The Ishihara color test, which consists of a series of pictures of colored spots, is the test most often used to diagnose red–green color deficiencies. A figure (usually one or more Arabic digits) is embedded in the picture as a number of spots in a slightly different color, and can be seen with normal color vision, but not with a particular color defect. The full set of tests has a variety of figure/background color combinations, and enable diagnosis of which particular visual defect is present. The anomaloscope, described above, is also used in diagnosing anomalous trichromacy.

Because the Ishihara color test contains only numerals, it may not be useful in diagnosing young children, who have not yet learned to use numerals. In the interest of identifying these problems early on in life, alternative color vision tests were developed using only symbols (square, circle, car).

Besides the Ishihara color test, the US Navy and US Army also allow testing with the Farnsworth Lantern Test. This test allows 30% of color deficient individuals, whose deficiency is not too severe, to pass.

Another test used by clinicians to measure chromatic discrimination is the Farnsworth-Munsell 100 hue test. The patient is asked to arrange a set of colored caps or chips to form a gradual transition of color between two anchor caps.

The HRR color test (developed by Hardy, Rand, and Rittler) is a red–green color test that, unlike the Ishihara, also has plates for the detection of the tritan defects.

Most clinical tests are designed to be fast, simple, and effective at identifying broad categories of color blindness. In academic studies of color blindness, on the other hand, there is more interest in developing flexible tests to collect thorough datasets, identify copunctal points, and measure just noticeable differences.

Optometrists can supply colored spectacle lenses or a single red-tint contact lens to wear on the non-dominant eye, but although this may improve discrimination of some colors, it can make other colors more difficult to distinguish. A 1981 review of various studies to evaluate the effect of the X-chrom contact lens concluded that, while the lens may allow the wearer to achieve a better score on certain color vision tests, it did not correct color vision in the natural environment. A case history using the X-Chrom lens for a rod monochromat is reported and an X-Chrom manual is online.

Lenses that filter certain wavelengths of light can allow people with a cone anomaly, but not dichromacy, to see better separation of colors, especially those with classic "red/green" color blindness. They work by notching out wavelengths that strongly stimulate both red and green cones in a deuter- or protanomalous person, improving the distinction between the two cones' signals. As of 2013, sunglasses that notch out color wavelengths are available commercially.

There is generally no treatment to cure achromatopsia. However, dark red or plum colored filters are very helpful in controlling light sensitivity.

Since 2003, there is a cybernetic device called eyeborg that allows people to perceive color through sound waves. Achromatopsic artist Neil Harbisson was the first to use such a device in early 2004, the eyeborg allowed him to start painting in color by memorizing the sound of each color.

Moreover, there is some research on gene therapy for animals with achromatopsia, with positive results on mice and young dogs, but less effectiveness on older dogs. However, no experiments have been made on humans. There are many challenges to conducting gene therapy on humans. See Gene therapy for color blindness for more details about it.

Dichromacy ("di" meaning "two" and "chroma" meaning "color") is the state of having two types of functioning color receptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights. By comparison, trichromats require three pure spectral lights to match all colors that they can perceive, and tetrachromats require four.

Dichromacy in humans is a color vision defect in which one of the three basic color mechanisms is absent or not functioning. It is hereditary and sex-linked, predominantly affecting males. Dichromacy occurs when one of the cone pigments is missing and color is reduced to two dimensions.

According to colour vision researchers at the Medical College of Wisconsin (including Jay Neitz), each of the three standard colour-detecting cones in the retina of trichromats – blue, green and red – can pick up about 100 different gradations of colour. If each detector is independent of the others, simple exponentiation gives a total number of colours discernible by an average human as their product, or about 1 million; nevertheless, other researchers have put the number at upwards of 2.3 million. Exponentiation suggests that a dichromat (such as a human with red-green color blindness) would be able to distinguish about 10,000 different colours, but no such calculation has been verified by psychophysical testing.

Furthermore, dichromats have a significantly higher threshold than trichromats for coloured stimuli flickering at low (1 Hz) frequencies. At higher (10 or 16 Hz) frequencies, dichromats perform as well as or better than trichromats.. This means such animals would still observe the flicker instead of a temporally fused visual percept as is the case in human movie watching at a high enough frame rate.

In segmental heterochromia, sometimes referred to as sectoral heterochromia, areas of the same iris contains two completely different colors.

Segmental heterochromia is rare in humans; it is estimated that only about 1% of the population have it.

Heterochromia is classified primarily by onset: as either genetic or acquired.

