Metrological apparatus and a confocal sensor for use in such apparatus are described. The confocal sensor (1) has an optical pinhole (1 1) adapted for letting through a light beam (2). An optical assembly of the sensor has a first lens (12) having a refractive profile (121) and a diffractive profile (122) and a second lens (13) having at least a refractive profile (131). The refractive profile (121) of the first lens (12) and the refractive profile (131) of the second lens (13) focus the light beam (2) into a focused beam (21 ). The diffractive profile (122) of the first lens (12) creates longitudinal chromatic aberration so that the focused beam (21) has a focal zone with a longitudinal depth (R).
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