A dichroic filter or thin-film 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. (See dichroism for the etymology of the term.)
Used before a light source, a dichroic filter produces light that is perceived by humans to be highly saturated (intense) in color. Although costly, such filters are popular in architectural [1] and theatrical applications.
Used behind a light source, dichroic reflectors commonly reflect visible light forward while allowing the invisible infrared light (radiated heat) to pass out of the rear of the fixture, resulting in a beam of light that is "cooler". 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.
Dichroic filters use the principle of 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 an oil film, alternating layers of optical coatings with different refractive indexes 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 deposited in a vacuum. By controlling the thickness and number of the layers, the frequency (wavelength) 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 (which attempts to absorb all energy except for that in the passband). (See Fabry-Pérot interferometer for a mathematical description of the effect.)
Where white light is being deliberately separated into various color bands (for example, within a color video projector or color television camera), the similar dichroic prism is used instead.
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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 (saturation)
- 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 (unlike for example, gel filters)
- Filter will not melt or deform except at very high temperatures (many hundreds of degrees Celsius)
- Capable of achieving extremely high laser damage thresholds (dichroics are used for all the mirrors on the world's most powerful laser, the National Ignition Facility)
Disadvantages of dichroic filters
- Higher initial cost (sometimes much higher)
- Glass dichroic filters are more fragile than plastic conventional filters
- Glass dichroic filters are harder to work with than plastic conventional filters
- Can reflect light back into an optical system
- Specific bandpass depends on incidence angle (can be an advantage in some applications where in-situ tuning is desirable)
Other uses of dichroic filters
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 (for example) earrings swing. Another interesting application of dichroic filters is spatial filtering.[2][3]
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.[4]
Further reading
- H. A. Macleod, Thin Film Optical Filters, (Bristol, England; Philadelphia, PA: Institute of Physics Pub., 2000)
- I. Moreno, et al., "Thin-film spatial filters," Optics Letters 30, 914-916 (2005)
See also
- Fabry-Pérot interferometer or Etalon for a mathematical description.
- Dielectric mirror
- Thin-film optics
- Holographic Versatile Disc
- Color gel
- Filter (optics)
References
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