Conclusion

In this course, we have seen the different techniques and elements used for recording a hologram. We have established that in “analogue” holography, the principal of reconstructing the object wave front follows a “physical” approach, by using the recording laser.

As far as reconstruction by digital holography is concerned, it is necessary to correctly simulate the diffraction of the generally plane reference beam on the discretised hologram. In this case, the Fresnel transform can be implemented using direct or even by simulating a convolution.

Certain approaches based on the use of wavelets have been presented in other sources [] but have not been mentioned in this course.

Since the 1970s, “analogue” holography has been the object of a large number of studies which cover a large field of investigations: non-destructive control, study of fluids, study of particles, and live science . Research into holography is carried out notably for its use for non-contact and non-invasive measurement []. In this context, the research comprises comparison by interferometric channels of a hologram of an object said to be in a reference state with a hologram portraying its actual state. It seems that by this method it is possible to study structures which have been submitted to pneumatic, thermal or mechanic loadings in static, stationary or transitory conditions.

Holography allows for a global evaluation which is both qualitative, through the simple visualisation of the fringes which encode the displacement of the image and quantitative through the clearing of the fringes.

Over the last few years, Fresnel's “digital” holography has been much developed and has been used with some success in numerous fields. Indeed fascinating possibilities have been presented: imagery through a scattering medium [], digital colour holography [], the measurement of the surface outline [], the measurement of micro-composant parameters [], imagery by aperture synthesis [], suppression of the object aberrations [], and the mechanical measurement of sptial-division multiplexing [].

Digital holography is also a technology of the future which will be used in the comparison and the recognition of 3D objects as recent works on the subject has shown [] [].