Racial Differences in Visual Acuity

Abstract

Visual acuity will be determined at a distance of 20 feet or the mirror equivalent under standard conditions of illumination. The illumination of the target chart shall be between 12- and 18-foot candles. This degree of illumination may be obtained by a 200-watt lamp, 5 feet diagonally from the 20/20 line in the target, and incident to this part of the chart at an angle of 450. All lamps must be shielded from the direct vision of the examinee by an opaque shade. The individual to be tested, if wearing glasses, will remove them before entering the examining room, and then will be seated without viewing the test chart. Individuals awaiting the test must be kept out of hearing distance. The examiner holds the occluder and covers the candidate's left eye, while instructing the examinee to keep both eyes open without squinting. The occluder must not be permitted to touch any part of the eye to be shielded, but will be held in contact with the side of the nose. The examinee is then directed to begin with the first (visible) line and to read as many as possible. 'The acuity for the left eye is then tested, using a different chart or by having the examinee read the lines backward. An individual who normally wears glasses is tested again with them in place, following the same procedure. Where there is a suspicion that the examinee has memorized the charts, he is directed to read the letters or targets in reverse order or will be shown a different chart. The individual is expected to read the letters: promptly. No precise time limit will be applied, but 1 or 2 seconds per letter is ample time. When an individual fails a letter or target, he will not be asked to read it again. If the individual is a rapid reader and his mistakes are obviously careless ones, he will be cautioned to slow down and the test will be repeated on another chart. Vislion is recorded in the form of a fraction. The upper number is the distance in feet from the target, and the lower number is the value of the smallest test chart line read correctly. Thus a person reading the 30-foot test chart line at a distance of 20 feet is given a score of 20/30. A score of 20/20 indicates the person reads test chart line marked 20 at a disitance of 20 feet. Similarly, 20/200 means that person reads only the test chart line marked 200 from a distance of 20 feet.