Double monochromators

Double monochromators

When using a single monochromator such as that shown in figure 1-9, it is possible that light, entering from the entrance slit, be scattered off the walls and structures constituting the monochromator, reach the exit slit. Therefore, at a given wavelength,  , an artificially high signal is measured.
This is termed stray light and is of concern where low light level measurements are performed where there exists a significant light component at other wavelengths.
Classical examples are measurements of UV sources and high optical density filter transmission.
Consider for example the measurement of a quartz halogen lamp, a lamp often used as calibration standard.
The following figure demonstrates the effect of scattered light in measuring the lamp UV output where there exists a significant amount of visible light.

Figure 1-13:- Measurement of QH lamp with single and double monochromator

A double monochromator situates a second single monochromator at the exit of the first.
Entering the first monochromator is all the light from the source to be measured; entering the second monochromator is the wavelength selected and a level of stray light, which one desires to reduce.
In the second monochromator, the desired wavelength re-selected; at the exit slit one finds that the level of the stray light has reduced to the square of the case of the single, for example a factor 103 down in a single, a factor 106 down in a double. 
There are two possible configurations of double monochromator; with additive or subtractive dispersion.
With additive dispersion, the first monochromator is followed by a device of similar type.The band of light transmitted from the first to second is further dispersed, resulting in twice the dispersion of a single system; for a given required bandwidth therefore, the monochromator slits may be doubled in size with respect to a single monochromator, which increases the system throughput.
At the exit of the first monochromator, the light to be transmitted to the second monochromator is dispersed across the slit; at the exit of the second monochromator this dispersion does not exist and all the wavelengths are combined.
The dispersion of a subtractive double monochromator therefore is the same as that of a single monochromator.
The subtractive configuration is often employed in such systems as primary transfer standard where the uncertainty of dispersion across the detector slit is unacceptable (yet for most applications of no real consequence).
A further important point is that of the slits of the double monochromator.
With additive dispersion, it is the entrance and exit slits which define the system bandwidth, the middle slit between the two monochromators being employed to reduce the stray light being transmitted to the second element. The middle slit should be at least twenty percent larger than the largest slit of the system to prevent tracking problems (beating) between the two component monochromators.
With subtractive dispersion, it is the entrance and middle slits which define the system bandwidth; the exit slit is employed to reduce the system stray light and again should be at least twenty percent larger than the largest slit of the system.