A1.1.1 A two-field instrument must show two illuminated
areas of equal size and shape, one filled with light transmitted
by color standard, the other with light transmitted by the
sample. These illuminated areas shall be disposed symmetrically
about a vertical median line and shall be separated in a
horizontal direction so that the horizontal separation of the
closest portions subtends the eye of the observer not less than
2° nor more than 3.6°.