A dichroic filter, thin-film filter, or interference filter is a very accurate
color filter used to selectively pass light of a small range of colors while
reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors
tend to be characterized by the color(s) of light that they reflect, rather than
the color(s) they pass. Dichroic filters can filter light from a
white light source to produce light that is perceived by humans to be highly
saturated in color. Although costly, such filters are popular in
architectural and theatrical applications.
Dichroic reflectors are commonly used behind a light source to reflect visible
light forward while allowing the invisible infrared light to pass out of
the rear of the fixture, resulting in a beam of light that is literally cooler.
Such an arrangement allows a given light to dramatically increase its forward
intensity while allowing the heat generated by the backward-facing part of
the fixture to escape. Many quartz halogen bulbs have an integrated
dichroic reflector for this purpose, being originally designed for use in
slide projectors to avoid melting the slides, but now widely used for interior
home and commercial lighting. This improves whiteness by removing excess
red; however, it poses a serious fire hazard if used in recessed or enclosed
luminaires by allowing infrared radiation into those luminaires. For
these applications non cool beam lamps must be used.
Theory Dichroic filters use the principle of
thin-film interference, and produce colors in the same way as oil films on
water. When light strikes an oil film at an angle, some of the light is reflected
from the top surface of the oil, and some is reflected from the bottom
surface where it is in contact with the water. Because the light reflecting from
the bottom travels a slightly longer path, some light wavelengths are
reinforced by this delay, while others tend to be canceled, producing the
colors seen. In a dichroic mirror or filter, instead
of using an oil film to produce the interference, alternating layers of
optical coatings with different refractive indices are built up upon a
glass substrate. The interfaces between the layers of different refractive index
produce phased reflections, selectively reinforcing certain wavelengths of light
and interfering with other wavelengths. The layers are usually added by vacuum
deposition. By controlling the thickness and number of the layers, the frequency
of the passband of the filter can be tuned and made as wide or narrow as
desired. Because unwanted wavelengths are reflected rather than absorbed,
dichroic filters do not absorb this unwanted energy during operation and so
do not become nearly as hot as the equivalent conventional filter.
Where white light is being deliberately separated into various color bands, the
similar dichroic prism is used instead. For cameras, however it is now more
common to have an absorption filter array to filter individual pixels on a
single CCD array. Applications
Recessed or enclosed luminaires that are unsuitable for use with dichroic
reflector lights can be identified by the IEC 60598 No Cool Beam symbol.
In fluorescence microscopy, dichroic filters are used as beam splitters to
direct illumination of an excitation frequency toward the sample and then at
an analyzer to reject that same excitation frequency but pass a
particular emission frequency. Some LCD projectors use dichroic filters
instead of prisms to split the white light from the lamp into the three
colours before passing it through the three LCD units.
They are used as Laser Harmonic Separators. They separate the various
harmonic components of frequency doubled laser systems by selective spectral
reflection and transmission. Advantages of dichroic filters
Much better filtering characteristics than conventional filters
Ability to easily fabricate a filter to pass any passband frequency and block a
selected amount of the stopband frequencies
Because light in the stopband is reflected rather than absorbed, there is
much less heating of the dichroic filter than with conventional filters
Much longer life than conventional filters; the color is intrinsic in the
construction of the hard microscopic layers and cannot "bleach out" over the
lifetime of the filter Filter will not melt or deform except at
very high temperatures Capable of achieving extremely high
laser damage thresholds Disadvantages of dichroic filters
Higher initial cost Glass dichroic filters are more fragile
than plastic conventional filters Small scratches or patches of skin oil
can badly degrade performance of a dichroic, instead of the graded loss of
performance for a conventional plastic filter
Can reflect light back into an optical system
Specific bandpass depends on incidence angle
Other uses Artistic glass jewelry is occasionally
fabricated to behave as a dichroic filter. Because the wavelength of light
selected by the filter varies with the angle of incidence of the light, such
jewelry often has an iridescent effect, changing color as the earrings swing.
Another interesting application of dichroic filters is spatial filtering.
With a technique licensed from Infitec, Dolby Labs uses dichroic filters for
screening 3D movies. The left lens of the Dolby 3D glasses transmits specific
narrow bands of red, green and blue frequencies, while the right lens
transmits a different set of red, green and blue frequencies. The projector uses
matching filters to display the images meant for the left and right eyes.
Long-pass dichroic filters applied to ordinary lighting can prevent it from
attracting insects. In some cases, such filters can prevent attraction of other
wildlife, reducing adverse environmental impact.
Further reading H. A. Macleod, Thin Film Optical
Filters, I. Moreno, et al., "Thin-film spatial
filters," Optics Letters 30, 914-916 See also
Color gel Dielectric mirror
Fabry–Pérot interferometer or Etalon for a mathematical description
Filter Holographic Versatile Disc
Thin-film optics References