Digital Holography vs. Display Holography - What are their differences and what do they have in common?

Holography is a two-stage imaging process in which the wave field of the object is recorded in a first step so that it can be reconstructed in a second step. It involves the physics of diffraction and interference to record and reconstruct optical wavefields or 3D objects. The connecting element between the recording and reconstruction stages is the hologram itself in which the holographic code is stored. While in the early days holography was a purely experimental and analog technique, the building blocks of holography were later digitalized step by step. Holograms were first simulated and later reconstructed by computer, the hologram storage medium became discretized optical elements, pixelated sensors, and light modulators. Due to different approaches and use cases, the language of holography has evolved in diverse and sometimes confusing ways. In this paper, we address the differences and similarities between digital holography and display holography. Both techniques are digital, but their meanings in the community are sometimes different. In general and common understanding, the term digital holography (DH) refers to a digital hologram recording of a wave field emanating from a 3D object, followed by a numerical reconstruction of that object. On the contrary, the term computer-generated display holography (CGDH) describes the numerical calculation of the hologram and its physical representation, followed by an experimental reconstruction of the 3D object by optical means. Thus, it is the purpose that distinguishes the two techniques: digital holograms are used to numerically reconstruct and measure previously captured (unknown) objects or object changes, whereas computer-generated display holograms are utilized to visualize (known) 3D objects or scenes in a way that best mimics natural vision. The purpose of this paper is to clarify the terminology of holography, contrasting digital holography and computer-generated display holography. In particular, we will explain how each method works, emphasize their specific characteristics and mention how they are used in different applications. We will also provide some examples of how the two technologies are used.