A1.1.3 Each illuminated area in the two-field instrument
shall cover a circle of diameter subtending at least 2.2° and
may be enlarged to any size provided that no two illuminated
points in the field of view are separated by a distance
subtending more than 10°. In the case of the three-field direct
viewing instrument, the subtending angles become 2.6° and
6.4°, respectively.