Abstract: Apparatus for measuring contamination in a hot melt flow. The apparatus includes a flow cell tube adapted to pass the melt flow. Apparatus is provided for directing laser energy through the tube and the melt flow passing through the flow cell tube to a laser energy detector producing an electrical signal representative of the size of a contaminant in the flow melt. A heater is provided for heating the melt flow as such melt flow passes through the flow cell tube. The apparatus is adapted to measure contamination in a hot melt where the temperature of the melt must be maintained as the melt passes through the flow tube. The flow tube, laser, detector and heater are disposed within a housing. The housing has a cooling chamber. The detector is disposed in the cooling chamber to thermally isolate the detector from the heating of the flow.