Influence of the collimation of the reference wave in off-axis digital holography

In digital holography, the novel concept of the digital reference wave has opened in numerous opportunities for various fields to which digital holography may be applied. Oftentimes, a plane wave is utilized in digital holography experiments to operate as the reference wave; thus, the simulation of the plane wave is necessary for performing a numerical reconstruction. Conventionally, perfect collimation producing a perfect plane wave impinging on the reference should be carefully adjusted with a shear interferometer in order to obtain acceptable results. In this paper, we claim that it is not necessary to use a perfect plane wave in digital holography experiments. We experimentally proved that by using the Michelson interferometer, no influence of imperfect collimation of the reference wave in an off-axis digital holography exists, as has been previously claimed. Our deductions were verified by applying perfect and imperfect collimations to three different surfaces (flat, spherical and step) for height inspection and the results were almost consistent. The consistencies of the measured height values for the three tested surfaces at both perfect and imperfect collimations prove that there is no influence of imperfect collimations in off-axis digital holography.