Terahertz phase retrieval is a promising technique able to assess the complex diffracted wave properties through an iterative processing algorithm. In this letter, we demonstrate the implementation of this technique in reflection geometry with a continuous wave acquisition system working at 0.287 THz. To ensure a high signal-to-noise ratio in the measured dataset, we proposed a double parallel recording scheme with one detector and two lock-in amplification operating with the complimentary sensitivity setting. This provided a higher numerical aperture than conventional rasterscanning focal plane imaging. A specialized digital interferometric post-processing procedure was applied to obtain a surface heights map from the reconstructed phase distribution in the object's irradiated area.
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