Measuring the froth height and depth in froth flotation for the separation of valuable mineral particles is crucial for process optimization. The froth depth is linked to the flotation performance and product quality as it influences various flotation subprocesses such as particle entrainment. Further, it is strongly related to the operating conditions of the flotation cell. Therefore, froth height monitoring is widely used in industry, and can also provide valuable insights for laboratory-scale batch flotation testing. To address the need for precise measurement of the froth surface height and the pulp level in opaque three-phase systems, non-invasive optical or laser-based techniques are required. They allow to investigate the effects of various operating variables on the froth depth in laboratory-scale batch flotation. This experimental study presents a method of optical imaging through the transparent sidewall of a laboratory-scale flotation cell to determine the froth depth by means of advanced image post-processing and analysis. We have demonstrated the performance of this method in a comprehensive study following a full-factorial design of experiments, showing the effects of both hydrodynamic and chemical operating conditions on the froth depth in a batch flotation process. Supplementary measurements using a Dynamic Foam Analyzer confirmed the influence of the chemical conditions on the froth depth. Simultaneously to the optical imaging, in order to provide a comparison to industrial measurement tools, a lidar sensor monitored the froth surface height from above the flotation cell. In conclusion, the proposed imaging method was found to robustly detect the froth phase under the varying experimental procedures of laboratory-scale batch flotation tests, and is similarly applicable to other applications and processes involving foam and froth.