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At Deposition Sciences you’ll
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OPTICAL FILTERS continued
Traveling further down the edge of the
filter to the 0.1–0.01% region, a “shoul-
der” or a “sideband measurement arti-
fact” may appear. This feature is unique
to very steep transition measurements
from high blocking to high transmission,
much like this Raman longpass edge fil-
ter. It arises from the non-monochro-matic illumination source and SBW ef-
fect discussed above.
The instrument scans over a wave-
length on the very steep edge and with
a non-zero SBW. The finite spectral
bandwidth means that instead of be-
ing attenuated at the actual OD level
of the filter, additional periphery wave-
length noise is transmitted by the filter
within the band, registering as a signal
on the detector. The sum of primary
and periphery signals leads to a high-
er intensity reading than what is actu-
ally present. In a commercial instru-
ment, there is little that can be done
to reduce this shoulder, except add
supplementary filtering in the mea-
surement path.
When there is not
much to measure
Lastly, the third measurement problem
arises from sensitivity limitations. As
mentioned, a filter having a high attenuation (OD > 5) has very little light reach-
ing the detector, so you are actually mea-
suring the absence of light. The optical
and electronic noise at the detector lim-
its the lowest signal that can be mea-
sured accurately.
Recall that the Raman signal we are
trying to measure is on the order of 1012
smaller than the excitation signal, and
most detectors have a dynamic range of
106, which is highly wavelength-depen-
dent. This effect is referred to as a “noise
floor.” The result is a flat, noisy spectrum
that bottoms out at some minimum, even
though the theoretical design is known
to perform much better.
The noise floor can be reduced by increasing the grating SBW, thus allowing
more light, which can allow for higher OD measurement. However, this in-
creased sensitivity results in reduced spec-
tral resolution. Another way to address
noise floor is to “baseline” the unit with
a rear beam attenuator, which helps bias
the dynamic range of the detector.
So, although it’s theoretically feasible to manufacture optical filters with
edge transitions <0.2% and blocking OD
> 10, the measurement is not yet credi-
ble. This is why you often see the fine
print “by design” on filter specs—
vendors are not reluctant to admit these lim-
itations. Experienced filter manufacturers have built custom in-house solutions
to address the measurement shortcomings, which might involve a monochromatic high-intensity light source to circumvent the sensitivity issues, in addition
to photomultiplier tube arrays for single
photon detection.
