Feature 25-3 Ruling Gratings

A dielectric is a nonconducting substance or insuiator. Such materiais are usuaiiy opticaiiy transparent. 

Filters have the advantages of simplicity, ruggedness, and low cost. Since one filter can only isolate a single band of wavelengths a new filter must be used for a different selection. Therefore, filter instruments are used only when measurements are made at a fixed wavelength or when the wavelength is changed infrequently. 

In the IR region of the spectrum, most modern instruments do not disperse the spectrum at all, although this was common with older instruments. Instead, an interferometer is used, and the constructive and destructive interference of electromagnetic waves is used to obtain spectral information through a technique called Fourier transformation. These IR instruments are further discussed in Feature 25-7 and in Section 26C-2. 

A transducer is a type of detector that converts various types of chemical and physical quantities into electrical signals such as electrical charge, current, or voltage. 

To obtain spectroscopic information, the radiant power transmitted, fluoresced, or emitted must be detected in some manner and converted into a measurable quantity. A detector is a device that indicates the existence of some physical phenomenon. Familiar examples of detectors include photographic film (for indicating the presence of electromagnetic or radioactive radiation), the pointer of a balance (for indicating mass differences), and the mercury level in a thermometer (for indicating temperature). The human eye is also a detector it converts visible radiation into an electrical signal that is passed to the brain via a chain of neurons in the optic nerve and produces vision. 

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A further feature of modern Raman spectroscopic microscopes is that the laser is polarized, allowing determination of, for example, depolarization ratios, and molecular orientation in crystals or liquid crystals. Care must be taken to account for the polarization response of the vertically ruled diffraction grating however. To date, there have been few or no polarization dependent studies of biological materials although recent studies have demonstrated that polarization dependent Raman can detect structural changes in the extracellular matrix associated with basal cell carcinoma. ... 

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