isopter

[ī-sŏptər]
n.
  1. A curve of equal retinal sensitivity in the visual field designated by a fraction, the numerator being the diameter of the test object, and the denominator being the testing distance.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.