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The region for precision.
Let’s say you need to evaluate a 785 nm
Raman longpass edge filter for integration into a spectroscopy setup for a flu-
orescence imaging system using confocal
Raman microscopy, similar to Figure 1.
The excitation and emission modes of the
fluorophore of interest can be similar in
wavelength, so the transition width of
the filter (distance in nanometers from
50% to 785 nm laser-line) often needs to
be <1% or better. In addition, the intensity of the returned Raman signal is inversely proportional to the fourth power of the excitation wavelength—that’s
on the order of 1012 smaller than the excitation intensity.
This means the filter requires very deep
attenuation at 785 nm to block any residual laser light emitted from the sam-
ple, typically requiring an optical density
(OD) > 6. Any superfluous light needs to
be completely attenuated to detect the ex-
tremely small emission signal. Likewise,
the transmission of the passband needs to
be optimized to transmit as much of the
signal wavelength as possible. The spec-
tral performance for the Raman long-pass edge filter would resemble Figure 2.
For components like this Raman precision filter, the measurement problem
1. Confirm transmission of the passband
2. Resolve and verify the steepness of the
nearly vertical edge is <1%
3. Measure the blocking to ensure >OD6
Confirming the transmission of the passband is straightforward and can be per-
formed with any well-calibrated spectro-
photometer. The difficulty comes when
moving toward the transition edge and
beyond to the verification of the OD of
the blocking. Determining these values
often requires more than one scan, with
tradeoffs as to what can be achieved with
each measurement. One could bump up
the integration time and scan at 0.25–
0.50 nm step intervals to achieve the accuracy needed, but time becomes a major factor and single scans could last up
to 30 minutes or longer depending on
the desired results.
Effects of spectrophotometer
When measuring the very narrow spectral
edge of precision filters, you need special
care to avoid misleading artifacts. First,
the resolution of the instrument can cause
a theoretical “square” edge to become a
“rounded” edge during the transition from
the high transmission region. This is directly related to the spectral bandwidth
and area of the diffraction grating—the
larger the area of diffraction, the higher
For a grating with a given number of
lines/mm, a larger grating area provides
higher resolution, but adds to the size
and cost of the instrument. Similarly, the
resolution can be increased by reducing
the spectral bandwidth (SBW) param-
eter, but this has the effect of reducing
the amount of light through the instrument, which, in turn, reduces the sensitivity at the detector.
The location of the edge in wavelength
space is also important—if the light source
is non-collimated and some f/# (cone angle) is present, the edge will exhibit a minor shift. Most spectrophotometers are
not perfectly collimated, so it’s best to use
very small apertures when measuring to
restrict the light to on-axis performance.
Again, this decreases the amount of light
in and the sensitivity of the system.
FIGURE 2. Spectral performance curve
for a typical 785 nm Raman longpass edge
filter with OD ≥ 6.0 coating performance.