Object Depending Measurement Uncertainty of Confocal Sensors

Introduction

Confocal gating is a very efficient tool to restrict light reaching the detector of an optical sensor to a certain volume. In fact, it was invented to facilitate imaging inside biological, strongly scattering media [1]. From there on a number of different sensor implementations relying on confocal gating were developed and so called confocal microscopes have been used extensively in many kinds of applications [2]. Meanwhile their popularity is so high, that confocal microscopy has been put on Natures list of milestones in light microscopy [3]. In classic confocal sensors, imaging is achieved by mechanically scanning the measurement object in axial direction relative to one or several optical measurement spots in parallel [4, 5]. This mechanical, axial scan has been replaced later by a spectral distribution of the measurement spot in axial direction both for point sensors [6] and areal sensor configurations [7]. More recently this chromatic confocal measurement principle has been combined with spectral interferometry in order to achieve a more constant lateral resolution over the measurement range while retaining an interferometric axial resolution [8]. Also it was realized recently that even in measurement systems, that utilize coherence gating [9], confocal gating is inherent and has to be taken into account [10]. Furthermore, it was shown that by properly aligning the properties of the coherence and the confocal gating, resolving power of coherence gating sensors can be improved [11].