most common is deuteranomaly 5% of all males. particularly important to screen young boys, doesn’t take place in school
• acquired colour defects can be ocular or systemic and can be caused by certain drugs like hydroxychloroquine in excessive amounts or in an allergic response. the drug is used to treat malaria, rheumatoid arthritis or systemic lupus erythematosus (LSE). acquired colour defects are associated with a loss of visual acuity and visual fields defects and monocular differences frequently occur
• • Acquired deficiency 1: red - green similar to protan defect and found in macular dystrophy
  • Acquired deficiency 2: red - green similar to deutan defect and found in retobulbar neurtitis
  • Acquired deficiency 3: blue similar to tritan defect and is found in many central and peripheral retinal lesions of the visual pathway like senile macular degeneration, central serous retinopathy, retinitis pigmentosa, diabetic retinopathy
• Acquired defects Often show a combination of characteristics associated with more than one type. Can be both dichromatic and anomalous trichromatic. Blue is vulnerable to acquired defects and Onset often insidious.
• An acquired Colour vision defect can be an early indication of pathology if symptomatic. Can be used to monitor pathology progression
• • Ishihara Standard test is the 38 plate version 24 plate abridged version most often found in practice
  • Ishihara Does not aim to grade severity it is a pass or fail test. Number of plates misidentified does not indicate severity of deficiency. Screening plates use to detect defect. Classification plates indicate type of defect. Studies show that six mistakes or more are made by colour deficiencies
• • Ishihara transformation plates: normal person sees one number and colour blind sees another
  • ishihara vanishing plates: normal sees a number and colour blind doesn’t
  • Ishihara appearing or hidden digit card is only seen by the colour defective
  • Ishihara classification plates are only used if screening plates are failed
• • Ishihara is disadvantageous because The efficiency of the screening plates is increased if the hidden digit plates are excluded from the assessment, No tritan detecting ability, doesn’t diagnose severity and Severity is the most important factor for vocational guidance
  • Screening tests are very sensitive and even mild colour vision defects can result in failure on several
• The city university test was designed to overcome some of the disadvantages experienced with the D-15, such as soiling of the colour caps and difficulties with the concept of sequencing the colours.
• The first and second editions of the city university test feature five coloured targets per plate, subtending 1.5 degrees at the test distance of 35 cm.
• Three of the surrounding colours in the first and second editions of the city university test represent the average isochromatic confusions of protanopes, deutanopes and tritanopes, and the remaining colour is the most similar to the central colour.
• Changes were made to the coloured elements for the third edition of the city university test.
• The task of detecting colours which differ from nearby colours was introduced in the third edition of the city university test.
• The third edition of the city university test has two sections, which may be used in sequence or separately.
• D - 15: An arrangement test, 50 cm viewing distance. 15 coloured caps to be placed in sequence by the patient. Clinician positions pilot cap first and patient positions the others and Order of caps is revealed by numbers on reverse side. More traverses across the circle the greater the severity of the colour vision defect
• • The aim of the D15 test is to divide patients into two groups

  1. Normal or slight colour defect
  2. Moderate or severe colour defect

  • This allows people with slight defects to work with colours when safety is not an issue
• • 100 hue test Mainly used in the hospital eye service. 85 coloured buttons as 100 hue too difficult
  • 100 hue test Takes time to conduct and to analyse the results. Some computerised versions with software analysis of results. Results obtained in the form of a score and also a polar diagram. Confusion lines relate to type of defect
  • Protan- errors 15 to 26 deg and 58 to 68 deg
  • Deutan- errors 12 to 17 deg and 53 to 60 deg
  • Tritan- errors 4 to 6 deg and 45 to 46 deg
• • Test each eye in turn and look for asymmetry in results
  • Use a test that detects yellow/ blue loss as well as red/ green loss
  • For example, loss of blue yellow discrimination is an early sign of glaucoma…this rules out the Ishihara
• • Conventional clinical assessments of VA are related to the eye’s resolving power
  • Alternative method of assessment is based on the eye’s sensitivity to luminance contrast
• • Patient shown grating of a certain spatial frequency, the contrast is adjusted until bars can only just be seen (i.e. contrast threshold)
  • gratings used as clinical test targets do not necessarily have uniform spatial frequency. Targets may vary logarithmically
• Clinically, if a sinusoidal grating is presented to an eye, its threshold of recognition as a grating is affected by both its spatial frequency and its luminance contrast (modulation) and the contrast sensitivity function will give information about a patients vision over a range of spatial frequencies and contrasts
• VISTECH has circular grating patches of 5 rows and 9 columns. the gratings have three orientations of vertical 90 degrees, left 105 degrees and right 75 degrees and some blank circles. Alternate forced choice is used and plotted before being compared with normal values
• constant contrast charts, variable spatial frequencies are like bailey - Lovie charts and extremely illuminated charts
• Pelli - Robson charts have a constant spatial frequency at 1m working distance, you must add +1.00DS for an older presbyopic patient. they also have variable contrast with contrast decreasing in 0.15 Log steps. it is externally illuminated
• To score on the pelli- robson chart you must score the last triplet where 2 of the three letters were correct or you can do letter by letter scoring with each letter equalling 0.05Log improving reliability. you can also have a working distance of 3m