Optical Spectroscopy Instrumentation Design
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conditions 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
contribute 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
.
• In-plane scattered light is light that is not wanted in the dispersion plane of the monochromator and is due mainly to variations in groove spacing or depth of the grating. The
intensity of diffuse scattered light exiting a monochromator is proportional to the slit area
and also to 1/λ
2
.
• Ghost light is primarily a scattering effected in mechanically ruled gratings and is caused
by periodic errors in the groove spacing at the time of ruling. It manifests as systematic
periodic spikes on the background signal. Holographic gratings do not, in general, suffer
from this potential scattering problem.
Stray light that is not caused by the scattered light from the grating is called instrumental stray light. Every monochromator will reflect light in the zero order and this must be
trapped to minimize its contribution to the overall instrumental stray light. Similarly light
from the other diffraction orders may also find its way to the exit slit and therefore contributes to the stray light. Careful instrument design, particularly with respect to baffles,
along with correct illumination of the optics, minimization of sharp edges, and “nonoptical” reflective surfaces will all contribute to minimizing stray light problems.
The key performance indicator for any instrument is the overall SNR, which evaluates
the ratio of diffracted light to unwanted light. Because this is an instrument function there
is no clear rule-of-thumb that indicates what grating type, ruled or holographic, might provide the higher SNR. It is the SNR of an instrument that will determine the system linearity
and dynamic range.
5.4.14 Cuvettes, Cleaning and Handling
There is a large range of optical cells available for spectroscopy measurements and they
vary in materials, size, shape, and spectral transmission characteristics. The most commonly used sample holder in fluorescence spectroscopy is a 10 mm × 10 mm × 45 mm
volume cuvette made from fused silica (for UV to near-IR operation), glass (visible), or a
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conditions 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
contribute 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
.
• In-plane scattered light is light that is not wanted in the dispersion plane of the monochromator and is due mainly to variations in groove spacing or depth of the grating. The
intensity of diffuse scattered light exiting a monochromator is proportional to the slit area
and also to 1/λ
2
.
• Ghost light is primarily a scattering effected in mechanically ruled gratings and is caused
by periodic errors in the groove spacing at the time of ruling. It manifests as systematic
periodic spikes on the background signal. Holographic gratings do not, in general, suffer
from this potential scattering problem.
Stray light that is not caused by the scattered light from the grating is called instrumental stray light. Every monochromator will reflect light in the zero order and this must be
trapped to minimize its contribution to the overall instrumental stray light. Similarly light
from the other diffraction orders may also find its way to the exit slit and therefore contributes to the stray light. Careful instrument design, particularly with respect to baffles,
along with correct illumination of the optics, minimization of sharp edges, and “nonoptical” reflective surfaces will all contribute to minimizing stray light problems.
The key performance indicator for any instrument is the overall SNR, which evaluates
the ratio of diffracted light to unwanted light. Because this is an instrument function there
is no clear rule-of-thumb that indicates what grating type, ruled or holographic, might provide the higher SNR. It is the SNR of an instrument that will determine the system linearity
and dynamic range.
5.4.14 Cuvettes, Cleaning and Handling
There is a large range of optical cells available for spectroscopy measurements and they
vary in materials, size, shape, and spectral transmission characteristics. The most commonly used sample holder in fluorescence spectroscopy is a 10 mm × 10 mm × 45 mm
volume cuvette made from fused silica (for UV to near-IR operation), glass (visible), or a