Monochromatic Light

Technologies using monochromatic light have a wide range of application, from astrophysics and astronomy to forensic science. The term monochromatic derives from the Greek words monos, meaning one or sole, and chromos, meaning color. Monochromatic light, or one-color light, is essentially electromagnetic radiation derived from photon emissions from atoms. Photons propagate, or travel, as energy wave fronts of different lengths and levels of energy. Energy levels determine the frequency of light, and the length of a wave determines its color. The bands of light wavelengths that humans can see are called visible light.

Visible light includes red light (in the lower energy level of the electromagnetic spectrum) and violet light in the higher visible energy level of the electromagnetic spectrum. As light propagates through different media, it interacts with atoms present in molecules, such as atmospheric gases, water, and organic matter. These interactions are known as atomic transitions, and consist of emission or absorption of specific wavelengths (or energy packages). The particular structure of isotopes (atoms or molecules of one element of the periodic table) as well as the structure of complex molecules (containing more than one element) defines their physical-chemical properties. Such properties will determine which wavelengths are absorbed and which ones are emitted. Absorption and emission of light by atoms occur in energy packages known as quanta. Absorption occurs when light excites atoms, making electrons suddenly jump to specific outer orbits. This is not a progressive movement between orbits, but a sudden change of energy state by which a given energy quanta is absorbed.

Emissions occur in the inverse manner, resulting in the release of the absorbed quanta. Monochromatic light and laser technologies take advantage of these atomic transitions as well as another atomic property known as ground state energy. Ground state energy refers to the tendency of electrons to return to the lowest energy level, therefore undergoing spontaneous emission of the energy quanta.

A monochromatic light beam is characterized by its brightness or light intensity, direction of propagation, and color (all visible characteristics) and by its state of polarization (an invisible characteristic). Light waves oscillate, or swing back and forth, perpendicularly to the direction of propagation. For example, if a light wave is propagating horizontally, it is oscillating vertically. The best example of monochromatic light is a laser beam. A laser light results from one atomic transition with a specific single wavelength, which results in a monochromatic light beam.

When a monochromatic light is directed to a substance or material, it induces transitions which are characteristic to the chemical properties of the constituent elements of such material. Optical spectroscopy instruments record the peaks and troughs of the resulting wave lights in a spectrometer that measures the changes in frequency and intensity of these transitions. The resulting wave patterns indicate the chemical composition of the sample. Scanning monochromators are optical instruments that disperse light, permitting the scanning of forensic samples or evidence, using one wavelength (or light color) at a time, and scan for the entire spectral range. Battery powered ultraviolet monochromatic devices are used to scan for evidence not easily detected by the naked eye at crime scenes. They allow hidden bloodstains, fibers, fingerprints, and lesions that are just beneath the skin on corpses to be visualized by the examiner.

Credit cards, currency, and important documentation are often marked with imprinted holograms on security stamping foils, which are created by monochromatic laser beams. Security standard holography represents the first generation of a security technology known as optically variable devices (OVDs). Other non-holographic OVDs technologies exist, and are detectable in marked materials with ultraviolet light devices.

See also Alternate light source analysis; Crime scene investigation; fluorescent light; Forensic science; Incandescent light; Light.

Sandra Galeotti