Optical Document Security, Third Edition
Containing over 150 illustrations, including actual security documents, this book delivers a comprehensive and cohesive treatment of all aspects of optical document security.
TABLE OF CONTENTS Optical Document Security, Third Edition
Chapter 6: Diffraction-Based Security Features
6.1 Optical Variability
As set forth in Chapter 1, our environment is optically invariable to a high degree. The colors of things surrounding us hardly or do not change with the angle of observation and illumination. This property of color constancy allows us to recognize objects more easily. This perceived constancy of brightness and color is the combined result of the diffuse scattering properties of most objects and our visual system, which automatically corrects for changes in the color of light (i.e., artificial light, daylight).
Optical variability, discussed in Chapter 2, is a property attached to objects that significantly change appearance, depending on the angle of illumination and observation. An important characteristic of optical variability is that it cannot be copied by photomechanical means, and OVDs therefore play an important role in document security.
6.1.1 Noniridescent Optical Variability
In Chapter 5, Section 5.2.5, optical variability was described as a property of transitory images. The shadow watermark also displays optical variability: a positive to negative contrast swap between observation in transmission and diffuse reflection (see Chapter 3, Figure 3.2). Therefore, the shadow watermark can be regarded as the oldest optically variable device. The laser engraved tilt image is another example of an optically variable device (see Chapter 3, Section 3.1.3). Furthermore, metallic foils and retroreflective foils can be regarded as noniridescent OVDs, and these are discussed in the next sections.
6.1.1.1 Foil Printing
Obviously, metallic foils are optically variable, as demonstrated in Figure 6.1. Foil printing, a straightforward hot foil...
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