High Optical Density Transmission
Whether a measurement be of a source, a filter etc, the spectral accuracy is dependant on the wavelength calibration, the optical setup, the rejection of higher diffraction orders and the stray light performance of the monochromator.
The latter aspect is of particular importance when it is question of measuring the transmission of filters of high optical density. When a monochromator is set to a given wavelength, λ, stray light is defined as that light which reaches the detector in addition to the contribution of the source at λ.
Stray light originates in that light in the monochromator which is scattered from the optics due to dirt and dust, and that scattered from structures within the device, such as mirror supports etc, and reaching the exit port. Good design minimises this effect, yet this is not sufficient in many applications where the stray light component is large with respect to that to be measured.
To reduce the effect of stray light upon a system, one may cascade two or more monochromators together, thereby re-selecting the desired wavelength, further dispersing the light, and on the way reducing the stray light measured; into the first monochromator enters the entire spectrum of the source; into the second merely a narrow bandwidth around the selected wavelength plus a broad spectrum of stray light etc.
With correct control of stray light measurement of filters with up to six decades of attenuation can be easily measured.