(b)
Upon the retirement of a Director who has elected coverage under this section and who has at least fifteen years of service and has attained the age of sixty-five years the Administrative Office of the United States Courts shall pay him an annuity for life equal to 80 per centum of the salary of the office at the time of his retirement.
Upon the retirement of a Director who has elected coverage under this section and who has at least ten years of service, but who is not eligible to receive an annuity under the first paragraph of this subsection, the Administrative Office of the United States Courts shall pay him an annuity for life equal to that proportion of 80 per centum of the salary of the office at the time of his retirement that the number of years of his service bears to fifteen, reduced by one-quarter of 1 per centum for each full month, if any, he is under the age of sixty-five at the time of separation from service.