Although a distinction is frequently made between heterochromia that affects an eye completely or only partially (segmental heterochromia), it is often classified as either genetic (due to mosaicism or congenital) or acquired, with mention as to whether the affected iris or portion of the iris is darker or lighter. Most cases of heterochromia are hereditary, caused by certain diseases and syndromes. Sometimes one eye may change color following disease or injury.

An odd-eyed cat is a cat with one blue eye and one eye either green, yellow, or brown. This is a feline form of complete heterochromia, a condition that occurs in some other animals. The condition most commonly affects white-colored cats, but may be found in a cat of any color, provided that it possesses the white spotting gene.

The odd-eyed coloring is caused when either the epistatic (dominant) white gene (which masks any other color genes and turns a cat completely white) or the white spotting gene (which is the gene responsible for bicolor and tuxedo cats) prevents melanin (pigment) granules from reaching one eye during development, resulting in a cat with one blue eye and one green, yellow, or brown eye. The condition only rarely occurs in cats that lack both the dominant white and the white spotting gene.

Achromatopsia (ACHM), also known as total color blindness, is a medical syndrome that exhibits symptoms relating to at least five conditions. The term may refer to acquired conditions such as cerebral achromatopsia, also known as color agnosia, but it typically refers to an autosomal recessive congenital color vision condition, the inability to perceive color and to achieve satisfactory visual acuity at high light levels (typically exterior daylight). The syndrome is also present in an incomplete form which is more properly defined as dyschromatopsia. It is estimated to affect 1 in 40,000 live births worldwide.

There is some discussion as to whether achromats can see color or not. As illustrated in "The Island of the Colorblind" by Oliver Sacks, some achromats cannot see color, only black, white, and shades of grey. With five different genes currently known to cause similar symptoms, it may be that some do see marginal levels of color differentiation due to different gene characteristics. With such small sample sizes and low response rates, it is difficult to accurately diagnose the 'typical achromatic conditions'. If the light level during testing is optimized for them, they may achieve corrected visual acuity of 20/100 to 20/150 at lower light levels, regardless of the absence of color. One common trait is hemeralopia or blindness in full sun. In patients with achromatopsia, the cone system and fibres carrying color information remain intact. This indicates that the mechanism used to construct colors is defective.

The fundus exam via ophthalmoscopy is essentially normal early on in cone dystrophy, and definite macular changes usually occur well after visual loss. Fluorescein angiography (FA) is a useful adjunct in the workup of someone suspected to have cone dystrophy, as it may detect early changes in the retina that are too subtle to be seen by ophthalmoscope. For example, FA may reveal areas of hyperfluorescence, indicating that the RPE has lost some of its integrity, allowing the underlying fluorescence from the choroid to be more visible. These early changes are usually not detected during the ophthalmoscopic exam.

The most common type of macular lesion seen during ophthalmoscopic examination has a bull’s-eye appearance and consists of a doughnut-like zone of atrophic pigment epithelium surrounding a central darker area. In another, less frequent form of cone dystrophy there is rather diffuse atrophy of the posterior pole with spotty pigment clumping in the macular area. Rarely, atrophy of the choriocapillaris and larger choroidal vessels is seen in patients at an early stage. The inclusion of fluorescein angiography in the workup of these patients is important since it can help detect many of these characteristic ophthalmoscopic features. In addition to the retinal findings, temporal pallor of the optic disc is commonly observed.

As expected, visual field testing in cone dystrophy usually reveals a central scotoma. In cases with the typical bull’s-eye appearance, there is often relative central sparing.

Because of the wide spectrum of fundus changes and the difficulty in making the diagnosis in the early stages, electroretinography (ERG) remains the best test for making the diagnosis. Abnormal cone function on the ERG is indicated by a reduced single-flash and flicker response when the test is carried out in a well-lit room (photopic ERG). The relative sparing of rod function in cone dystrophy is evidenced by a normal scotopic ERG, i.e. when the test is carried out in the dark. In more severe or longer standing cases, the dystrophy involves a greater proportion of rods with resultant subnormal scotopic records. Since cone dystrophy is hereditary and can be asymptomatic early on in the disease process, ERG is an invaluable tool in the early diagnosis of patients with positive family histories.

Cone dystrophy in general usually occurs sporadically. Hereditary forms are usually autosomal dominant, and instances of autosomal recessive and X-linked inheritance also occur.

In the differential diagnosis, other macular dystrophies as well as the hereditary optic atrophies must be considered. Fluorescent angiography, ERG, and color vision tests are important tools to help facilitate diagnosis in early stages.

