A robust sensor is developed for on-line measurements in paper production. The sensor is designed for optimal response to lignin in paper. A diode laser operating at 630 nm is used for excitation and fluorescence light in the wavelength region 660 to 740 nm is detected. A compact photomultiplier tube is used for fast detection with a high SNR. The measuring spot is 90 mum in diameter. All optical and electronics components are housed in a small 250X140X90 mm(3) box. Because the system can make up to 1.2 million measurements/s the distance between adjacent measuring points can be as short as about 20 mum, even at a paper speed of 25 m/s, as is common with a modern newsprint paper machine. The recording system can store data continuously for 45 min at this sampling rate and data from three more sensors can be recorded simultaneously with the same resolution. This high-capacity sensor system can monitor the behavior of a paper machine at much higher time and space resolution than has traditionally been possible. From test runs on four different newsprint paper machines we observed the existence of small spots of high fluorescence in the paper from one machine, characteristic behavior with a significantly lower level of fluorescence in periodical sections of about 200 m in another machine, and that wire marks and periodical patterns could be very well identified from the fluorescence recordings. Such data have proven to contain important information for the papermaker concerning, for example the relative shrinkage of the paper web in the drying section of the paper machine. © 2001 Society of Photo-Optical Instrumentation Engineers.