Suppression of DC and Twin-Image Terms

Abstract

The intensity distribution on a hologram contains four terms

$$ \begin{array}{lll} {I_{\text{H}}} = {\left| {{E_{\text{R}}} + {E_{\text{O}}}} \right|^2} = {\left| {{E_{\text{R}}}} \right|^2} + {\left| {{E_{\text{O}}}} \right|^2} + {E_{\text{R}}}^{*}{E_{\text{O}}} + {E_{\text{R}}}{E_{\text{O}}}^{*} \hfill \\ \quad \quad \quad \quad \quad \quad \; \equiv {I_{\text{R}}} + {I_{\text{O}}} + {\mathcal{E}_{\text{O}}} + {\mathcal{E}_{\text{O}}}^{*}. \hfill \\ \end{array} $$

(7.1)

Of these, normally only one of the last two terms yields the desired holographic image, while the other terms – the zero-order and twin-image terms – only contribute to blurring and interference of the image. This is especially true in in-line configurations where all four terms are superposed on top of each other, but even in off-axis configurations they can limit the number of usable pixels and cause degradation of images. It is therefore a major consideration in any holography system design and there have been developed a fairly large number of techniques addressing the “dc and twin-image problem.” Some of these remove the DC term only, while others can suppress the twin image as well.