Diagnosis of age-related macular degeneration rests on signs in the macula, irrespective of visual acuity. Diagnosis of AMD may include the following procedures and tests:

- The transition from dry to wet AMD can happen rapidly, and if it is left untreated can lead to legal blindness in as little as six months. To prevent this from occurring and to initiate preventative strategies earlier in the disease process, dark adaptation testing may be performed. A dark adaptometer can detect subclinical AMD at least three years earlier than it is clinically evident.

- There is a loss of contrast sensitivity, so that contours, shadows, and color vision are less vivid. The loss in contrast sensitivity can be quickly and easily measured by a contrast sensitivity test like Pelli Robson performed either at home or by an eye specialist.

- When viewing an Amsler grid, some straight lines appear wavy and some patches appear blank

- When viewing a Snellen chart, at least 2 lines decline

- Preferential hyperacuity perimetry changes (for wet AMD)

- In dry macular degeneration, which occurs in 85–90 percent of AMD cases, drusen spots can be seen in Fundus photography

- In wet macular degeneration, angiography can visualize the leakage of bloodstream behind the macula. Fluorescein angiography allows for the identification and localization of abnormal vascular processes.

- Using an electroretinogram, points in the macula with a weak or absent response compared to a normal eye may be found

- Farnsworth-Munsell 100 hue test and Maximum Color Contrast Sensitivity test (MCCS) for assessing color acuity and color contrast sensitivity

- Optical coherence tomography is now used by most ophthalmologists in the diagnosis and the follow-up evaluation of the response to treatment with antiangiogenic drugs.

Nyctalopia (from Greek νύκτ-, "nykt-" "night"; ἀλαός, "alaos" "blind, not seeing", and ὄψ, "ops" "eye"), also called night-blindness, is a condition making it difficult or impossible to see in relatively low light. It is a symptom of several eye diseases. Night blindness may exist from birth, or be caused by injury or malnutrition (for example, vitamin A deficiency). It can be described as insufficient adaptation to darkness.

The most common cause of nyctalopia is retinitis pigmentosa, a disorder in which the rod cells in the retina gradually lose their ability to respond to the light. Patients suffering from this genetic condition have progressive nyctalopia and eventually their daytime vision may also be affected. In X-linked congenital stationary night blindness, from birth the rods either do not work at all, or work very little, but the condition doesn't get worse.

Another cause of night blindness is a deficiency of retinol, or vitamin A, found in fish oils, liver and dairy products.

The opposite problem, the inability to see in bright light, is known as "hemeralopia" and is much rarer.

Since the outer area of the retina is made up of more rods than cones, loss of peripheral vision often results in night blindness. Individuals suffering from night blindness not only see poorly at night, but also require extra time for their eyes to adjust from brightly lit areas to dim ones. Contrast vision may also be greatly reduced.

Rods contain a receptor-protein called rhodopsin. When light falls on rhodopsin, it undergoes a series of conformational changes ultimately generating electrical signals which are carried to the brain via the optic nerve. In the absence of light, rhodopsin is regenerated. The body synthesizes rhodopsin from vitamin A, which is why a deficiency in vitamin A causes poor night vision.

Refractive "vision correction" surgery (especially PRK with the complication of "haze") may rarely cause a reduction in best night-time acuity due to the impairment of contrast sensitivity function (CSF) which is induced by intraocular light-scatter resulting from surgical intervention in the natural structural integrity of the cornea.

The most common tests perform to diagnose cerebral achromatopsia are the Farnsworth-Munsell 100-hue test, the Ishihara plate test, and the color-naming test. Testing and diagnosis for cerebral achromatopsia is often incomplete and misdiagnosed in doctor’s offices.

Remarkably, almost 50% of tested patients diagnosed with cerebral achromatopsia are able to perform normally on the color-naming test. However, these results are somewhat in question because of the sources from which many of these reports come. Only 29% of cerebral achromatopsia patients successfully pass the Ishihara plate test, which is a more accepted and more standardized test for color blindness.

A practical application of AMD-associated genetic markers is in the prediction of progression of AMD from early stages of the disease to neovascularization.

The long-term prognosis for patients with Stargardt disease is widely variable although the majority of people will progress to legal blindness.

Stargardt disease has no impact on general health and life expectancy is normal. Some patients, usually those with the late onset form, can maintain excellent visual acuities for extended periods, and are therefore able to perform tasks such as reading or driving.

Aulus Cornelius Celsus, writing ca. 30 AD, described night blindness and recommended an effective dietary supplement: "There is besides a weakness of the eyes, owing to which people see well enough indeed in the daytime but not at all at night; in women whose menstruation is regular this does not happen. But success sufferers should anoint their eyeballs with the stuff dripping from a liver whilst roasting, preferably of a he-goat, or failing that of a she-goat; and as well they should eat some of the liver itself."

