Monochromator Selection - A Guide

The Bentham Monochromator Selection Guide has been written to enable new users to quickly and easily identify the most appropriate monochromator configuration, detection electronics, and accessories needed for a given application.

When choosing the most suitable monochromator configuration and accessories for a given application, the following three principal, however not sole, considerations should be taken into account:

  1. The spectral range of operation
  2. The required spectral resolution
  3. The impact of stray light

Bentham has several monochromator models that are available as spectroradiometers, with integrated detection electronics, to offer a complete measurement solution.

Spectral Range and Diffraction Gratings

Diffraction gratings are specified by a line density in grooves per millimeter. The higher the density, the greater the light dispersed, leading to a relatively limited spectral extent. Moreover, the higher line density gratings are generally optimized for short wavelength use and vice versa. Before deciding on the number of gratings needed to cover a given range, the available grating(s) and the operation range must be taken into account.

A multiple-grating monochromator will enable the performance of a spectral measurement over a great range where multiple gratings are needed. On the other hand, a single (interchangeable) grating monochromator may be used with the measurement being carried out in two or more parts.

Interchangeable turrets can be purchased for multiple grating monochromators to carry out measurements over a very wide spectral range.

Order Sorting

Long-pass order sorting filters should be inserted at various points to ensure that only the first diffraction order is measured. The standard filter wheel has six positions (of which one is occupied by a blank disk to act as a shutter).

An eight-position filter wheel is available for wider spectral range applications. An external filter wheel may be used for very wide spectral range applications, and in certain cases of those having multiple entrance and exit slits. This filter wheel may also house band-pass filters to aid the improvement of stray light performance or neutral density filters for the purpose of optical attenuation.

Number of Entrance/Exit Ports

Where multiple inputs to the monochromator or multiple detectors/ multiple outputs are needed to cover a wide spectral range, the monochromator can be provided with up to two entrance and two exit slits, with a solenoid-based swing away mirror (SAM) positioned between the two for automated selection purposes.

Slits

Rectangular slits (defined by width) defines the bandwidth of the monochromator. The easiest option is the fitting of fixed rectangular apertures to the entrance and exit ports of which a set of three are provided. When a greater degree of freedom is needed, an option of motorized or micrometer adjustable slits is available.

The particular benefits of the motorized slits is that the instrument bandwidth can be varied through the spectral range of interest, and also these slits can be automatically modified to maintain a given bandwidth where multiple gratings are employed, or where a double as one migrates from grating to grating and the dispersion changes. The motorized slits can either replace the slit or be integrated into the monochromator.

Options are available on the position of the slits according to requirements in terms of space, minimizing the use of mirrors etc.

Stray Light Performance

Although the main purpose of a monochromator is to act as a tunable filter and to present the desired wavelength at the exit port, some of the broad band spectral input may be scattered due to imperfection or contamination on the optics in an uncontrolled manner, leading to the presentation at the exit slit of stray broadband light in addition to light at the chosen wavelength.

Due to the manufacturing technique used, holographic diffraction gratings present less stray light but still have lower efficiency than mechanically ruled diffraction gratings. Although this effect may be ignored in many cases, where the signal to be measured at the given wavelength may be small with regards to stray light contribution, a second monochromator must be used. This constitutes a double monochromator, in which the stray light and the selected wavelength are again diffracted and the stray light taken under control.

In certain conditions, a mid-point may be adopted between single and double monochromator stray light performance when using a single monochromator with a selection of band-pass filters. Such filters have a restricted use in the sense that they may be unavailable for the spectral range of interest, and they may have transmission windows at longer wavelengths which will affect their function to some extent.

Double Monochromator Mode

In additive dispersion mode (by far the most common), the dispersion of the diffraction grating in the second monochromator improves the dispersion of the diffraction grating in the first monochromator. In certain applications, the resulting dispersion of wavelengths across the exit slit cannot be tolerated; in such a case, the second diffraction grating is configured to act in opposition to the first to yield zero net dispersion at the exit slit.

Applications for which subtractive dispersion is significant include the measurement of detector spectral response, high echelon systems, such as those used by calibration laboratories, where the uncertainty contribution of dispersion across the exit slit is unacceptable and for the measurement of very fast pulsed sources since the path length between entrance and exit slit is the same for all wavelengths in this case.

Internal Optical Chopper

An optical chopper is employed to modulate the optical signal on a known carrier wave in circumstances where the optical signal to be measured may be confounded with a background optical signal, whether from ambient lighting or, in the infrared heat (infrared radiation) emitted by instrumentation and the background. The optical chopper enables discrimination of the two contributions. In certain cases, it is convenient when the optical chopper is mounted internally to the monochromator.

Other Considerations

Atmospheric gases which may absorb light must be removed from the monochromator in applications in the IR and in the UV below 200 nm. This can be achieved by passing nitrogen (or other) gas through the monochromator, for which, the option to adapt purge ports is provided.

Bentham Instruments Limited

This information has been sourced, reviewed and adapted from materials provided by Bentham Instruments Limited.

For more information on this source, please visit Bentham Instruments Limited.

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