Congenital Colour Vision Deficiencies  

 

                        

Types of Colour Deficiencies  

       Colour vision deficiencies are classified as either congenital and acquired.  Congenital deficiencies result from an inborn abnormality of the cone photoreceptors.  Acquired colour deficiencies can occur at any stage after birth and result from a disorder affecting the eye and/or brain.  In either type of deficiency individuals experience colour differently from those with normal trichromatic colour vision (see also component on Theories of Colour Vision).  The three types of congenital colour vision deficiencies  (dichromacy, anomalous trichromacy, and rod monochromacy) are described below.

 

Dichromacy  

      Individuals with a dichromatic deficiency are so named because they are able to match all colours using only two primaries rather than the normal three.  Dichromatics are missing one of the three cone types; it is replaced by one of the other two cone types.  The three types of dichromacy (protanopia, deuteranopia, and tritanopia) are named according the cone photopigment that is missing.  Protanopia and deuteranopia are sex-linked, inherited recessively through a gene located on the X chromosome.  If this deficiency is present males (XY) will manifest the deficiency because they have only one X chromosome.  Because females (XX) have two X chromosomes, they will be colour deficient only if the deficiency is carried on both X chromosomes.  They can pass the condition onto their male offspring without being colour deficient themselves. 

      One method for identifying the colour discrimination abilities of dichromats is to plot on the CIE chromaticity diagram the pair of colours confused.  A line joining the confused pair forms a confusion or isochromatic line. (see CIE diagram on the right).  All colours falling along this line cannot be discriminated by the observer.  Stimuli falling along and intersecting with white on the confusion line will be confused with white.  In other words, the appearance of these wavelengths will be white.  This is referred to as the neutral point.  The colours that are confused and the neutral points for protanopia, deuteranopia, and tritanopia are discussed below and are accompanied by their spectral distributions.

1. Protanopia

      In protanopia the long wavelength (L) photopigment (red or erythrolabe) is absent and is replaced by the medium wavelength (M) photopigment (green or chlorolabe).  Because of this, protanopes confuse red and green.  They see short wavelengths as blue.  As the wavelength is increased, the blue becomes less saturated until at 492 nm, the neutral point, white is perceived.  Wavelengths above the neutral point are seen as yellow.  Approximately 1% of males and .02% of females are protanopes. 

      The effects of a protanope's spectral distribution on colour perception in comparison to a normal trichromat is shown below.

 

2.  Deuteranopia

      Deuteranopes lack the M photopigment which is replaced with the L photopigment.  Like protanopes, they also confuse the colors red and green.  Blue is perceived below the neutral point at 498 nm, and yellow above it.  Notice how the spectral distribution for deuteranopes, as shown below, is similar to the spectral distribution for protanopes (shown above).  You will see that the deuteranopes' neutral point is at a higher wavelength (498 nm vs. 492 nm).  Like protanopia, deuteranopia is a sex-linked deficiency and affects about 1 % of males and .01% of females.        

                                     

      The effects of a deteranope's spectral distribution on colour perception in comparison to a normal trichromat is shown below.

 

3.  Tritanopia

      Although the basis of tritanopia is not clear, it may be due to the absence of the short wavelength (S) photopigment (blue or cyanolabe).  Tritanopes are insensitive to yellows and blues, seeing the world in shades of red and green.  As the diagram below illustrates, the tritanopes neutral point occurs at 570 nm; where green is perceived at shorter wavelengths and red at longer wavelengths.  Tritanopia is a very rare condition, affecting only about .002% of males and .001% of females.

 

      The effects of a tritanope's spectral distribution on colour perception in comparison to a normal trichromat is shown below.

 

Anomalous Trichromacy  

      Like normal trichromats, anomalous trichromats need three wavelengths to match any other wavelength.  But because their spectral sensitivities are somewhat different, they mix the primaries in different proportions.  As for dichromats,  the three types of anomalous trichromacies are named for the wavelengths to which their spectral sensitivity is shifted. 

1.  Protanomaly

      Protanomaly is associated with a defective L photopigment, causing the sensitivity spectrum to be shifted toward medium wavelengths.  Protanomalous trichromats are more likely to confuse red and green than are normal trichromats.  For these individuals, the least saturated point occurs at 492 nm.

2.  Deuteranomaly

      Deuteranomaly is thought to be caused by an abnormal M photopigment.  The absorption spectrum is shifted toward L wavelengths which also results in a confusion between red and green.  The least saturated point on the absorption spectrum is at 498 nm.  Deuteranomaly is the most common of the anomalous trichromacies.

