How to Specify a Monochromator
Wavelength Range
The UV-VIS-IR class of instrument is the least expensive to manufacture in that it has an air pathway and does not have to be vacuum sealed. Because it has an air pathway, its wavelength range is limited on the low side, by atmospheric absorption at about 185 nm. The upper limit is usually 40 microns or higher but users should be aware that in the Infrared are also absorbance bands from atmospheric gases.
A vacuum capable instrument requires more elaborate fabrication techniques. All seams need to be welded or O-ring sealed. All attached accessories also need to hold a vacuum or be isolated by a suitable window. A pumping system capable of achieving 10E-6 torr is required too. Throughout the instruments production, care is taken to minimize residual contaminants. Vacuum instruments of various designs are available for work at ~1 nanometer all the way to the Infrared. Vacuum UV instrument typically work from 30 nm to the Visible light region. Grazing Incidence types are used at shorter wavelengths.
Ultra-high Vacuum (10E-10 torr) instrument are fabricated from stainless steel to minimize contaminants and allow baking. They use exclusively metal seal flanges. UHV fabrication techniques may be applied to any monochromator design.
Resolution
A few parameters directly proportional to spectral resolution are grating groove density, diffracted order and instrument focal length. Higher grating groove density equals higher dispersion and lower apparent throughput efficiency, unless corrected for by slit width. Optical aberrations may also be introduced by the high angles of rotation required by high groove density gratings. While the price is not directly proportional to resolution, there is a close correlation. A higher resolution is larger, the necessary mechanisms of slits and drives must meet tighter optical and mechanical tolerances.
f Number
This is a measure of energy throughput. Lower f-number means better light gathering power and energy throughput. It becomes especially important in signal-limited experiments like Raman or luminescence or for fast measurements. Generally the input (aperture or slit size) is constant for a family of monochromators. As the focal length increases, f-number decreases.
Stray Light Rejection
If low stray light is important add aggressive filtering to your setup. You might also use a double monochromator. This doubles the number of instruments and increases the alignment tolerances significantly. Triple spectrometers even more so.
Wavelength Accuracy and Reproducibility
This parameter becomes important the spectral areas of interest are very narrow or when numerous measurements of the same region are required. If these requirements need to be tight, one should consider a system with a digital scanning drive directly coupled to a mechanical lead screw drive. The mechanical sub-step insures stability and repeatability and conveniently provides means of manually tuning wavelength.
Level of Automation
For every monochromator function that needs to be motorized or controlled remotely, a motor and a controller needs to be added. More often than not, this also means that software needs to be written. The more automation, the higher the price.
Number of Ports
A monochromator needs two ports to function: an entrance and an exit. Additional ports for mounting more than one source or detector may be added to most models. They will need turning mirrors and possibly automation. This greatly increases instrument flexibility and the price.
