Device for optically measuring fluorescence of test samples comprising: - a plurality of upright sample tubes made of a transparent material, having a side wall, a bottom end and containing one of the test samples, - a plurality of vertical sample wells each for receiving one of the upright sample tubes, - at least one excitation light source (24; 74, 76, 78, 80) for directing an excitation light beam through the side wall of a sample tube into the test sample, and - an optical fluorescence detector for capturing fluorescence emitted from the test sample upon excitation, wherein - the test samples contain nucleic acids and a plurality of fluorescent dyes, - the device comprises a plurality of excitation light sources (74, 76, 78, 80), disposed around each sample well, each excitation light source (74, 76, 78, 80) being disposed in the vicinity of the associated sample well for exciting a dye in a test sample in the sample well; - the excitation light beam from each of the plurality of excitation light sources (74, 76, 78, 80) has a different wavelength spectrum for exciting one of the plurality of fluorescent dyes; - the device comprises a plurality of optical fluorescence detectors, - underneath the bottom end of each sample tube is disposed one of the fluorescence detectors for capturing fluorescence emitted from the dye in the test sample upon excitation, - the axis of the sample tube received in the sample well is in alignment with the fluorescence detector, characterized in that - the sample wells are each provided in a single thermally conductive block, - each thermally conductive block is inserted into a through-hole of an optics block, - the optics block is provided with horizontal through bores, - each through bore houses one of the plurality of excitation light sources (74, 76, 78, 80) that are disposed around each sample well and - each through bore is aligned with a corresponding bore of the thermally conductive block that opens into the sample well. - a plurality of upright sample tubes made of a transparent material, having a side wall, a bottom end and containing one of the test samples, - a plurality of vertical sample wells each for receiving one of the upright sample tubes, - at least one excitation light source (24; 74, 76, 78, 80) for directing an excitation light beam through the side wall of a sample tube into the test sample, and - an optical fluorescence detector for capturing fluorescence emitted from the test sample upon excitation, - the test samples contain nucleic acids and a plurality of fluorescent dyes, - the device comprises a plurality of excitation light sources (74, 76, 78, 80), disposed around each sample well, each excitation light source (74, 76, 78, 80) being disposed in the vicinity of the associated sample well for exciting a dye in a test sample in the sample well; - the excitation light beam from each of the plurality of excitation light sources (74, 76, 78, 80) has a different wavelength spectrum for exciting one of the plurality of fluorescent dyes; - the device comprises a plurality of optical fluorescence detectors, - underneath the bottom end of each sample tube is disposed one of the fluorescence detectors for capturing fluorescence emitted from the dye in the test sample upon excitation, - the axis of the sample tube received in the sample well is in alignment with the fluorescence detector, - the sample wells are each provided in a single thermally conductive block, - each thermally conductive block is inserted into a through-hole of an optics block, - the optics block is provided with horizontal through bores, - each through bore houses one of the plurality of excitation light sources (74, 76, 78, 80) that are disposed around each sample well and - each through bore is aligned with a corresponding bore of the thermally conductive block that opens into the sample well.