7.1 Introduction

As shown in Figure 7.1, iridescent OVDs can be divided into DOVIDs, discussed in Chapter 6 and ISISs, discussed in this chapter. ISISs and DOVIDs have opposite physical characteristics (Chapter 2, Table 2.1): DOVIDs are composed of fine adjacent diffraction lines, while ISISs are composed either of one single or of multiple thin interference films.

Figure 7.1: Iridescent optically variable devices are based on light diffraction by gratings or light interference in thin film structures.

Thin film interference security structures are extensively discussed by Dobrowolski [1 3], whose publications are considered classic treatments of the subject. An overview of the various types of ISIS is presented in Figure 7.2. As Figure 7.2 shows, two different classes of multilayer ISISs are distinguished: single-layer and multilayer structures. While multilayer structures are abundant in security applications, only one type of single layer structure is known: pearl luster ink, which is discussed in Section 7.1.3.

Figure 7.2: Interference-based security structures are divided into single layer structures and multilayer structures.

Two different classes of multilayer structures are subsequently distinguished: multiple coatings consisting of a limited number of layers generally three to five in security applications and Bragg structures consisting of tens to hundreds of layers. Multilayer structures are discussed in Section 7.2.