Choose a monochromator/spectrograph based on
- A system that will allow the largest entrance slit width for the bandpass required.
- The highest dispersion.
- The largest optics affordable.
- Longest focal length affordable.
- Highest groove density that will accommodate the spectral range.
- Optics and coatings appropriate for specific spectral range.
- Entrance optics which will optimize etendue.
- If the instrument is to be used at a single wavelength in a non-scanning mode, then it must be possible to adjust the exit slit to match the size of the entrance slit image.
Note: f/value is not always the controlling factor of throughput.
For example, the light may be collected from a source at f/1 and projected onto the entrance slit of an f/6 monochromator so that the entire image is contained within the slit. Then the system will operate on the basis of the photon collection in the f/l cone and not the f/6 cone of the monochromator.
What are the criteria for choosing my monochromator?
Top criteria: Dispersion, resolution, bandpass
The dispersion shows the capability to disperse light. It gives the usable bandpass of a monochromator or indicates the spectral range of a spectrograph equipped with a multichannel array detector such as a CCD camera or InGaAs array. Changing the width of the slit aperture can adjust the bandpass.
Spectral resolution is inversely proportional to the linear dispersion of a monochromator. The resolution requirement of the experiment is often the key performance requirement of any application. For narrow structure analysis (resolution better than 0.1 nm in the visible range), large monochromators are the best choice because they offer increased spectral dispersion and thus a higher spectral resolution.
If the application’s most important requirement is to acquire a large spectral range in one shot, small spectrographs are better, as in a process application. In fact, single timonochromators with a focal length below 0.3 m are suitable where most stray light is not considered a major problem.
Laboratory researchers who don’t need high resolution and wide range at the same time, however, should choose larger spectrographs. These users can change gratings (i.e., to a higher or lower groove density) or acquire multiple spectra to achieve complete results.
Second criteria: Accuracy, speed
Monochromators work by scanning through the spectral features of the optical signals, step by step. As a result, the measurement process is generally slower than that of spectrographs with multichannel array detectors that operate in a fixed grating position and directly acquire a full spectrum according to their dispend mainly on the grating motor drive. Instruments with 0.5-m or higher focal length are usually equipped with sine bar mechanisms that give excellent accuracy (better than 0.1 nm) and repeatability (better than 0.01 nm). But the trade-off is speed: It can take several minutes to scan a large spectral range with high spectral resolution in monochromator mode. Smaller devices commonly use direct, or worm drives, because their resolution specifications are lower. In this case, they set the grating position within a few seconds.
Third criteria: Throughput, imaging quality
Most of the time, small monochromators and spectrographs have better throughput than large ones because of their larger numerical apertures (f numbers) and simpler design (often with fewer optical components).
However, the numerical aperture is not the final consideration in optical throughput. The linear dispersion is also important because it defines the input aperture sizes for a particular spectral resolution.
A more useful figure of merit for comparing monochromators for imaging applications is light-gathering capability. Manufacturers of small instruments usually find that to preserve spectral resolution, they can produce only a small image.
Producing a larger image, especially across a large spectral range, is very difficult because of the spatial corrections required.
The commercial introduction of imaging spectrographs has partially corrected the spatial imaging quality issues. These instruments use toroidal gratings or toroidal mirrors to correct for astigmatism in the image plane and to improve image quality, while keeping the numerical aperture at the same level as non-imaging devices. This correction requires complex calculations. The choice of the toroid, the optical incident angles of the device and wavelength optimizations shows the expertise of the manufacturer.
Well-corrected, fixed, compact spectrographs can provide excellent image quality. Some can discriminate three or four spectral channels over a 6-mm-tall focal plane. Interesting axial spectrograph configurations using prisms and lenses also offer excellent image quality over a few hundred nanometers in the visible range.
Some 30-cm or larger imaging spectrographs allow more than 10-channel analysis with a minimized channel overlap.
Final criteria: Stray light, design, focal length
Stray light relates mainly to the quality of the device’s optical components (mirrors and grating). The user is generally not aware of stray light or improper internal reflection, which can produce poor results.
Because of their slit/slit configuration, monochromators have less stray light or reentrant light than do spectrographs, which have no exit slit. However, when the stray light is important in an application, large-focal-length instruments, or double monochromators, are the best choice. Small devices present more risk of stray light than larger ones.
In terms of optical design, most large monochromators/ spectrographs use the asymmetric Czerny-Turner configuration. Smaller instruments tend to use an asymmetric “V” con-figuration as a compromise.
Weighing the trade-offs
No one device can cover all spectroscopic applications. However, a user who carefully analyzes the spectral and performance requirements of an application can weigh the tradeoffs involved in choosing between small- and large-focal-length monochromators and spectrographs.
If you need to analyse a short spectral range at low resolution, you can probably use an inexpensive, compact monochromator or spectrograph. Even if these devices have stray light, chemometric calibration methods can correct it without influencing the results. However, if you need high resolution, accuracy or versatility, large monochromators and spectrographs are often the safest buys. That is why these are generally the best instruments for research or high-technology industries.
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