As a human, one of the first things that newborn babies experience when they get through a mother’s womb is facing the light. Light is one of the most important sources of life because it allows plants to photosynthesize in order to make their own energy source. In addition, light can also trigger proteins in the human body to develop vitamin D. Lack of vitamin D can affect a human’s health. Most importantly, light provides vision to not only humans but to many other multicellular organisms.
Light acts like particles and behaves like an energy wave (Brynie, 2009). The highest peaks on the waves are called crests and the distance between the peaks can vary which is called a wavelength (Thomas, 2011; Brynie, 2009). The sun is the primary source of light on earth and there are various types of light which make up the electromagnetic spectrum. The electromagnetic spectrum consists of many different wavelengths. They are in ascending order from the shortest to the longest wavelength, cosmic rays, gamma rays, X – rays, Ultraviolet (UV), visible spectrum, and infra-red (IR), microwaves, TV, and radio (“Electromagnetic Radiation,” 2013).
The human eye is limited to the visible spectrum which is composed of red, orange, yellow, green, blue, indigo, and violet in descending order from the longest wavelength to the shortest wavelength. Yellow and green are the most sensitive wavelengths for the human eye (Thomas, 2011). The visible spectrum runs from 380 nm ~ 780 nm and violet has the shortest wavelength while red is the longest wavelength. This visible spectrum gets through the eye in order to see different colors. However, not everyone can see the same colors or all colors. Some may have color vision deficiencies or color blindness. Many have the misconception of color blindness as being able to see only black and white. However, color blindness is the complete absence in any color sensation and perceives everything just in shades of gray which is known as monochromacy (Waggoner, 2000). On the other hand, color vision deficiencies contain many different types and degrees of severity. This paper is going to discuss about the anatomy of the eye, description of color, different types and degrees of color vision deficiencies, causes, diagnosis, and treatment.
A physicist will state that there is no such thing as color in the real world because it is the brain that differentiates the wavelengths (Bryine, 2009). Objects reflect different wavelengths, but the truth is that wavelengths have no color (Bryine, 2009). Humans can perceive the visible spectrum which falls in the range of 380 ~ 780 nm (nanometer). The human brain differentiates color variation within this range. Light travels through many different components in the eye through the brain in complicated pathways. The light enters through the transparent cornea, aqueous humor, crystalline lens, and vitreous humor to stimulate the photoreceptors of the retina which sends signals through the optic nerve via the optic chiasm, LGN (lateral geniculate body), optic radiation, and occipital lobe part of the brain in an orderly manner (Chalkely, 2000).
When the light falls on the retina, it passes through the photoreceptors. The retina is the innermost layer of the eye that is composed of nerve fibers such as the rod and cones that are responsible for perceiving light (Thomas, 2011). The central portion of the retina is called the macula which contains almost all of the cone cells (Chalkely, 2000). This area surrounds the fovea centralis and provides the most distinct vision. Cones are the photoreceptors that are responsible for perceiving a good, clear, colorful, fine detailed visualization under a well-illuminated environment (Thomas, 2011; Chalkely, 2000). On the other hand, rod cells make up most of the periphery of the retina. They are more numerous and sensitive than cones. However, they are not sensitive to color. Rods aid in scotopic and peripheral vision. They function well under low illumination (Chalkely, 2000).
The cones that are responsible for color sensitivity come in three types based on their wavelengths. There are S (short wavelength), M (medium wavelength), and L (long wavelength). S is most sensitive around 420 nm, M responds best around 530 nm, and L has an optimal response at 560 nm (Birch, 2013). Furthermore, these cones are sensitive to blue (S), green (M), and red (L), although their color responsiveness is not limited to these specific colors (Bryine, 2009). People with normal cones and light sensitive pigments can see different colors which are known as trichromacy. Trichromacy is also known as normal color vision (Waggoner, 2000).
Color has its own metalanguage, signs, and conventions. Mixture of colors creates color variations depending on which photopigment is stimulated by the strong responses from the cones. Furthermore, the arrangement of cones is not uniform or predictable. Color can be described along with four crucial terms, hue, saturation, tone, and brightness (Biggam, 2012). According to metalanguage in color, ‘/’ implies range of color or mixed colors. In addition, ‘-’ indicates mixture of both hues and first color predominates the following color (Biggam, 2012).
Hue or chromatic color refers to visible spectrum according to their wavelengths such as red, green, and blue that can be seen by a human. The classic example of hue in nature is the color of the rainbow. Saturation refers to purity or the amount of grey contained in a hue. Saturation creates the range from vivid hue that has no grey at all, to grey which contains no hue at all. For example, a red color can be described as vivid red or bright red which contains no grey at all compared to dull red, grey-red, or dirty red that contains grey in some degree. As a result, saturation is another term to distinguish color classification (Biggam, 2012).