3.  Tritanomaly                                                    

      Tritanomaly is very rare and its basis is unclear.  What is known about protanomaly and deuteranomaly suggests that tritanomaly is caused by defective S photopigment.  It is known that persons affected by this condition have difficulty distinguishing between yellow and blue.

   

Rod Monochromacy

      Rod monochromacy is an extremely rare hereditary condition that is due to the absence of cones in the eye.  Vision is dependent on rod photoreceptors which are not sensitive to colour.  As a result hues are matched solely on the basis of brightness - any wavelength in the spectrum can be matched by adjusting the intensity of any single light source.  Since rods are responsible for seeing in low illuminations, rod monochromats experience glare in bright conditions that is so blinding they seek to avoid it (i.e., photophobia).  Their experience might be like the dazzling blinding glare that you experience from emerging from a darkened theater into bright sunlight.  The absence of cones also means that acuity is very poor - at its best, approximately 20/200.  The absence of cones is also associated with poor fixation, erratic eye movements (nystagmus), visual field defects (namely a central scotoma due to impaired cone functioning), and serious refractive errors (especially myopia).

      The effects of a rod monochromat's spectral distribution on colour perception in comparison to a normal trichromat is shown below.      

Colour Vision Deficiencies and Everyday Life

      Many tasks depend on our ability to discriminate colour.  Selecting products at the grocery store, matching paint colours or items of clothing, or connecting colour-coded wiring all depend on efficient colour vision.  Colour vision deficiencies can seriously affect an individual’s ability to learn, to work at a chosen occupation and move effectively in the world.

      Young children are expected to learn colour names early in their educational experience and colour is frequently used to categorize educational materials.  Good colour vision is also important later on for subjects such as art, chemistry, biology, geology and geography.  A child with deficient colour vision will be disadvantaged on such tasks as colour naming, coding, and matching.  Colour vision testing should be done for all children as early as possible, and certainly prior to starting school (see section below on Testing Colour Vision).

      If a colour deficiency is present, the child's school, teacher, and parents should be informed so that methods of instruction can be modified to meet their visual needs.  Teachers and parents can help in a number of different ways.  First, images and utensils such as crayons, pencil crayons, and pens can be labeled with words or symbols.  Second, discrimination between items of different colour can be facilitated by the use of high luminance contrast.  For example, it would be better to use white chalk on a black or green chalkboard or a dark marker on a white board than combinations that provide less luminance contrast.  The level of illuminance contrast in coloured materials can be determined quite easily by making a black and white photocopy of them or by converting then to black and white on your computer.  Third, children should be taught common objects by their usual colour (e.g., "bananas are normally yellow and the sky blue").     

      Occupations vary in their requirement of colour identification.  For some, good colour judgment is desirable but not necessary.  For others, knowledge of one's colour vision is critical.  Examples where good colour judgment can be critical for careers include a painter, safety officer, dermatologist, pharmacist, cartographer, coroner, chemist, buyer of textiles, food inspector, electrician, and marine navigator.  Colour perception failures in such jobs could be costly, even disastrous.

      Colour is used extensively for communicating safety information.  Because colour has a real "pop-out" effect for those with good colour vision, colour-coded targets are easily noticed and quickly detected.  Notice the picture on the left.  The red area really stands out from the green background.  This feature makes colour effective for coding many types of safety information.  Red is often used to symbolize fire, danger, or stop, yellow, amber and blue to indicate the need for caution, green to signal safety, orange to communicate potential danger, and purple to warn about radiation hazards.  Such colour coding can be seen in hazard warnings, signals and flags, coding of the contents of pipes, cylinders, gas, and medical containers, and electric cables and wires, just to name a few.  Some examples of colour coding are shown below.  Problems can arise when such coding is inconsistent or absent, and also when the individual can't distinguish colour effectively.  For example, a protanope or deuteranope who sees only in shades of blue and yellow could experience problems with red-green coding.  Therefore, it may be helpful to also include luminance contrast in the same materials. 




