An optical sensing system for detecting welds and defects in metal brightly illuminates a broad section of the metal and images a swath of the metal onto a multi-element detector array, generating voltage signals representing the reflectivity of the swath of the metal as it passes under the detector. A processor system detects areas of low reflectivity and indicates a defect. A slit mask placed in front of the detector array narrows the imaged swath to reduce false accepts. A color filter in front of the detector array reduces ambient light interference. The processor system incorporates an auto-calibration function to compare the current signal to an averaged reference signal.