Introduction

Holography was invented in 1947 by the Hungarian physicist Dennis Gabor [] while he was carrying out research on electronic microscopy. D.Gabor received the Nobel Prize for Physics in 1971 for this invention. However, it wasn't until 1962 [] and the invention of the first lasers that this technique had a concrete use.

Holography comprises the successful combination of interferences and diffraction. The interferences encode the amplitude and the contours of a three-dimensional object and the diffraction acts as a decoder which reconstructs a wave which seems to be formed from the previously illuminated object. This encoding as the etymology of the word ‘holography' would suggest (“holo” = whole, “graph” = writing/drawing), contains all the necessary information: the optic phase and therefore the depth and contours of the object.

In practice, the analogical process that has been known about for 40 years can be broken down into 3 stages :

• The first stage pertains to the reading of the interferences on a photosensitive plate.

• The second stage involves a chemical process in order to develop the plate and typical lasts for a good quarter of an hour for silver photo plates.

• The last stage is the process of physically reconstructing the object wave during which a laser is diffracted onto the sinusoidal network encoded within the photosensitive plate.

Considering the restraints related to the processing of holograms (an essential stage in the development process), which makes it difficult to use them in industry (for example in quality control on a production line), it was considered as early as 1972 [ ] to replace the silver photo plate with a matrix of holographic discrete values. The idea was to replace the analogue recording/decoding of the object with a digital recording/decoding process simulating a diffraction onto a digital network made from the recorded image. Holography had gone “digital”.

Work presented by Konrod et al. in 1972 was the first attempt at reconstruction by calculation of an object encoded in a hologram. At the time, it took 6 hours of calculation to reconstruct a field of pixels, using the Minsk-22 computer. The discrete values were obtained from a plate hologram by 64 bit digitization with a scanner. However, it wasn't until the 1990s that matrix detector-based digital holography became a reality.[]

Indeed, important evolutions have taken place in two areas of the technology :

• From that time period on, microtechnological processes have enabled detector matrices with sufficiently miniaturized pixels to be obtained in order to respond to the Shannon criteria concerning the discretisation of the spatial distribution of light.

• The computerised processing of images became accessible thanks, in the most part, to the notable improvement in the performance of microprocessors, particularly in their central processing unit (CPU) as well as in their virtual memory space.

This course will first tackle so-called “analogue” holography before moving on to so-called “digital” holography. These two sections will deal with the recording of a hologram as well as with the reconstruction of the encoded object, considering the types of photosensitive plate which are used. The case study section will introduce the results of experiments into the “digital” reconstruction of holograms. The exercise section will put forward several sets of problems linked to the experimental implementation of holography.