Method for investigating biological cells or cell cultures in a microplate reader, wherein the method comprises: a) providing a microplate reader, which comprises: - at least one measuring device (6,7,8) having an optical axis (21', 21", 21"'); - an illumination source and an imaging camera with an own optical axis, which runs in parallel to the optical axis (21', 21", 21'") of the measuring device (6,7,8), - a receiving device for receiving at least one microplate and for positioning a received microplate with respect to the optical axis (21', 21", 21"') of the measuring device (6,7,8) or with respect to the optical axis of the illumination source and the imaging camera of the microplate reader; and - a processor (11',11"); b) positioning a well of a received microplate with respect to the optical axis (21', 21", 21'") of the measuring device (6,7,8) of the microplate reader using the receiving device, and detecting an integral signal in the well of the microplate using at least one of the measuring devices (6,7,8), wherein the integral signal is selected from a group comprising: a fluorescence, a luminescence, and an absorbance; characterized in that the method comprises the following further steps: c) positioning the well of the microplate with respect to the optical axis of the illumination source and the imaging camera using the receiving device, transilluminating, and generating a transmission image of a bottom of said well using the illumination source and the imaging camera; d) determining a confluence of biological cells or cell cultures on the transmission image of the bottom of the well generated in step c), e) comparing the detected integral signals in the well of step b) with the determined confluence of the biological cells or cell cultures of step d) using the processor (11',11"); and f) normalizing the integral signal of the well of step b) with the confluence of the biological cells or cell cultures in said well determined in step d). a) providing a microplate reader, which comprises: - at least one measuring device (6,7,8) having an optical axis (21', 21", 21"'); - an illumination source and an imaging camera with an own optical axis, which runs in parallel to the optical axis (21', 21", 21'") of the measuring device (6,7,8), - a receiving device for receiving at least one microplate and for positioning a received microplate with respect to the optical axis (21', 21", 21"') of the measuring device (6,7,8) or with respect to the optical axis of the illumination source and the imaging camera of the microplate reader; and - a processor (11',11"); - at least one measuring device (6,7,8) having an optical axis (21', 21", 21"'); - an illumination source and an imaging camera with an own optical axis, which runs in parallel to the optical axis (21', 21", 21'") of the measuring device (6,7,8), - a receiving device for receiving at least one microplate and for positioning a received microplate with respect to the optical axis (21', 21", 21"') of the measuring device (6,7,8) or with respect to the optical axis of the illumination source and the imaging camera of the microplate reader; and - a processor (11',11"); b) positioning a well of a received microplate with respect to the optical axis (21', 21", 21'") of the measuring device (6,7,8) of the microplate reader using the receiving device, and detecting an integral signal in the well of the microplate using at least one of the measuring devices (6,7,8), wherein the integral signal is selected from a group comprising: a fluorescence, a luminescence, and an absorbance; c) positioning the well of the microplate with respect to the optical axis of the illumination source and the imaging camera using the receiving device, transilluminating, and generating a transmission image of a bottom of said well using the illumination source and the imaging camera; d) determining a confluence of biological cells or cell cultures on the transmission image of the bottom of the well generated in step c), e) comparing the detected integral signals in the well of step b) with the determined confluence of the biological cells or cell cultures of step d) using the processor (11',11"); and f) normalizing the integral signal of the well of step b) with the confluence of the biological cells or cell cultures in said well determined in step d).