Abstract: A device for characterizing the formation of a sheet material is described. The device comprises a basis weight sensor for measuring the basis weight of the sheet material as the material passes through the sensor and signal processing circuitry which receives a signal from the basis weight sensor indicative of the measured basis weight and which produces three types of outputs, which outputs characterize: (1) the magnitude of variation in the sheet basis weight; (2) the strength of the weakest portion or portions of the sheet; and (3) the size of the flocs comprising the sheet. The basis weight sensor includes a light pipe which is held against the sheet, as the sheet moves through the sensor, and which directs a small spot of light transmitted through the sheet to a light detecting device. The magnitude of variation in the sheet basis weight is computed to be proportional to the true root-mean-square variation in the basis weight of the sheet.