Tone indicates the mixture of white and black with a hue in a range from pale to dark (Biggam, 2012). As black is added, the hue is perceived darker. Furthermore, there is a special tone range for achromatic colors. Achromatic means without a hue, and it refers to white, black, and the greys (Biggam, 2012). Finally, brightness is considered as the amount of light reaching the eye but it can vary depending on the source of light. For instance, brightness may play an important role in hunting, and in avoiding danger either in daytime or night time. All of these elements mentioned above allow humans to have color vision. However, not everyone can see the colors how normal human would perceive them (Biggam, 2012).
It is extremely rare to be totally color blind. There are different types and degrees of color vision deficiencies. Humans have a total of three types of photopigments which contain red, green, and blue. Indication of one photopigment differs from the normal or shift in pit sensitivity is known as anomalous trichromat. It is considered a mild color deficiency. This impaired color vision can be divided into three different categories such as protanomaly, deuteranomaly, and tritanomaly. Each category is a result of weaknesses in photopigments of red, green, and blue respectively. Many people with anomalous trichromat can have a routine daily life with no trouble except for the color vision test (Waggoner, 2000).
In addition, color vision deficiencies can be further separated by the degrees of color vision deficiencies. Dichromat is a more advanced deficiency. Hence, it is color blindness. As the name indicates, there are two photopigments impaired. Dichromat can be subdivided into three different categories just like anomalous trichromat. There are protanope, deuteranope, and tritanope. Once again, each represents the blindness in color of red, green, and blue respectively. Dichromat is considered to be a severe color deficiency. It is problematic to those who cannot distinguish among different colors without a significant difference compared to other colors. Protan and deutan are unable to distinguish between red/orange, yellow/green, and brown/green (Waggoner, 2000). Furthermore, protan and deutan have some common confusion between green/white and red/white at threshold saturation. On the other hand, protan and deutan also differ in confusing colors. Protan gets confused with blue-green/grey/red-purple, and red/black, whereas deutan has green/grey/blue-purple, and green/black confusion (Waggoner, 2000).
Acquired color deficiencies are caused by many different forms of ocular pathology or medication. Most color deficiencies are inherited and congenital, but disease and injury can also result in color vision loss. One of the most common forms of inherited conditions is red-green deficiencies due to sex linked X-linked recessive genes (Judd, 2013). Red-green are generic term for four types of color vision deficiencies as following specifically: protanopia, protanomaly, dueteranopia, and deuteranomaly. Moreover, chromosome 7 is responsible for tritanopia and tritanomaly (Judd, 2013).
Human have a total number of 23 pairs of chromosomes (46 chromosomes). However, one pair of sex-chromosomes differentiates between men and women. The sex-chromosomes are made of X and Y, whereas male chromosome consists of XY compared to female chromosome which have a combination of XX. Men are mainly affected because males have only one X chromosome. If one male X chromosome is color defective, he will have color deficiencies, whereas female must have two defective X chromosomes in order to have a color deficiency. 8 percent of the male and 0.5 percent of the female in the world have some degrees of color deficiencies. That is 1/12 males and 1/250 females approximately (Wade & Swanston, 2001).
In contrast, diseases or damage in the optic nerve or retina can also affect in color vision loss, such as diabetes, medication, aging, chemical exposure (Judd, 2013). Many illnesses can lead to acquired color blindness. Blue color deficiency is often linked to physical disorders such as diabetes mellitus. Furthermore, medications that treat high blood pressure, heart disease, infections, and psychological problems can affect color vision (Judd, 2013). Aging decreases the clarity of color vision. For example, a cataract occurs due to age-related changes in the crystalline lens getting cloudy which makes it harder for a light to enter (Thomas, 2011). Direct contact with chemical exposure such as fertilizer and styrene has been reported to cause loss of color vision (Judd, 2013).
As a result, color deficiencies must be diagnosed in a proper manner. Fortunately, there are many different types of color vision tests. For example, Nagel amaloscope, pseudoisochromatic testing plates, Waggoner HRR, Farnsworth lantern, Farnsworth D-15, Ishihara, Dvorine, and Color Vision Testing Made Easy are all tests available for screening color vision deficiencies. Light source is very important factor in pseudoisochromatic color vision test. Most of tests were designed using Commission Internationale de I’Eclairage (CIE) illuminant C (Waggoner, 2000).
The Macbeth Easel lamp was considered a gold standard lamp in color vision testing; however, this is no longer available. Fortunately, there are several commercial lamps that are available for color vision testing. In addition, printer’s cabinets are an excellent light source or natural daylight lamps work well. Most offices use fluorescent light which can give erroneous results during advanced color vision testing (Waggoner, 2000). Furthermore, incandescent yellowish light is never recommended as a proper light source and any test results gained when using this light source are invalid (Waggoner, 2000).
The spectral amaloscope is the gold standard in color vision testing for congenital deficiencies and validates most other color vision tests. It is one of the tests that can differentiate between anomalous trichromat and dichromat (Waggoner, 2000). Nagel amaloscope has its own light source. When one look through the eyepiece, there is a circle split in half. The top half of the circle is adjusted with the left knob from one end that is red on one end and green on the other end (Waggoner, 2000). On the other hand, the bottom half of the circle is always yellow. The right knob makes the yellow on the bottom half to either be brighter or darker. The goal is to match the upper and lower halves with the same brightness and color (Waggoner, 2000).
