The medical and physical requirements for getting a Merchant Mariner Credential are contained in Code of Federal Regulations, Title 46 CFR 10.215; paragraph b contains the requirements for the vision test. General vision requirements are:
- A minimum of 20/200 uncorrected vision correctable to at least 20/40.
- An applicant that has lost vision in one eye must wail six months before they can apply and supply a statement from a doctor that documents that their ability to function visually with one eye.
- Normal color vision using an accepted color sensing test.
The ability to distinguish between red and green color at night is important for the mariner. These colors are used to mark the right and left sides of the channel, and the red and green sidelights are used to determine the zone of approach for applying the Rules of the Road. The short version is if you can distinguish between the color red and green you are not color blind. To prove this the Coast Guard requires that you pass one of the accepted color sense exams; there are eight excepted exams. If you can pass any one of the eight you are good to go.