Theory of Spectroscopy

Spectroscopy pertains to the study of light by dispersing it into its component colours. Light may be regarded as a wave and each distinct colour of light has its own wavelength. The wavelength of light is usually described in units of nanometres (nm). Visible light exists between the wavelengths 380 nm to 780 nm. The energy of light is related to its wavelength, with shorter wavelengths having higher energy than longer wavelengths.

Ultraviolet radiation is light that has wavelengths between 100 nm and 380 nm. This has much higher energy than visible lights and can cause eye and skin damage, for instance, sunburn.

Infra red radiation is light that has wavelengths above 780 nm, up to 5000 nm. Any hot object emits infra red radiation, so infra red imaging techniques can be used in thermal imaging applications such as motion thermography.

Spectrographic Techniques

Absorption spectroscopy is the measurement of the spectrum of a beam of light before and after interaction with a sample. This technique is used in materials and chemical analysis. The principles are the same as that used in reflection and transmission spectroscopy. The absorption, reflection or transmission of light with a sample can affect the shape of a spectrum as well as its power. This makes materials analysis possible as a material can be identified by its spectral lines. Infra red light is particularly useful in this regard and can be used to pick out the water content of food and pharmaceutical products, plastic type for recycling, together with active ingredient content and concentration in powdered products.

Fluorescence Spectroscopy

Low wavelength light can be absorbed by a sample and re-emitted at lower wavelengths. If the emission spectrum for a particular material is known, this technique can be used to identify the presence of that material. This technique is very useful in the food industry, as ultra violet light absorbed by fat in meat samples is re-emitted as blue light.