Another form of color vision testing is done by Ishihara which comes in a variety of editions: the 14 plate editions, standard 24 plate editions, and full 38 plate editions. The standard 24 edition screens for congenital protan and duetan which is red-green defects. However, this test is not designed to detect for tritan blue defects. Each plate has a specific purpose as follows transformation plates give different responses from anomalous color observers. Vanishing plates will be observed only by normal people. Hidden numbers are seen only by anomalous observer. Qualitative plates are used as classification for protan and deutan defects along with estimated degree of color vision deficiencies (Waggoner, 2000).
There is another test called Waggoner HRR which contains 28 plates. It screens for congenital and acquired protan (red), duetan (green), and tritan (blue) color vision deficiencies (Waggoner, 2000). This test is designed with vanishing symbols including a star, circle, triangle, cross, and square. The first eight plates contain one symbol that will be seen by every individual. The first 11 plates contain all circles that need to be found. This test also estimates the degree of color vision deficiencies (Waggoner, 2000).
Color Vision Testing Made Easy is a test for pediatric patients and is made of 14 plates. The test is designed with vanishing symbols such as a star, circle, and square. It also contains a dog, boat, and car because children do not need to know the numbers. The first six plates have one symbol that everyone can see in order to avoid the malingering, to see if the subject understands the test, and to keep the subject from becoming embarrassed (Waggoner, 2000). The first nine plates consist of a circle that needs to be found (Waggoner, 2000).
The Dvorine pseudo isochromatic 4th edition is composed of 23 plates. 15 plates contain numbers and the remaining 8 plates present pathways. This test screens for congenital protan, deutan red-green defects but no tritan blue defects (Waggoner, 2000). It is designed with vanishing numbers and has classification plates for protan and deutan which estimates the grades of color deficiency. This test also contains a color naming test with eight saturated and 8 desaturated colors (Waggoner, 2000).
Standard pseudoisochromatic plates (SPP) 1st edition has 19 plates. The test was intended to screen for congenital protan, deutan with red-green defects. The first edition was not designed to test tritan blue defect; however, the second edition was designed to test for tritan defects (Waggoner, 2000). This test uses vanishing numbers and dot patterns. The first edition has a classification for protan and deutan but it does not estimate the degrees of defect (Waggoner, 2000).
There are still some other tests designed for screening color vision deficient patients but are no longer available. The Farnsworth lantern was developed in World War II and became Navy standard 1954 (Waggoner, 2000). There are two signal lights one on top of the other; with a total of three colors; red, green, and white. These three colors can show up in nine different combinations. Each presentation is shown in two seconds at a distance of eight feet with normal illumination. The Farnsworth lantern is now substituted by Optec 900 by Stereo Optical from IL, Chicago. It has the same principle with a two second built in timer (Waggoner, 2000).
The Farnsworth D-15 test is another test developed by Dean Farnsworth. This test is made of a reference cap and 15 removal caps. The test is done by having a subject find the color that is closest to the reference cap starting from the left to the right in an orderly fashion to match the color closest to the previous one. Once the test is completed, the Farnsworth D-15 case will be flipped over to plot the numbers on the bottom of the caps on a score sheet which contains 3 axes for protan, deutan, and tritan. Even though this test is useful to detect what type of defect the patient has, it is not recommended as a routine color vision screening because it fails about 3 ~ 5 percent of the time (Waggoner, 2000).
Unfortunately, despite the wide array of diagnostic tools, there is no cure or treatment for inherited color vision deficiencies. However, awareness of color vision deficiency may help to improve one’s lifestyle. If the cause is an acquired color vision deficiency such as an illness or eye injury, vision could improve with proper treatment (Judd, 2013). Many of those with a color vision deficiency claim that tinted lenses enhance their color vision (Simunovic, 2010). There is also a theory that a monocular filter could improve color discrimination in dichromat (Simunovic, 2010). Moreover, successful early detection can prevent a color deficient individual from entering a dangerous working environment which involve colors. A color deficient patient may not be eligible for certain occupations such as those involving electronics, vocational driving, graphic design and interior decorating (Shute, 1991).
Some studies show that color significantly improves the speed with which images can be recognized and improves the memory of those images. This kind of evidence supports the belief that color vision plays a critical role in one’s life. A color vision deficiency can be either congenital and/or acquired. A wide variety of diagnostic tools will help the eye health care professional to determine the severity of the color vision deficiency. Although, there is no cure to congenital color vision deficiency, the quality of an affected individual’s life can be improved by having an awareness of their condition. Fortunately, acquired color deficiency may be cured with proper treatment. As a result, it is highly recommended to get a periodic checkup by eye health care professionals.
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Hyun S. Lee, a graduate of Rutgers University with continuing study in opticianry at Raritan Valley Community College. He is a member of Phi Theta Kappa honor society, received a scholarship from OANJ and was awarded a certificate for excellence in ophthalmic science. He is very passionate about improving the quality of patients’ life in health care profession.