Dichroic filter
Halogen_bulb.jpg
A dichroic 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 Dichroic for the etymology of the term.)
Used in front of 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 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". Modern quartz halogen incandescent light bulbs frequently contain an integrated dichroic reflector (see picture).
Dichroic filters operate using the principle of interference. Alternating layers are built up upon a glass substrate, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. The layers are usually deposited using a process carried out 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 don't absorb much energy during operation and so don't become nearly as hot as the equivalent conventional filter (which attempts to absorb all energy except for that in the passband).
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 can't "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 100's 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 (glass can't be cut with scissors!)
Fun with 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.