In this paper, we present the concept and prototype of the Augmented Reality (AR) app InfoGrid for mobile devices, which enables museum visitors to experience digital media as an overlay for physical exhibits. With InfoGrid, it is possible to view 3D objects, videos, and animations, or listen to audio re-cordings prepared by museum professionals. InfoGrid interfaces with our web-based framework, the Network Environment for Multimedia Objects (NEMO), which stores and handles all data in a cloud-based semantic database. In this contribution, we present the usability field evaluation of InfoGrid in a nature museum, using the System Usability Scale (SUS) questionnaire and additional observations.