Interference filter
An interference filter, dichroic filter, or thin-film filter is an
An interference filter consists of multiple thin layers of
Dichroic mirrors and dichroic reflectors are the same type of device, but are characterized by the colors of light that they reflect, rather than the colors they pass. Dielectric mirrors operate on the same principle, but focus exclusively on reflection.
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
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. For cameras, however, it is now more common to have an absorption filter array to filter individual pixels on a single CCD array.
Applications
applications.Dichroic reflectors known as
In
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.
Older DLP projectors typically transmit a white light source through a color wheel which uses dichroic filters to rapidly switch colors sent through the (monochrome) Digital micromirror device. Newer projectors may use laser or LED light sources to directly emit the desired light wavelengths.
They are used as laser harmonic separators. They separate the various harmonic components of frequency doubled laser systems by selective spectral reflection and transmission.
Dichroic filters are also used to create gobos for high-power lighting products. Pictures are made by overlapping up to four colored dichroic filters.
Photographic enlarger
Artistic glass
With a technique licensed from
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.[4]
Advantages
Dichroic filters have a 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). They can be fabricated 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. Dichroics are capable of achieving extremely high laser damage thresholds, and are used for all the mirrors on the world's most powerful laser, the National Ignition Facility.
See also
- Color gel
- Dielectric mirror
- Filter (optics)
- Holographic Versatile Disc
- Thin-film interference
- Thin-film optics
References
- ^ "The Copenhagen Opera House". Archived from the original on 2009-05-10. Retrieved 2009-09-04.
- ^ Optics Letters
- CBS Interactive. Archived from the originalon 2012-02-24. Retrieved 2016-12-08.
- ISSN 1092-194X.
Additional sources
- This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22.
- M. Bass, Handbook of Optics (2nd ed.) pp. 42.89-42.90 (1995)
Further reading
- MacLeod, H. Angus (2010). Thin-Film Optical Filters (4th ed.). Taylor & Francis. ISBN 978-1420073027.
- Moreno, Ivan; Araiza, JJ; Avendano-Alejo, M (2005). "Thin-film spatial filters". Optics Letters. 30 (8): 914–6. PMID 15865397.