HOW TO SPECIFY A MONOCHROMATOR


There are a number of basic parameters that need to be examined in order to draw up an informed monochromator specification:

How does each parameter affect the configuration...

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 sealed to contain a vacuum. Because it has an air pathway its wavelength range is limited, on the low side, by oxygen absorption at about 185 nm. The upper limit is usually 50 microns or higher.

The VUV class of instrument is more expensive than the UV-VIS-IR class because it needs to hold a vacuum.  This means that all seams need either to be welded or O-ring sealed and all accessories need to hold a vacuum or be isolated.  A pumping system needs to be added that can bring the whole system down to 10e-6 torr.  Also, care has to be taken to minimize contaminants throughout production.  A VUV instrument has a typical wavelength range of 30 nm to 1 micron.

The UHV class of instrument is the most expensive to make because: stainless steel is used to further minimize contaminants, metal seals are need and they are not mass produced.  When this type of system is evacuated to 10e-10 torr, it has a typical wavelength range of less than 1 nm to about 200 nm.

Resolution
There are two parameter that directly affect resolution. Grating groove density and instrument focal length. Resolution is directly proportional to each of these parameters.  If one doubles the groove density, the resolution doubles. If one doubles the focal length, the resolution dimension halves. However, the higher the groove density, the higher the dispersion and the lower the apparent reflective efficiency, unless corrected for by slit width. While the price is not directly proportional to resolution, there is a close correlation. A higher resolution system costs more because: it is larger and optical and mechanical tolerances are tighter.

f Number
This is a measure of energy throughput. The lower the f number is, the higher the energy throughput is.  It becomes important when the energy source is at a lower level. Generally the input size (aperture - slit size) is constant for a family of monochromators.  Thus, as the focal length grows, the f number proportionally decreases. Because the f number is a measure of optical size, the lower the f number (smaller value of focal length divided by aperture), the higher the price.

Stray Light Rejection
If stray light requirements are tight, one can do two things: add a predisperser or use a double monochromator. Not only does this double the number of instruments, it also greatly increases the alignment tolerances. Thus, it will normally (at least) double the price.

Wavelength Accuracy and Reproducibility
This parameter becomes important the spectral areas of interest are very thin. If these requirements need to be tight, one should consider a system with a digital scanning drive which is directly coupled to a mechanical lead screw wavelength drive. The mechanical sub-step insures stability and repeatability and provides a means of manually tuning the 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 slit and an exit slit. If one needs additional ports for mounting more than one source or detector, they need to be added. This increases the flexibility and the price.

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