A Robust Method for Full Chromatic Error Compensation in Digital Color Holography

Abstract

An increasing number of important applications rely today on the possibilities of using digital color holography to record and reconstruct colored objects at high precisions using a simple optical setup. However, using multiple wavelengths in the same optical system can lead to severe chromatic aberrations. Several actions have been taken in the last ten years in order to correct them, especially in the field of digital color holography [1]. Different wavelengths can be used for recording colored objects, but due to the chromatic aberrations, a classical optical system, not only will image the same object at different planes, but these images will have different sizes regarding the wavelength. Furthermore, in a simultaneous color detection scheme [2], the reference beams cannot be perfectly aligned, resulting in the occurrence of a lateral shift between the different images. Finally, in a usual Fresnel configuration, the recording distance only can be increased so far, ruling out the study of large objects due to the Shannon theorem, which reduces considerably the area of possibilities. In 1996, Schnars proposes to use a negative lens so as to reduce the spatial frequency spectrum of the object, as well as the recording distance [3]. Unfortunately, the use of a negative lens introduces aberrations that modify both the sensor-to-object distance and the virtual object size along each wavelength. The opportunity provided by digital color holography is that chromatic aberrations can be corrected by a pure and simple numerical compensation. Figure 1 describes the basic scheme for a digital color holographic set-up. Three laser wavelengths in the red, green and blue domains [2] are used to illuminate the object. The three color beams are combined before the reference and object beams are separated. Then, the object wave illuminates the useful area with a unique or several dissociated illumination directions. The reference beam is extended and spatially filtered to produce an inclined reference plane wave (off-axis holography) that is combined with the diffracted object wave, using a beam splitter. The sensor records simultaneously the three colors, providing real-time capabilities to the holographic setup. For a large object, the negative lens is placed just in front of the beam splitter, in the object optical path. Thus, a virtual object is produced in front of the sensor at a smaller distance than the initial one, and this enables the Shannon conditions to be fulfilled.