MCS260B
CORNERSTONE 260B MONOCHROMATORS
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7 MONOCHROMATOR RESOLUTION
Gratings are available in various groove densities (i.e. lines/mm). Higher groove densities give higher
reciprocal dispersion and therefore higher resolution. The monochromator bandpass with a 1200
lines/mm grating is half that of the same arrangement with a 600 lines/mm grating. Note that this simple
relationship is not accurate for slit widths below 50 µm, as the optical aberrations begin to play a role in
the bandpass and resolution.
Using a grating with a high groove density may increase resolution, but the spectral range narrows. The
dispersion of a grating changes inversely with the groove density. If the groove density is halved, the
dispersion is doubled. A monochromator mechanism can only rotate the grating through a limited range of
angles. The angle and groove density determine the transmitted wavelength.
The gratings can be rotated to 0 degrees, so the lowest possible wavelength for a UV grating is set by the
transmittance of air at about 180 nm. The ability to output wavelengths below 180 nm is also dependent
on the efficiency characteristics of the grating.
The resolution of a monochromator is generally determined by grating and slit width. The slit width is
usually selected to achieve the spectral resolution required. To optimize the throughput, always use the
widest slits you can, while maintaining other system requirements.
If the source image on the input slit is large and uniform, then doubling the width of both slits gives about
four times as much radiation into the monochromator for broadband source. If the slit is bigger than the
source image on the slit, then opening the entrance slit has no effect on the power through the
monochromator. In this case, doubling the width of the exit slit only increase the power by about 2 for a
broadband source.
7.1 DETERMINING RESOLUTION
Micrometer-adjustable and motorized slits allow for continuous variation of the width and height.
The resolution can be determined by multiplying the micrometer-adjustable slit width setting (mm)
by the reciprocal dispersion (nm/mm) to calculate the resolution for each grating.
Monochromators featuring fixed slit holders require fixed slits to be installed at the input and output
ports. Fixed slits are ordered separately and should be the same size at the input and output ports
(Fig 22).
Resolution is calculated for each grating at the grating's blaze wavelength, i.e. the wavelength with
the greatest efficiency. Actual performance is determined by the monochromator wavelength
accuracy, precision and calibration. Newport suggests having the monochromator recalibrated
annually by a qualified service technician.
For slits with widths of 50 µm or less, aberrations begin to play a role in the actual achievable
resolution.
Figure 22: Some of the Fixed Slit Sizes Available.
