What do fringe patterns from interference testing reveal?

Человек открывает коробку с логотипом Beckman Coulter

If the two beams pass through identical substances, the resulting fringe pattern is relatively constant across the length of the image, as shown here:

Fringe pattern for air no cell with AUC

If the beams pass through different substances, and the concentration of one of them varies across the radial length, the fringe pattern shows interference as seen here:

Fringe pattern for changing concentration with AUC