**3.1 Large object investigation**

Whenever large object recording and reconstruction is needed, digital holography with CO2 laser (Allaria et al., 2003) has many advantages, with respect to digital holography in the visible region. First of all, a long wavelength radiation has a lower sensitivity to sub micrometric vibrations and this peculiarity provides a higher fringe visibility when large samples are investigated. A second advantage is related to the high output power of CO2 lasers allowing to irradiate more efficiently and uniformly the surface of large samples (Pedrini et al.,1996). The last advantage derives directly from Eq. 9. Working with a longer wavelength, it is possible to use larger angles and, ultimately, smaller distances between the object and the recording device. In Fig. 7, a configuration to realize Infrared Digital Holography (IDH) of large objects is shown (Pelagotti et al., 2010). In this case we used a CW CO2 laser emitting in the TEM00 mode at 10.6μm with 110W output power. Interferometric patterns where recorded by means of an ASi microbolometric infrared camera with a resolution of 640x480 pixels and 25μm pixel pitch. In this configuration the object is placed at the minimum distance allowed by Eq. 9. In this set up the laser beam is first divided by a ZnSe beam splitter (BS1) which reflects 90% of the impinging radiation and transmits the remaining 10%. The transmitted beam, which constitutes the reference beam, after passing through a variable attenuator (VA), is enlarged by a ZnSe spherical lens of 1 inch focal length (L2) in order to reach the thermocamera with a suitable low intensity. The object beam is enlarged by means of a spherical ZnSe lens (L1) of 1 inch focal length and directly sent to the object under investigation.

Fig. 7. Single beam setup: M1, M2 and M3 are plane mirrors; BS1 is a ZnSe 90/10 beam splitter; L1 and L2 are ZnSe spherical lenses 1in focal length; the object is a plastic mannequin 1.90cm high.

The speckle interferogram, whose visibility is controlled by means of the variable attenuator (VA), is collected by the recording device. With this configuration we recorded holograms of the bronze statue 30cm high shown in Fig. 8(a) (Paturzo et al., 2010). The hologram and its numerical amplitude reconstruction are shown in Fig 8(b), Fig 8(c), respectively.

Whenever large object recording and reconstruction is needed, digital holography with CO2 laser (Allaria et al., 2003) has many advantages, with respect to digital holography in the visible region. First of all, a long wavelength radiation has a lower sensitivity to sub micrometric vibrations and this peculiarity provides a higher fringe visibility when large samples are investigated. A second advantage is related to the high output power of CO2 lasers allowing to irradiate more efficiently and uniformly the surface of large samples (Pedrini et al.,1996). The last advantage derives directly from Eq. 9. Working with a longer wavelength, it is possible to use larger angles and, ultimately, smaller distances between the object and the recording device. In Fig. 7, a configuration to realize Infrared Digital Holography (IDH) of large objects is shown (Pelagotti et al., 2010). In this case we used a CW CO2 laser emitting in the TEM00 mode at 10.6μm with 110W output power. Interferometric patterns where recorded by means of an ASi microbolometric infrared camera with a resolution of 640x480 pixels and 25μm pixel pitch. In this configuration the object is placed at the minimum distance allowed by Eq. 9. In this set up the laser beam is first divided by a ZnSe beam splitter (BS1) which reflects 90% of the impinging radiation and transmits the remaining 10%. The transmitted beam, which constitutes the reference beam, after passing through a variable attenuator (VA), is enlarged by a ZnSe spherical lens of 1 inch focal length (L2) in order to reach the thermocamera with a suitable low intensity. The object beam is enlarged by means of a spherical ZnSe lens (L1) of 1 inch focal length and

Fig. 7. Single beam setup: M1, M2 and M3 are plane mirrors; BS1 is a ZnSe 90/10 beam splitter; L1 and L2 are ZnSe spherical lenses 1in focal length; the object is a plastic

numerical amplitude reconstruction are shown in Fig 8(b), Fig 8(c), respectively.

The speckle interferogram, whose visibility is controlled by means of the variable attenuator (VA), is collected by the recording device. With this configuration we recorded holograms of the bronze statue 30cm high shown in Fig. 8(a) (Paturzo et al., 2010). The hologram and its

**3.1 Large object investigation** 

directly sent to the object under investigation.

mannequin 1.90cm high.

Fig. 8. (a) Photo of the Perseus statue; (b) acquired interferogram; (c) numerical reconstruction.
