The detector (100) to detect a fluorescent dye (F) in a sample (P) has a light beam unit (110) aligned at the sample and a detection unit (120) within a hollow zone (130) with a highly reflective inner surface (131). It has one opening (132) aligned at the sample, and a second opening (133) opposite the detection unit. The hollow zone (130) has a generally spherical shape. The highly reflective surface contains barium sulfate or Spectralon (RTM). The light unit (110) emits monochromatic light, and especially a laser light. The light beam can be expanded. The light beam can be scanned at a carrier, where the light beam is carried through. The light scanner is a focusing lens, to focus the light beam on to the sample (P), using a F/[theta] lens.; Part of the light beam is deflected towards the sample, and part through the fluorescent dye (F) to be detected for the fluorescent emissions to be directed into the hollow zone. The light is separated by a dichroitic beam divider. At least one further detection unit can be located within the hollow zone, with its own entry opening, to register the fluorescent emission from the dyestuff to be registered, with a different wavelength from the first detection unit. The additional detection unit can register the presence of at least one additional fluorescent dyestuff. Each detection unit (120) is mounted in its opening, sealed against light. Each detection unit has a photomultiplier, and has a color filter for the wavelength of the fluorescence to be registered. Each detection unit has a collimation lens.; A barrier filter is in front of the first opening which is impermeable to the light used for excitation, and permeable to the emitted fluorescent light, using an interference and/or a starting filter.