Digital Holography
Digital Holography
Shape measurement is very important in many fields which are interested in the 3D surface of objects, for example, car manufacturing and medical inspection. It helps to control the product quality and make the correct diagnosis. Holographic interferometry is a popular non-contact and non-destructive method for shape measurement. It can produce an image of a three dimensional object modulated by a fringe pattern corresponding to contours of constant elevation with respect to a reference plane.
Holography is a unique technique to record the complete wave field, i.e. both the amplitude and the phase of the light wave scattered by the object. It was performed only on photographic plates more than ten years ago. However, with the emergence of high resolution Charge Coupled Device (CCD) and the improvement of calculation capacity of PCs, digital holography, which is a way of digitally recording and numerically reconstructing a hologram, is taking the place of conventional holography in many applications. Because there’re no wet chemical processing and other time consuming procedures, digital holography can be done in almost real time. Through numerical reconstruction, it offers great flexibility on controlling some parameters, such as focusing, image size and resolution.
Fig. 1: Optical set-up of two-wavelength contouring method
In our project, we’re using digital holography to get the 3D surface information of the object. Two-wavelength method, two-illumination-point method and two-refractive-index technique are applied to generate the holographic contour interferograms. Two-wavelength method has been used more often than other two methods in many practical applications. The principal set-up of the two-wavelength method is shown in figure 1. The back scattered light interferes with the plane reference wave at the CCD sensor. Two holograms with different wavelengths λ1 and λ2 are recorded by CCD and are numerical reconstructed in the computer, respectively, to get the phase information Φ1 and Φ2. The phase difference ΔΦ = Φ1 - Φ2 is related to the height distribution of the object surface.
But in our research, we would like to capture one hologram with wavelength λ1 by CCD, then use different wavelengths λ1 and λ2 to reconstruct it. Using λ2 to reconstruct the hologram of recording wavelength λ1 introduces displacement and error. We’re going to find solution to correct this deviation so that we can get the phase difference which is related to the height distribution of the object surface by only one hologram. We’re making effort to do this job and at the moment it’s in progress but without suitable results to show.
Fig. 2: A hologram of a die
Fig. 3: The numerical reconstruction of Fig.2
Here is an example of digital holography. Fig. 2 is a hologram of a die and Fig. 3 is the numerical reconstruction of Fig. 2. The clear image in Fig. 3 is the real image on which the dots on each face of the die can be recognised clearly. The bright area in the centre is the zero-order term and the blurred area in the above is the out-of-focus virtual image.
Yan Li.
