The invention relates to an interferometric test arrangement for testing the
surface shape of a
test object, comprising an illumination device (105, 205, 305) for generating an input wave from
electromagnetic radiation generated by a
light source, and an interferometer (150) in which, after the input wave has been split into at least one test wave directed onto the
test object (170) and a
reference wave, the
surface shape of at least one partial surface of the
test object (170) can be tested by interferometrically superimposing the at least one test wave and the
reference wave, wherein a diffusing plate (130, 230, 330) is arranged in the illumination device, wherein the illumination device has a first
optical path (110, 210, 310) and, separate therefrom, a second
optical path (120, 220, 320), wherein the input wave can be coupled into the interferometer via both the first
optical path and the second optical path, wherein, when the input wave is coupled in via the first optical path, a
light spot generated on the diffusing plate has a first
diameter (D1), and wherein, when the input wave is coupled in via the second optical path, a
light spot generated on the diffusing plate has a second
diameter (D2) which differs from the first
diameter (D1). (Figure 1a)