ALIGNMENT MARK DETECTION SIGNAL PROCESSING METHOD

PURPOSE:To conduct an alignment operation in a highly precise manner by a method wherein, in the alignment of the projection exposure of a color liquid crystal substrate against a mask plate, a differential pulse, which is obtained by differentiating each detected signal after the detected signal of an alignment mark has been properly processed. CONSTITUTION:The detection pulse p1... of the mask mark on the side of +pole, based on the light-receiving level VB of a liquid crystal substrate, and the detection pulse pG< of the substrate mark on the side of -pole are amplified by a fixed amplification factor. The detection pulse P1... is saturated to an amplifier by increasing the intensity of an illumination light, and at the same time, the waveform of each detection pulse is steepened. Each detection pulse is differentiated, two differential pulses p1U, p1d..., pgd, pGU... are generated respectively against the rise and the fall points, the center of the two differential pulses is computed, and the above-mentioned alignment is conducted. The intensity of illumination light for the above-mentioned alignment is set by selecting a plurality of light-dimming filters provided in a light source part.