1. Light and Radiation
Spectral analysis
In the previous section we have seen that the human eye detects photons with the help of three different receptors, the S, L and M cones. The L cones, for example, cannot distinguish photons having energies which correspond to the wavelengths 560 nm (yellow) and 650 nm (red), but they detect them in quite the same way. Other types of cones are needed to decide if it is yellow or red light: the M cones detect 560 nm light much better than 650 nm light, and the S cones do not detect any of these wavelengths. The brain finally differentiates yellow and red light from the count rates of the cones. This is true with all colours that we perceive. These colours are therefore denoted to belong to the physiological colours.
How can we measure intensities of light versus the wavelength as a continuous curve? I.e., the optical power or the photon numbers at each wavelength of light? How can we obtain such a physical spectrum? This is the challenge of spectral analysis or spectroscopy. An instrument which is used to measure spectra is called spectrograph or spectrometer.
Radiation is absorbed and scattered by matter. Absorbance and scattering are substance and wavelength dependent. The colours we perceive in nature and observe with satellites in space are produced this way. Measuring physical spectra makes it possible to obtain data on vegetation on land and at sea, to detect pollutants in the atmosphere, to retrieve the temperature of the ocean surface and to map the ice cover in the Artic, to mention only a few.
How does a spectrometer split light into colours? Obviously this requires an optical component which acts upon light in different ways; for example, by deflecting light into different directions depending on its wavelength. There are different physical effects which make this possible: absorption, refraction and interference of light.
Source: NASA
Optical components which are based on such effects are said to be wavelength selective. In the next pages, three methods of wavelength selection are presented:
- Optical filters can be used to select narrow spectral bands by means of transmitting radiation through these bands and the exiting radiation is either absorbed or reflected.
- The velocity of light in transparent media (like glass or water) depends on the wavelength, also otherwise known as dispersion of light. This results in a wavelength-dependent index of refraction and is practically used with prism spectrometers.
- The diffraction and interference of light depends on the wavelength. This makes it possible to use optical gratings as wavelength-dependent optical elements in grating spectrometers.
