5.1 Introduction

Security printing inks and printing techniques are discussed in the previous chapter. This chapter treats printed security images and patterns, which are divided into three main groups: classic images, digitally watermarked images, and screen-decoded images. Reference is made to several excellent contributions on security printing [1 3] for further reading.

• Classic images. The somewhat vague term "classic" may be loosely interpreted in this context as "predigital age." Classic security patterns and images raise a variety of barriers for the counterfeiter. This chapter treats a few of the most important examples of these classic features in Section 5.2:

  1. High-resolution images with microprinting and fine guilloche patterns;

  2. Images with accurate front-to-back register (see-through register);

  3. Shapes that are difficult to originate (guilloches and portraits);

  4. Patterns that require printing techniques that are unavailable to counterfeiters, such as intaglio printing and latent images;

  5. Images that present printing difficulties, such as positive-negative guilloche line types.

• Digitally watermarked images. Digital watermarking involves the embedding of additional information in digital media (i.e., images, video, and audio). Such embedded information can be visible, such as in proprietary images, without seriously disrupting image content. The digital watermark can also be made imperceptible to the human senses, even under magnification, and require machine reading and dedicated image and signal-processing techniques to be uncovered. Digital watermarking is discussed in Section 5.3 of this chapter.

• Screen-decoded images. This type of image typically contains embedded information that is invisible or illegible to the naked eye but that can be visualized...