|Obtaining the highest image contrast is probably the most important factor in designing any machine vision system or HMI ( Human Machine Interface ). Increasing system performance by upgrading lighting or lenses can add significant cost to such a system. However, if designers evaluate the spectral characteristics of the objects to be imaged, they may find the optical filters can improve imaging performance with minimal impact to other design elements.There is a range of filter types at ones disposal and they are often defined by the structure of their transmission curves. Optical filters are devices that selectively transmit light of different wavelengths, usually implemented as pane glass or plastic devices in the optical path / as windows in front of electronic displays. They are either dyed or coated to get desired characteristics.
optical filters are defined by their transmission curve
There is a range of filter types at ones disposal and they are often defined by the structure of their transmission curves. Optical filters are devices that selectively transmit light of different wavelengths, usually implemented as pane glass or plastic devices in the optical path / as windows in front of electronic displays. They are either dyed or coated to get desired characteristics.
An optical filter selectively transmits light in a particular range of wavelengths (colors), while blocking the remainder.
The passband may be narrower or wider; the transition of cutoff between maximal and minimal transmission can be sharp or gradual. There are filters with more complex transmission characteristics, for example with two peaks rather than a single band.
Finding the right optical filters
Types of optical filters that are used in industrial applications would be:
Absorptive filters are made from glass, acrylic or polycarbonate to which various inorganic or organic compounds have been added. These compounds absorb some wavelengths of light while transmitting other.
Monochromatic filters made of colored glass, acrylic or polycarbonate only allow a narrow range of wavelengths (a single color) to pass. The spectral discrimination of monochromatic filters is due to the selection of dopants added. Transmission wavelengths and attenuation of a given filter are determined by the selection of dopants and their concentration. They offer a cost-effective solution for improving contrast in applications with relatively broad spectral requirements and their performance is angle-independent, i.e. their optical transmission does not shift, even when used with wide-angle lenses or when tilted with respect to the optical axis of the system. However, colored glass / plastic filters typically feature slow transitions between blocking and transmission wavebands, with transmission curves that are not as steep as coated interference (dichroic) filters:
⦁ Although colored glass bandpass filters are relatively simple in concept and design, they can drastically improve image quality. They efficiently narrow the waveband that is visible by the vision system and work best when used to block our colors on the opposite side of the color wheel:
TTV offers a large variety of colored filters for electronic displays / display windows in cell cast acrylic (LUXACRYL, ANTIFLEX-AC, and POLYCARBONATE)
Dichroic filters (also called “reflective” or “thin film” or “interference” filters) can be made by coating a glass or plastic substrate with a series of optical coatings. Dichroic filters usually reflect the unwanted portion of the light and transmit the remainder.
Dichroic filters use the principle of interference. Wavelength-selective interference filters consist of alternating dielectric layers of high and low indices of refraction deposited on a specific substrate. The substrate surface quality and uniformity establishes the baseline optical quality for the filter, as well as defining wavelength limits based on the transmission characteristics of the substrate. The dielectric layers produce the detailed spectral characteristics of the filter by creating destructive interference between wavelengths of light incident upon the filter that are not in the transmission band, blocking these wavelengths from transmitting through the filter.
The interference effects responsible for the high performance of hard-coated filters are also responsible for their primary limitation. The characteristics of interference depend upon the relationship between light of a given wavelength and the length a wave of light travels through a medium. When a light wave travels through an interference coating at an angle other than the one for which it was designed, the path length through each layer changes, modifying the wavelength selectivity of the filter (Figure a).
Interference filters function based on the distance that the light incident upon the filter travels. (a) At the correct angle of incidence, the light waves incident on the filter destructively interfere, disallowing them from making it through the filter. At a different angle, the destructive interference is not as effective, changing the way the filter will behave. (b) An example of blue shift, shown with a bandpass filter used at a 15 degree angle of incidence. Note not only the shift toward a lower center wavelength, but that the slope is not as steep. The dashed curve is ideal, when the filter is used at a 0 degree angle of incidence.
Interference filters are much more precise than colored glass filters but are usually more expensive and delicate than absorption filters.
the term “infrared filter” can be ambiguous, as it may be applied to filters to pass infrared (blocking other wavelengths / visible light) or to block or reflect infrared wavelengths but pass visible light.
If we refer to IR-Filters in our product documentation we always talk about the first kind, i.e. filters that pass infrared and block visible light (see LUXACRYL-IR)
Longpass (LP) filters are optical interference or colored filters that attenuate shorter wavelengths and transmit (pass) longer wavelengths over the active range of the target spectrum. (see tinted LUXACRYL and ANTIFLEX)
Shortpass (SP) filters are optical interference or colored filters that attenuate longer wavelengths and transmit (pass) shorter wavelengths over the active range of the target spectrum.
Bandpass filters transmit a central wavelength band while blocking shorter and longer wavelengths. The width of such a filter is expressed in the wavelength range it lets through and can be made by combining an LP- and an SP filter.
Polarizers or polarization filters block or transmit light according to its polarization. They are made from anisotropic films or dielectric surfaces. Reflections, especially from water and wet road surfaces, but also the light from a clear blue sky are polarized, the filter will block some of the reflected light allowing for a better view of the object / display behind the filter.
TTV offers one of the world’s largest portfolios of optical filters (in glass, acrylic and polycarbonate) for a full spectral solution for your requirements.
Sources: Schott AG, Advanced Optics “Interference Filters”
Schott AG, Advanced Optics “Optical Filter Glass”
Nicholas Sischka, Edmund Optics “Using optical filters in enhancing image contrast”