Monochromator, linear dispersion

The radiation from the flash is passed through a Bausch Lomb 500-mm monochromator equipped with a 1200-grooves per mm grating. The linear dispersion is 16.5 A/mm in the first order. Additional colored glass or interference filters can be used in conjunction with the monochromator to... 

The linear dispersion. This is the spectral width in nm transmitted through the exit slit of the monochromator. It is given in units of nm/mm. For example 5 nm/mm would mean that the nominal bandwidth of the monochromatic light is 5 nm if the slit width is 1 mm. This defines the purity of the monochromatic light. 

One of the advantages of a monochromator with an echellette grating is that in contrast to a prism monochromator, the dispersion of radiation along the focal plane is, for all practical purposes, linear. Figure 25-9 demonstrates this property. The linear dispersion of a grating greatly simplifies the design of monochromators. 

The lituar dispersifocal planes as shown in P iguro 7-18. If /is the focal length of the monochromator, the linear dispersion can be related to the angular dispersion by the relationship... 

A giatfng monochromator wiih a reciprocal linear dispersion of 1.2 nm/mm i.s to be used to separalc the sodium lines at 5SS.9950 nni and. 89.5924 nni. In the ory. what slit width would be required ... 

By the same token, an important criterion of monochromator performance is reciprocal linear dispersion, commonly expressed in Ang-stroms/millimeter. If the reciprocal linear dispersion of a given monochromator is, say, lOA./mm., then at a slit width of 1 mm., the resolution of the unit is lOA. For best energy, the reciprocal linear dispersion... 

Dispersion of a Grating Monochromator. The resolution of a monochromator measures its ability to separate adjacent wavelengths from each other. Resolution is related to a useful quantity called the reciprocal dispersion, or reciprocal linear dispersion,... 

The reciprocal linear dispersion dX/dx) is the function of the geometric slit width S) and spectral bandpass (AA ,) of the monochromator ... 

AAS measurements are almost exclusively carried out in the first order, and a good grating has 2000 to 3000 grooves mm. The reciprocal linear dispersion of 1 nm mm or less can easily be obtained with the usual focal lengths employed. Comparable reciprocal linear dispersions are achieved with a conventional prism monochromator only in the far UV region, while in the near UV and visible regions grating monochromators are superior. 

At the output side of the monochromator, the light exits through a slit. The width of the slit is variable (usually between 10 pm and 3 mm) and determines the intensity and wavelength range of the light. An important parameter in this regard is the linear dispersion of the monochromator. Its value can be estimated with the expression... 

The effective bandwidth (also called the spectral bandpass or spectral slit width, which is one half the bandwidth when the two slit widths arc identical, is seen to be the range of wavelengths that exit the monochromator at a given wavelength setting. The effective bandwidth can be related to the reciprocal linear dispersion by writing Equation 7-9 in the form... 

By substituting Equation 7-7 into Equation 7-9 we have the reciprocal linear dispersion for a grating monochromator ... 

Thus, for all practical purposes, if the angle r is small, the linear dispersion of a grating monochromator is constant, a property that greatly simplifies monochro-malur design. 

Quimby et al used a prototype version of the Spectraspan III echelle grating monochromator (Spectrametrics, Inc., Andover, Mass.) with a reciprocal linear dispersion of 0.122 nm/mm at 400 nm. The entrance slit width was 50 m slit height, 500 m exit slit width,... 

Grating monochromators provide a linear dispersion, i.e. distances at the focal plane correspond to the wavelength scale. Reflection gratings are free from adverse effects caused by Hght absorption as they are encoimtered with prism monochromators. 

---