Apparatus for analyzing the formation of a paper web

On-line apparatus for analysing the relative uniformity of fiber distribution of a moving web of paper comprising:

• (a) a source of light which in operation directs a narrow beam of light onto the moving web,
• (b) a detector positioned so as to receive the beam of light after it has passed through the web and having photodetector means,
• (c) a circuit connected to the photodetector means and having amplifier means connected to receive decoupled A.C. signals from the photodetector means, a tunable band pass filter connected to the output from said amplifier means, a demodulator connected to receive signals from the tunable band pass filter and indicator means connected to the output from the demodulator, said tunable band pass filter being tuned, in relation to the web speed, to give a maximum response at a floc wavelength of around 3 cm. and being arranged to attenuate floc wavelengths of less than 2.3 cm. or more than 3.7 cm. to about 10% of said maximum response, wherein said indicator means includes an analog to digital converter circuit, means for averaging the signal received from said converter circuit, means for computing the standard deviation of said digitized signal, and means for continually computing a formation index which is the sum of said time averaged digitized signal and the absolute value of said standard deviation.