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photonicsproducts
OPTICAL FILTERS
Multiband coated filters redefine
performance standards for
scientific applications
ALANNAH JOHANSEN, RANCE FORTENBERRY, PETER EGERTON,
MIKE SCOBEY, and AMBER CZAJKOWSKI
Multiband filters can be categorized
into a variety of classes that solve
unique optical problems. Each class
presents its own set of fabrication chal-
lenges, placing limits on what is practically achievable and affecting the reli-
ability of the thin-film manufacturing
process. By understanding the scientif-
ic and industrial applications for multiband filters, the various filter class-
es and manufacturing possibilities are
better understood.
Multiband filter applications
Common to all multiband filters is the
ability to allow multiple but distinct
wavelength regions—typically ranging
from the ultraviolet (UV) to the mid-
wave infrared (MWIR), or approx-
imately 280 nm to 5 μm—to trans-
mit through the filter while blocking
the bands in between. 1 Multiband fil-
ters exhibiting this comb-like spectral
structure are a subset of hard-coated,
dielectric thin-film op-
tical filters made by
depositing alternat-
ing layers of materi-
als with varying indi-
ces of refraction onto a
substrate.
Multiband optical fil-
ters are critical to ap-
plications in the life
sciences. In fluorescence microscopy,
multi-bandpass excitation and emis-
sion filters are used in conjunction with
multichroic beamsplitters to allow for
the simultaneous detection of multiple
fluorescent tags in a single
sample. Ever-increasing ad-
vances in this field, such as
new classes of switchable
monochromatic LED and
laser light sources, have
increased the demand for
high-performance mul-
tiband filters having ex-
tremely high transmission
(93–98%), deep blocking
(optical density >OD5.5
by measurement and >OD8
by design), and steep tran-
sitions between bands
(near-vertical slopes ap-
proaching < 1%).
Additional advances
in laser fluorescence and
Raman spectroscopy applications have
driven the demand for high-perfor-
mance multi-notch filters that main-
tain broadband transmission while
concurrently blocking multiple la-
ser lines. The amount of laser light
required to energize a fluorophore
into an excited state is much higher
than that of the returned Raman sig-
nal, which is inversely proportional
to the fourth power of the excitation
wavelength. In some cases, this means
detecting a fluorescence marker 1012
smaller than the excitation intensity. It
becomes critical to block superfluous
Advances in thin-film technology have
given rise to a new class of multiband
coatings that task manufacturers
to offer improved performance at
competitive prices, enabling multiband
filters that redefine the performance
standards and drive innovation across a
variety of disciplines.
