### Optical Multimode Online Probe  
The Optical Multimode Online Probe (OMOP) is an image-based shadowgraphic measurement technique, with an opposite based array [22] as depicted in Figure 1. One of the probe tubes contains an illumination unit that generates parallel light via a LED and a plan convex lens. The light passes out through an inspection window at the front end of the probe tube and afterwards through the measuring volume between the two diametrical probe tubes. On the opposite side, a camera with a telecentric lens acquires the resulting shadowgraphic image. The arrangement of illumination and lens result in images having high contrast and significantly better image quality compared to commercial incident light probes. In addition, at equal magnification, a significantly higher depth of field is achieved by telecentric lenses than by conventional entocentric lenses [23]. Telecentric optics use an aperture at the image-side focal point, whereby the image is formed exclusively by parallel light. This results in images with a distance-independent and constant magnification. In contrast, when images are taken with entocentric lenses, objects are projected at different sizes depending on their distance to the optics, which either requires calibration or leads otherwise to erroneous measurements. With telecentric systems, on the one hand, calibration is not necessary due to the constant magnification [24], and, on the other hand, the high contrast and greater depth of field make it much easier to evaluate images using simple image evaluation algorithms. With entocentric systems complex image processing algorithms are required even for simple evaluations at low holdup. The drawback of the first version of OMOP are a roughly four- to eightfold increase in installation space with the opposite illumination unit compared to incident light probes. Therefore, telecentric endoscope probes were developed to overcome the disadvantages [25], as a one-sided installation via a flange reduces the required installation space, but retains the complete measuring principle with adjustable size of the measurement gap and its position in the apparatus. In recent years, a probe for industrial-scale apparatuses has been developed that is inserted via a DN80 flange [26] and can be used for apparatus sizes from about DN450 to DN1000. In addition, the probe has been scaled down, and a laboratory probe [27] has been developed for a small apparatus between DN80 and DN600 and can be inserted via a DN40 [28] or DN50 [27] flange. In this work, the laboratory probe is used, which is depicted in Figure 2.  
Although the method of probe insertion has been further developed, both a two-sided measuring flange and an endoscope probe can be usefully applied. Endoscopic probes are particularly useful for a large (<DN500) and existing apparatus, while two-sided measuring rings are mainly used at small scales (DN25-DN450), e.g., in pipes and small vessels. Figure 1: Scheme of a OMOP in a DN100 extraction column. The invasive tubes are mounted eccentric and do not interfere with the impeller shaft. In addition to these design improvements the application of the measurement technology has been extended to other multiphase flows and apparatus, from extraction columns to mixer-settlers [29; 30], reaction pumps [31], bubble columns [24; 25; 27; 32], crystallization processes [28], evaporation [33] and spray regimes in distillation and absorption columns [34; 35], where for the latter minor adaptions were necessary [36]. In order to determine the DSD from the experimental OMOP data, an image analysis using conventional image processing or neural networks (NN) is necessary. This includes binarization and, if necessary, watershed segmentation to segment touching or overlapping particles [32; 37] as well as Hough transformation [38], which detects specific shaped objects in an image [38; 39; 40]. Unfortunately, conventional image processing algorithms are limited to images at low and medium phase fractions [41].