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Home > Pages perso > Mees Loic > In-Line Holography

3D lens less imaging

Holography consists in recording the interference pattern between the light wave scattered (or diffracted) by an object and a reference wave. In line holography, or Gabor holography, is a special single beam set-up in which the lightning beam is also used as the reference beam

Digital in-line holography can be viewed as a lens less imaging technique. In back light imaging, the camera faces the incident light. The lens collects the light transmitted through the medium and forms an image on the camera’s sensor. This light is composed of the lighting beam and the light scattered (or diffracted) by the object. To obtain a sharp image of the object, it must be placed at a certain distance, entirely determined by the distance lens - sensor and the focal length f of the lens.
Digital In Line holography in based on a similar set-up, exepted that a coherent light source is requiered and that the lens is removed. The camera sensor is placed instead to record directly the superposition (intereference pattern) of the incident beam and the scattered light. Now, the optical operation made by the lens in classical imaging is replaced by a computation. The main advantage of this "numerical lens" compared to the real one, is that its focal length can be tuned to recontruct sharp images in several planes, at various distances from the sensor, and from on a single hologram. For example, sharp images of several objects spread out in a full volume can be obtained.
The main drawback of the technique is the twin image noise. Light is a wave and must be described by a modulus and a phase. In the classical imaging system, the lens made a transformation onto this complex quantity to form the image after propagation to sensor. In holography, the sensor records only the modulus of the interference pattern and the loss of the phase leads to the twin image formation. In cases where this twin image noise is limiting, it can be completely or partly removed, by using a reference beam ( in a more standard holography scheme), by using algorithms, more or less complex and efficient, or by following an inverse problem approach.