The analysis of fingerprints is important for biometric identification. Two-wavelength digital holographic
interferometry is used to study the topography of various types of fingerprints. This topography depends on
several conditions such as the temperature, time of the day, and the proportions of eccrine and sebaceous sweat.
With two-wavelength holographic interferometry, surface information can be measured with a better accuracy
compared to single-wavelength phase-retrieving techniques. Latent fingerprints on transparent glass, a
forensically relevant substrate are first developed by the deposition of 50–1000-nm-thick columnar thin films,
and then analyzed using the transmission-mode two-wavelength digital holographic technique. In this technique,
a tunable Argon-ion laser (457.9 nm to 514.5 nm) is used and holograms are recorded on a CCD camera
sequentially for several sets of two wavelengths. Then the phase is reconstructed for each wavelength, and the
phase difference which corresponds to the synthetic wavelength (4 μm to 48 μm) is calculated. Finally, the
topography is obtained by applying proper phase-unwrapping techniques to the phase difference. Interferometric
setups that utilize light reflected from the surface of interest have several disadvantages such as the effect of
multiple reflections as well as the effects of the tilt of the object and its shadow (for the Mach-Zehnder
configuration). To overcome these drawbacks, digital holograms of fingerprints in a transmission geometry are
used. An approximately in-line geometry employing a slightly tilted reference beam to facilitate separation of
various diffraction orders during holographic reconstruction is employed.