To meet the Third-Class standard, a candidate must exhibit a visual acuity of 20/40 in either eye. These metrics can be applied with or without a corrective lens.
For the intermediate vision metric, a candidate must demonstrate a visual acuity of 20/40 in either eye, at a distance of 16 inches.
To secure a Third-Class Medical Certificate, there is no requirement for distance testing.
Third-Class color accuracy is based on a candidate's ability to see and identify colors, in order to operate an aircraft safely. Namely, the candidate should be able to identify, and respond to, certain color-coded placards in and outside of an general aviation aircraft. Also, depending on the octane rating of the fuel, different colored dyes are blended with base formulas. For the candidate, this offers a color-based way to make sure that a pilot has the right fuel, in the right airplane; for this reason, it is critical that the candidate can see colors accurately.
For the exam, the candidate will take part in a series of practical tests. For a Third-Class medical, the candidate will initially undergo what is referred to as a Standard Snellen Test. This places the candidate 20 feet from what is referred to as a Snellen Card target. The card's configuration is based on lines of block letters, that are reduced in size line by line, while looking from the top to the bottom. Next, the examiner will typically test eye muscle response using an ophthalmoscope, to ensure that the eye is responding within a normal range. Subsequently, various machine-based tests will be used for focal measuring, and finally the candidate will undergo various color tests.