Geoscience Reference
In-Depth Information
as multiple peaks or small shoulder effects, then the bandpass should be adjusted to an
appropriate size to accurately record and maintain the integrity of these spectral features
The bandpass appropriate for the features of the spectrum that one wishes to resolve
should be used. This is determined by the photo-physics, chemistry, and biology of the
samples, as well as the closeness (in terms of spectral proximity) of the excitation and
emission peak wavelengths and any scattering caused by the sample. Ideally, the widest
bandpass that does not distort the spectral features should be used to ensure the best possi-
ble SNR in the measurement. Finally, it is important to note that bandpass and resolution
are not the same quantity. If the bandpass is equal to the slit width multiplied by the recip-
rocal linear dispersion and the slit becomes so narrow that no improvement in the bandpass
can be observed, then, in effect, the bandpass is equal to the resolution.
5.4.13 Stray Light
Stray light in a monochromator or spectrometer system is all light that reaches the image
plane of the monochromator from anywhere other than from diffraction by the grating
(according to the grating equation), that is, any light that is passed by the monochromator
that is outside of the interval
λ
0
±
Δλ
, where
λ
0
is the wavelength setting and
Δλ
is the spec-
tral bandpass. Usually, stray light is expressed as a ratio of the total light passing through
the exit slit at a specified wavelength compared to another wavelength. In some cases, the
stray light is specified by the relative amount of light that is passed
x
nm from a specified
laser line. Other methods of demonstrating stray light performance can be made using
near-end or far-end methods using either narrow line or broadband spectral sources. What
is clear is that manufacturers of optical systems measure and specify the stray light perfor-
mance of their products differently. Thus it is often difficult for users to directly compare
the performance of one device against another. In the end, the practical demonstration of
the monochromator stray light performance in the specific illumination and spectral condi-
tions of the user's application is the best means to determine its fitness for a given purpose.
There are many possible causes of stray light. All components in an optical system con-
tribute to the problem, including baffles, apertures, partially reflecting surfaces, scattering
from internal walls, and the fluorescence of optical materials. Ambient light from the room
where the system is physically housed is also a source of stray light.
Scattered light
in a monochromator is light that is neither diffracted nor absorbed by the
grating. Such light can arise from imperfections in the spacing and shape of the diffraction
grating grooves and also from the roughness of the grating surface. Three main processes
can classify scattered light from a grating:
•
Diffuse scattered light
emanates into a hemisphere in front of the grating and is due to the
micro-roughness of the grating surface. It is the primary scattering process for holographic
gratings. The intensity of diffuse scattered light is higher near the diffraction orders for a
particular wavelength than between the orders. Therefore the intensity of diffuse scattered
light exiting a monochromator is proportional to the slit area and also to 1/
λ
4
.
