In chapter 4 the so-called classical optical elements were introduced in some detail. Advanced scientific and technological optical systems require more sophisticated elements, in particular interface devices that provide a conversion between electronic and optical signals. The photographic camera and the various television recording devices record optical information while transparencies and light modulators convert electronic or other signals into optical signals. While electronic recording of optical information is mainly a subject for electrical engineering, the display end of the process is considered as an optical element. These optical elements that can be categorized as transparencies are the main subject of this chapter. The term “transparency” will be applied here to a variety of elements, starting from the conventional photographic transparency and ending with various electro-optical devices used for real time light modulation. Each specific recording-display process has its special characteristics with its limitations and attributes. Some of these are discussed in this chapter and also in chapter 11. However, the relationship between the actual optical information recorded and the information displayed on a transparency have certain aspects that can be considered in a generic way. The most important of these aspects are the overall response to light and the resolution. Other parameters to be considered for specific applications include the thickness of the recording medium, its specific transfer characteristics (whether amplitude modulation, phase modulation or both), temporal behavior, recycling capability and so on.