 

Enhancing Colour Deficient Performance with Filters

      The colour performance of those with colour vision anomalies can be sometimes enhanced using coloured filters.  By absorbing wavelengths selectively, these filters help the observer to differentiate stimuli based on their relative brightness.  For example, a red object viewed through a green filter or a green object viewed through a red filter will appear much darker.  For example the X-chrom lens is a red contact lens worn on one eye that absorbs shorter wavelengths and passes longer ones.  By comparing the relative brightness in eye with the X-chrom lens to that in the eye without it, a dichromat's ability to distinguish red from green can be enhanced.  While such monocular comparisons may be useful in specific applications, the user remains a dichromat and is unlikely to find the approach practical for everyday use.

 

Testing Colour Vision

      Colour vision tests to identify colour vision anomalies vary greatly in complexity, time to complete, and the colour vision problems they detect.  Four such tests are described below: colour matching, pseudoisochromatic plates, colour arrangement, and the anomaloscope.  

1.  Colour Matching Tests

      Colour matching involves the adjustment of one to three light sources so that the test stimuli produced matches the colour appearance of an adjacent comparison stimulus.  This is usually done by having the observer adjust the light falling on one-half of a “bipartite” display to match the colour of the "comparison" half of the display.  The number of wavelengths as well as the proportion of each needed to match the comparison stimulus indicates the type of colour deficiency, if any.  For example, a rod monochromat could match all comparison colours using only one of the three wavelengths.  A dichromat will need two wavelengths to match any colour; an anomalous trichromat will need three wavelengths, but mixed in different proportion than those used by a normal trichromat.  

2.  Pseudoisochromatic Plate Tests

      Pseudoisochromatic plate tests are the most commonly used tests of colour vision because they are inexpensive, simple and quick to administer.  The test plates consist of a coloured number or a visual path embedded in a background that is the same brightness but different in overall colour.  The colours of the figure and background fall along a dichromatic confusion line. A person with normal colour vision can correctly identify the figure based on colour differences whereas an individual with dichromatic vision cannot.  The most extensively used pseudoisochromatic test is the Ishihara test (see sample plate on the right).  For young children and others who cannot read, tracing paths are used instead of numbers.  A limitation of pseudoisochromatic plates is that they cannot adequately differentiate dichromatic from anomalous trichromatic vision.  





3.  Colour Arrangement Tests

      On colour arrangement tests, the observer arranges numbered colour chips in order of similarity, starting from a fixed reference chip.  For a respondent with normal colour vision, a line joining the chips in order of increasing number would follow the circumference of the hue circle, from blue through green, yellow, orange and red.  Colour deficient arrangements result in crossovers on the hue circle; the orientation of the axis of the crossover (i.e., deutan, tritan or protan) indicates the type of colour deficiency.  The colour arrangement space for normal trichromatic vision, protanopia, deuteranopia, and tritanopia are shown below.  The most common colour arrangement tests for those with congenital deficiencies are the Farnsworth-Munsell 100-Hue Test and Farnsworth Dichotomous Test (D-15).  For those with more subtle acquired deficiencies, the Lanthony’s Desaturated 15-Hue Test can be used.

 

 

1.  Farnsworth-Munsell 100-Hue Test

     The 100-hue test consists of 85 chips, divided into four sets.  The test is both time-consuming and expensive.  Another drawback is that the 100-hue test cannot determine dichromacies from anomalous trichromacies.

      2.  Farnsworth Dichotomous (D-15) Test  and Lanthony’s Desaturated 15-Hue Test

     The D-15 and the Desaturated D-15 both consist of a set of 15 chips that are arranged in order of similarity beginning with a reference chip. The D-15 is useful for detecting dichromacys, in particular, tritan defects which are often associated with eye disease and drug toxicity.  The disadvantage with this measurement is that minor defects are not detected.  The Lanthony desaturated D-15 is similar to the Farnsworth D-15 except that the colour on chips is much less saturated.  This makes the hue circle smaller and the arrangement task more difficult.  It is especially useful for detecting subtle acquired colour deficiencies.   

3.  The Anomaloscope Test

      The anomaloscope is an instrument used in the diagnosis of red-green deficiencies.  The Nagal anomaloscope, the most common of these devices, assesses the observer’s ability to make a specific colour match.  The observer looks into the anomaloscope via an eyepiece, to view a bipartite colour field.  A mixture field composed of red and green wavelengths is presented in the top half of the display, the bottom half of the test field is yellow.  The observer’s task is to adjust the mixture field to match the colour of the test field.  A major advantage of the Nagal anomaloscope is that it can distinguish between dichromatic and anomalous trichromatic vision by measuring the balance of red and green wavelengths in the mixture field.    

 

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