OPTICAL CHARACTERISTICS OF MATERIALS IN THE INFRARED
During the course of this book, mention has been made of the manner in which many materials transmit and reflect the infrared. A knowledge of this is necessary for the intelligent application of infrared photography and satisfactory interpretation of the results. In this chapter are brought together data concerning other materials which might be of interest to the photographer, including a somewhat fuller discussion of the properties of lenses, filters, safelights, and common materials. It is not the intention to give extensive tables of the optical properties of materials in the infrared, but merely to select those which are of particular interest. For data concerning other materials the reader should consult the books of chemical and physical constants, particularly the "International Critical Tables," Volume V, Landolt- Börnstein's "Physikalisch-Chemische Tabellen," "Tables Annuelles de Constantes et Données Numériques," "Smithsonian Physical Tables," and references at the end of this chapter.
Most colorless glasses, including window and plate glass, ultra- violet-transmitting glass and optical glass, transmit freely out to about 26,000 A. Beyond this point they absorb in a manner determined by their composition. Crown glasses show the strongest absorption and flint glasses the least--with the exception of quartz, which has still less. The transmission of colored glasses in the near infrared depends on the nature and amount of the metallic oxides, silicates, sulphides, and colloidal metals which are added to the basic glass.32, 62, 78 Some of them have very