Infrared, Visible and Ultraviolet Radiation

Abstract

Sir William Herschel's discovery of “obscure rays,” extending beyond the red end of the visible spectrum, launched the exploration of the electromagnetic spectrum outside the visible range in the year 1800. The following year Johann Ritter demonstrated that invisible rays beyond the violet end of the spectrum are capable of chemical action. These three adjacent portions of the electromagnetic spectrum: infrared (IR), visible (vis), and ultraviolet (UV) are collectively known as optical radiation. Although infrared and ultraviolet radiations are invisible to the human eye, they are considered to be “optical” because they share some propagation and interaction characteristics with visible. As does the rest of the electromagnetic spectrum, optical radiation obeys the laws of electrodynamics and can be described both as electromagnetic waves and as energy corpuscles. All known electromagnetic radiations are customarily arranged monotonically according to their energy in a continuum called the electromagnetic spectrum . The electromagnetic spectrum spans many orders of magnitude in energy and, correspondingly, in frequency and wavelength. The optical radiation range is located between microwave radiation and X‐rays. The optical radiation range is composed, in order of increasing energy, of infrared, visible, and ultraviolet radiation. Although there are no sharp, well‐defined boundaries in the electromagnetic spectrum, the optical radiation range is conventionally defined as extending from 1 mm at the bottom end of the infrared to 100 nm at the upper end of the ultraviolet. The optical range is divided as follows: Ultraviolet 100–400 nm Light 380–400 to 760–780 nm Infrared 760–780 nm to 1 mm The reason for the “fuzzy” boundaries for the visible range is that they are defined by the physiological process of vision, which has some intrinsic variability. The main mechanisms that produce optical radiation are incandescence, electrical discharge, and lasing. The wavelengths in the optical radiation range have limited penetration into the human body. Therefore, the main organs affected by optical radiation are the skin and the eyes, although systemic effects have also been identified. Evolving in an environment where the sun is the main source of optical radiation, humans have developed adaptive characteristics, such as skin pigmentation, a hairy scalp, receded eyes, and aversion responses to bright lights and to excessive heat. These characteristics, however, provide only partial protection against optical radiation. Optical radiation can act on biological tissue through thermal and photochemical processes. The extent of damage depends on the intensity of the radiation, the wavelength, the exposure time, and the optical and physiological characteristics of the tissue exposed. The variability in biological effectiveness of different wavelengths (three orders of magnitude within the ultraviolet range) is particularly striking. This has led to the definition of “biologically effective” quantities, which are obtained by using a biological spectral effectiveness function. Both the eye and the skin are at risk of acute and chronic injury from optical radiation. The ocular media transmit visible and near‐infrared radiation to the retina, but most ultraviolet and far‐infrared radiation are absorbed in the cornea and the lens. The response of the skin depends strongly on its albedo and pigmentation.