Experiments of a shock hitting a curtain of particles were conducted at the multiphase shock tube facility at Sandia National Laboratories. These are studied in this paper for quantifying the epistemic uncertainty in the experimental measurements due to processing via measurement models. Schlieren and X-ray imaging techniques were used to obtain the measurements that characterize the particle curtain with particle volume fraction and curtain edge locations. The epistemic uncertainties in the experimental setup and image processing methods were identified and measured. The effects of these uncertainties on the uncertainty in the extracted experimental measurements were quantified. The influence of the epistemic uncertainty was significantly higher than the experimental variability that has been previously considered as the most important uncertainty of experiments.