EXAMINATION AND DIFFERENTIATION OF MATERIALS BY INFRARED
Infrared photography owes most of its value to the fact that many materials reflect and transmit infrared and visible radiations to different extents. It is thus possible by its means to see through some materials which are visually opaque, and to show contrast between things which appear equal both to the eye and in an ordinary photograph. This has made it a very helpful implement in many fields of special investigation. In this chapter we shall deal with its application in the textile and graphic arts industries, in the examination of paintings and documents, and in criminology. Much relevant material also may be found in Chapter XII, and particularly in Chapter XIII dealing with camouflage detection and in Chapter XVI which treats of the optical characteristics of a number of materials in the infrared.
In the textile industry, irregularities in the dyeing or weaving of cloth and damage to fibers as the result of excessive action of finishing solutions are difficult to study visually when the cloth or yarn is dyed in dark colors. Photography by means of panchromatic materials and filters usually does not help very much. Most dyes, however, exhibit characteristics of reflection and transmission in the infrared which are different from those in visible light. Infrared photography, therefore, should be of assistance in the examination of them, and it has been much studied in England and Germany in connection with textile problems.
In general, it may be stated that there are no dyes so far reported which appear light to the eye and dark in the near infrared, whereas there are many cases of dyes which are dark to the eye and which reproduce as light in an infrared photograph.