Historically, nyctalopia, also known as moonblink, was a temporary night blindness believed to be caused by sleeping in moonlight in the tropics.

In the French language, and have inverse meanings, the first naming the ability to see in the dark as well as in plain light, and the second the inability to do so. It is thought that this inversion from Latin happened during the 2nd century AD, even though the ancient greek νυκτάλωψ ("nuktálōps") has been used in both senses.

Distortion of vision refers to straight lines not appearing straight, but instead bent, crooked, or wavy. Usually this is caused by distortion of the retina itself. This distortion can herald a loss of vision in macular degeneration, so anyone with distorted vision should seek medical attention by an ophthalmologist promptly. Other conditions leading to swelling of the retina can cause this distortion, such as macular edema and central serous chorioretinopathy.

An Amsler grid can be supplied by an ophthalmologist so that the vision can be monitored for distortion in people who may be predisposed to this problem.

Tunnel vision implies that the peripheral vision, or side vision, is lost, while the central vision remains. Thus, the vision is like looking through a tunnel, or through a paper towel roll. Some disorders that can cause this include:

Glaucoma - severe glaucoma can result in loss of nearly all of the peripheral vision, with a small island of central vision remaining. Sometimes even this island of vision can be lost as well.

Retinitis pigmentosa - This is usually a hereditary disorder which can be part of numerous syndromes. It is more common in males. The peripheral retina develops pigmentary deposits, and the peripheral vision gradually becomes worse and worse. The central vision can be affected eventually as well. People with this problem may have trouble getting around in the dark. Cataract can be a complication as well. There is no known treatment for this disorder, and supplements of Vitamin A have not been proven to help.

Punctate Inner Choroidopathy - This condition is where vessels gro (( material is missing ))

Stroke - a stroke involving both sides of the visual part of the brain may wipe out nearly all of the peripheral vision. Fortunately, this is a very rare occurrence

Xanthopsia is a color vision deficiency in which there is a predominance of yellow in vision due to a yellowing of the optical media of the eye. The most common cause is digoxin's inhibitory action on the sodium pump, and the development of cataracts which can cause a yellow filtering effect.

It has been suggested that Digitalis-derived digoxin, used to treat heart failure, induced xanthopsia responsible for the yellow tinting exhibited by many of Van Gogh's works.

Xanthopsia is also a rare side-effect of jaundice, in which bilirubin may be deposited into the eye in sufficient quantity to produce a yellow tint to the vision.

Though there is no treatment for Cone dystrophy, certain supplements may help in delaying the progression of the disease.

The beta-carotenoids, lutein and zeaxanthin, have been evidenced to reduce the risk of developing age related macular degeneration (AMD), and may therefore provide similar benefits to Cone dystrophy sufferers.

Consuming omega-3 fatty acids (docosahexaenoic acid and eicosapentaenoic acid) has been correlated with a reduced progression of early AMD, and in conjunction with low glycemic index foods, with reduced progression of advanced AMD, and may therefore delay the progression of cone dystrophy.

Imaging studies such as ultrasonography (US), Computerized Tomography (CT) and Magnetic Resonance Imaging (MRI) can aid diagnosis. On ultrasound, Coats' disease appears as a hyperechoic mass in the posterior vitreous without posterior acoustic shadowing; vitreous and subretinal hemorrhage may often be observed.

On CT, the globe appears hyperdense compared to normal vitreous due to the proteinaceous exudate, which may obliterate the vitreous space in advanced disease. The anterior margin of the subretinal exudate enhances with contrast. Since the retina is fixed posteriorly at the optic disc, this enhancement has a V-shaped configuration.

On MRI, the subretinal exudate shows high signal intensity on both T1- and T2-weighted images. The exudate may appear heterogeneous if hemorrhage or fibrosis is present. The subretinal space does not enhance with gadolinium contrast. Mild to moderate linear enhancement may be seen between the exudate and the remaining vitreous. The exudate shows a large peak at 1-1.6 ppm on proton MR spectroscopy.

Some examples of entoptical effects include:

- Floaters or "muscae volitantes" are slowly drifting blobs of varying size, shape, and transparency, which are particularly noticeable when viewing a bright, featureless background (such as the sky) or a point source of diffuse light very close to the eye. They are all shadow images of objects suspended just above the retina. Some may be individual red blood cells swollen due to osmotic pressure or chains of these cells stuck together and diffraction patterns can be seen around these. They may also be "coagula of the proteins of the vitreous gel, to embryonic remnants, or the condensation round the walls of Cloquet's canal". Floaters may collect over the fovea (the center of vision) and therefore be more visible when lying on your back looking upwards.

