Gratings, diffraction concave

Figure 3 One-dimensional spectrographs used for Raman spectroscopy. Several variations on these designs are available. FP, spectrograph exit focal plane ES, entrance slit CM, concave mirror TG, transmission diffraction grating L, multielement lens XG, convex diffraction grating CG, concave diffraction grating.

The dispersing element is a diffraction grating preferably used under conditions of grazing incidence (6 in Equation 3.9 about 89°) to improve the reflectance. The grating may also be concave to avoid the use of a focusing mirror. 

The relationships in Figure 4-11 are strongly reminiscent of those that exist for ordinary light diffracted from a concave Rowland grating.24 To understand this process, it is best to look first at the reflection of light from a concave mirror. 

Modem gratings may be etched on concave surfaces so that they will serve a dual purpose of diffracting light and also focusing the radiation. This decreases the number of parts in a spectrometer and also decreases losses in intensity by having fewer optical parts. 

Rowland circle spect A circle drawn tangent to the face of a concave diffraction grating at its midpoint, having a diameter equal to the radius of curvature of a grating surface the slit and camera for the grating should lie on this circle. ro.lsnd, s3r-k3l ... 

Many modern spectroscopes employ the diffraction grating instead of the prism. In the concave grating spectroscope, developed by Rowland, the collimating lens and telescope or camera objective are unnecessary because of the focusing effect of the grating itself. See Fig, 2,... 

Polychromators. Polychromators are multichannel spectrometers with PMTs as detectors. Instmments with up to 64 PMTs are commercially available. These instruments generally use a concave diffraction grating as the dispersion device, as shown schematically in Fig. 7.13. As can be seen, a concave grating focuses light of different wavelengths, Ai, A2, A3, and so on, at different points on the circumference of a circle. 

The principle of this method is depected in Fig. 6.85. The signal pulse at the frequency co and its frequency-shifted replica 2A(o + Sa> pass with a time delay x through a beam of noble gas atoms. The high harmonics at and n(concave grating and reach the detector. 

Fig. 3.8 Schematic diagram of a Czemy-Tumer monochromator illuminated by a focused beam of light. The diffraction grating consists of a highly reflecting planar surface in which sawtoothshaped grooves are etched. Mirrors Ml and M2 are concave-shaped, mirror M3 is a planar mirror that directs the light to the exit slit The aperture in front of the lens prevents overfilling of the mirror, which would result in stray light inside the monochromator...

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