Extension of longitudinal measuring range of a confocal surface profilometer using a gradient-intensity probe beam

A confocal surface profilometer with a long vertical range was developed by using a gradient-intensity probe beam. In conventional confocal microscopy, vertical scanning of a tested surface by either stage height movement or the shifting of objective focus is time-consuming, thus making unacceptable measurement efficiency for in situ inspection. To overcome this, in this research, a quasi-Bessel beams (QBB) with the properties of narrow lateral spot and gradient-intensity axial distribution is adopted as a probe beam to generate an accurate intensity-to-height conversion for 3-D profile measurement. The influences of the incident Gaussian beam and the real axicon lens with an oblate-tip shape on the axial probe beam are calculated and analyzed. By switching the axicon lenses with different base angles, the measurable height of the surface profilometer can be ranged from a few millimeters to several hundred millimeters. According to the measured results, it was found that by using the axicon lens with a base angle of 20°, the measurable height reached at least 2 mm. By integrating a high-speed acquisition unit, the proposed profilometer can further increase the measurement efficiency, especially fitting in situ automatic optical inspection (AOI).