Library of Congress regulations (36CFR 701.6(b)(1)

(i) Blind persons whose visual acuity, as determined by competent authority, is 20/200 or less in the better eye with correcting glasses, or whose widest diameter if visual field subtends an angular distance no greater than 20 degrees.
(ii) Persons whose visual disability, with correction and regardless of optical measurement, is certified by competent authority as preventing the reading of standard printed material.
(iii) Persons certified by competent authority as unable to read or unable to use standard printed material as a result of physical limitations.
(iv) Persons certified by competent authority as having a reading disability resulting from organic dysfunction and of sufficient severity to prevent their reading printed material in a normal manner.

•Competent authority is defined in 36 CFR 701.6(b)(2) as follows:
(i) In cases of blindness, visual disability, or physical limitations: doctors of medicine, doctors of osteopathy, ophthalmologists, optometrists, registered nurses, therapists, professional staff of hospitals, institutions, and public or welfare agencies (e.g., social workers, case workers, counselors, rehabilitation teachers, and superintendents).
(ii) In the case of a reading disability from organic dysfunction: doctors of medicine who may consult with colleagues in associated disciplines.