The electromagnetic spectrum
Having read the first chapter, you will now have an understanding of the history and applications of infrared photography. However, there is a lot of important theory behind this technique.
The chart that you can see below shows the range of the electromagnetic spectrum. The electromagnetic spectrum comprises of numerous waves that fall in to one of the ranges in the spectrum – radio, microwaves, infrared, visible, ultraviolet or gamma rays. The distance between two waves is called the wavelength and is measured in nanometers (a millionth of a meter). Using light’s wavelength, we can tell what color it is and which range of the electromagnetic spectrum it is in.
Humans can see light with wavelengths of between 400nm and 700nm, as shown on the chart below. This range is called visible light. Humans cannot see any light with wavelengths above or below this range without special equipment. This section of the chapter explores a few of the different ranges of radiation– ultraviolet, visible and infrared.
Ultraviolet light has a frequency of 10nm to 400nm, meaning that it is not visible to the human eye. Its wavelength is longer than that of X-Rays, but shorter than visible light. Ultraviolet has a large number of uses at various wavelengths. Bug zappers use UV at 350-370nm because flies are most attracted to ultraviolet light at 365nm, whereas 250-300nm UV is used for forensic analysis and drug detection.
Ultraviolet is also heavily used for security purposes. Many sensitive documents, such as passports or credit cards, include watermarks or images that can only be seen under UV light. This is to prevent counterfeiting and to show that these documents are valid. The picture on the right shows the Visa Bird, which is only visible under ultraviolet Light.