- Blue field entoptic phenomenon has the appearance of tiny bright dots moving rapidly along squiggly lines in the visual field. It is much more noticeable when viewed against a field of pure blue light and is caused by white blood cells moving in the capillaries in front of the retina. The white cells are larger than the red cells and must deform to fit. As they go through a capillary, a space opens up in front of them and red blood cells pile up behind. This makes the dots of light appear slightly elongated with dark tails.

- Haidinger's brush is a very subtle bowtie or hourglass shaped pattern that is seen when viewing a field with a component of blue light that is plane or circularly polarized and rotating with respect to the observer's eye. If the light is all blue, it will appear as a dark shadow, if the light is full spectrum, it will appear yellow. It is due to the preferential absorption of blue polarized light by pigment molecules in the fovea.

- Purkinje images are the reflections from the anterior and posterior surfaces of the cornea and the anterior and posterior surfaces of the lens. While these first four reflections are not entoptic, Becker described how light can reflect from the posterior surface of the lens and then again from the anterior surface of the cornea to focus a second image on the retina, this one much fainter and inverted. Tscherning referred to this as the sixth image (the fifth image being formed by reflections from the anterior surfaces of the lens and cornea to form an image too far in front of the retina to be visible) and noted it was much fainter and best seen with a relaxed emmetropic eye. In a dark room, with one eye closed and looking ahead with the other eye, move a light back and forth under your gaze – you should see a dimmer image moving in the opposite direction.

- The Purkinje tree is an image of the retinal blood vessels in one's own eye, first described by Purkyně in 1823. It can be seen by shining the beam of a small bright light penlight through the pupil from the periphery of a subject's vision. This results in an image of the light being focused on the periphery of the retina. Light from this spot then casts shadows of the blood vessels (which lie on top of the retina) onto unadapted portions of the retina. Normally the image of the retinal blood vessels is invisible because of adaptation. Unless the light moves, the image disappears within a second or so. If the light is moved at about 1 Hz, adaptation is defeated, and a clear image can be seen indefinitely. The vascular figure is often seen by patients during an ophthalmic examination when the doctor is using an ophthalmoscope. Another way in which the shadows of blood vessels may be seen is by holding a bright light against the eyelid at the corner of the eye. The light penetrates the eye and casts a shadow on the blood vessels as described previously. The light must be jiggled to defeat adaptation. Viewing in both cases is improved in a dark room while looking at a featureless background. This topic is discussed in more detail by Helmholtz.

- Purkinje's blue arcs are associated with the activity of the nerves sending signals from where a spot of light is focussed on the retina near the fovea to the optic disk. Look at the right edge of a small red light in a dark room with your right eye (left eye closed) after dark-accommodating your eye for about 30 seconds and you should see two faint blue arcs starting at the light and heading towards the blind spot. Look at the left edge and you will see a faint blue spike going from the light to the right.

- A phosphene is the perception of light without light actually entering the eye, for instance caused by pressure applied to the closed eyes.

A phenomenon that could be entoptical if the eyelashes are considered to be part of the eye is seeing light diffracted through the eyelashes. The phenomenon appears as one or more light disks crossed by dark blurry lines (the shadows of the lashes), each having fringes of spectral colour. The disk shape is given by the circular aperture of the pupil.

Oguchi's disease is unique in its electroretinographic responses in the light- and dark-adapted conditions. The A- and b-waves on single flash electroretinograms (ERG) are decreased or absent under lighted conditions but increase after prolonged dark adaptation. There are nearly undetectable rod b waves in the scotopic 0.01 ERG and nearly negative scotopic 3.0 ERGs.

Dark-adaptation studies have shown that highly elevated rod thresholds decrease several hours later and eventually result in a recovery to the normal or nearly normal level.

The S, M and L cone systems are normal.

Entoptic phenomena (from Greek ἐντός "within" and ὀπτικός "visual") are visual effects whose source is within the eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.)

In Helmholtz's words; "Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called "entoptical"."

Grossly, retinal detachment and yellowish subretinal exudate containing cholesterol crystals are commonly seen.

Microscopically, the wall of retinal vessels may be thickened in some cases, while in other cases the wall may be thinned with irregular dilatation of the lumen. The subretinal exudate consists of cholesterol crystals, macrophages laden with cholesterol and pigment, erythrocytes, and hemosiderin. A granulomatous reaction, induced by the exudate, may be seen with the retina. Portions of the retina may develop gliosis as a response to injury.