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An in-line holographic particle counter concept is presented and validated where multiple micrometer sized particles are detected in a three dimensional sampling volume, all at once. The proposed PIU is capable of detecting holograms of particles which sizes are in the lower <inline-formula> <math display="inline"> <semantics> <mi mathvariant="sans-serif">&#956;</mi> </semantics> </math> </inline-formula>m- range. The detection and counting principle is based on common image processing techniques using a customized HT with a result directly relating to the particle number concentration in the recorded sampling volume. The proposed counting unit is mounted ontop of a CNM for comparison with a commercial TSI-3775 CPC. The concept does not only allow for a precise in-situ determination of low particle number concentrations but also enables easy upscaling to higher particle densities (e.g., <inline-formula> <math display="inline"> <semantics> <mrow> <mo>&gt;</mo> <mn>30</mn> <mo>.</mo> <mn>000</mn> <mspace width="0.166667em"></mspace> <mfrac> <mo>#</mo> <mrow> <mi>c</mi> <mi>c</mi> <mi>m</mi> </mrow> </mfrac> </mrow> </semantics> </math> </inline-formula>) through its linear expandability and option of cascading. The impact of coincidence at higher particle densities is shown and two coincidence correction approaches are presented where, at last, its analogy to the coincidence correction methods used in state-of-the-art CPCs is identified.