The Discovery of Infrared Radiation

The field of radiometry examines the measurement and transport of electromagnetic radiation, regardless of wavelength. The term often encompasses the detection and assessment of this radiation’s quantity, quality, and effects. The discovery of infrared radiation traces back to 1666 when Sir Isaac Newton (1642 -1727) first demonstrated the dispersion of electromagnetic energy by passing sunlight through a glass prism, splitting it into the visible spectrum of colours. Sir William Herschel (1738 – 1822) made a significant breakthrough in 1800, advancing this exploration.

While investigating new optical materials, Herschel used a mercury thermometer with a blackened tip to measure the heating properties of the different colours refracted through a prism. During his experiment, he noticed that the temperature rose progressively as he moved the thermometer beyond the visible red portion of the spectrum. Remarkably, the highest temperature was recorded in the unseen region just beyond red light—now known as the infrared wavelength range. Sir William Herschel, in addition, not only discovered infrared radiation but attempted to draw the first distribution of energy as a function of wavelength.

Next, Johann Lambert (1728 – 1777) noted that the amount of radiated energy in a solid angle is proportional to the cosine of the angle between emitter and receiver. By the early 20th century, scientists like Max Planck, Josef Stefan, Ludwig Boltzmann, Wilhelm Wien, and Gustav Kirchhoff enhanced the theoretical framework for understanding the electromagnetic spectrum. Gustav Kirchhoff (1824 -1887) discovered that the emissivity of a surface is equal to the absorption. Joseph Stefan (1835 – 1893) determined the total radiant exitance from a source at all wavelengths to be equal to the emissivity multiplied by a constant time the temperature raised to the fourth power.

Nevertheless, the leading architect of modern radiometry is Max Karl Ernst Ludwig Planck (1858 – 1947). Planck is most noted for describing the blackbody radiation in a simple equation and developing the quantum theory of energy. He noted that the Rayleigh-Jeans approximation agreed well with experimentations for long wavelengths, and the Wien law worked well at shorter wavelengths and devised a mathematical approximation that incorporates both. William Coblentz (1873 – 1962) was the first to experimentally verify the Planck’s equation. Historically, the blackbody radiation was often crudely measured by observing glass melts.

In 1918, Planck was awarded the Nobel prize for this work. Interestingly, Planck himself remained sceptical of his own work. Often not appreciated today, describing the blackbody radiation represents a monumental achievement in science and engineering. Although instrumental, Planck was not alone in the development of the foundations of radiometry. Many others, like Newton, Melloni, Bouguer, and Leslie, contributed greatly and expanded the theoretical framework for understanding the electromagnetic spectrum. [1,2,3]