The monochromator bandwidth, defined in nm, is the range of wavelengths seen by the detector at one time, and is directly linked to the monochromator slits in use.
This is an important quantity to take into account, particularly when measuring sources have fine spectral features such as line emission- for example the measurement of a source having two spectral lines one nanometre apart with a system bandwidth of five nanometres, will result in the measurement of a single line. In many instances this is of no concern, since the power measured shall nevertheless be correct.
The effect of monochromator entrance and exit slits on monochromator bandwidth can be viewed in two manners.
In the first instance, the monochromator is an imaging system; the input port is imaged at the exit port; the dimension of the monochromator entrance slit defines the image size at the exit port.
Furthermore, at the exit of the monochromator, since the light incident thereupon is dispersed, one can imagine the wavelength axis running along parallel to the wall of the exit slit, and the size of this slit determines how many wavelengths can be seen at one time.
One can imagine therefore an infinite number of images of the entrance slit, of incrementally differing wavelength, presented parallel to the exit slit; whichever of the two are the largest, defines the bandwidth of the system.
In a double monochromator, a further slit is included, the middle slit (in the case of a system having additive dispersion).
The purpose of this slit is to reduce the amount of stray light going from the first to second monochromators and should at all times be set to at least 20% larger than the largest slit in the system, else tracking problems between the component monochromators shall result.
The slit function of a monochromator provides interesting information with regards the device performance and the system bandwidth.
The slit function may be determined by the measurement of a source of narrow spectral width, such as a laser.
One should perform a measurement at smaller steps than the system bandwidth (for example 0.1nm), over a spectral range of around four times the expected bandwidth, centred on the expected wavelength of the emission line, for example 632.8nm for the HeNe laser.
The full width half maximum (FWHM) of this spectrum provides the bandwidth of the system.
Inspecting the signal at one bandwidth, two bandwidths etc. relative to the peak, provides information of the stray light performance of the system.
If the entrance and exit slits are of the same dimension, the slit function shall have a triangular profile, otherwise, the function shall be flat-topped.
It is worthy to note that care should be made in making this measurement- it is not sufficient to shine a laser in the entrance slit of the monochromator.
This measurement should ideally be performed by filling the entrance slit, for example with the use of an integrating sphere, and illuminating the sphere with the source.
Finally, it follows of course that slit dimension has an impact of the light throughput of the monochromator, and in certain instances where a reduction in signal is required, either the entrance or exit slit is reduced, whilst maintaining the same system bandwidth.It is preferable that the slit to be reduced be the exit slit to avoid any conflict with the input optic.