An apparatus for detecting organic molecules, especially biomolecules and polymers, in a probe substance, comprises a test system (300) comprising an array of test sites (T) to which the probe substance can be supplied, each test site having specific probe molecules (320), an illumination system (100) for optically illuminating a sub-array of test sites (T), and a detection system (400) for identifying those test sites (T) whose probe molecules (320) interact with the organic molecules (350) to be detected, the illumination system (100) comprising an array of independently addressable illumination sources (B) arranged such that to each test site (T) of the sub-array is assigned at least one illumination source (B) that illuminates substantially only it.