# Holostereosynthesis: An Improvement to [PERSON]'s Photostereosynthesis Technique
## 1 Introduction  
[PERSON] [PERSON], often referred to as the [PERSON] brothers, are French engineers and industrialists who played a leading role in the history of cinema and photography. Their best-known joint contributions among 196 patents are Cinematographe [1], a motion picture system invented in 1895, and Autochrome [2], a color photographic process in 1903. One of the later inventions of [PERSON] alone, presented to the French Academie des Sciences on 8 November 1920, was photostereosynthesis--_la Photo-stereo-synthesize_--an early photography technique that could recreate three-dimensional (3D) images [3]. To obtain 3D images without a specific artifice, [PERSON] proposed a technique with a concept not based on photographs with a horizontal shift, as is the case for conventional stereoscopy [4], but on the axis. Photostereosynthesis required a series of six to eight photographs of the same subject, usually a portrait, each recorded with a different focal length (Figure 1a) thanks to a unique large-format camera built by [PERSON] that mechanically narrowed the depth of focus of the image. The photographs were then reassembled, maintaining the same distance as the initial object, affected by a coefficient corresponding to the adopted scale. Each lightly developed positive photographic plate was printed on a glass plate as transparent positive, mounted in a wooden frame, and stacked to produce a translucent 3D image several centimeters thick (Figure 1b) when illuminated by backlighting through frosted glass for an observer placed just in front. Theoretically, photostereosynthesis requires the recording of an infinite number of images. However, [PERSON] found that, in practice, a small number of photos with an extremely shallow depth of field of a few millimeters was sufficient to obtain a convincing result. Owing to the complexity of recording images at the time of [PERSON], photo-stereosynthesis remained a laboratory experiment without follow-up. Its implementationwas difficult from both the photograp's and the subject's perspectives. To obtain a perfect final 3D image, the model had to remain still during successive exposures. The final device was a complex assembly of glass plates mounted on a wooden frame, and it was fragile, bulky, heavy, and difficult to build and observe. However, [PERSON]'s concept laid the foundation for the current techniques used in medical scanners [5], 3D printers [6], and auto-stereoscopic multi-layer displays [7], and was a precursor to plenoptic cameras [8], also known as light-field cameras, that will soon be available on smartphones [9]. Today, only a dozen works are known, and the original camera used to capture them has disappeared [10]. The portraits recorded by [PERSON] were limited to his entourage and a few famous people, such as the president of the French Republic at the time, [PERSON] (see Figure 2a), World War I Supreme Allied Commander [PERSON] (Figure 2b), and the French engineer and inventor [PERSON] (Figure 2c). Modern observers of these historical portraits notice a strong resemblance to holography [11; 12], a 3D technique based on the interference and diffraction of light waves invented by Hungarian-British physicist [PERSON] in 1948 [13]. For [PERSON] in 2020 [14], this style of assemblage created a holographic illusion, and for [PERSON] in 2021 [15], the depth impression was similar to a holographic display. The purpose of this research is to replace the complex assembly of glass plates that comprise [PERSON]'s photo-stereosynthetic device with a single reflection hologram, thus creating 'holostereosynthesis.' Figure 1: Principle of photostereosynthesis illustrated by the portrait of [PERSON], [PERSON]’s brother (circa 1920). A series of photographic glass plates numbered from 1 to 8 and recorded with a progressive shift in focus (**a**) were viewed together through transparency and illuminated by backlighting through the frosted glass to reconstruct a three-dimensional portrait (**b**).