an acuity of 6/60 (20/200) or worse, and often leave a person with complete blindness. Non-genetic causes
Physical trauma can cause color blindness, either neurologically – brain trauma which produces swelling of the brain in the occipital lobe – or retinally, either acute (e.g. from laser exposure) or chronic (e.g. from ultraviolet light exposure). Color blindness may also present itself as a symptom of degenerative diseases of the eye, such as cataract and age-related macular degeneration, and as part of the retinal damage caused by diabetes. Vitamin A deficiency may also cause color blindness.Color blindness may be a side effect of prescription drug use. For example, red–green color blindness can be caused by ethambutol, a drug used in the treatment of tuberculosis. Blue-yellow color blindness can be caused by sildenafil, an active component of Viagra. Hydroxychloroquine can also lead to hydroxychloroquine retinopathy, which includes various color defects. Exposure to chemicals such as styrene or organic solvents can also lead to color vision defects. Mechanism
Color blindness is any deviation of color vision from normal trichromatic color vision (often as defined by the standard observer) that produces a reduced gamut. Mechanisms for color blindness are related to the functionality of cone cells, and often to the expression of photopsins, the photopigments that catch photons and thereby convert light into chemical signals. When an individual does not satisfy the requirements for trichromatic vision, they will express dichromacy or monochromacy and be colorblind.
It is now qualified as a condition that could potentially compromise driver safety, therefore a driver may have to be evaluated by an authorized ophthalmologist to determine if they can drive safely. As of May 2008, there is an ongoing campaign to remove the legal restrictions that prohibit colorblind citizens from getting drivers licenses. In June 2020, India relaxed its ban on drivers licenses for the colorblind to now only apply to those with strong CVD. While previously restricted, those who test as mild or moderate can now pass the medical requirements. Australia instituted a tiered ban on the colorblind from obtaining commercial drivers licenses in 1994. This included a ban for all protans, and a stipulation that deutans must pass the Farnsworth Lantern. The stipulation on deutans was revoked in 1997 citing a lack of available test facilities, and the ban on protans was revoked in 2003. All colorblind individuals are banned from obtaining a drivers license in China and since 2016 in Russia (2012 for dichromats). There are several features available that help the colorblind compensate for their color vision deficiency:

British Rail signals use more easily identifiable colors: the red is blood red, the amber is yellow and the green is a bluish color. The relative position of traffic lights is fixed internationally as red, amber, green from top to bottom. Horizontal lights will differ depending on the country, but right hand traffic typically follows a "red light always on the left" pattern.