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a) b) c)
so for collimated input light, rotating the angle of incidence of
one of these filters serves to smoothly tune its transmission spec-
trum. And, because there are two of these filters, they can be used
to independently define the short- and long-wavelength edges of
the overall transmission curve. In this way, both the center wave-
length and bandwidth of the transmitted light from any collimat-
ed source are fully adjustable. Just as important, a third glass
plate is simultaneously rotated to offset any slight lateral walk-
off in the beam path as the angles of incidence are set or swept.
The bandwidth of TwinFilm products can be user- or fac-tory-adjusted anywhere between 1. 5 and 20 nm, and the center wavelength can be chosen to be at virtually any visible
wavelength (350–900 nm). Moreover, because they work with
collimated light, the infinity space (the collimated space) in
a microscope is an ideal location for compact devices based
on this technology. In addition, the collimated input/output
allows for simple fiber coupling, as is already offered in some
commercial devices based on this technology.
This construction gives TwinFilm filters several key advan-
tages. Like a grating-based device, the out-of-band extinction is
very high ( 10-6). But, like a traditional filter (and unlike a grating monochromator with a narrow slit geometry), the spectral
performance of a TwinFilm device is uniform across 95% of the
large (up to 10 mm diameter) clear aperture. These characteristics
make it well suited for spectral filtering in imaging applications.
Automated, manual, and
Currently, there are three different embodiments of this core
technology. These provide automated wavelength scanning,
manually adjustable wavelengths, and custom-select fixed-wavelength characteristics.
The greatest functionality is provided in the form of a flexible wavelength selector, which is equivalent to a fully featured
monochromator, but with a large, uniform imaging aperture.
Here, rotation of the internal optics is motorized, and all control is via USB interface through software with a user interface
(as well as through a set of serial commands). The center wave-
length can be scanned (or set) with a resolution of ±0.1 nm,
and the bandwidth can be varied from 1. 5 to 20 nm. This type
of device will enable researchers to fully optimize every spec-
tral-imaging application, particularly those involving new fluoro-phores or varying background matrices, such as new fluorescent
FIGURE 3. An example of bandpass tuning in action shows wavelength-filtered images of a sample of HeLa cells; the blue image (a) captures
DAPI (a nucleic acid label), the red image (b) is fluorescence from Dil (a red membrane label), and the third image (c) is a composite of the two