Infrared Technology Fundamentals

Monroe Schlessinger

• Publisher: Routledge
• Year: 2019
• ISBN:
  9781351438346
• Edition: 2
• Topic: Naturwissenschaften
• Subtopic: Optik & Licht

This work provides a basic understanding of the physical background and engineering considerations required for the design of IR systems, examining all components and combining them into examples of current surveillance systems. This second edition presents: new coverage of state-of-the-art optical systems, including lightweight mirrors and adaptiv

1

What Is Infrared?

I. HOW IT ALL BEGAN

In the year 1800, Sir William Herschel, the royal astronomer to the King of England, was conducting an experiment with a prism in sunlight. The prism spread the sun’s rays into a spectrum from violet to red, as shown in Fig. 1. Herschel placed a thermometer in the violet color and recorded the temperature. He moved the thermometer through the colors from blue to red and noticed that the temperature increased progressively. He then moved the thermometer beyond the red end of the visible region and the temperature continued to increase. Thus he found energy beyond the red; this energy has come to be known as infrared (IR). More important, Herschel demonstrated that there is a relationship between temperature and wavelength.

For scientific purity, Herschel placed two other thermometers on the same moving platform as the one in sunlight. They showed no change in temperature throughout the experiment. Figure 2 shows the visible and heat spectra produced by Herschel in the year 1800 [1]. The extent of the S curve is probably out to about 1.0 to 1.2 μm.

II. THE ELECTROMAGNETIC SPECTRUM

This experiment was conducted within a limited portion of the overall electromagnetic spectrum, which includes gamma rays and x-rays; ultraviolet, visible, and infrared radiation; and microwaves and radio waves. All of these radiations have in common two properties: they propagate through space as transverse waves and their frequency is equal to the speed of light in vacuum divided by their wavelength.

Figure 1 Herschel’s experiment.

Figure 2 These spectra, drawn by William Herschel, indicate the heating and illuminating power of the solar spectrum. The spectrum R was prepared from Herschel’s observations of the illuminating power. The spectrum S was prepared by observing the temperature rise of a thermometer placed at that point in the dispersed radiation. (From Ref. 1.)

Figure 3 shows the electromagnetic spectrum from wavelengths of 0.1 Å (10⁻¹¹ m) to 100 km. These correspond to frequencies of 3 × 10¹⁹ down to 3 × 10³ Hz. The portion from 0.4 to 30 μm has been expanded in the lower band in Fig. 3. In this band the wavelengths in reciprocal centimeters (also call...