Laser diode 12 or the LED excitation source: fluorescence is imaged on the detector is a silicon photodiode (16 116) derived from the (勵 起 原 excitation source) (optionally using the lens (14 114)). An optical filter (18 118) is located between the sample Cell (20 120) and a photodiode detector to remove scattered excitation light. Output from the photodiode is amplified to generate an output voltage proportional to the amount of light impinging on the type (22 122) a photodiode detector. Fluorescence is the amount of chemical treatment agent or other additive which is proportional to the concentration of the fluorescent substance present in the sample flow, and further, because the concentration of the phosphor is proportional to the concentration of the chemical treating agent or other additives present, and thus continuous monitoring of the output voltage is present in the system Real-time (實 時間) on may be performed to allow quantification.